Category: Geobiology, palaeontology, and evolution

There is a Gaelic proverb, which loosely translated goes: “There are more ways of killing a cat than drowning it in butter”.  That seems apt for mass extinctions, particularly the most severe, at the end of the Palaeozoic.  A new hypothesis points the finger towards breathing problems, but not those likely from massive, ground-hugging emissions of sulphur dioxide from the Siberian flood basalts that coincide with the P-Tr extinction: “everyone knows” that they resulted in the universal coughing reflex in all surviving land vertebrates…..  Raymond Huey and Peter Ward of the University of Washington reckon a major contributing factor for terrestrial extinctions was a fall in atmospheric oxygen (Huey, R.B. & Ward, P.D. 2005.  Hypoxia, global warming and terrestrial Late Permian extinctions.  Science, v. 308, p. 398-401).

For most of the Carboniferous and Early Permian Earth flipped in and out of glacial conditions that dominated the southern supercontinent of Gondwana.  Tropical latitudes were cloaked in dense vegetation for the first time.  Rapid sedimentation buried vast amounts of carbon in the form now taken by the world’s largest and most extensive coal deposits.  Net carbon burial for 90 to 100 Ma resulted in extraordinary oxygen concentrations in the atmosphere. One line of evidence for that is the huge size of Carboniferous and Early Permian insect fossils, such as those of dragonflies.  Insects do not breathe, but take in oxygen by a diffusive process through spiracles on the underside of their bodies.  The more oxygen the larger they can grow.  Carbon burial also links in with the global cooling that made the Carbonierous and Early Permian susceptible to astronomic forcing of glacial-interglacial cyclicity: CO₂ fell.

The present-day oxygen concentration in the air is about 22%, whereas estimates for the Carboniferous Permian peak are around 30%.  Most land animals today, including ourselves, have an altitude limit to permanent life of around 4 to 5 km, though the vast majority live much lower.  In the Early to Middle Permian, the availability of oxygen for respiration corresponding to that at sea level today would have been around 6 km altitude, and at the top of a mountain the height of Everest breathing would be easy.  The limit to altitude range of animals would have been temperature rather than oxygen availability.  So, given sufficient warmth, the area available for animal life would have been very high.  Estimates of the oxygen level at the end of the Permian are as low as about 16%.  Even living at sea level would have demanded an ability to survive at about 2.7 km today, and at 6 km during the oxygen-rich Early and Middle Permian.  Evolution of land animals during the 100 Ma long “global winter” would have adjusted to elevated oxygen availability, which Huey and Ward believe would have led to at least a limited altitude stratification of available ecosystems, governed by temperature.  Their hypothesis is that declining oxygen forced extinctions by reducing the habitable range severely, and increased competition among those taxa able to live in the reduced, low-altitude land area: probably patches of “refugia”.

The decline in oxygen was accompanied by global warming.  Permian and Triassic sedimentary records show a dramatic increase in red terrestrial sediments, coloured by iron oxide.  Iron had been released and oxidised to insoluble iron(III), possibly by increased continental weathering, which would have sequestered oxygen by the formation of iron oxide coatings to sedimentary grains.  Increased oxidation would also have encouraged biodegradation by aerobic bacteria, which may have run-away to help boost atmospheric CO₂ levels.  One testable outcome of such events is the rate of extinction during the Late Permian, which should have risen slowly, rather than plummeting at the P-Tr event.  Another is that survivors might show signs of adaptation to low oxygen levels, and indeed some Triassic reptiles do.  All in all, those times were stressful on land.  Yet the extinctions were just as severe in marine ecosystems, where the fossil record is more complete.  Less oxygen and warmer seas would have resulted in similar hypoxia for aquatic animals.

The late Jack Sepkoski did a lasting service for those who study life’s record by combing the literature to compile the first and last appearance of each marine fossil genus.  It is from this archive that we have been able to visualise mass extinctions and those less in magnitude numerically.  As well as the “Big Five” there are other die-offs, particularly through the Mesozoic and Cenozoic record.  To some extent the extinction patterns also appear among terrestrial taxa that have been less well documented, partly because few have had Sepkoski’s determination and partly because land organisms leave fewer traces.  It quickly became apparent to him and other palaeontologists that extinction occurred sharply, which is why the biologically-determined division of Phanerozoic time since 542 Ma is so well defined world-wide.  What also emerged from inspection of the time series of genus and family numbers was a pulse in the timing of significant extinctions, which appears to have been between 25 and 30 Ma.  That struck a chord with specialists in volcanic activity, and there is a good correlation between the occurrence of flood-basalt outpourings and extinctions.  But at least one of the five largest extinctions, at the K-T boundary, coincides with abundant evidence for a major impact by an extraterrestrial body.  Planetary scientists then began looking for a pulsed variation in the intensity of bombardment of the Inner Solar System.  There is no tangible evidence of that, although there are theoretical arguments that suggest that the Sun in its ~250 Ma orbit around the galactic centre wobbles through dust arranged in bands close to the galactic plane every 30 Ma.

