Category: Geobiology, palaeontology, and evolution

The Late Devonian saw sufficient extinctions (around 55 % of all genera) for it to rank among the Big Five, but most genera that disappeared were shallow-water marine, particularly rugose and tabulate corals.  Although the Woodleigh impact structure, just north of Perth in Western Australia, has been suggested as a possible culprit, its age is not reliable.  Another possible cause is climatic cooling at low-latitudes, because the extinction was followed by the spread to tropical localities of high-latitude faunas.  The key to supporting a climatic influence is temperature data from areas most affected by the extinctions.  Unusually, a recent study selected phosphatic conodonts (tooth-like microfossils) for oxygen-isotope investigations – carbonate-shelled creatures are the usual choice.  Michael Joachimski and Werner Buggisch, of the University of Erlangen in Germany, found prominent oxygen isotope excursions close the Frasnian-famenian boundary (Joachimski, M.M & Buggisch, W. 2002.  Conodont apatite d¹⁸O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction.  Geology, v. 30, p. 711-714).  Their data are well controlled stratigraphically, because the rapid evolution of conodonts in the Devonian allows fine biostratigraphic division.

The extinction event is bracketed by two espisodes of sea-surface cooling, estimated to involve a drop of 6°C from an otherwise constant ambient temperature of around 32°C.  They coincide with significant positive shifts in d¹³C of seawater, interpreted by the authors as evidence of the burial of much organic carbon debris.  Therein lies a possible cause for the cooling.  Carbon burial would have drawn down atmospheric CO₂ levels.  The extinction does seem to have been a response to temperature stress, tallying with the colonization of low-latitude seas by high-latitude faunas.  However, that still begs the question of why carbon burial underwent two spurts.  Was there an increase in sediment supply to the oceans that might augment burial rates, or are the positive carbon-isotope excursions reflections of the extinctions themselves?  The second still leaves open the possibility that the undoubted cooling events may have had other causes, such as an increase in stratospheric aerosols, resulting either from major explosive volcanism or perhaps impacts that are yet to be found.

Proterozoic

Accepted biogeochemical wisdom suggests that about 2000 Ma ago, the terrestrial environment changed from one in which oxygen was a rare free element to an increasingly oxygenated world.  One line of support for this involves the first appearances around that time of redbeds and lateritic palaeosols, that signify a surge in the O₂ content of the atmosphere.  The other pointer is the disappearance of banded iron formations (BIFs), suggesting that soluble iron-2 was no longer available in the oceans due to its oxidation near its main source at mid-ocean ridges. The first unambiguous microfossils of eukaryotes, which need oxygen for their metabolism, also appeared some two billion years ago.

There is, however, a different view; that there was a transition between the anoxic world of the Archaean and Early Palaeoproterozoic and that marked by pervasion of atmosphere and hydrosphere by oxygen.  It stems from studies of sulphur isotopes in Proterozoic marine sediments by Donald Canfield of Odense University Denmark (Canfield, D.E., 1998.  A new model for Proterozoic ocean chemistry.  Nature, v. 396, p. 450-453).  Canfield found evidence for steadily increasing sulphate ions in seawater from 2300 Ma, which he suggested would have led to increasing production of hydrogen sulphide in the deep oceans by sulphate-reducing bacteria.  He proposed that it was combination with deep-ocean sulphide ions that shut off the supply of soluble iron-2, essential for the production of shallow-water BIFs.  Today, sulphide precipitation is restricted to hydrothermal vents and most iron is removed by combination with oxygen in sediments on the main ocean floors.  In short, Canfield proposed a transitional ocean akin to the Black Sea, with an oxic near-surface zone but anoxic at depth.  Not only iron would have been removed in sedimentary sulphides, but many other metals, leading to their depletion in seawater.  Ariel Anbar of the University of Rochester and Andrew Knoll of Harvard examine the biological repercussions of this transitional ocean (Anbar, A.D. & Knoll, A.H. 2002.  Proterozoic ocean chemistry and evolution: a bioinorganic bridge?  Science, v. 297, p. 1137-1142).

Iron and molybdenum are crucial elements for eukaryotes, albeit only in small quantities, because they are central to the enzymes that fix nitrogen.  Insufficient quantities would put early eukaryotes at an evolutionary disadvantage to prokaryote life.  Moreover it would reduce ocean productivity.  This, they propose, can help explain the lack of evolution among eukaryotes until the late Proterozoic.  The carbon isotope record of seawater (derived from limestones) shows a strange pattern that supports a period of biological stasis from 2000 to about 1200 Ma.  From the end of the Archaean until 2 billion years ago, there are huge fluctuations (to highly positive and negative values) in the proportion of heavy ¹³C, and so too in the Neoproterozoic.  The period in between shows no significant carbon-isotope fluctuation, d¹³C remaining at around zero, which Anbar and Knoll attribute to very low biological productivity.  In their model, it was the release of massive amounts of metals by continental erosion during the “Snowball Earth” glacial periods of the Neoproterozoic that was able to kick start life, especially that of the eukaryotes.  Emergence of the efficient, multicelled algal photosynthesizers drove up oxygen levels, eventually to oxygenate the deep oceans.

A cautionary note needs to be thrown in, however, especially when using analogies with the modern Black Sea (see Analogue of Archaean carbon cycle in Black Sea reefs).  Biogenic carbonates on the Black Sea bed show huge negative excursions in their d¹³C, because organisms that formed them metabolized methane, thereby incorporating methane’s strong depletion in heavy carbon.  As well as there being little direct evidence for Anwar and Knoll’s idea, the methane part of the carbon cycle needs to be factored into interpretations of the carbon-isotope record.

See: Kerr, R.A. 2002.  Could poor nutrition have held life back?  Science, v. 297, p. 1104-1105.

Isotopic evidence for early life may be from metamorphic processes

Controversy has surrounded reports of carbon-isotope evidence from the oldest recognisable sedimentary rocks that can be interpreted as signs of life 3800 Ma ago.  The problem is that the data came from carbon trapped in resistant minerals, such as apatite, in the metamorphosed Isua supracrustal rocks of west Greenland.  A detailed study of carbon in various forms in the Isua metasediments (van Zullen, M.A. et al. 2002.  Reassessing the evidence for the earliest traces of life.  Nature, v. 418, p. 627-630) strongly suggests that the isotopic evidence for life is flawed.  It seems likely that both graphite and carbonates in the Isua rocks originated by chemical reactions that took place during metamorphism; they are probably metasomatic in origin.  The wide range of d¹³C values found in both graphites and carbonates could have formed by isotopic exchange between graphite and carbonate during metamorphism.  Graphite inclusions in apatite, the source of carbon isotopes claimed to reflect the earliest biological activity, are petrographically no different from inclusions in other minerals.  Indeed, the sample originally used to suggest the isotopic influence of early life is of metasomatic origin.

All is not lost, however, for graphite that is highly depleted in heavy carbon-13 (a sign, albeit ambiguous, for organic processes) also occurs in turbidites that show graded bedding.  These rocks show no petrographic signs of metasomatism, and may contain signs of life.  Ominously, the US, Norwegian and Estonian co-workers, having looked in detail at carbon found in low concentration within BIFs and cherts from Isua, conclude that at least some is recent organic matter that groundwater flow has carried into the rocks.

Whatever happened at the Triassic-Jurassic boundary (around 200 Ma ago), the palaeontological shifts then coincided with eruptions of flood basalts of the Central Atlantic Province and the start of Atlantic opening (see And now, the Tr-J boundary, Earth Pages May 2002).  Although questioned as a mass extinction event, the boundary contains extremely high proportions of fern spores, that may signify the land being cloaked by rapidly spreading ferns after it had been wiped clean of other vegetation.  New evidence suggesting the influence of an impact at the time emerges from a geochemical study of the fern-rich boundary layer (Olsen, P.E. and 9 others 2002.  Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary.  Science, v. 296, p. 1305-1307), which revealed anomalously high levels of iridium.  High iridium is only one pointer to possible extraterrestrial influences, and the clinching factor of shocked mineral grains has yet to be shown convincingly.

The novel feature of the paper by Paul Olsen of the Lamont-Doherty Earth Observatory and colleagues from the USA, Canada, Italy and Austria is how they used trace fossils to reach a remarkable conclusion.  They combed eastern US terrestrial sediments either side of the boundary for reptilian foot prints.  They tracked time using evidence for climate change paced by Milankovich cycles.  Their records of 10 thousand sets of tracks show a decline in non-dinosaur footprints, and a jump in the proportion left by dinosaurs from 20 to 50% of the total, as the boundary is crossed.  Those of some Triassic reptiles that had survived for 20 Ma end abruptly at the boundary.  It seems that, whatever the boundary event was, early dinosaurs were able to adapt to change better than evolutionarily more primitive reptiles, so that they could speciate rapidly when their Triassic companions bit the dust.  Dinosaur evolution seems to have been similar to that of the mammalian adaptive radiation that followed the K-T extinction event. 

Gigantic claims for “geogenomics”

Fossils and their stratigraphic ages no longer offer the only clues to biological evolution, now that is possible to judge the degree of relatedness between living organisms from sequences of genes and proteins that their cells contain.  The molecularly inferred family trees of modern animals, plants and micro-organisms help scientists to visualize the relative antiquities of the sharing of a common ancestor by different pairs of a living group.  By assuming constant rates for genetic mutation and protein evolution, some palaeobiologists have asserted that they are able to assign absolute ages to evolutionary divergences.  If that were so, then it would be possible to correlate evolutionary milestones with transformations brought on by geological and climatic upheavals, and also with other past changes in the biosphere.  Good examples would be linking fossil and genetic changes in ruminant mammals to the rise of grasses, or the rise and divergence of corals following the end-Permian mass extinction.  The inter-linkage between palaeontology and genomics is in its infancy.  That it promises a great deal by way of insights, as well as possible bloomers, is nicely brought out by a recent review (Benner, S.A. et al. 2002.  Planetary biology – paleontological, geological and molecular histories of life.  Science, v.  296, p. 864-868).  Whether charting the “planetary proteome” will become “a civilization-wide enterprise”, as Steven Benner and his colleagues predict, is something that I would not care to comment on during the 2002 World Cup.  As Bill Shankly once observed, some things are far more important than matters of life and death.

Too much iron, too little phosphorus delayed an oxygen-rich atmosphere

The age of the earliest blue-green bacteria hinges on the imagination of some palaeobiologists and how well they can focus a microscope (Doubt cast on earliest bacterial fossils, Earth Pages April 2002).  Without doubt, it was blue-greens that first began breaking the chemical equilibrium of water to release free oxygen to the environment, yet it was some 2½  billion years after the Earth had formed that atmospheric oxygen had a tangible effect on the Earth’s bare surface.  In rocks around 2.2 to 2.0 Ga old geologists find the first evidence for that in soils that are rich in oxidized Fe-3.  For iron to lose an electron and change from soluble Fe-2 to Fe-3, whose oxides and hydroxides are highly insoluble, demands the abundant presence of an electron acceptor, or oxidizing agent.  The most likely of these in the atmosphere and hydrosphere is oxygen.  However, there are sedimentary rocks that form vast repositories of Fe-3 and oxygen that predate the first well-accepted oxygen-rich atmosphere.  They are known as banded iron formations or BIFs, whose minuscule layering seems to signify that they formed as precipitates from water, when dissolved Fe-2 met a source of oxygen to produce hematite – Fe₂O₃ – and goethite – Fe(OH)₃  BIFs signify deep ocean water devoid of oxygen, to enable soluble Fe-2 to circulate abundantly, yet a sizeable supply of oxygen where they were precipitated. Since only organic photosynthesis is capable of breaking the powerful bond in water, some kind of photosynthetic bacteria are implicated in the formation of BIFs.  Whether or not palaeobiologists and geochemists can demonstrate evidence for the first appearance of such bacteria, BIFs more or less prove their existence, in the absence of any other plausible means of formation.

Until recently, the huge delay in the Earth’s surface environments becoming oxygenated has been ascribed to the mopping up of any biogenic oxygen by its reaction with a vast excess of dissolved Fe-2.  However, once blue-green bacteria evolved photosynthesis, their chemical trick of splitting water molecules to provide hydrogen for processes at the cell level should have meant that they would have spread like wildfire across the ocean surface.  In that respect they are unique among bacteria, most of which exploit very narrow ecological niches.  Oxygen should have quickly come to dominate both oceans and atmosphere.  That is, unless there was some check on the living ocean biomass.  It turns out that BIFs may contain the answer, for they are rich in phosphates, adsorbed onto the surfaces of their iron minerals (Bjerrum, C.J. and Canfield, D.E. 2002.  Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides.  Nature, v. 417, p. 159-162).  Phosphorus is vital in any organism, being an essential component of nucleic acids and phospoholipids.  By working out the partition coefficient between water and iron oxide, and estimating the production rate of BIFs before 1.8 Ga when their production ceased, Bjerrum and Canfield conclude that phosphorus was an order of magnitude less abundant in sea water until then.  Such a deficiency in a vital nutrient would have limited the scope of blue-greens, and the rate at which they produced oxygen. 