Extinctions are not, of course, the only features of the fossil record.  Primarily it charts variations in diversity, of which suddenly lowered numbers are one aspect in broader fluctuations.  Each extinction eventually precedes an increase in diversity as adaptive radiation from surviving taxa fills ecological niches left vacant or under-populated.  That part of the record has its fascinations, as complexity seems to have emerged in three great pulses, through the Palaeozoic, Mesozoic and Cenozoic Eras, each producing more diverse forms than its predecessor.  There are also slackenings in the pace and periods of apparent stasis.  Getting to numerical grips with the full record requires analysis that uses similar mathematical techniques to that which unlocked proof of Milankovich’s theory of astronomical pacing of climate from finely calibrated oceanic-sediment records.  It is possible to analyse time series in terms of discrete frequencies from which the curves can be reconstructed.  Physicists Robert Rohde and Richard Muller of the University of California have used this Fourier analysis on the 36 thousand strong catalogue published after Sepkoski’s death, with some recalibration of the time scale and some pruning of data – they removed genera with only a single record or whose age is poorly known (Rohde, R.A. & Muller, R.A. 2005.  Cycles in fossil diversity.  Nature, v. 434, p. 208-210).  There are definitely distinct frequencies that dominate the record, and they cannot be present by chance, although that is a purely statistical view.  But to their surprise, and everyone else’s, they are completely unexpected ones at 62 and 140 Ma.  It is proving exceedingly difficult to come up with plausible Earthly or extra-terrestrial explanations.  There are two interesting features: the 62 Ma periodicity dominates the record of relatively short-lived genera; and the “Big Five” seem to fit neatly into the patterns of diversity, albeit at unequally spaced intervals, when the effects of background fluctuations have been removed.  That filtering may allow for increasing preservation towards recent times.  One major control over diversity is, logically, a mixture of the number of potential niches and their geographic isolation, and both are probably related to plate tectonic activity.  Unfortunately, fluctuations in 2 and even 3 geographic dimensions have only the broadest calibration to time.  Added to that is the complex way in which global sea level has changed with time.  So we can expect a great deal of head scratching, and it may come as a relief that the crowing of some volcanologists and impact theorists may have been silenced at a single stroke!

See also:  Kirchner, J.W. & Weil, A. 2005.  Fossils make waves.  Nature, v. 434, p. 147-8.

The Doushantuo Formation of southern China dates from just before the Cambrian Explosion, and has become a source of astonishing information about animals that preceded the appearance of those with hard parts.  It contains fossil embryos, algae, achritarchs, and small bilaterians that are purportedly the Earth’s earliest animals.  Moreover the formation rests on the cap carbonates of a diamictite reckoned to represent a late Neoproterozoic glacial epoch, and provides a variable trend of carbon-isotope variation that extends up to the base of the Cambrian in southern China.  Because the sequence contains a number of volcanic ash beds it is potentially dateable.  Using a single-zircon U-Pb method, Daniel Condon of MIT and colleagues from the Chinese Academy of Science have established the ages of both top and base of the Doushantuo Formation with considerable precision (Condon, D. et al. 2005. U-Pb Ages from the Neoproterozoic Doushantuo Formation, China.  Science Express, 24 February 2005).  Sedimentation is bracketed between 635 and 550 Ma, the oldest age coinciding with that for the Ghaub tillite in Namibia.  Time-calibration of the carbon-isotope record allows it to be matched with others in Namibia, Oman and Newoundland.  There is one snag; within the sequence is a formation boundary that signifies non-deposition, which the authors correlate with a glacial epoch recognised in Newfoundland (the Gaskiers diamictite), citing sea-level withdrawal as the cause of non-deposition in China.  The well-constrained correlation suggests a major, global increase in the burial of ¹²C that produced a marked negative excursion in d¹³C that spans around 90% of the Ediacaran Period that saw the rise of large soft-bodied animals shortly before the emergence of shelly faunas.  The interpretation placed by the authors on this signature of burial of dead organic matter, which relates to no sign of glaciation, is that it would have elevated oxygen levels in the Late Neoproterozoic oceans.  That might have increased productivity by primitive eukaryotes, and possibly opportunities for predation.  The uppermost part of the Doushantuo Formation broadly coincides with the first appearance of complex trace fossils and mollusk-like bilaterians, and elsewhere there are signs of the first reef formation by weakly calcified metazoans at around that time.  Clearly, it is well-dated sections such as these that may hold the key to what exactly prompted the general secretion of skeletal material; the hallmark of the 10 Ma later explosion in fossil animals.

No graphite in Akilia apatites, no sign of life?

In the first EPN of 2005 evidence was reported that weighed against a sedimentary origin for the ~3.8 Ga ironstones of West Greenland from which isotopically light carbon had been claimed to indicate the earliest signs of life (see Iron isotopes enter the Archaean life debate January 2005 EPN).  The original work that claimed a biological signature in carbon from the oldest known metasedimentary rocks focussed on carbon-isotope analyses of apatites in them, in the belief that they would have withstood intense metamorphic alteration because of the resistance of that mineral to chemical reactions.  Following close on the heels of that revelation comes one a great deal more worrying for aficionados of biogeochemistry.  Geoscientists from Estonia, France, the US and Sweden have systematically made petrographic observations on apatite grains from the rocks of the Akilia Association, including those originally reported as carrying geochemical signs of life existing at that time (Lepland, A. et al. 2005.  Questioning the evidence for Earth’s earliest life – Akilia revisited.  Geology, v. 33, p. 77-79).  Of the 190 individual apatite grains examined in 17 rocks, not one showed the slightest trace of carbonaceous material.  It seems that apatite is unlikely to have been the host for the low d¹³C that caused such a stir in palaeobiological circles when it was first announced, and may well not be a good place to look for biomarkers.  It also throws into question what did produce the signal.  If it was the bulk rock, then the depletion in ¹³C could have resulted from temperature induced isotopic fractionation.  Another possibility is that the samples were contaminated with modern biological materials, despite the precautions taken to avoid that.