Just why the Fe-P checks and balances on oxygen production collapsed around 2.2 to1.8 Ga is something of a mystery.  One possibility is that the iron concentration in sea water fell, perhaps as sea-floor spreading waned from its high early rates; basalt magma provides the main input of iron through ocean-floor hydrothermal activity.  Less production of BIFs would leave more phosphorus in solution, helping greater biological productivity, whose oxygen output would eventually remove soluble iron from sea water.

See also Hayes, J.M. 2002.  A lowdown on oxygen.  Nature, v. 417, p. 127-128.

Despite the downgrading of the raw data for mass extinctions by their stratigraphic weighting (see The “Big Five” become the “Big Three”? Earth Pages, January 2002), which cast doubt on the magnitude of the Triassic-Jurassic boundary event, the Tr-J draws attention because something out of the ordinary did happen then.  In terms of changes in fauna and flora, the boundary is globally recognisable.  The giant Manicouagan crater in Quebec formed almost at the boundary, and the Central Atlantic Magmatic Province (CAMP) had about the right timing too – 202 to 198 Ma ago.  The Camp was the magmatic expression of the first substatially break up of Pangaea.  Carbon isotopes in sediments reflect changes in the inorganic and organic parts of the C-cycle, and it seems that a significant excursion from the norm characterizes the change from Triassic to Jurassic biota at four important sections in western Canada, Hungary, Britain and East Greenland (Hesselbo, S.P. et al., 2002.  Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism?  Geology, v. 30, p. 251-254).

Around the palaeontologically defined boundary, each section records a sharp decrease in the proportion of heavy ¹³C to light ¹²C (d¹³C), followed by a protracted period dominated by isotopically light carbon in the earliest Jurassic.  Such a shift must indicate some kind of global change in the C-cycle.  One explanation is an increase in the amount of CO₂ expelled from the mantle by major volcanic activity, which tallies with the CAMP.  If that was the reason, then d¹³C would be a proxy for all manner of other effects of volcanic activity – acid rain, atmospheric dust as well as volcanic enhancement of the “greenhouse effect”.  However, as several long-running sagas have shown, isotopically light carbon recorded in sediments can also be explained by methane escape from deep sediments, and even by a massive reduction in biological activity that would otherwise sequester light carbon through metabolism.  More confusion arises from the carbon that is isotopically analysed.  That in carbonate sediments records the isotopic composition of carbon dissolved in seawater, while that found as carbon in preserved organic matter tells a different story.  The Tr-J data are from bulk organic carbon in reduced sediments, and relates to the reservoir of carbon on which cell metabolism has drawn – CO₂ dissolved in seawater, and that in the air for marine and terrestrial life respectively, both of which are in equilibrium.  It looks like the excursion stemmed from an increase in volcanic emissions to the atmosphere, and the CAMP.  However, the study by Stephen Hesselbo and colleagues from the Universities of Oxford and Copenhagen, and the Geological Survey of Denmark and Greenland, reveals a glitch.  The appearance of the ammonite genus Psiloceras has generally been taken to mark the start of the Jurassic.  In Canada, such fossils occur just after the first major shift in carbon isotopes, whereas in Britain Psiloceras appears considerably later.  One takes ones’ choice: either ammonite species appeared synchronously everywhere (assuming that they were pelagic ocean-crossers like modern Nautilus), or signs of change in the C-cycle are truly global.  Because of very rapid mixing of air and water, on geological timescales, the latter is like to be true and faunal zones are not so reliable as stratigraphers believed over the last century.  It seems that as well as tying whatever happened at the Tr-J boundary to massive volcanism, the study marks a turn from traditional palaeontology to geochemical markers as the “golden spikes” in stratigraphy.

There are other means of linking changes in the pace of volcanism and the surface environment, that emerge by careful choice of geochemical proxy data.  A long used, but imprecise approach depends on the much slower rate at which radiogenic ⁸⁷Sr builds up in the mantle than in the continental crust, because of the much lower rubidium content of the mantle.  Oceanic strontium isotopes, measured for the past by analysing marine carbonates, reflect the derivation of strontium by erosion and weathering of continental crust, and addition of strontium to sea water by its hydrothermal interaction with newly emerged volcanic rocks at constructive margins or on oceanic plateaux (submarine flood basalts).  Since the ⁸⁷Sr/⁸⁶Sr ratio of seawater is a commonly used proxy for varying rates of continental weathering, identifying signs of massive increases in submarine volcanism is only possible when carbonates reveal extremely low values of the ratio.  The mantle is enormously richer than continental crust in elements likely to have entered the core preferentially, such as gold and especially the platinum group elements.  Partial melting allows unusually high amounts of such elements to enter basalt magmas, and the lavas and ejecta that enter the surface environment.  Potentially, the environmental abundance of such normally exceeding rare elements is a reliable proxy for major magmatic events.  Osmium isotopes are particularly promising, even in tiny concentrations, because two (¹⁸⁷Os and ¹⁸⁸Os) are daughters of the decay of unstable rhenium isotopes.  The details of their use is complex.  As well as the exceedingly light carbon isotopes in sediments at and around the Tr-J boundary, marine shales also reveal a sudden increase in osmium abundance, the ¹⁸⁷Os/¹⁸⁸Os ratio and the abundance of rhenium (Cohen, A.S. and Coe, A.L. 2002.  New geochemical evidence for the onset of volcanism in the Central Atlantic Province and environmental change at the Triassic-Jurassic boundary.  Geology, v. 30, p. 267-270).  The most likely source for globally distributed anomalies of this kind are the volcanic rocks associated with the CAMP, and their reaction with both rainfall and hydrothermal fluids.  However, the data do not rule out an extraterrestrial influence by a major impact.  That the Tr-J boundary is associated with both the CAMP and the Manicougan impact seems likely to vex geochemists hoping to tie things down to any single trigger.

It’s a measure of the resistance to events controlled by processes outside of Earthly ones that evidence in support of an impact cause for mass extinctions has assumed monumental dimensions.  The iridium anomalies at the K-T boundary, found by Alvarez and Son in 1980, were never enough for a great many palaeontologists.  Nor, for that matter, were the co-occurrences of glass microspherules, shocked quartz grains and soot, discovered by later investigators at 30 to 100 sites worldwide.  Even the remains of the 180 km-wide Chixculub impact crater that formed at the same time as the extinction event, off Yucatan in the Mexican Gulf, was insufficient for the most intransigent sceptics.   That the sooty material contained massive carbon molecules in forms akin to Buckminster Fuller’s geodesic dome, and moreover those fullerenes contained trapped noble gases in proportions that could never have been present on Earth, formed the smoking siege gun for most sensible scientists.  The fullerenes contain helium, neon and argon with isotopic proportions comparable with those in carbonaceous chondrites and interplanetary dust, probably created by processes in a supernova that preceded accretion of the solar nebula.  The hypothesis that such odd materials were delivered to the K-T boundary layer by an extraterrestrial object was amply confirmed by Luann Becker’s discovery that carbonaceous chondrites, never affected by extreme events since they formed, also contain fullerenes (Becker, L. 2002.  Repeated blows.  Scientific American, v. 286(3), p. 62-69).  The latest occurrence of such convincing evidence for impact control of mass extinction comes from Permian-Triassic boundary deposits in China, Japan and Antarctica, that coincide with the most severe disruption of eukaryote life – around 90% of marine and continental families failed to survive it (see Land vertebrates snuffed at the end of the Permian in February 2002 Earth Pages).

It now seems that palaeontologists and a great many others, including creationists who envisage some kind of design within the fossil record, will be compelled to face up to an unearthly influence over the shaping of life on our planet.  There are many impact structures that are candidates for having affecting the biosphere from the Mesoproterozoic onwards, yet no pattern to their timing and energy of formation.  Such is the complexity of gravitational fluctuations that fling asteroids and comets into Earth-crossing orbits, that aside from the inevitability that, given time, they will strike with devastating consequences, they are essentially random events.  Our species is a late development from a vast concatenation of events, both from outside and within the Earth system, that spanned the entire 4.5 billion-year physical evolution of our home world.  No-one has yet turned statistics to estimate the likelihood of such chance occurrences being repeated, with one outcome being conscious beings.  If that were possible, then for the seekers of extraterrestrial intelligence, it might well be as welcome as a Semtex suppository on a wide-bodied jet!

Dinosaur digest

Having suffered vivid nightmares about dinosaurs when a kid – I did not even dare watch Jurassic Park alone as an adult – it comes as a huge relief to learn that the scariest of all monsters, T. rex, was about as agile as I am.  Indeed, it seems highly likely that you or I could outrun one.  The reasoning behind this welcome news (Hutchinson, J.R and Garcia, M. 2002.  Tyrannosaurus was not a fast runner.  Nature, v. 415, p. 1018-1021) stems from scaling up the sprinting powers of chickens (a chicken is surprisingly fast!) to the estimated 6 tonne weight of an adult T. rex.  The analysis involves two factors.  First, muscle proteins have the same capacity for powering movement, and the total power of musculature depends on its cross-sectional area, but while body mass and volume grows, this area and potential power falls behind.  Secondly, the bearing capacity of bone decreases with size too, because this also depends on area rather than volume.  Hutchinson and Garcia’s scaling hens to 6 tonnes, and calculating the necessary mass of leg muscle to propel them in their fearsome dashes to grab a tidbit (you or me), resulted in the absurd vision of a creature with 86% of its body mass in its legs.  Tyrannosaur modelling from their skeletons falls a very long way short of that, and they would be hard pressed to clock much more than 5 ms^-1, which I think I could manage quite easily, for a short while.  That they would ever break into more than a fast walk is unlikely, for the second factor poses a limit.  One wrong pounce would be curtains, for they would break a leg.  Two possible life styles seem to emerge from the analysis.  They may have subsisted on carrion.  Alternatively, the far bigger herbivorous dinosaurs would have been even more stately, for the same mechanical reasons, which generates the absurd vision of large carnivorous dinosaurs ambling down their prey.

See also:  Hecht, J. 2002.  T. rex was a lumbering old slow coach.  New Scientist, 2 March 2002, p. 6;  Biewener, A.A. 2002.  Walking with tyrannosaurs.  Nature, v. 415, p. 971-973.

That dinosaurs could survive high-latitude winters, in near total darkness, if not glacial conditions, was first suspected in 1960 when their footprints turned up in Spitzbergen.  Since then, palaeontologists have found fossils of a wide variety of dinosaurs in areas that would have been near-polar during the Jurassic and Cretaceous Periods (Rich, T.H. et al. 2002.  Polar dinosaurs.  Science, v.  295, p. 979-980).  Surely, these dinosaurs must have been warm-blooded, as their containing sediments sometimes show signs of the effects of permafrost.  There are signs in some of the fossils for heightened visual powers too.  In the case of Australian faunas, it seems certain that the abundant dinosaurs there did not migrate to high latitudes in summer, because seaways blocked passage to lower latitudes.

Extremophiles and possibilities for extraterrestrial life

Bacteria can survive extremes of temperature (-10 to 110°C) and chemistry, and the biosphere extends to crustal depths in excess of 2 km, as shown by thriving communities in deep wells.  So far as biologists are aware, temperature forms the limit to life’s range, because of the instability of crucial molecules and of course the boiling point of water.  Since temperature increases with depth in the Earth, due to its self-heating by radioactive decay, the biosphere has a depth limit too, depending on the geothermal gradient.  However, recent experiments on two common bacteria show that life can survive at extremely high pressures (Sharma, A. et al. 2002.  Microbial activity at gigapascal pressures.  Science, v. 295, p. 1514-1516).  By compressing bacterial films on ice in diamond anvil cells, a team from the Carnegie Institute in Washington, DC have shown that simple life can survive pressure as high as 1.6 Gpa, that is equivalent to crustal depths of 50 km or an ocean bed160 km below the surface.  Because subduction takes cold lithosphere downwards, and the associated geothermal gradient is low in such environments, the deepest biosphere may be below volcanic arcs.  However, the most significant implication of the experiments is that probing the icy crusts of Europa, Ganymede or Callisto (and liquid water that might be present at great depths there) and the Martian ice caps, conceivably could reveal living organisms, if life ever evolved on these bodies.  Whereas this possibility encourages various plans for such exploration, what the experiments did not show was replication by the bacteria, and that is central to any living organism.

Without doubt, the mass extinction at the Permian-Triassic (P-T) boundary was the most important biological event in the history of Phanerozoic evolution.  Around 80-90% of families disappeared, and perhaps more than 50% of species diversity.  But, the evidence stems largely from marine records.  Marine organisms went down at a hasty rate, as evidenced by the superb boundary sequence in China.  However, such is the inconsistency of preservation on land that matching evidence is sparse from the terrestrial realm.  The best chance of examining the response of land animals to whatever wrought such havoc at sea lies in the Karoo sediments of southern Africa.  Roger Smith of the South African Museum and Peter Ward of the University of Washington have combed the mainly fluvial sediments for evidence (Smith, R.M.H. and Ward, P.D.  2001.  Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa.  Geology, v. 29, p. 1147-1150).  Rather than supporting the general view that terrestrial P-T extinctions took a few million years, they have been able to show that Permian vertebrates disappeared abruptly, to be replaced by a very different fauna equally suddenly in the lowermost Triassic.  Only one genus (Lystrosaurus) spans the boundary, and the boundary itself contains no evidence of life.  Calculations based on estimates of the rate of sedimentation point to around 50 thousand years for the extinction event, about the same as that affecting marine organisms.  Interestingly, the event sharply separates very different sediments, that Smith and Ward interpret as products of perennially wet Permian flood plains and those experiencing ephemeral flow in the Triassic (see End-Permian devastation of land plants in Earth Pages October 2000).  Whatever its cause, the stresses placed on land vertebrates seem to have included the sudden onset of aridity.

Mesozoic fossil hunting in Madagascar

Most papers on palaeontology report the details of years of research on what the fossil hunters have found, with mentioning the months of patient searching.  John Flynn and André Wyss have provided an insight into the tribulations of palaeontological field work in difficult terrains, as well as a broad account of the context of their finds (Flynn, J.S. and Wyss, A.R. 2002.  Madagascar’s Mesozoic secrets.  Scientific American, v. 286, February 2002, p. 42-51).  Madagascar lingered at the heart of the Gondwana supercontinent until it finally began to split into drifting segments during the early-Triassic.  It lay on the eastern flank of an evolving rift basin that filled with mainly terrestrial sediments until the late-Jurassic.  This particular basin remained uninterrupted by volcanism or erosion, and so is a repository for organic remains trapped in a continuous sedimentary sequence.  This period in geological history, particularly the Triassic, spans the emergence and development of both the dinosaurs and primitive mammals.  The wealth of vertebrate fossils that geologists are beginning to unearth suggests that Madagascar may well become the site where the mysterious origins of both are resolved.

The simplest living ecosystem

Hugely complex as life is, at the cell level it has a profound simplicity, at least as regards its fundamental chemistry.  Cell metabolism receives its power from the transfer of electrons from a high to a low energy level.  High-energy electrons stem from chemically active molecules, atoms or ions able to release them; electron donors or reducing agents.  The metabolic path ends in oxidizing agents accepting these electrons.  This process of donating and accepting electrons takes the form, in most cell types, of “pumping” hydrogen ions, or protons back and forth across the cell wall to create an electrochemical gradient that is continually charged and discharged.  Biochemistry reflects this by the ADP-ATP cycle at life’s core, in many different versions.

The simplest provision of electrons is by hydrogen, and arguably a supply of hydrogen gas is a highly likely precondition for the origin of life.  Surprisingly, hydrogen is generated by many geological reactions, although little survives some form of oxidation for long.  In a few places hydrogen gas escapes abundantly, as in the weathering of ultramafic rocks by groundwater.  The essential process is the breakdown of iron and magnesium silicates to various kinds of clay, by the interaction of hot water with fresh igneous rocks.  Geochemists and microbiologists from the USA  analysed such a hydrothermal system 200 m beneath a volcanic area in Idaho, and found a thriving and diverse ecosystem dominated by simple organisms that do depend on hydrogen (Chapelle, F.H. et al. 2002.  A hydrogen-based subsurface microbial community dominated by methanogens.  Nature, v. 415, p. 312-315).  More than 90% of the organisms are methane-producing Archaea, which reduce carbon dioxide to methane, using hydrogen.  No other hot-spring system comes close to this probably highly primitive community.  It is a handy analogue for the kind of ecology that may have developed if life has arisen deep beneath the icy surface of Europa – a target for future NASA missions.

Incidentally, the exploitation of electron and proton transfer that underpins cell metabolism potentially forms a source of electrical power.  Younger readers may have experimented with using fruit as the basis of a simple galvanic battery, thereby exploiting low pH conditions.  Investigation of the potential of bacteria for direct electricity generation recently made a breakthrough (Bond, D.R. et al. 2002.  Electrode-reducing microorganisms that harvest energy from marine sediments.  Science, v. 295, p. 483-485).  A member of the family Geobacteraceae (Desulfuramonas acetoxidans)  has been found to readily produce excess electrons as it metabolises organic material in oxygen-free muds.   Daniel Bond and co-workers from the University of Massachusetts and the US Naval Research Laboratory introduced graphite electrodes into airless fish-tank muds and the upper oxygenated sediments.  Even with such a crude experiment, sufficient current flowed to power a small calculator.  Moreover, D. acetoxidans colonised the electrodes within a matter of days, showing that they were directly involved in the oxidation-reduction system at the root of such a fuel cell.  As well as raising the possibility of powering submarine monitoring devices using bioelectricity, such geobacteria are able to metabolise a range of common organic pollutants.  Marine organic sediments are virtually limitless, so it is not inconceivable that the process may result in yet another renewable power source, albeit difficult to convert to high-power supplies, with the blessing of pollution control as a sideline.

That mass extinctions mark several fundamental boundaries in the stratigraphic column (late-Ordovician, late-Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary) seemed to have become a well established feature of geology, thanks to the vast compilation and analysis of marine and terrestrial  organisms by the late John Sepkoski and David Raup.  However, it is very much a numerological exercise matching extinctions, new arrivals and their precise timing.  Although not exactly “lies, damned lies and statistics”, analysing the fossil record depends on both data and algorithms.  A new crunching of Sepkoski and Raup’s data, by Richard Bambach and Andrew Knoll of Harvard University, casts doubt on two of the formerly outstanding extinctions.  They see a distinction between true mass extinctions – lots of genera popping their collective corks very quickly, and mass depletions, when a more general rate of extinction fails to be matched by newly evolved taxa.  According to Bambach and Kroll, the late-Devonian and end-Triassic events fall in the latter category, leaving only three “big ones”.

Palaeontologists seem quite relaxed about these demotions and an earlier degradation of the Cenomanian-Turonian extinction, but one wonders about those who have beavered away at possible causes.  Impactophiles have congregated lately on both boundaries, studying signs of correlation with large cratering events (Woodleigh in Western Australia and Manicouagan in Canada, respectively).  Because the fossil record has a great deal to do with where collectors have been (and that has usually been close to their home bases in Europe and North America), it is anthropogenically biased.  So far, new collections from further afield have failed to numerically overcome this skew, but the demise of the late-Devonian event stems largely from recent work in China.

Personally, I fail to see the distinction.  The failure of evolution to repopulate niches abandoned by extinct genera seems equally as odd as spikes in the rate of extinctions.  However, I have always been worried that the humble graptolite’s disappearance at the end of the Silurian hasn’t been recognised as a sign of dreadful times.  These meek and co-operative creatures spread far and wide as plankton throughout the Ordovician and Silurian, evolved with an unmatched enthusiasm, and yet failed to inherit the Earth as their meekness should have guaranteed.  Still, few now seem concerned with the vast panoply of graptolitic thecae and stipes.

Source:  Kerr, R.A. 2001.  Paring down the Big Five mass extinctions.  Science, v. 294, p. 2072-2073.  Report on November 2001 Annual Meeting in Boston of the Geological Society of America.

Relating mass extinctions to the effects of impacts by comets or asteroids is now a major industry, and a great number of geologists who sneered at early suggestions of extraterrestrial influences over evolution are finding ever new ways to cook and eat their headgear.  Oddly, however, many of those who bore the brunt of such mean-spirited, and somewhat premature scorn still cling to the safe old K-T event.  Soon all the thin K-T boundary material will have been consumed by these cautious, if meticulous scientists.  Thankfully, some have ventured to seek evidence for other catastrophes that came out of the blue. In comparison with the end-Permian extinction, the K-T event is a mere bagatelle.  However, attaching it to an extraterrestrial cause has proved difficult.  It has attracted as many opponents of impact theories as “whizz-bang” aficionados, with much talk of the effects of sea-level changes, volcanism, ocean anoxia and climate shift.  They may be in for a big surprise.

The Permian-Triassic boundary in Meishan, China is at first sight a nondescript sequence of shallow marine strata, albeit complete.  The last occurrence of Permian marine genera there, with typical signs of mass extinction, coincides with a 20-fold increase in nickel concentrations.  Closer examination reveals other brusque geochemical and mineralogical anomalies, including magnetic grains of iron-silicon-nickel alloy, but no iridium anomaly (the popular target for detecting asteroidal impact horizons) or examples of shocked quartz and feldspar (Kaiho, K. et al. 2001.  End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle.  Geology, v. 29, p. 815-818).  Most significant is a sudden drop in ³⁴S due to a large increase in the amount of isotopically light sulphur in the environment.  Kaiho et al. attribute this to vast emission of sulphur from the mantle.  A coincident fall in the ⁸⁷Sr/⁸⁶Sr ratio could also result from entry into the oceans of lots of mantle-derived strontium.

The P-Tr boundary also coincides with the time of eruption of the largest continental flood-basalt province, the Siberian Traps.  No doubt other scientists will seek to account for the chemical anomalies at Meishan as distant effects of the Siberian volcanism alone, as they have for the K-T boundary anomalies because of their coincidence with Deccan volcanism.  The authors prefer to suggest a causal link between impact and massive volcanism.

Surviving the Archaean with a UV jacket

Earth’s dominance, for at least the last half billion years or so, by oxygen-dependent and oxygen producing life forms stems from the evolution of photosynthetic organisms whose cell metabolism involves breaking the strong bonds in water molecules with solar energy.  Chemo-autotrophic life that exploits other energy sources has been consigned to niches that are very much narrower than they were at the biosphere’s outset.  The earliest primary producers using oxygenic photosynthesis were the cyanobacteria – arguably the predecessors of modern plants’ chloroplasts, in Lyn Margulis’ endosymbiotic model for the origin if the Eucarya.  Carbon isotopes from the early Archaean do suggest their presence close to the start of recordable geological history, and at around 3.5 Ga the first known stromatolites were almost certainly secreted by blue-green bacteria (See Carbonates and biofilms, Earth Pages August 2001).

To thrive and colonise ocean surface waters, the shallows and perhaps even the continental surface – their water-splitting, solar powered metabolism opened up those opportunities – cyanobacteria, more than any other prokaryotes, had to resist massive damage from ultraviolet radiation.  Lack of atmospheric oxygen, and therefore ozone, left Earth’s surface with no shield to the most biologically damaging, short-wave UV.  Despite the fact that modern “blue-greens” can survive climatic extremes from the frigidity of Antarctica’s Dry Valleys to superheated water in hot springs, as regards UV damage they are wimpish.  This is partly due to its bleaching effect on the light-harvesting pigment on which chlorophyll depends.  Cyanobacteria cells do have some biochemical protection against radiation damage, but it is of no avail when bathed in the “hardest” UV likely to have characterized Archaean surface environments.

A widely held view is that “blue-greens” survived and prospered because of another function common to many single-celled organisms; their tendency to promote nucleation of inorganic compounds outside their cell walls.  Stromatolites themselves are good examples of the production of biofilms, being made of minute laminae of carbonates, whose secretion helps cyanobacteria avoid calcium stress.  In modern hot springs that contain dissolved silica, these organisms often help generate sinters made of silica.  A team from the University of Leeds (Phoenix, V.R. et al.  2001.  Role of biomineralization as an ultraviolet shield: Implications for Archaean life.  Geology, v. 29, p. 823-826) has performed controlled experiments on living cyanobacteria from Icelandic hot springs to check their defences against short-wave UV.  With a biofilm screen (in the experiment they used wafers made from associated iron-silica sinter, as well as colonies with a biofilm) the organisms easily survived and continued to photosynthesize.  Exposed “naked” they succumbed after only a few days exposure.  It seems that traces of iron incorporated in the films dramatically enhance the UV-screening, without reducing photosynthesis.  Archaean iron-rich cherts are massively abundant in banded iron formations, and the first definite remains of cyanobacterial cells come from such silica-rich material.  However, the ubiquitous stromatolites in limestones of early Precambrian times are the main signs of life.  It remains for the UV-screening properties of carbonate biofilms to be assessed.

New phyllum from Chinese Cambrian

Incompleteness of the fossil record is partly a result of the bias towards organisms with hard parts and against soft tissue, during sedimentary processes.  For preservation of soft-bodied animals, together with that of intricate parts of the usual fossils, palaeontologists look to site where preservation is exceptionally good – lagersttätten.  An example is the Solenhöfen Limestone, famous for Archaeopterix.  Mudstones formed under highly reducing conditions, which excluded bacteria that complete oxidize flesh, provide similar opportunities.  Work through the last two decades by Simon Conway Morris of the University of Cambridge has resulted in working and interpretative methods that permit extremely detailed analysis of physiologies, beginning with the most famous lagersttätte, the Middle Cambrian Burgess Shale of British Columbia.  Conway Morris and others unearthed beasts so strange that they had little choice other than to erect new Linnaean Classes and Phylla to classify them.  Equally as important, such sites help fill in the details of early members of those which survive today, including the elusive penis worms.

Conway Morris has been part of a team based at the Northwest University in Xi’an China, which has discovered lagersttätten in the Lower Cambrian, closer in time to the explosive development and radiation of animals at the end of the Precambrian.  Once again, unsuspected novelty has turned up (Shu, D.-G. et al.  2001.  Primitive deuterostomes from the Chenjiang lagersttätte (Lower Cambrian, China).  Nature, v. 414, p. 419-424).  Along with excellent examples of agnathan fish and many familiar soft-bodied animals, the prize in this case are remains that warrant a new, extinct Phyllum, the Vetulicolia.  The organisms are small but complex, with two main body chambers that reveal mouth, innards and gill slits.  The last helps place them within the deuterostomes; an “umbrella” that groups chordates (sea squirts and vertebrates) and echinoderms (they have lost such slits, but are genetically closer to chordates than any other group).  Critical to the evolutionary significance of the vetulicolians is a groove that floors what is interpreted as the anterior part of their alimentary canals.  Such a groove characterizes the pharynx of chordates, where it serves as “gutter” for various glands – the endostyle, also involved with iodine in metabolism.  If the vetulicolian groove is an endostyle, then they are chordates.  However, lacking an axial stiffening rod (notochord of the chordates in general, and vertebral column in vertebrates) they must be primitive.  Occurring with true vertebrates, in the form of jawless fish, the vetulicolians are a relic of some earlier stage in vertebrate evolution.  Shu et al. take the cautious view that they are early deuterstomes from which echinoderms and chordates emerged – close to the fundamental division among animals into deuterostomes and protostomes.