 

In December 2004 EPN commented on what appears to be a serious challenge to claims of geochemical evidence that would support a major impact associated with the largest of all mass extinctions in the Phanerozoic, that at the close of the Permian Period and the Palaeozoic Era, around 251 Ma ago.  Newly published analyses from two other well-constrained P-Tr boundary sites found no signs of the elements that would be expected from a major collision with a metal or silicate-rich asteroid (Koeberl, C. et al. 2004.  Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component.  Geology, v. 32, p. 1053-1056).  Koeberl of the University of Vienna and colleagues from the US and UK focussed on platinum-group elements (PGEs), and osmium and helium isotopes. Both sites are stratigraphically similar and dominated by carbonate sediments, with evidence from one site for deepening water that laid down organic-rich marls.  Sure enough, there is a “spike” in iridium at the level of these marls, which had been documented at the Austrian site in 1989, and there is another 50 m higher in the sequence.  The new work confirmed both, and also found the marl-related “spike” in Italy. But the reason why iridium has been used to suggest extraterrestrial impacts is because, of all the PGEs, it is the easiest to analyse at very low concentrations. That can give rise to “false positives”, for there are purely terrestrial processes that can concentrate PGEs.  An unambiguous arbiter between these processes and impacts lies in the isotopic composition of the metal osmium.  Rocks of the Earth’s crust have high rhenium (Re) and low osmium (Os) contents, whereas in meteorites the Re/Os ratio is very much smaller.  The unstable isotope ¹⁸⁷Re decays to produce a daughter ¹⁸⁷Os that adds to the common ¹⁸⁸Os isotope. Consequently, terrestrial rocks acquire high ¹⁸⁷Os/^!88Os rapidly after they crystallise from magmas and that “signature” is imparted to the entire surface environment through weathering and solution. On the other hand, meteorites have low ¹⁸⁷Os/^!88Os ratios, so the two influences on the geochemical record can be distinguished – if you have good enough analytical facilities.  The two iridium spikes fail that test, as regards an impact origin.  It seems likely that they originated through precipitation of PGEs from sea water under reducing conditions on the deep sea floor.  The helium isotope data carry the same negative message; they are typically terrestrial.

Impact-induced extinctions, particularly ones that wipe out a sizeable proportion of all organisms, are likely to be unremittingly sudden – direct effects being felt within hours over the whole planet, and secondary effects such as “nuclear winter” and acid rainfall over a matter of a few years or decades. Radiometric dating is incapable of resolving such short periods, and at the age of the P-Tr boundary probably not even several hundred millennia. Faunal sequences can give a better indication of abruptness. To most intents the marine record at the time does look as if extinction was very sharp, but it does not indicate anything by way of clear evidence for an impact, such as glass spherules, shocked quart grains and other tell-tale signs.  The continental record is pretty sparse, so has not figured much in the debate.  However, the Karoo basin of South Africa contains thick continental sediments that span the boundary, and is famous for its primitive reptile fauna, some of which became extinct around the time of the P-Tr event.  Incidentally, this die-off created the genetic conditions for the adaptive radiation in the Mesozoic that led not only to the dinosaurs but also the mammals and birds.  Charting the timing of the Karoo extinctions has proved difficult, although it appears not to have been sudden in a stratigraphic sense.  New age data has emerged from studies of palaeomagnetic field reversals in the sediments, together with variations in carbon isotopes, that allow timing to be better assessed through comparison with magnetic and carbon records from other sections (Ward, P.D. et al. 2005.  Abrupt and gradual extinction among Late Permian land vertebrates in the Karoo Basin, South Africa.  Science [soon to be published, currently available on Sciencexpress at www.sciencemag.org/sciencexpress/recent.shtml]).  The signs are that the proto-reptiles died off over tens to hundreds of thousand years due to some protracted crisis, probably connected with the giant continental flood basalt eruptions that formed the Siberian Traps. Those lavas overlap the timing of the P-Tr boundary, and would certainly have added sufficient CO₂ to give substantial global warming and also massive emissions of SO₂ that would have created chemically hazardous conditions on a global scale.

New predators on the Mesozoic block

Most people have been led to believe that, although the earliest mammals appeared in the Triassic fossil record, throughout the Mesozoic they were tiny and meekly scurried and skulked while the dinosaurs reigned supreme over land, sea and air.  They had to wait for the K-T extinction to develop their full ecological potential.  That is now a myth, for Chinese strata (yet again) have revealed much larger mammals than ever thought possible, and some of them ate dinosaurs (Hu, Y. et al. 2005.  Large Mesozoic mammals fed on young dinosaurs.  Nature, v. 433, p. 149-152).  One indisputable mammal skeleton contained the bones of young dinosaurs in its body cavity.  In fact so many that one wonders if it met its end through greed.