(See also:  Gee, H.  2001.  On being vetulicolian.  Nature, v. 414, p. 407-408)

Popular animations of dinosaurs in Jurassic Park and Walking with Dinosaurs are palaeontologically speaking “state of the art”.  That is, except for the beasts’ noses.  A close observer will have seen Tyrannosaurus and Triceratops with nostrils high on their snouts, and appealing brachiosaurs apparently breathing through the tops of their heads.  Such reconstructions rely on the position of the nasal passages where they enter the skull, and in dinosaurs such bony nostrils are large and complicated.  Traditionally, dinosaur reconstructors have gone for the rear of the cavity for the positions of the fleshy nostrils.

Despite their extinction at the end of the Cretaceous Period, dinosaurs have many living close relatives, such as birds, crocodiles and some primitive lizards.  All of them have fleshy nostrils situated at the front of the bony openings.  For that matter, so do mammals.  For several years Lawrence Witmer of the College of Osteopathic Medicine at Ohio University (Athens) has been pondering on this, even setting up the DinoNose project.  Not only did Witmer apply the principle of parsimony to this intriguing issue, but noted the marks left on skulls by the blood vessels that supply the muscles that enable land vertebrates to snuff the air in many interesting and useful ways.  Such marks appear on dinosaur skulls, towards the forward end of the nasal openings.  The outcome is a fundamental revision of  dinosaur physiognomy (Witmer, L.M. 2001.  Nostril position in dinosaurs and other vertebrates and its significance for nasal function.  Science, v. 293, p. 850-853).  The next logical step is to seek signs that carnivorous dinosaurs did indeed snarl.

Cambrian Explosion:  Shropshire hits the news

As if by magic, nearly all animal phyla suddenly appear in the fossil record around 545 Ma, at the base of the Cambrian period.  The most famous of these are trilobites, a group within the phylum Arthropoda, for enthusiasts of which the Cambrian of Shropshire has long been a happy hunting ground.  Temporary excavations into the Protolenus Limestone of Comley have revealed a somewhat diminutive, though nonetheless startling relative that helps resolve the long-running debate over the origins of animals (Siveter, D.J., Williams, M. and Wlaoszek, D.  2001.  A phosphatocopid crustacean with appendages from the Lower Cambrian.  Science, v. 293, p. 479-481).  Superbly preserved in calcium phosphate, the tiny beast reveals great detail of its body parts, peeping from between a two-valve, spherical carapace.  It is possibly an early ostracod, and certainly a crustacean.  That such an advanced arthropod occurs close to the base of the Cambrian lends support to the view that animal diversification into extant phylla, and some vanished ones too, may have gone on far back into the Neoproterozoic.  The other view is that this radiation was explosive, beginning only 10 Ma or so before the base of the Cambrian.

The “long-fuse” hypothesis for the emergence and diversification of the animals is also supported by differences in the molecular biology of distantly related modern animals.  Assuming that accumulation of genetic change is steady, and can be calibrated by the coexistence of such groups as far back as the Cambrian, the “molecular clock” for animals probably started between 700 to 1500 Ma ago.  The problem, of course, is that only animals with hard parts or which miraculously had soft tissue rendered preservable by mineralization can assist palaeobiologists resolve the issue.  That is unfortunate, as such fossils occur only after about 5 Ma before the start of the Cambrian, and the large ones are exclusively Cambrian or younger.  The “explosion” was the sudden appearance of skeletal material, using calcium compounds such as carbonates or complex phosphorus-bearing material.  Such is the fascination with the detail of phyllogeny, that the trigger for the explosive emergence of hard parts is often overlooked.

See also:  Fortey, R.  2001.  The Cambrian Explosion exploded?  Science, v. 293, p. 438-439.

BBC-2’s Live from Dinosaur Island (4-16 June 2001) brought palaeontology into Britain’s living rooms.  Centred on a frantically excavated series of Jurassic sites on the Isle of Wight, and fronted by the irrepressible Bill “Birdman” Oddie and genuinely excited (and sometime irascible) professional palaeontologists, the series used the now familiar approach of Channel 4’s Time Team, with the added frisson of being unedited and live.  The BBC was in debt to its viewers after the truly dreadful, if visually astonishing, Walking with Dinosaurs, and has repaid them handsomely by showing the bone-people working in their natural habitat.  It should help repopularize geology after a century of our being the brightly coloured anoraks seen dimly in the drizzle.

Dinosaurs are perhaps the main link between the popular imagination and the Earth’s past.  However, leaving them at the level of awesome animals that a comet strike snuffed out 65 Ma ago may enthuse, but does not really educate.  Live from Dinosaur Island began to break the T rex – My Little Pony connection, by also showing how we can recreate the environments that long-dead creatures inhabited, and how they changed.  Climate and life (above), hints that dinosaur breath may even have affected climate during the Mesozoic.

Barely a month passes without dinosaur news.  The latest concerns the rediscovery of the Egyptian site, from which Ernst Stromer von Reichenbach gathered  a rich collection of animal fossils between 1911 and 1936.  Stromer’s collection, housed in the Bayerische Staatssammlung museum in Munich, was destroyed by wartime bombing.  Because Stromer left no clues regarding the precise location of his site, except that it was near the Baharyia Oasis in the Western Desert, it seemed unlikely ever to be found again.  A team from the University of Pennsylvania, let by Josh Smith, more or less tripped over the site by luck, when combing the area for coastal Upper Cretaceous sedimentary outcrops, after Smith’s inspiration by Stromer’s monographs (Smith, J.B. and 7 others 2001.  A giant sauropod from an Upper Cretaceous mangrove deposit in Egypt.  Science, v. 292, p. 1704-1706).  The highlight of their excavations is Paralititan stromeri, a sauropod reckoned to be the second most massive animal that lived, after South America’s Argentinosaurus.  The tidal sediments also yielded a diversity of lesser animals that matches and will certainly transcend Stromer’s destroyed collection.

See also:  Stokstad, E.  2001.  New dig at old trove yields giant sauropod.  Science, v. 292, p. 1623-1624.

Doubts cast on the increase in diversity with time

The late John Sepkoski of the University of Harvard painstakingly spent 20 years trawling the palaeontological literature to build an archive of the duration of every marine fossil known.  Others did similar work for terrestrial fossils, but Sepkoski’s database stands out, head and shoulders, for its comprehensiveness.  It is largely from his work that the record of extinction events took on semi-quantitative form.  Plotted against Phanerozoic time, his counts of genera also seem to show patterns that chart the fluctuations of biodiversity; rapid rise from the Cambrian Explosion to plateau in the mid-Palaeozoic, a decline in the late Palaeozoic and early Mesozoic, and then a post-Jurassic explosion in diversity.  Much speculation has hung on Sepkoski’s empirical data, such as the influence of “modern” evolutionary designs on the number of ecological niches that life can exploit.

Enormously important as Sepkoski’s work was, inevitably it rested on the selective nature of fossil collecting, itself partly determined by the variable quality and quantity of preservation, but also by the limited numbers of active palaeontologists, the manner in which they worked and their selection of sites.  There are gross biases in fossil collections, but how can archivists possibly allow for their influence?  Without a superhuman effort to re-collect more intensively, to plunder every conceivable stratum wherever it crops out and perhaps standardise what is meant by a genus, the only available means is through statistics.  Palaeontologists at the universities of California (Santa Barbara) and Harvard, led by John Alroy and Charles Marshall respectively, are compiling information along more comprehensive lines than did Sepkoski, including the dimension of geographic occurrence as well as duration, in the Palaeobiology Database.  Their first attempts to allow statistically for the welter of biases, published in the 25 May 2001 issue of Proceedings of the National Academy of Sciences, all point in the same direction.  The Cretaceous to Tertiary genera show patterns of change that are little different those for the Silurian to Carboniferous, compared with Sepkoski’s suggestion of explosive diversification in the first and a plateau in the second.  The main problem remains; vast as they are, fossil collections are not truly representative of life in the past.

Source:  Kerr, R.A. 2001.  Putting limits on the diversity of life.  Science, v. 292, p. 1481.

Continually improving resolution of telescopes is now beginning to reveal signs of planetary systems around other stars.  Because their gravitational effects on stellar motion are detectable, the 60 or so known planets in distant stellar systems are all gas-giants, similar to but bigger than Jupiter.  Surprisingly, calculations show that such massive planets are in very different orbits than those in the Solar System.  Their orbits are highly eccentric, and bring them remarkably close to the star, unlike the almost circular orbits in the Solar System.   Yet, if they are mainly gaseous, they must have formed far from the warming influence of their companion star, as did Jupiter, Saturn, Uranus and Neptune.  Somehow, they have been gravitationally perturbed over the billions of years of evolution of the stellar systems.

How, then, did such bodies move inwards?  One possibility is that they exchanged angular momentum with smaller, rocky planets, forcing both into eccentricity.  For the smaller bodies the effect would be more dramatic, potentially either flinging them into interstellar space or into collision with their star.  Spanish and Swiss astronomers using spectroscopes at an observatory on the Canary Islands have discovered a large lithium anomaly in the spectrum of one star with such an aberrant gas giant (Israelian, G.  et al. 2001.  Evidence for planet engulfment by the star HD82943.  Nature, v. 411, p. 163-166).  Because the anomaly is accompanied by greater than usual abundances of many elements heavier than helium, and because lithium is quickly consumed as stars “ignite”, Israelian and colleagues conclude that the star has engulfed an Earth-like planet.

If such processes are common, and theory suggests that it may be, our Solar System could be one of very few in which potentially life-building and sustaining planets had sufficient time to develop a biosphere.  It seems that the more small planets there are between a star and an outer gas-giant, the more likely it is for such perturbations to take place.  The Solar System has only four, and calculations using Jupiter’s mass and orbit point to a minute tendency for such eccentricities to evolve.  Looking on the bright side, at least for those committed to a view of life pervading the cosmos, current observational resolution is only able to detect giant planets in wildly eccentric orbits.  Many planetary could be more stable.

Se also:  Samuel, E.  2001.  Banished forever.  New Scientist, 12 May 2001, p. 15.

Late-Palaeocene red tides?

About 55 Ma ago, in the late-Palaeocene, the carbon-isotope record shows a sudden drop in ¹³C, signifying a sudden release of methane from ocean-floor gas hydrates or clathrates.  That period also reveals evidence of s brief global warming, against the general trend of cooling through the  Tertiary.  Since the discovery of this massive discharge of the “clathrate gun”, palaeontologists have looked for ecological effects in sea-floor sediments.  For them to be significant, it is important that climate-related ecological effects occurred at the same time in widely separated parts of the globe.

Geologists from the Netherlands, Denmark, New Zealand, Austria and Sweden have examined the microfossil record from two late-Palaeocene sequences in Austria and New Zealand, and show such synchronicity (Crouch, E.M. et al.  2001.  Global dinoflagellate event associated with the late Palaeocene thermal maximum.  Geology, v. 29, p. 315-318).  Exactly at the time of the d¹³C dip in both sections, the abundance of cysts of single-celled phytoplankton known as dinoflagellates rose dramatically, only falling when carbon isotopes recovered to usual levels.  The authors link this to exceptionally high surface-water temperature and photosynthetic productivity.  Over the same period, the fossil record shows a mass extinction of benthonic organisms, and noticeable turnover and diversification of plankton and mammals, though not as dramatic as other biological events.

Today, dinoflagellates explode in numbers, along with other phytoplankton, under similar conditions and when nutrients increase in surface waters, to create phenomena known as “red tides”.  Because some species of dinoflagellates produce potent neurotoxins, “red tides” often result in massive death of marine animal life.  The effects linger as such toxins build up in the cells of animals, such as bivalves, which survive the bloom.  The air above such blooms is filled with stinging, choking aerosols, not far different from nerve gas.  Rotting of dead organisms causes oxygen levels in local seawater to drop, further adding to the death tool at deeper levels.  Red tides that result from human input of nutrients in sheltered embayments often sterilize them for long periods.

Although it is impossible to tell if such neurotoxins built up during the late-Palaeocene thermal maximum, that is not an impossibility.  Such biological “warfare” (no-one knows why some dinoflagellates produce the toxins) might explain the biological crisis that accompanied methane release.

A broader view of the Permian-Triassic mass extinction

That the Palaeozoic Era ended in the greatest mass extinction is well know, although why it happened is still a topic of fierce debate.  Part of the problem is that its effects on land and in the oceans emerge from studies of widely separated P-Tr sections, and many of these are extremely thin.  Such condensed sequences are notoriously difficult to resolve in terms of relative and absolute timing, as well as to correlate from place to place.