At the end of the Triassic Period, around 200 Ma ago, life underwent a major crisis that so far has not been believably connected to either extraterrestrial or geological causes.  Previous studies have shown that the mass extinction was accompanied by an decrease in ¹³C in sediments that suggests a short-lived global warming of  between 2-4 °C at the Tr-J boundary.  That CO₂ levels rose is suggested by a decrease in the density of pores (stomata) on fossil leaves.  It has been suspected for some time that the largest known continental igneous event, which accompanied early rifting of the modern Atlantic Ocean basin may have been responsible, but so far the dating of this Central Atlantic magmatic province (CAMP) has not been tied to the boundary conclusively.  A large consortium of Italian, French, US, Moroccan and Swiss has addressed the sedimentary and igneous record around Tr-J times in the High Atlas of Morocco (Marzoli, A and 14 others 2004.  Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis.  Geology, v. 32, p. 973-976).  There, one of the few uneroded continental flood basalt sequences of CAMP (most preserved CAMP magmas are in the form of sills and dykes in offshore basins) occurs among Triassic and Jurassic sediments.  Their base deforms the underlying sediments, suggesting that eruption was onto unlithified sediments, shortly after their deposition.  Fossils from the sediments are of little help in tying down the age of eruption, however, Ar-Ar ages of the lavas are all within error of 200 Ma, and tally with magnetic stratigraphy from the Tr-J boundary elsewhere.  Both age and geochemistry of the flows are remarkably similar to those of flood basalts from the other side of the Atlantic.  Magmatic duration, like that in other large igneous provinces was of short duration, no more than a couple of million years.  So it now seems that three of the “big five” mass extinctions (the others are end-Permian, connected with the Siberian Traps, and the K-T boundary and associated Deccan Traps) have at least a partial cause from CO₂ release by massive volcanism.

Iron isotopes enter the Archaean life debate

Some years ago geochemists obtained carbon-isotope data from 3.8 Ga rocks in Greenland that seemed at the time to be persuasive evidence for the emergence of life during or shortly after Earth’s most traumatic period.  Up to 3.8 Ga the Moon was bombarded by huge projectiles, and its companion Earth would have received at least 13 times the flux of destruction.  The carbon was within sturdy apatite grains from supposed iron-rich metasediments, and may have been preserved from later high-grade metamorphism.  Doubt has been cast on that hypothesis, either because of the unlikelihood of any carbon remaining unfractionated by heating, or because some aspects of the rocks’ geochemistry suggested that they we of igneous origin rather than sediments.  Readers will have seen in previous years’ EPN that a controversy rages over even tangible signs that suggest cellular material from rocks half a billion years younger.  Geochemists from France and the US have taken a different tack with the ancient Greenlandic rocks that ought to at least resolve the igneous versus sedimentary origin of the banded iron-rich rocks (Dauphas, N. et al. 2004.  Clues from Fe isotope variations on the origin of Early Archean BIFs from Greenland.  Science, v. 306, p. 2077-2080).  They found that the heavy iron isotope ⁵⁷Fe is more enriched in the ironstones than in any igneous rocks, with little chance that the difference was induced by thermal fractionation.  They are metasediments.  But therein lies a surprise.  The heavy-iron signatures are greater than in less aged banded ironstones.  One way in which that could have arisen is from biogenic precipitation of soluble reduced Fe-2, perhaps involving anoxygenic photosynthesisers – because of the strong capacity of photosynthesis for setting electrons in motion, all such organic reactions create local oxidising conditions, whether or not oxygen itself is produced.

Studies of the organic chemicals in meteorites and in “space snow” that falls continually on the Earth, show that amino acids and nucleotides (the CGAT building blocks of nucleic acids), together with other moderately complex compounds, were widespread in the solar nebula as it formed.  They can form in the absence of life.  Life’s dependence on DNA and RNA for its necessary self-replication marks a chemically complex step that assembled such building blocks by a process of polymerisation.  That presupposes an awful lot of chance reactions, none more so than the formation of the peptide bond that dominates genetic material and proteins.  Lots of mechanisms have been tested, but none work sufficiently well in a test tube to be plausible candidates for processes on the early Earth.  Perhaps the simplest, first proposed more than 30 years ago is the operation of a simple gas called carbonyl sulphide (COS).  Experiments that expose amino acids to carbonyl sulphide in water at “room temperature” yield lots of peptides in a matter of a few minutes to hours (Leman, L. et al. 2004.  Carbonyl sulphide – mediated prebiotic formation of peptides.  Science, v. 306, p. 283-286).  The more metal ions, such as those of iron, lead and cadmium, that are in the solution, the more efficient the reactions.  The likeliest place for such processes to go on would be near submarine hydrothermal vents, as COH quickly breaks down once emerged from a volcanic source.  Its role could have been crucial in the complex molecular evolution that many biochemists believe to have been intimately associated with the structures of clays and sulphide minerals that hydrothermal activity produces in abundance.