As with much else, Greenland promises to throw light on the end-Palaeozoic events, thanks to a 700 metre sequence of siliciclastic sediments in East Greenland that spans the Permian-Triassic boundary without a break.  Its most exciting feature is the way in which marine and non-marine sediments interleave with one another.  Geologists from the USA, the Netherlands, Australia and Britain have pieced together the evidence of biological change from a small part of this little described occurrence (Twitchett, R.J. et al.  2001.  Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis.  Geology, v.  29, p. 351-354).

In marine sediments, the Permian biota collapse, together with evidence for disturbance of the sediment structure by burrowing , in a mere 50 cm of the almost 40 metre sequence that the authors analysed.  Over the same interval, pollens of Permian land plants also fall dramatically, but all the pollen types linger through the overlying 15 metres.  Only at a level 25 metres above the biotic collapse do  fully Triassic faunas and floras appear.  From estimates of the rate of sedimentation the marine and terrestrial collapse appears to have taken between 10 and 30 ka.  Oddly, the now well-known fall in ¹³C does not coincide with that in the biota.  The authors visualize two possibilites: that it resulted from the collapse itself, or reflects an external factor that played little or no role.  One interesting scenario that they suggest is that it may indicate a major release of methane by breakdown of gas hydrates (a now increasingly popular mechanism!).

In the manner of Count Dracula’s dogged refusal to shed his mortal coil by hiding from sunlight, is it conceivable for primitive organisms to be immortal by being protected from UV radiation?  That it might be possible emerged from the revival of bacteria trapped in fluid inclusions in Permian rock salt by Russell Vreeland and William Rozenzweig of West Chester University in Pennsylvania (see Earth Pages, November 2000, The undead).  Despite taking stringent precautions to avoid any contamination of their samples by modern bacteria, Vreeland and Rozenzeig’s claim has been fiercely challenged.  It is possible that the dormant bacteria could have entered the salt in much younger solutions permeating the deposit (incidentally one of the most stable tectonically and hydrogeologically – it is the prospective site for burial of US radioactive wastes).

Vreeland’s team  found 4 bacterial strains – all salt-tolerant halobacteria – but have genetically fingerprinted only one so far.  It is related to a modern genus living in the Dead Sea that forms spores.  The minute fluid inclusions from which samples came have insufficient energy and nutrients to have sustained cell growth and division.  The inactivity involved in spore formation, combined with the slowing down of biological processes by dense brines in the inclusions, might just allow immensely long survival for 250 Ma without breakdown of the DNA essential for revivable dormancy.  Hydrogen diffusing into the salt and biological materials could have played a role in maintaining DNA’s integrity.  One snag is that the DNA sequence of the revived bacteria is 99% identical to that of its closest modern relative.  Using the theory of molecular clocks, they should have been different by 5 to 10%.  Yet, says Vreeland, salt deposits continually add to the surface environment, being soluble.  Any dormant bacteria within them would replenish fully living stocks in similar environments to those which formed the salt originally.  Such continual addition might preserve ancient genetics, that would otherwise evolve steadily.

Aside from giving comfort to proponents of life spreading throughout the universe as spores adrift on dust driven in the manner of a solar sail, the results encourage probing of older salt deposits, which go back in almost undisturbed form to the Mesoproterozoic.

(Source:  Knight, J.  The Immortals.  New Scientist, 28 April 2001, p. 36-39).

Most of the sulphide mineralization involved in base-metal ore bodies formed by reaction between metal ions and those of sulphur released by bacteria that reduce sulphate ions in water.  They do that while oxidizing organic matter or hydrogen in their metabolism, under completely anaerobic conditions.  Like other biological processes, sulphide production at the cell level fractionates the isotopes of sulphur so that it becomes possible to chart sulphate-reducing bacteria through time.  Depletion of ³⁴S in sedimentary sulphides relative to that in co-existing sulphates (such as baryte) was previously known with certainty back to 2.7 Ga.  Danish and Australian bio-geochemists have now pushed this particular bacterial metabolism back by 750 Ma (Shen, Y.  et al. 2001.  Isotopic evidence for microbial sulphate reduction in the early Archaean era.  Nature, v. 410, p. 77-81).

The data from the Pilbara Craton of Western Australia helps calibrate the evolutionary bush of the prokaryotes, which is based on comparisons between RNA in different living organisms.  The trouble is, sulphate-reducing species with very primitive genetics and similar lifestyles (hyperthermophilic) occur among both the Bacteria and Archaea.  Shen et al. go for the Bacteria Thermodesulfobacterium as the most likely organism responsible.  Their argument is that the mineralization replaces originally sedimentary gypsum, formed at low temperatures, and probably represents hydrothermal processes in which thermophilic organisms could have thrived.  Bacteria that reduce sulphate ions at low temperatures – gram-positive and purple bacteria – are genetically more advanced than their candidate.

See also:  Slime to the rescue Earth Pages December 2000

“Piltdown” bird

Fragmentary remains of vertebrates in particular are notoriously prone to misguided reconstruction – Gideon Mantell placed the Iguanodon’s thumb on its nose, thereby obscuring evidence for the first hitchhiking dinosaur for many decades.  The forger of fossils has two possible motives – spite in the case of Piltdown Man, or profit.  The skilled forgeries of Silurian trilobites by quarrymen from Dudley in Britain’s West Midlands are now more valuable because they were made for rapacious Victorian antiquaries, than bona fide Calymene specimens.  Missing links sought by professional palaeontologists and archaeo-biologists are in a field of their own.  It has long been suspected that birds evolved from small carnivorous dinosaurs, and the early Cretaceous of China has provided spectacular transitional fossils.  Archaeoraptor was announced as the final missing link in 1999.  Within a year it was denounced as a forgery that combines very skilfully the bones of a primitive bird with those of a non-flying dromeosaurid dinosaur.  How it was assembled has finally been revealed using X-ray tomography, which shows that as many as 5 different specimens were “cut and pasted” together (Rowe, T. et al. 2001.  The Archaeoraptor forgery.  Nature, v. 410, p. 539-40).

Cretaceous water lilies

Readers of Earth Pages will be delighted to learn that fossil flowers of Nymphaeales (water lilies) have been found in the Lower Cretaceous of Portugal.  (Friis, E.M. et al. 2001.  Fossil evidence of water lilies (Nympaeales) in the Early Cretaceous.  Nature, v. 410, p. 357-360).

When modern corals emerged

Fossil corals  fall into three taxonomic groups or Orders: tabulate, rugose and scleractinian.  Only the last group is alive today.  Scleractinian corals have been central to the “carbonate factories” that have drawn down CO₂ from the atmosphere throughout the Mesozoic and Cainozoic Eras to form reef limestones.  They are major regulators of long-term climate fluctuation.  However, there is something very odd about their appearance in the fossil record, as discussed recently by George Stanley and Daphne Fautin (Stanley, G.D. and Fautin, D.G. 2001.  The origins of modern corals.  Science, v. 291, p. 1913-1914).

The rugose and tabulate corals were exclusively Palaeozoic colonial, carbonate-secreting organism.  Their record ends abruptly with the end-Permian mass extinction.  No examples of scleractinian corals have been found in rocks older than Triassic.  The oddity is a 14 Ma gap in known coral fossils in the earliest Triassic.  Scleractinians secrete calcium carbonate as aragonite, whereas rugose corals formed from calcite; an important difference in processes at the cellular level.  It is hard to avoid the conclusion that the ancestors of scleractinians did not secrete carbonate and were entirely soft-bodied taxa during the Palaeozoic Era.  If Permian Rugosa and Tabulata happily secreted carbonate, while proto-Scleractinia did not, there ought to be a biochemical or geochemical explanation for the last taking on a reef building role in Mesozoic times.

Molecular evidence suggests that scleractinian ancestry goes back to the Late Carboniferous, and that there is a complex “lawn” (as opposed to tree or bush) of genetic relationships between modern hard corals and soft-bodied organisms that are closely related.  The puzzle can potentially be resolved if modern corals and their ancestral lines lost and regained skeleton building several times in the Mesozoic and Cainozoic.  Exploring that requires more understanding of how carbonate is secreted at the cell level, and the geochemical conditions in seawater that underpin the need for secretion.

Following the greatest ever mass extinction at the end of the Permian, early Triassic oceans were almost sterile and anoxic.  Global CO₂ levels were high, yet little carbonate was deposited in the marine environment.  That would have increased the amount of calcium and bicarbonate ions in sea water.  Many corals harbour algal symbionts that are involved in calcification.  As calcium carbonate saturation drops so too does carbonate secretion, and vice versa.  Calcium is a two-edged sword in cell metabolism.  On the one hand it is vital in “information” transfer, yet above a threshold it combines with CO₂ to form crystalline carbonate within the cell wall, that spells cell death.  In Palaeozoic oceans rugose and tabulate corals, as well as a host of other carbonate secreting animals, would have buffered calcium concentrations below levels tolerable by other, soft-bodied animals.  Their sudden demise 251 Ma ago, along with most everything else, would have left calcium to build up in the early Triassic “Strangelove” ocean.  Survivors of the holocaust would have had a fierce task coping with potential calcium toxicity, and the scleractinians may well have adopted calcification as a survival mechanism.  Thereafter, oceans restocked with reef building organisms would have had tolerable calcium concentrations for most organisms, those now able to secrete carbonate having the benefit of armour against predation and a solid substrate for colony building.

Reconstructing an environment devoid of multicellular life requires some stretch of the imagination.  Before the appearance of the first metazoans in the Proterozoic ecology might seem to have been somewhat tedious.  However, discovery from molecular biology of the antiquity of living prokaryotes and detailed analysis of their highly diverse metabolism makes such a venture fascinating.  In a review of early life and habitats, Euan Nisbet and Norman Sleep (Nisbet, E.G. & Sleep, N.H. 2001.  The habitat and nature of early life.  Nature Insight, v.  409, p. 1083-1091) weave an intricate fabric of biology, geochemistry and tectonics that serves to enthuse undergraduates and professional Earth scientists alike.  The review is understandably speculative, dealing as it does with proxy evidence for Archaean life forms themselves and their possible precursors.  But it presents a useful logic for seeing the Archaean Aeon as having a highly diverse biosphere, albeit one that is probably as alien as any that humans are likely to find…. even if they get to Mars!

Buckyballs and the end-Palaeozoic extinction

The largest mass extinction in the 600 Ma history of multicellular life took place 251 Ma ago, at the close of the Palaeozoic Era.  The end-Permian event wiped out up to 90 percent of marine animals and 70 per cent of land vertebrates.  Spanning the Permian-Triassic boundary are the vast Siberian Traps (continental flood basalts), that have been the most widely suspected trigger for the extinction.  Their emissions of sulphur dioxide may have created acid rain and stratospheric aerosols that cooled conditions through the event.  The boundary shows up in ocean-floor sediments incorporated in a Japanese ophiolite, which suggest less than 100 000 years saw the massive die off.  The popular notion of an impact cause for mass extinctions seemed to be a non-starter for the P-Tr boundary until late February.  There had been sporadic reports of iridium anomalies from the boundary, but not so believable as that at the K-T boundary.

Another tell-tale sign of extraterrestrial causes for extinctions is the presence of peculiar molecules in which more than 60 carbon atoms are bonded in a structure similar to a geodesic dome, called fullerenes after the creator of  this architectural structure, Buckminster Fuller (they are nicknamed “buckyballs”).  Fullerenes are thought to be created in the aftermath of supernovae, and therefore likely to occur in comets from the outer Solar System, where the most primitive material resides.  Their structure allows them to act as immensely strong and impermeable “cages” for gases around at the time of their formation.

In 1996 US geochemists discovered fullerenes in rocks formed in a huge impact crater near Sudbury, Ontario that must have come from space nearly two billion years ago and arrived on Earth intact. Last year the same team showed that even more complex carbon molecules, with as many as 200 atoms, had survived an impact from space at the same time as an impact wiped out the dinosaurs at the K/T boundary.  Sensitive measurements of isotopes of helium ad argon locked within the carbon cages reveal that their proportions are uncharacteristic of more common Solar System materials and must have been formed by nucleosynthesis far off in space.

Samples from the Permian-Triassic boundary in China, Japan, and Hungary contain fullerenes with these unusual combinations of helium and argon isotopes (REF).  This is incontrovertible evidence for an impact influence.  As yet, no candidate crater has been found, though with 70 percent of the Earth being occupied by recyclable ocean floor, it may have vanished down a subduction zone (the oldest sea floor now is late Triassic).  However, the coincidence of impact, massive flood basalt eruptions and a mass extinction is familiar.  The long-running debate about the K-T event is fuelled by such a triple coincidence – the death of the dinosaurs and much else, the Deccan Traps and the Chicxulub structure in the Gulf of Mexico.

Some authorities believe that extinctions big enough to be adopted as the principal boundaries in the stratigraphic column may need a “double whammy” to occur.  But there is also evidence that links the timing of flood basalt events to other extinctions that have yet to reveal a correlation with impacts.  Undoubtedly an outcome of mantle upwelling in superplumes that might start from the core-mantle boundary, the seeming regularity of flood basalt events (around every 30 Ma) poses a conundrum.  Linking two major basalt floods with impacts raises the possibility that superplumes might be triggered by major impacts.  One idea is that seismic energy released by major impact travels to the core, to trigger dislodgement of core-mantle boundary material into a rising superplume at the opposite side of the planet.  The Decccan Traps are at almost the exact antipode of the Chicxulub structure.  Using this logic, the place to look for the P-Tr culprit would be at the antipode of Siberia, when it was part of Pangaea.  That conveniently places the possible site in the huge ocean that encompassed Pangaea at the end of the Permian – it would ultimately be subducted as Pangaea broke up and continents began their latest round of drifting.