The volcanism versus impact debate about the K-T boundary runs and runs, as newshounds tend to say.  Things are not so evenly balanced for the biggest of all mass extinctions at the end of the Permian.  Although signs have been reported, a link with an impacting extraterrestrial body has not convinced a decisive majority.  On the other hand, there is a 1-2 Ma mismatch between the well-determined age (around 253 Ma) of the Siberian Traps and previous dates for the end of Permian stratigraphy in sections that have no depositional break with the Triassic.  The extinction has all the hallmarks of a catastrophe, by definition a sudden event, so tying down its age and that of a plausible cause is essential.  Not being able to do that for the K-T event and the Deccan Traps, and with uncertainties about the relationship of impact rocks to signs of extinction at the Chicxulub site, add fuel to that long-running debate.  The accepted “golden spike” or GSSP for the Permian-Triassic boundary is at Meishan in eastern China, and there are other sites in China that run it close.  The sections contain several volcanic ash layers, so zeroing in on a date for the extinction would seem straightforward, using U/Pb zircon dating.  There is a problem.  Some of the zircons in the ashes are xenocrysts rather than having formed during the various magmatic episodes, and they are microscopically indistinguishable from those that should give precise dates.  All the zircons also show signs of having lost radiogenic lead during later alteration of the beds.  The last could explain the mismatch with the Ar-Ar age of the Siberian Traps, the generally favoured culprits for the extinction.  US and Australian geochemists have taken a new tack in dealing with these problems (Mundil, R et al. 2004.  Age and timing of the Permian mass extinction: U/Pb dating of closed system zircons.  Science, v. 305, p. 1760-1763).  They have “aggressively” treated zircon grains to remove outer parts from which radiogenic lead has been lost, so leaving isotopically undisturbed cores of the grains.  Their U/Pb data are mainly from a boundary section in central China (Shangsi), dating 8 separate ash layers, plus one from the boundary clay itself at the Meishan GSSP.  The dates agree well with the stratigraphic sequence of the ashes, and hare high precision.  Judging the actual age of the boundary at Shangsi relies on statistical analysis of the sequence of ages from the different ashes, and gives a date of 252.6±0.2  Ma.  That is within error of the accepted Ar-Ar age of the Siberian Traps.  As usual, this is not cut and dried, because there are other ages for the Siberian Traps, including one using the same U/Pb zircon method that suggests a 251.4 Ma age.  Clearly the mismatches for the end-Permian events will be a meaty bone of contention, when all respected geochronologists turn up for a meeting early in 2005 to thrash out the conflicts that continually inflame their passions.

As with several major extinction events, the Permian-Triassic boundary is characterised by a major excursion in carbon isotopes of marine towards negative d¹³C.  This is often taken to indicate a reduction in the burial of dead organic matter, perhaps because of low global biomass.  US, Chinese and Canadian geoscientists have added great detail to the P-Tr carbon-isotope record  from analysis of three continuous sections through carbonate-dominated sequences in an Early Triassic reef system in southern China (Payne, J.L. et al. 2004.  Large perturbations of the carbon cycle during recovery from the end-Permian extinction.  Science, v. 305, p. 506-509).  This is no ordinary reef, for it was built by carbonate secretions by micro-organisms, either algae or bacteria.  The tabulate coral reef builders of the Palaeozoic became extinct at the end of the Permian (251 Ma), and their successors, scleractinian corals, do not appear until about 10 Ma later.  The Early Triassic was undoubtedly characterised by low animal diversity, before adaptive radiation could “re-stock” a devastated biosphere.  The authors found a remarkable series of ups and downs in d¹³C within the reef carbonates, some of the negative excursions being even more severe than that just after the mass extinction.  Some of the positive peaks go far beyond the d¹³C levels in preceding and following times, and could be due to periods of extremely high burial of organic matter.  But the fossil record shows that such burial probably involved a restricted number of taxa, so perhaps there were huge “blooms” among a few groups that filled vacant ecological niches only to collapse.  As suddenly as this see-sawing of the carbon cycle had begun, at about 246 Ma it settled to a more or less constant level, just after the start of the Middle Triassic.  There are two reasonable explanations for the fluctuations.  One is that biotic recovery from the mass extinction was set back three of four times by further environmental upheavals, thereby dashing diversification.  The other is that the fluctuations reflect instability in the simple ecosystems of the Early Triassic and their control on carbon burial.

Calcium in the ocean and the Cambrian Explosion

If ever there was a geoscientific topic that would “run and run”, it would be explaining why creatures with hard parts just popped into being 542 Ma ago.  Physiologically, members at the phylum level of the Cambrian fauna have little in common apart from hard parts made from calcium compounds, either carbonate or phosphate.  Calcium carbonate was secreted as stromatolites by blue-green bacteria as far back as the Archaean, but not in an organised form linked to their function.  In the very latest Precambrian, the Ediacaran, there are tiny shell-like bits and pieces in its very uppermost strata (the “small shelly fauna”) but they suggest no obvious function and no association with any of the various soft-bodied metazoans that define that Period.  The Cambrian Explosion has no rudimentary precursor.  Because calcium is an element with a very narrow tolerance in cells, from the level needed for viable function (it has a “messenger” function) to that at which it is fatally toxic, and it is a common element in all environments, adoption of calciferous hard parts seems very likely to have a risen as a means of avoiding toxicity, without any other role.  Once established in large animals, hard parts provide a means of and a defence against predation, so losing the ability to secrete hard parts would be an evolutionary risky strategy; once established it cannot be lost except when substituted by other effective defences or mealtime tackle.  There were times in the Precambrian record when calcium compounds exceeded their solubility, and they are marked by inorganically precipitated crystalline forms in sediments.  The early Archaean was one such period, but if levels of Fe-2 are high in water those solubilities are enhanced.  Therein lies a link between Archaean and Palaeoproterozoic stromatolites, banded iron formations and the oxidation potential of seawater.  In fact precipitation of BIFs seems to link nicely with the abundance of stromatolites, because the production of oxygen by blue-green bacteria would locally have consumed electrons to oxidise soluble Fe-2 to Fe-3 that has insoluble oxides and hydroxides.  This connection returned several times in the Neoproterozoic, oddly at the times of so-called “Snowball Earth” episodes, first noted by Preston Cloud.  Could the last of these have triggered adoption of calcium secretion by the early metazoan animals?  That is hard to judge, because it preceded the Cambrian by several tens of million years.  Geochemists from the US Geological Survey, the State University of New York and the US Oak Ridge National Laboratory have taken a cunning route to shedding some light on the biggest of all palaeontological mysteries (Brennan, S.T. et al. 2004.  Seawater chemistry and the advent of biocalcification.  Geology, v. 32, p. 473-476).  They sought crystals of evaporitic halite that spanned the Precambrian-Cambrian boundary, and which usually contain fluid inclusion containing samples of the brine from which they formed, hopefully seawater.  So far, they have two sets of suitable halites that can be assigned to a marine environment, from Siberia and the Oman, and their measurements of calcium concentrations are very precise.  The first is dated around 515 Ma the other set from 544 Ma.  Two sample points are not enough to prove a role for elevated calcium levels in the ocean, but the results are encouraging.  Calcium concentrations (with suitable corrections for changes during evaporation of restricted seas) jumped by a factor of 3 from the very end of Precambrian to Cambrian times.  Over the same period, it is thought that global sea-floor spreading rates were much higher than at present, and there is also strontium-isotope evidence for an increase in ocean-floor hydrothermal activity that adds elements derived from oceanic basalts to seawater.  That, however post-dates the start of the Cambrian by about 15 Ma.  With a CO₂-rich atmosphere and elevated continental weathering calcium is likely to have been supplied from the continents.  Whatever, the results fit with models based on variation of continental and oceanic additions to seawater with changing spreading rates (Hardie, L.A. 2003.  Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas.  Geology, v. 31, p. 785-788).  Hardie suggested that calcium in seawater fell to very low levels during the Neoproterozoic from an unprecedented high at its outset at 1000 Ma.  That is a time when metazoans were probably not around, while the period when they appear in the later Neoproterozoic record was one of calcium-poor conditions.  Large animals may have evolved when there was little danger of calcium shock, only to face it once they were well established.  Then would have had to rid their cells of it very efficiently. Studies of fluid inclusions from marine precipitates seem likely to grow following Brennan et al.’s important discovery, though suitable samples are likely to be few and far between.  One important role they need to play is verifying Hardie’s model for secular variation in seawater chemistry, which depends on difficult interpretations of rates of sea-floor spreading and continental erosion.

Ancestral animal?

The significant feature of the first appearance of widespread, large fossils during the Cambrian Explosion about 542 Ma ago was really the adoption of hard parts by most of the existing (and some now extinct) phylla of animals.  The preceding Neoproterozoic Ediacaran Period witnessed lots of large life forms, but preserved them only as imprints; they were soft  bodied.  Superficially, the outset of the Cambrian appears to marked the simultaneous emergences of the rough blueprints of all subsequent animals.  In reality, this was probably not a faunal explosion, but one of biochemical processes, wherein many phylla turned the fundamental cell process of excreting excess calcium as carbonate and phosphate to generating functional parts of their bodies.  Why that happened explosively is still a mystery.  Looking for the origin of animals requires going further back in geological time, and an element of luck as regards exceptional preservation of soft tissue.  The other way is using a molecular clock approach to the genetic differences among modern phylla, but that is fraught with uncertainties and gives a very large time range (possibly 1500 to 600 Ma) in which to find tangible evidence.  The maximum limit is around 2200 Ma, when oxygen became significant in the atmosphere and the upper ocean – the prime condition for eukaryote life.  A rather dull carbonaceous fossil, with a spiral form and thought to be the first known multicelled eukaryote (Grypania) appears in the record about 1500 Ma ago, but what it was is unclear.  The best place to look for ancestral animals is in known repositories of well preserved organisms.  One such lagerstät is the Doushanto Formation in SW China.  This goes back to the last “Snowball Earth” event at 600 Ma, and has been heavily mined for primitive life forms.  Chinese palaeontologists, teamed up with others from the USA have indeed found something intriguing (Chan, J-Y. et al. 2004.  Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian.  Science, v. 305, p. 218-222).  Only about 0.2 mm across, 10 specimens seems to show microscopic signs of all the basic elements of many members of the Animal Kingdom: bilateral symmetry, a mouth and gut, skin tissue and possible sensory organs.  The layers from which they were extracted are between 580 to 600 Ma, well before the Cambrian Explosion.  However, micropalaeontologists in general subscribe to the “once bitten, twice shy” outlook, especially following controversies over even earlier evidence for small organisms and those purported to occur in Martian meteorites, which are as likely to be results of inorganic mineralisation as fossils.  Various mineral crusts and films, formed inorganically, can mimic organic structures.  The one feature that persuades Chen and colleagues is that the same features show up in all the specimens, and they are all the same size.  That is highly unlikely from some inorganic process.

Source:  Stokstad, E.  2004.  Controversial fossil could shed light on early animals’ blueprint.  Science, v.  304, p. 1425.