BSE in reverse?

Some say that we are witnessing and are even the source of the latest mass extinction.  If so, it is not entirely a product of modern society.  The late-Pleistocene of Australia and the Americas saw massive losses among large marsupial and more advanced mammal species from around 70 thousand years ago, i.e. since the first arrival of humans.  A widely accepted view is that this selective extinction was because the vanished species were eaten because they were naïve and easy prey.  Certainly the disappearances were sudden.  A huge diversity of large American mammals, including several elephants, camels, giant sloths and sabre-toothed cats (about 30 species), was decimated from around 11 to 9 thousand years ago, as humans spread quickly southwards through two continents when climate emerged from the last Ice Age.  Gluttony on such a scale is difficult to comprehend.

In an article in the February issue of Scientific American, Ross McPhee of the American Museum of Natural History in New York introduces an alternative hypothesis, that the extinctions resulted from infectious diseases crossing species barriers.  His idea stems not from evidence for epidemics, but the lack of it for massive butchery in the form of cut marks on bones of these extinct beasts.  Isolated for millions of years from both humans and the diseases that evolved in the Old World, American and Australian mammal populations would have had no immunity to viruses or pathogenic bacteria brought in by the human colonisers.  The crash in population of native Americans following European colonization was mainly due to epidemics.  The fact that several human diseases originally evolved in other species – poxes among cattle, ‘flu in birds and AIDS in African apes, for example – points to mutations possibly occurring in the opposite direction.  Add to that the fact that human immigrants would have been accompanied by dogs and almost certainly rats and infesting insects, and the idea become plausible.

(Source:  McKie, R. Man’s germs wiped out mammoths.  The Observer, 28 January 2001)

Cretaceous owl?

Earth Pages has charted over the last 9 months a tendency for publication in the “journals of record” of fossil arcana.  February 2001 adds yet another.  Early Cretaceous sequences of Cuenca in Spain include lagerstätten (horizons of exquisite preservation).  One provided a near perfect example of a regurgitated pellet, similar to those coughed up by owls (Sanz, J.L. et al. 2001.  An early Cretaceous pellet.  Nature, v. 409, p. 998-999).  In it are the remains of four chicks, including evidence of feathers, of different bird species, whose bones show clear microscopic evidence of having been partially digested.  Being 23 cm² in flattened form, the pellet is presumably from some predator approximating the dimensions of a modern owl.  That does not necessarily call for Cretaceous owls, for any small predator, such as a pterosaur or small theropod dinosaur may well have encountered difficulty passing bony debris to dung, and resorted to regurgitation.

For small, shelly faunas that just preceded the Cambrian Explosion, outcrops that span mass extinction events, the evolution of vertebrates and much else besides, the huge diversity of Chinese geology has become a hive of palaeontological activity.  Perhaps this is due to an astonishing run of good fortune through the Phanerozoic as regards excellence of preservation, or the patience, ingenuity and skill of Chinese fossil experts.  The embarras de richesse is probably a blend of both with the fact that for decades following the Cultural Revolution little work was possible for political reasons.  Pent-up enthusiasm and curiosity is a marvellous driving force in research when released.

Such is the degree of interest that the 12 January 2001 issue of Science devotes 10 pages (Stokstad, E., Normile, D. and Lei, X. 2001.  Paleontology in China.  Science v.  291, p. 232-241) to a summary of discoveries so far, how Chinese palaeontologists are organising and funding their work, the in-fighting that goes on (not so different from anywhere else!) and the dangers of unique material being looted in the manner of rare works of art.  One difference in fossil hunting between developed and poor countries that are geologically well-endowed, is that in the former most of it is by professionals or well-heeled amateurs seeking entertainment.  In China it is a potential source of extra income for rural people, in the same manner as artisanal gold working, widespread in Africa.  That is double edged: while leaving no stone left unturned where fossils crop up in soil, it is the source of semi-legal international trade in treasures like dinosaur eggs containing embryos, and untutored fossickers make no records of stratigraphy.

The most important issue discussed in the revue concerns how essential overseas resources focus on scientific potential in less well-heeled countries.  There is a tendency, which has tempted most scientists with access to funds to pay lip-service to transnational collaboration, merely to add names to proposals and publications of individuals who for various reasons have not played a full, or sometimes any role at all.  That is a device to attract funds with an air of philanthropy, and to get official access to material.  It has no benefit for transfer of knowledge, skills and technology.  Most Chinese palaeontologists now rightly demand to participate fully in order to boost and widen expertise in their community.

The Chinese experience offers plenty of lessons for Earth scientists in other poor countries.  For one thing, it has focussed the government’s attention on reversing the previous drain of excellence by earmarking affordable funds for research.  Another is that it shows how curiosity and plain hard work can open up entirely new knowledge from the previously overlooked.  There is no reason why their application in other poorly-known geological scenarios shouldn’t uncover crucial threads for many other problems of the Earth’s evolution – about 75% of the continental surface still remains to be mapped at scales better than 1:1 million.

Oh Dear, another weird dinosaur!

China isn’t the only new frontier for palaeontologists.  It looks as though Madagascar is on the fossil map, because of fine preservation in late-Cretaceous, terrestrial sediments there.  The latest find there is a somewhat diminutive (~1.8 m long), but nontheless strange abelisaurid theropod – the group best known for having T. rex as a member (Sampson, S.D. et al. 2001.  A bizarre predatory dinosaur from the late Cretaceous of Madagascar.  Nature, v. 409, p. 504-506).

Masiakasaurus knopfleri  (the expedition crew included the few surviving fans of Dire Straits) had nimble teeth; in fact a whole gob-full of them.  Not a beast on whose snout to place a little kiss, for lots of pointy and serrated fangs protrude in a most alarming manner.  “It shows there’s still more to theropod lifestyles than we thought”, observed Tom Holtz of the University of Maryland; something with which we can all agree.  But upon what victims did it prey?  There are similarly equipped fossil crocodiles, and M. knopfleri certainly seems well-equipped to snaffle the odd passing trout.  However, the late Cretaceous greenhouse world had an atmosphere with high oxygen levels due to much greater rates of photosynthesis than now.  It probably teemed with large flying insects, because oxygen levels determine the maximum size compatible with the high metabolism needed for flight.  The discoverers plump for an insectivorous lifestyle.

But just what constitutes “weirdness”, the adjective “bizarre”?  To me, they are appropriately applied to living beetles that boil formic acid and spit it on a predator, giant squid whose sexuality involves males injecting packets of sperm under high pressure into the tentacles of females, who, at their leisure, rip off the skin that heals the wounds to impregnate themselves, and, of course, the recently discovered phyllum that lives exclusively on the lips of lobsters.

The most bizarre US presidential election in history also had a geological twist.  Kansas fossil dealer Alan Dietrich asked voters in Barton County to write in the name of a large Cretaceous fish on their ballots, where local politicians were running unchallenged – no chads for this enlightened county!  Xiphactinus, one specimen of which contains the fossilized remains of its last hapless victim and which is peculiar to the Cretaceous marine sequence of Kansas, polled 235 votes, only 15 fewer than the Green Party presidential candidate Ralph Nader, and, arguably, enough to have returned the Democrat Gore to the White House, had they been cast in Florida.

Dietrich is best known for peddling a Tyrannosaurus rex with a US$20 million price tag, but is intent on Xiphactinus becoming the state fossil – a tradition that we Britons must surely have taken up at the shire or metropolitan level long ago, but for the abundance of even more archaic, bizarre and living human candidates.    Jest not about Alan Dietrich, however, for his campaign is a celebration of the collapse of creationism in Kansas.

Source:  Holden, C. (Ed.) 2000.  Random Samples.  Science, v. 290 (8 December issue), p. 1887.

About 33.7 Ma ago, at the Eocene-Oligocene boundary marine invertebrates suffered their largest downturn in the Cainozoic.  Marine-core oxygen isotope records suggested that this coincided with a major cooling, when East and West Antarctica both possessed ice sheets.  Deep ocean water temperatures, recorded by the oxygen isotopes of benthonic forams, fell by 3-4°C, yet surface waters at low latitudes appear to show little detectable change in the isotopics of planktonic forams.  Data from cores become less well resolved in time, the older the sediments are, for a variety of reasons.  Tying down a climatic cause for the E/O extinction demands much better precision.

From an astonishing piece of ingenuity and technical skill, we are closer to an answer.  Lida Ivany and colleagues, from the Universities of Michigan and Syracuse, USA, collected the tiny ear bones or otoliths of fossil fish from a boundary section on the Gulf of Mexico.  Because these grow with the fish and contain growth layers, potentially they can give resolution to the level of a single season.  The trick is to get samples on a layer by layer basis and then analyse the tiny masses so extracted for oxygen isotopes.  That is what the team managed to do (Ivany, L.C. et al.  2000.  Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary.  Nature,  407, 887-890).  Comparing the fine detail from Eocene and Oligocene fish ears shows that the local climate was much more seasonal in the early-Oligocene.  While summer temperatures stayed at much the same level as in the immediately preceding Eocene, early-Oligocene winters were much colder.  That would account for the inability of marine core data to detect any significant global cooling, and seasonal contrasts could have knocked out marine invertebrates evolved to more equable conditions.

News and Views in the same issue of Nature includes a fascinating look at these novel data in the context of wider knowledge of what was happening at the E/O boundary (Elderfield, H.  2000.  A world in transition…  Nature,  407, 851-852

Primordial slime

A timeless phrase from the film One-eyed Jacks is Marlon Brando’s, “You ain’t nothin’ but a ball o’ spit”, to the oppressive and corrupt lawman played by Slim Pickens.  Some molecular biologists would come close to agreeing, though not in anyway to mock that fine actor.  In Lyn Margulis’ theory of endosymbiotic origin for the Eucarya, of which we are a multicellular one, a candidate for the organism that played host to several others that went on to become eucaryan organelles is a slimy beast.  It is Thermoplasma acidophilum, a member of one of the three fundamental domains of living things, the Archaea.  Thermoplasma has no proper cell wall, contains DNA with proteins like those which bind nucleic acid in eucaryan cells, and it thrives in burning coal heaps.  It is pretty much slime that needs both highly acid and very hot conditions to metabolise, and both result from the spontaneous oxidation of sulphides in coal exposed to air.  Its very sliminess makes it worth considering as the original envelope for the baggage of the first Eucarya, so that they could get in.  It is also an anaerobic fermenter – a methanogen – on whose waste products aerobic Bacteria might live while protecting the host from oxygen that would be highly toxic to it and perhaps supplying it with useful chemical products.  Very roughly, that is how Margulis explained mitochondria, the organelles that are common to all eucaryan life.  For a symbiosis to become a cellular unit from which all animals, plants etc descended demands an exchange of genetic material between all the participants, so that they become incapable of independent reproduction.

A few months after gongs were beaten to announce the completion of the human genome sequencing, Andreas Ruepp and colleagues from Germany and the USA laid out the genome of the loathsome Thermoplasma (Ruepp, A. and 9 others  2000.  The genome sequence of the thermoacidophilic scavenger Thermoplasma acidiphilum.  Nature, 407, 508-513).  Thermoplasma, being an “extremophile” is also a candidate for having evolved in the hot environment of sea-floor, hydrothermal vents.  It comes equipped with so-called heat-shock proteins, that eucaryan cells have turned to a multiplicity of other uses in their later, cooler, oxygen-loving evolution.  The astonishing feature of its genome is that it is either a molecular thief or prone to being burgled.  Many of its genes are identical to those in the sequences of other bacteria species whose habitats overlap with that of Thermoplasma.  As well as offering little hindrance to large molecules entering it, the archaean seems not to generate enzymes that in many other cells detect and destroy alien DNA.  The fact that Thermoplasma shows less affinities with eucaryan genetics than with that of Bacteria, suggests that it probably was not our ultimate ancestor.  But that is hardly surprising, since such an organism would have had to share an environment with aerobic ancestors of organelles, one very different from the high temperatures and low pH of Thermoplasma and its fellows.  To me, the new information serves to show strongly that an endosymbiotic origin of the Eucarya was indeed possible, given this mixture of larcenous and tolerant metabolism.

See also: Cowan, D.  2000.  Use your neighbour’s genes. Nature, 407, 466-467

Eve never met Adam

A bit of molecular biology never did Earth scientists any harm, and new research on connectedness in DNA between people now living in different parts of the world sheds new light on the origin of fully modern humans.

All humans are, at most, one tenth of a percent different in their genetic make up; we are ten times more closely related than are chimps from different bands in the forests of West Africa.  This low variance almost certainly results from the origin of fully modern humans in very recent times.  The well-known comparison between DNA in mitochondria (mtDNA) of people across the world points to a divergence in our “bush” of descent about 140 000 years ago.  Because mtDNA passes through the female line, this aspect of modern human origins has been said to stem from a mitochondrial “Eve” living in Africa at the time.  This does not mean that only one fully-modern woman was alive at the time, but that lines of descent from others died out subsequently.