The case of the stranded, tiny mammoths

It does seem likely that our ancestors ate all the mammoths (Mammuthus primigenius), a species that had wandered over the northern tundras bordering the Northern Hemisphere ice sheets through several glacial-interglacial periods.  But some of them did escape to survive into the Holocene.  They were stranded on high-latitude islands off NE Siberia and Alaska as sea levels rose.  The last of them died on Wrangel island about 4 thousand years ago.  A common tendency in small populations of large mammals that are restricted to islands is that they become smaller and smaller with each generation.  This happened to the stranded mammoths of the Bering Straits islands, remains of which are often dwarfs.  (Guthrie, R.D. 2004.  Radiocarbon evidence of mid-Holocene mammoths stranded on an Alaskan Bering Sea island.  Nature, v. 429, p. 746-749).  St Paul Island is now only 91 km² in area, too small to support even tiny, woolly elephants, but it was probably much larger when sea-level rise first isolated it from the vast Bering steppe across which mammoths roamed.  It was that isolation about 13 thousand years ago that probably helped the stranded mammoth population avoid the hunters who colonised the Americas, until 7 900 when the last mammoth there died.   The even later population on much larger Wrangel Island fell to human colonisation, but there are no signs of human intervention on St Paul.  The earlier extinction there was probably a result of shrinking browse as sea level steadily rose., when St Paul would have been 5 to 10 times larger than it is now.

The four largest extinction events of the Phanerozoic (late Devonian, 370 Ma; end-Permian, 251 Ma; end-Triassic 201 Ma; end-Cretaceous , 65 Ma) each coincide with periods of rapid and voluminous continental flood-basalt volcanism.  There is also evidence from the extinction horizons that each coincided with a major impact event as well, most widely accepted for the end-Cretaceous event.  Geological time is so long that pure chance cannot be ruled out entirely to explain coeval impacts and CFB events, but is unlikely (a 1 in 8 chance for one coincidence, but 1 in 3500 for four).  So there has been a long-running controversy over a volcanic or an extraterrestrial cause for extinctions, together with speculation that large impacts can somehow trigger CFB events.  The last does not work for the end-Cretaceous extinction, because the Deccan volcanism began somewhat before the formation of the “smoking-gun” Chicxulub crater, and a linking mechanism is not clear.  Taking into account lesser extinctions and CFB events, there is a rough periodicity of 30 Ma and similar ages for both.  Geoscientists at the Geomar Institute of the University of Kiel in Germany have stoked up the controversy by taking a very different view of events (Phipps Morgan, J. et al. 2004.  Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link?  Earth and Planetary Science Letters, v. 217, p. 263-284) albeit not a completely new one.  They consider the processes at depth that presage CFB events, where rising mantle material impacts at the base of thick continental lithosphere.  Each of the CFB provinces linked in time to the four large extinctions lies on an ancient craton, devoid of tectonic activity for over a billion years, and greatly depleted in heat-producing elements.  Lithosphere beneath them is over 300 km thick and might have acted in the manner of the lid on a pressure cooker, building up gas pressure during the delay in breaking through overlying rock.  Eventually pressure would be sufficient to breach the lithosphere, and gases (CO₂ and SO₂) would be explosively vented, perhaps creating globally toxic conditions.  Release of the pressure would lead to collapse above the plume head that would propagate upwards, at hypersonic speeds according to the authors.  Maybe that would fling enormous amounts of rock into the stratosphere.  Some chunks might be large enough to cause big impact structures at the surface when they fell back, so explaining the coincidence.  They account for the pre-extinction start of CFB outpourings, as in the case of the Deccan traps, by lateral and upwards migration of part of the plume to locally thinned lithosphere.  The power involved in such an event extending through the entire lithosphere could account for the shocked grains, microspherules and fullerenes in known extinction horizons.  Being sourced in mantle rock that may once have resided near the core-mantle boundary, such a process could also eject high iridium concentrations that were the signs that first led to the Alvarez’ hypothesis of impact-induced extinctions, but without an extraterrestrial culprit.  Despite the attractions of the impact theory, no sign of meteoritic debris has been found in any of the ejecta horizons or the craters themselves.  On Phipps Morgan and colleagues’ account that is not surprising, because the impacting objects would have been common Earth rock.  The authors decided to dub these hypothetical events “Verneshots” after Jules Verne’s book From the Earth to the Moon, which involved a giant gun firing the space craft moonwards.  If there is anything in the idea, then surely there would be spectacular evidence of the source of the blasts, but perhaps they are conveniently buried by later CFBs.  Geophysical studies do show signs of circular features beneath both the Deccan and Siberian Traps.  However, the associated seismic shock waves would pervade large volumes of crust outside the blast vent, and signs of that, such as shatter cones, are perhaps an easier target.  As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.

That gas build-up might lead to catastrophic crustal collapse gets some support from a modelling study on the processes involved in volcanic collapse (Reid, M.E. 2004.  Massive collapse of volcano edifices triggered by hydrothermal pressurization.  Geology, v. 32, p. 373-376), albeit in miniature.  Mark Reid of the USGS focuses on those volcano collapses that occur without any warning signs from eruptions and seismicity.  His study examines the effects of deep intrusion of magma on the groundwater systems within stratovolcanoes.  This could promote increases in gas pressures deep within the edifice.  Their upward propagation would destabilise the entire volcanic structure, leading to its collapse in extreme situations.  The modelling indicates increased likelihood of over-pressuring where permeability is low; a crude analogy to Phipps Morgan and colleagues’ pressure lid of inert cratonic lithosphere.  Gas-rich magmas can emerge explosively in continental flood basalt provinces, normally regarded as forming by episodic, quiet outpourings from fissure systems.  That is well demonstrated by the Ethiopian-Yemeni CFB province.  The main basaltic trap sequence is followed by very widespread felsic ignimbrites on both sides of the Red Sea that formed by lateral blasts of incandescent debris and felsic lava shards.  Only one example of an ignimbrite centre is known from the province.  Lying about 60 km south of Sa’ana, near the small town of Mabar, it is a circular structure about 18 km across with clear concentric zoning.  Interestingly the zones dip steeply towards the centre of the structure, in an inverted cone, that is possibly due to collapse even more dramatic than in the calderas that sourced the more familiar ignimbrites of the Andes.