The other side of the evolutionary coin is descent worked out through the male line.  Molecular biologists have focussed on DNA in Y-chromosomes that only men possess and pass on to their sons.  A team at Stanford University in California used cell material from over a thousand men from 24 widely separated regions to investigate relatedness and divergence with the highest precision yet.  Their results point to a time of divergence between 50 and 70 000 years ago; half that for female inheritance.  While the mismatch certainly knocks creationism and its literal reading of the Old Testament still further out of the park, how the mismatch arose is hard to fathom.  One possibility is that a mutation affecting Y-chromosome DNA only imparted such an advantage to the males who carried it that their descendants survived, while those not so favoured had their lines snuffed out.  Alternatively, it may simply have been that some important technological discovery, or maybe even a cultural change, such as art that seems to first appear in Africa around 70 000 years ago, gave a very small family group the potential for only their descendants to survive through 3 to 4 000 generations.  Whatever, the “bottleneck” through which all our genes passed at the time was in Africa.

Source:  Cohen, P.  2000, Eve came first.  New Scientist, 4 November 2000, p. 16.

The undead

The notion of bringing to life ancient organisms carries overtones of Jurassic Park, and more scientifically those of contamination by modern organisms.  But has it been done?   Russell Vreeland and colleagues from West Chester University, USA, claim to have cultured bacteria preserved in fluid inclusions from a Permian salt deposit (Vreeland, R.H. et al.  2000.  Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal.  Nature, 407, 897-900).  The stringent conditions of sampling suggest that indeed this is an old bug, as does the fact that it seems to be a salt-tolerant bacterium.  However, it is hard to believe that living organic material can survive without apparent damage for so long.

In the accompanying News and Views pages, John Parkes, of the University of Bristol, UK, discusses the ramifications, and that surrounding claimed revival of bee-dwelling bacteria from Miocene amber.  Some are worrying. Bacterial spores might survive indefinitely, to be released on an ill prepared world that has lost any shred of resistance to pathogens.  Others bring a spark to some dormant ideas, particularly that of life spreading galactically by meteorite transportation.

In the United States’ legal system I believe there is a statute of limitations.  It doesn’t apply to the Cretaceous Period.  More precisely, the most complete and fierce-looking specimen of a Tyrannosaurus rex skeleton has been the subject of legal wrangles from the moment she – a female named Sue after Sue Hendrickson of the Black Hills Institute of Geological Research (BHIGR), South Dakota who found her – was excavated.  The legal saga is the subject of a new book by a lawyer, Steve Fiffer (Tyrannosaurus Sue, Freeman, New York, ISBN 0-7167-4017-6).  The trouble started when the owner of the land on which Sue was discovered in 1990 was paid a paltry US$5000 for the privilege of seeing the awful fossil removed.  The rancher’s subsequent claim on her was matched by another from the Cheyenne River Sioux, because the owner had placed his land in trust with the US Department of the Interior, and that conveys certain advantages to Native Americans….  The plot indeed thickened.  The FBI and the local sheriff pounced on the hapless saurischian in 1992, and the National Guard supervised her impoundment, pending due process of law.  Five years of hearings and criminal proceedings later –  a raft of 148 felonies and 6 misdemeanours fell on the owners of BHIGR and one was jailed for 18 months – Sue became probably the oddest lot at Sotheby’s auction rooms.  To add further insult, the auction price of US$8.36 million was partly raised by Disney and McDonald’s, and the landowner made US$7.6 million after commission.  Sue now entertains in Chicago’s Field Museum of Natural History.

Source: Pojeta, J., 2000.  Fossils, G-men, money and museums.  Science, v. 289 8 September 2000, p. 1695-1696.

The most familiar photosynthesis is that associated with green plants, members of the Eucarya, in which organelles known as chloroplasts play a crucial role.  Lyn Margulis’ theory of endosymbiotic incorporation of various bacteria in the origin of the eukaryote cell, sees cyanobacteria as the most likely progenitors of chloroplasts in plants.  Aspects of the genetic material in chloroplasts are sufficiently similar to that of blue-green bacteria to make this a robust view.  Tracking down when that melding of bacterial ancestors took place is a difficult task, both for molecular biologists and palaeontologists, partly because the record of cell material similar to that of cyanobacteria goes cold about 2.5 billion years ago.

Stromatolites, which today grow through the action of cyanobacteria excluding calcium from their cells in hypersaline environments, go back into the Archaean 3.46 billion years ago, but there is no guarantee that stromatolite forms were always confined to oxygenic photosynthesisers.  However, the manner in which photosynthesis by blue-greens fractionates carbon isotopes possibly gives a signal in the d¹³C record of ancient hydrocarbons.  Sadly, the overlaps between carbon-isotope fractionation oxygenic photosynthesisers, chemoautotrophs and anoxygenic photoautotrophs are too broad for this kind of study to give a definitive answer.  Nonetheless, some researchers have claimed an Archaean origin for the cyanobacteria using this approach.

The advance of molecular biology, which compares gene sequences among living organisms to seek degrees of relatedness (phylogenies), steadily moves towards widely accepted molecular “clocks” that might resolve the timing of emergent life processes.  A joint US-Japan team of molecular biologists have compared the photosynthetic genes of two modern photoautotrophs – green sulphur and green nonsulphur bacteria, neither of which are oxygen producing – with those of other photosynthetic bacteria (Xiong, J. et al., 2000.  Molecular evidence for the early evolution of photosynthesis.  Science, v.  289 8 September 2000, p. 1724-1730).  Their results firmly place oxygenic photosynthesis, as in cyanobacteria, as descendent from earlier anoxygenic photoautotrophy, purple bacteria likely being the first to emerge by developing pigments capable of using solar energy to fuel proton pumping across cell walls.  Jin Xiong and co. do not derive any timing for this phylogeny, but palaeobiologists are suggesting from their evidence that the six major photosynthetic bacterial lineages were around in the mid-Archaean (2.8 to 3.0 billion years ago) and maybe earlier.  This comes nowhere close to the greater antiquity of stromatolites, but tagging purple bacteria as the first photosynthetic organisms, albeit not producing oxygen, gives a helping hand.  Organic molecules originating in them are sufficiently distinct to already have shown up in kerogen from ancient shales, and such precursors to petroleum are present in Archaean sediments.

The interest in the emergence of photosynthesis is understandable, because of the huge increase in opportunities that it presented, by comparison with chemoautotrophic metabolism that seems likely to have been the first life strategy.  The latter depends on chemical tricks with reduced materials, such as S, Fe²⁺ and methane delivered by sea-floor hydrothermal vents.  Assuming appropriate rates for Archaean magmatism, that could sustain about 10¹² moles of carbon fixing in cells per year.  The anoxygenic photosynthetic pathway would have multiplied that by ten times.  However, it is oxygenic photosynthesis that exploded life’s potential for interaction with the inorganic world, and that stemmed from the chemical-physical process at the root of what blue-greens did.  The essence of oxygenic photosynthesis is that the pigments (like chlorophyll in plants) involved in transforming photon energies into electron flows, which are essential in the reduction of CO₂ and water to carbohydrates, actually break the very strong bond between hydrogen and oxygen in water; that is why it releases free oxygen as a by-product.  That feat involves a combination of the processes used by green sulphur and purple bacteria, which in itself implies the later emergence of cyanobacteria as confirmed by Xiong et al’s work.  By using water molecules in this way, however, oxygenic photosynthesis opened up the whole near-surface of the hydrosphere, increasing potential bioproductivity by a further two or three orders of magnitude at least.  It can be said that such a development truly brought life onto the front stage from hiding in obscure nooks and crannies.  But we still have little precise idea of when that happened.

See also:  Des Marais, D., 2000.  When did photosynthesis emerge on Earth? Science, v.  289 8 September 2000, p. 1703-1705.

The mass extinction that marks the boundary between the Palaeozoic and Mesozoic Eras snuffed out more than 90% of marine animal species and about 70% of terrestrial vertebrates.  The most complete record of the Permian-Triassic boundary is in marine sediments atop an obducted ophiolite in Japan (Isozaki, Y., 1997.  Permo-Triassic boundary superanoxia and stratified superocean: records from lost deep sea.  Science, v. 276, p. 235-238).  These record a 20 million-year period when deep ocean water was lacking in oxygen, and the anoxia reached extreme conditions for about 4 million years across the boundary.  All the palaeontological signs are that shallow marine faunas dwindled slowly in the 10 million years before the P-T event.  Carbon isotopes from hydrocarbon-rich boundary strata in Canada suggest that over a period of  only 1000 years the oceans were almost devoid of life.  The open oceans had become dead from top to bottom; a scenario graphically expressed by Ken Hsu as a “Strangelove” ocean.  Whatever the pace of preceding extinctions the boundary event was a catastrophe, and the Japanese and Canadian sections suggest that maybe a half-million years passed before surviving organisms began to recover and diverge.

The much-studied K-T boundary’s association with abundant evidence for an associated giant impact, prompted geologists to look for a similar story for the near end of Earth’s life 190 million years earlier.  Supporting evidence has yet to emerge, although the boundary includes the period when huge volumes of continental flood basalts poured over what is now Siberia.

Terrestrial records are far less easy to divide into fine time divisions, partly because they record both deposition and erosion, and partly because fossils are less well-preserved than in marine sediments.  Continental sediments spanning the P-T boundary are particularly frustrating, because of the wide extent of arid to semi-arid conditions then.  The Karoo Basin of South Africa does record wonderfully the fate of vertebrates (only 6 out of 44 genera survived the  boundary event), but less so that of plants.  Abrupt changes in plant-life are equally as important as those of animals, simply because they are at the base of the terrestrial food chain.  One way of addressing vegetation shifts of the most general kind is to look for evidence of how river systems changed their patterns of deposition, and this is what a team from the University of Washington (Seattle) and the South African Museum have done in the Karoo Basin (Ward et al., 2000.  Altered river morphology in South Africa related to the Permian-Triassic extinction.  Science, v. 289 8 September 2000, p. 1740-1743).

Peter Ward, David Montgomery and Roger Smith examined sedimentary structures produced by river channels in the sandstone members of the Karoo sedimentary pile.  Permian rivers seem to have flowed in distinct, meandering channels, whereas those of Triassic age laid down sands that show consistent evidence for intricately braided  channel systems.  The shift from one to the other type falls right at the P-T boundary.  Meanders of large river channels typify land surfaces with abundant vegetation that binds alluvium.  Where vegetation cover is sparse, there is little to constrain river flow and alluvial erosion, and wide braided river courses develop.  The authors conclusion is that vegetation suffered a catastrophic die off at the P-T boundary, leaving formerly lush plains as sandy wastes.  Such a loss of plants that would previously have contributed to balancing the atmosphere’s CO₂ levels and the proportions of light and heavy carbon isotopes in the global environment would have helped produce the “Strangelove” signal in the ocean sediments.  The land was seared, and evidence from similar sediments in Australia and Antarctica suggests a global loss of plant life.  Incidentally, the boundary in many places shows a leap in the abundance of fungal spores, so the Mesozoic began with decay on a grand scale.

See also: Kerr, R.A., 2000.  Biggest extinction his land and sea.  Science, v. 289 8 September 2000, p. 1666-1667.

Organisms at the base of the food chain, autotrophs that synthesise biological compounds directly from carbon dioxide, water and other fundamental materials in their environment, favour incorporating the lighter of the two common isotopes of carbon, ¹²C, as opposed to ¹³C.  Consequently, one of the prime signatures of life in the carbon found in rock is a depletion in ¹³C, usually expressed as d¹³C with a negative value.  It is this signature that has allowed the origin of life to be pushed back almost to the age of the oldest rocks on Earth (around 3.9 billion years ago) from carbon isotope studies of carbonaceous compounds (kerogen) in ancient sediments.

Different organisms alive today, particularly among the ecologically diverse bacteria, use different biochemical reactions in synthesising living material.  Each of these have different effects on d¹³C.  Potentially these differences could be used to identify roughly the kinds of bacteria that lived in the distant past.  Up to now, however, isotopic studies of organic carbon have only been possible for bulk extracts from rock.  That enables some bold conclusions, such as the current suggestion that oxygen-producing blue-green bacteria were around 3.5 billion years ago, but whole-rock results are ambiguous because of mixing of carbon originating from different metabolic pathways. 

Being able to analyse carbon isotopes from individual fossil cells is a major breakthrough, and a team of palaeobiologists from the universities of California and Regensburg, Germany has done just that (House, C.H. et al., 2000.  Carbon isotope composition of individual Precambrian microfossils.  Geology, v. 28, p. 707-710).  They used an ion microprobe that allowed the discovery of biological carbon encapsulated in resistant materials from 3.8 billion-year old metamorphosed iron formations from West Greenland.  That involved probably mixed carbon of biological origin.  In the new work, the isotopic analyses are from individual bacterial cells preserved in 850 and 2100 Ma banded iron formations, and suspected to be blue-green bacteria.  The results clearly distinguish one metabolic pathway – the Calvin cycle used by blue-greens – from other possibilities.

Tangible bacterial fossils go back, albeit rarely, to more than 3 billion years ago.  It is the older life forms that are most intriguing, because by 2100 Ma ago the Earth’s atmosphere had become oxygen bearing, thereby allowing the rise of the Eucarya from which we stem.  Older material might give clues to the more primitive Bacteria and Archaea that were the exclusive rulers of the biosphere before about 2200 Ma, and controllers of the Earth’s atmospheric composition and thereby its climate, which remains a mystery.

Searching for sudden events that might explain the disappearance of sizeable proportions of fossil taxa is a growing cottage industry among geologists.  Until 1980, with Alvarez’ discovery of geochemical evidence for a comet or asteroid impact at the Cretaceous-Tertiary boundary, such tumbles in life’s diversity and volume were merely palaeontological markers which geologists chose to divide the stratigraphic column of the Phanerozoic into Periods and Stages.  Mass extinctions now take on a much greater importance through the hunt to explain them.  The popular vision of herds of dinosaurs writhing in the inferno following the Chixculub bolide strike at the K-T boundary dwarfs to a large degree the equally certain knowledge that at the same time vast basalt floods in what is now north-western India may have had an equally doleful outcome.

Super-large volcanic events, akin to the Deccan Traps, are a great deal simpler to spot than the subtle signs of impacts in the rock record.  Improved precision in dating such basalt piles shows that three of the “Big Five” mass extinctions occurred within the 1 to 2 million-year life spans of flood-basalt paroxysms: the Deccan Traps at the K-T; The Parana Basalts at the Triassic-Jurassic; and the Siberian Traps at the Permian-Triassic boundaries.  A similar correlation exists for the lesser Palaeocene-Eocene boundary event at 55 Ma, which implicates the North Atlantic large igneous province responsible for flood basalts in north-west Scotland and Greenland.

The scales have tilted further towards a terrestrial cause for mass death with the recent discovery that the Karoo and Ferrar flood-basalt provinces of South Africa and Antarctica formed at a time (183.6+ 1 Ma) that brackets a lesser extinction event in the early Jurassic Period.  Jósef Pálfy of the Hungarian Natural History Museum and Paul Smith of the University of British Columbia (Pálfy, J.  and Smith, P.L., 2000. Synchrony between Early Jurassic extinction, oceanic anoxic event, and the Karoo-Ferrar flood basalt volcanism. Geology; v. 28, p. 747–750) use U-Pb dating of thin volcanic ash layers in the Jurassic sedimentary pile of North America to calibrate the ages of individual ammonite Zones of the Pliensbachian and Toarcian Stages of the Jurassic.  At that time, about 25 % of organisms at the family level became extinct globally over a period of about 4 million years – the Pliensbachian-Toarcian event was not abrupt.  The record in the British Jurassic for extinction of marine animal species shows a marked change at around 183 Ma, within the time span of the Karoo-Ferrar eruptions.

This correlation ties in well with the Toarcian ocean-anoxia event, recorded in the British and Swedish Jurassic (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb) which seems to have coincided with a huge gush of methane into the atmosphere, released by methane hydrate layers in ocean-floor sediments.  Methane, a greenhouse gas in its own right, oxidizes to carbon dioxide.  What may have happened is that the Karoo-Ferrar volcanism injected massive amounts of CO₂, leading to global warming.  This, transmitted to deep ocean water, could have triggered breakdown of methane hydrate to give a massive positive feedback to global climate.  The heat itself might have driven species and families to extinction, or changed ocean circulation to induce stagnation and anoxia.

Important as Pálfy and Smith’s findings are, they by no means resolve the complexities of interwoven terrestrial events.  The 90 million-year old Cenomanian-Turonian ocean-anoxia and extinction event had an associated methane burst, but no flood basalts.  That at the Palaeocene-Eocene boundary has no associated anoxia.  The largest basalt flood known, beneath the Pacific to form the Ontong-Java Plateau about 120 Ma ago, induced methane release and anoxia, but has no associated extinction peak

Despite well-funded attempts to link mass extinctions, other than the K-T event, to impacts, there is little tangible sign of such a connection using precise radiometric dating.  Still, the focus of high-profile stratigraphic research is on boundaries rather than what lies between them.

Putting numbers on ecological effects

In the same issue of Geology a team of American palaeoecologists (Droser, M.L.. Bottjer, D.J., Sheehan, P.M. and McGhee, G.R., 2000. Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions. Geology; v. 28, p. 675–678) assess the degree to which ecologies change after mass extinctions.  They focus on the Late Ordovician and Late Devonian events (two of the “Big Five”).  Although both involved similar levels of loss of taxonomic diversity (about 22% decline in marine families), marine ecosystems underwent no significant change after the Ordovician event.  Following that towards the end of the Devonian, however, marine ecology changed drastically.  One example is reefs colonized by tabulate corals.  The early corals were devastated by both extinctions, losing about 75% of taxa.  Coral-rich reefs continued after the Ordovician, but virtually disappear from marine ecosystems after the Devonian, until much later in geological time.  The most likely explanation for this is that Palaeozoic reefs formed mainly from organisms known as stromatoporoids, which gave the 3-D structure required for tabulate corals.  Stromatoporoids lost 50% of their diversity after the Devonian event, and did not recover as reef-formers.  The main implication of this study is that the effects of extinctions do not simply depend on the quantity of taxa that are snuffed out, but on specific components of the ecosystems involved.

Following last month’s item on vast moth beds in Denmark, yet another bizarre result of painstaking palaeontological research has surfaced in the July 14 issue of Science.

Palaeobotanist Peter Wilf of the University of Michigan, and colleagues, have collected extensively from the late-Cretaceous and early Tertiary terrestrial sediments in Wyoming and North Dakota.  Among their specimens of early angiosperm (flowering plants) leaves are a number showing evidence of insect damage.  The authors matched chew marks on what are probably ancestral leaves of the ginger plant with those of living beetles.  Amazingly, Wilf and co. showed that the damage is near-identical to that created by larvae of rolled-leaf beetles that still prey on the ginger plant (Wilf, P. et al., 2000.  Timing the radiations of leaf beetles: hispines on gingers from latest Cretaceous to Recent.  Science, v. 289, p 291-294).  Larvae take up residence in the curled, young leaves of gingers.  The young beetles then chew leaf tissue in highly distinctive patterns.  Only when the leaves unfurl do the bite marks reveal themselves, the beetles being long gone.

Curled-leaf beetles are extraordinarily loyal to their favourite plants among the gingers and heliconias, so that beetle-plant pairings generally involve only one beetle- and one plant species   Quite probably beetles and other insects underwent an evolutionary explosion at the time of the radiation of the angiosperms, because of the diversity of forms and metabolic pathways followed by flowering plants compared with other members of the Plant Kingdom.  The find helps confirm the hypothesis proposed by insect evolutionist Brian Farrell of Harvard University, that most plant-eating beetles evolved in parallel with flowering plants.

Everyone has heard of the demise of the dinosaurs around 65 million years ago, and has probably seen a trilobite.  Moths are not so common in the geological record.  Jes Rust of the University of Göttingen is one lucky palaeontologist.  In the lowermost sediments of the Tertiary Period in Denmark, about 55 million years old, he found a huge swarm of lepidopterans with representatives of at least seven species.  (Rust, J., 2000.  Fossil record of mass moth migration.  Nature, 405, p. 530-531.

Rust reckons that the 1700 specimens bedded in marine sediments represent mass migrations over the precursor of the modern North Sea.  They are not just in a single layer, but several horizons.  The find probably records annual, summer migrations much like those occurring today when winds are calm and land temperatures high.  Rust’s analysis suggests no major climatic or environmental shifts took place during deposition the 30 metres of sediment of the evocatively named Fur Formation.  To add to the oddity of the local geology, he has also found that slightly older sediments in the area contain giant ants, damsel flies and crickets, that by any stretch of the imagination could not have flown far.  They represent near-shore sedimentation, whereas the moth beds, devoid of such feeble fliers, formed in deeper water.  Stratigraphers should note that this is the first case where insects have traced a marine transgression.

Our feathered friends

Notwithstanding Sir Fred Hoyle’s contention that the famous Archaeopterix fossils from the 145 million-year old Solenhofen Limestone are forgeries, feathers are found as fossils.  But a recent find throws a lizard among the pigeons (precisely a small, squat reptile from the Triassic of central Asia) as regards the not unpleasant view that birds are the surviving descendants of the last dinosaurs.  (Jones, T.D. et al., 2000.  Nonavian feathers in a late-Triassic archosaur.  Science, 288, p. 2202-2205; Stokstad, E., 2000.  Feathers, or flight of fancy. Science, 288, p.2124-2125).

Fossils of Longisquama insignis have appendages that are remarkably like feathers, though less well-preserved examples were first regarded as long scales, hence the beast’s name.  If they are feathers, Longisquama is far too old to be a dinosaur, but may have begun a line of feathered reptilians from which the birds eventually evolved.  The authors of the new interpretation argue that feathers are unlikely to have evolved more than once.  Most vertebrate palaeontologists cite the very close skeletal similarities between theropod dinosaurs and birds as evidence for a close evolutionary relationship, sometime in the Cretaceous Period.  Feather specialists are dubious, suggesting the similarity is superficial and that Longisquama‘s ‘plumage’ are more like ribbed membranes.

Under no circumstances should readers tease or otherwise annoy ducks.  Australian palaeontologists have unearthed fossil evidence that a family of enormous, flightless birds – the dromorthinids or ‘thunder birds’ – which roamed Australian rain forests from 24 Ma to as recently as 50 ka, were not related to emus as previously thought, but were ducks.  “Fine”, you might think.  “Pretty big ducks”.  “Quack, quack”.  This is an unwise attitude.

Newly discovered in Queensland, 15 Ma old fossils of the giant and fondly named Bullockornis – estimated at 3 m tall and weighing a third of a ton – include its beak.  This is not akin to the beak of Daffy Duck – it was other anatomical details that placed dromorthinids among the anseriforms – but a serious pair of biting shears with immense musculature, fronting a head about the size of a horse’s.  The even taller, though lighter moas of New Zealand had small heads in proportion to body size, and, like ostriches and emus, were undoubtedly herbivores.  Bullockornis was either a fearsome predator or a pretty awesome scavenger.  The only Australian mammalian predator that might conceivable have been a competitor was the 15 Ma old marsupial lion, Wakaleo vanderleueri; about Rottweiler size, but better equipped in terms of fangs.

Full proof of its predatory habits awaits discovery of remains that preserve the stomach contents of this dangerous duck.  Should that materialize, and the excellence of preservation in the Miocene limestones of Queensland suggests that it is possible, Bullockornis would have been the largest land predator since the demise of the dinosaurs.

Source:  Stephanie Pain, The Demon Duck of Doom.  New Scientist, 27 May 2000

Life sneaked through ‘Snowball Earth’

The awesome magnitude of glacial epochs in the late-Precambrian from about 850 to 590 Ma was first brought to popular attention by the late Preston Cloud in his book Oasis in Space.  More recent work than his centred on the position of the continental masses that underwent repeated glaciation at that time.  One puzzle was the close association in time and place of glacigenic sediments with thick sequences of biogenic carbonates, as well as the fact that every continent preserves evidence for glaciations during this lengthy episode.  Carbonates today are manufactured at tropical latitudes, but that cannot be certain for all geological time.  So the key technique in checking for low-latitude ice sheets was using magnetic field evidence, in particular the inclination of remanent magnetism preserved in rocks of that age.  This gives a good approximation for their latitude at the time.

Repeatedly, investigators found evidence that large Neoproterozoic ice sheets able to extend to sea level did indeed occur on continents straddling the equator at that time.  That presents a major climatic problem.  Ice reflects incoming solar energy extremely well – and at that time solar power was probably somewhat less than its present value.  Ice at the equator implies ice everywhere and runaway cooling, so that the oceans would freeze over too.  This would seem to be a  situation from which there could be no thermodynamic escape, except by slow build up of volcanic carbon dioxide to give global warming by the ‘greenhouse’ effect.  Clearly, the Earth did emerge from a ‘snowball’  state, but even a short period of complete ice cover would annihilate marine life forms dependent on photosynthesis.  The whole of the Eucarya would quickly disappear, though bacterial forms depending on chemical and thermal  energy sources could have survived in the depths, kept liquid by geothermal energy.  Eucarya did survive, at least some did, for following the so-called ‘Cryogenian’ period the fossil record properly begin with a vengeance in the Cambrian Explosion.  Quite possibly the enormous stress placed on primitive, small Eucarya by repeated long periods of global glaciation helped accelerate the pace of evolutionary change.  But that demanded at least some ice-free parts of the oceans.

William Hyde, Thomas Crowley, Steven Baum and Richard Peltier (25 May 2000,Nature  vol. 405, p 425) have modelled the climate when Earth had its continents clustered mainly in the southern hemisphere in the late Precambrian.  For the first time they build into a late-Precambrian climate model the effects of ice sheets themselves, as well as the mathematics of energy balance and general air and ocean circulation.  Even with reduced solar input and no build-up of CO2 they found that air temperatures could have been high enough to sustain a permanent belt of open water at tropical latitudes, while clustered continents were ice bound.  A spin-off from this result is that isolated, ice-free continental fragments in the tropics of the time may preserve fossils of those few metazoa that did make it through the big freezes- the long sought missing ancestors for the Cambrian Explosion of life as we know it.