See also:   Ravilious, K. 2004.  Four days that shook the world.  New Scientist * may 2004, p. 32-35.

How, when and under what circumstances vertebrates got limbs to take them charging across the forested land of the late Palaeozoic form a central issue in our own evolution, as well as that of the other four-footed land animals.  By negative analogy with the functional though rather rudimentary enlarged fins of various modern fish that flop from pond to pond during dry seasons, many vertebrate palaeontologists have considered limbs as evolutionary adaptations in air-breathing fish once they made this a habit.  As so often, the fossil record has not given up enough evidence for that to be certain.  Well, an upper foreleg bone (humerus) has turned up in Late Devonian rocks from Pennsylvania at a time and in a context that strongly suggests it was carried by a fish (Shubin, N.H. et al. 2004.  The early evolution of the tetrapod humerus.  Science, v. 304, p. 90-93).  While not able to ride a bicycle, the advanced fish probably used what became limbs to hold itself motionless while lying in ambush for its prey.  That would provide a plausible point of departure from which walking might develop.

Early biomarkers in South African pillow lavas

It is now established that various kinds of bacteria infest rocks down to depths of 2 km or more, one particularly favourable habitat being in sea-floor basalts though which hydrothermal fluids travel.  Although the majority probably inhabits cracks and joints, some seem to work actively to corrode rock, especially volcanic glass, thereby obtaining mineral nutrients.  Signs of this microbial corrosion in modern volcanic glasses are radiating tubes on a scale of a few micrometres, that show up in micrographs, and many may have been overlooked by petrographers in all kinds of rock.  That they are definitely formed by organic activity is demonstrated by the presence of nucleic acids, carbon and nitrogen in the tubules.  Carbon isotopes from them show the strong depletion in ¹³C that is the hallmark of organic fractionation of natural carbon.  A team of geoscientists, from Norway, Canada and the USA, who have steadily accumulated evidence for biological rotting in modern oceanic basalts, turned their focus to the oldest, well- preserved pillow lavas in the 3.5 billion-year old Barberton greenstone belt of north-eastern South Africa (Furnes, H. et al. 2004.  Early life recorded in Archean pillow lavas.  Science, v. 304, p. 578-581).  Virtually identical microtubules seem common in them too, particularly in hydrated glasses that are now tinged with the low-grade metamorphic mineral chlorite.  Indeed, chlorite seems to have grown preferentially from clusters of the holes, which suggests that they formed before metamorphism of the basalts.  Micro-geochemical studies confirm the presence of hydrocarbons with low d¹³C.  The bulk of the tubules occur in the inter-pillow debris, that probably formed as glassy rinds as magma protruded on the Archaean sea floor.  As well as adding to evidence for ancient terrestrial life, the find has inevitably opened up the search for such signs in meteorites reckoned to have come from Mars.  In two, olivine grains show similar structures, although why the olivine hadn’t broken down in the presence of water that is essential for life makes such observations worth taking with a pinch of salt. A number of studies have stymied claims for early bacterial fossils (see Artificial Archaean “fossils” and Doubt cast on earliest bacterial fossils, April 2002 and December 2003 issues of EPN) and inorganic processes conceivably might create structures that can be mistaken for ones formed by biological action.  The Fischer- Tropsch  process is capable of producing hydrocarbons, and produces depletion in ¹³C abiogenically.  In the on-line April edition of Science Express (www.sciencexpress.org) experiments are reported that highlight the possible influence of chromium-bearing mineral catalysts in hydrothermal generation of hydrocarbons from inorganic carbon dioxide(Foustoukos, D.I. & Seyfried, W.E. 2004.  Hydrocarbons in hydrothermal vent fluids: the role of chrome-bearing catalysts.  Science Express, April 2004).  The Barberton greenstone belt is well known for ultramafic lavas rich in chromium, as are most early volcanic sequences.

See also:  Kerr, R.A. 2004.  New biomarker proposed for earliest life on Earth.  Science, v. 304, p. 503.

China is proving to be the repository of a vast wealth of well-preserved ancient faunas, thanks to several lagerstãtten, the most famous being that which hosts early ancestral birds that show links with dinosaurs.  But Chinese strata with exceptional preservation also occur in Cambrian sediments, close enough to the first appearance of preservable life forms to make any out-of-the-ordinary finds especially revealing.  Ten years ago many palaeontologists scoffed at reports of trilobite embryos being unearthed in southern China, yet there has been a steady flow of material that opens up what might be called “palaeoembryology”.  Being able to describe and analyse an entire life cycle of an organism is vital in studies of the inter-relatedness of living metazoans.  The lack of data on fossil life histories to some extent thwarts attempts to place extinct animals accurately within an evolutionary scheme.  Palaeontologists from the University of Bristol and Peking University have therefore put such studies on the map through finding exquisitely preserved Cambrian embryos of what is now a rare and bizarre animal group, but one thought to lie at the root of the explosive radiation of the arthropods, which includes insects (Dong, X. et al.  2004.  Fossil embryos from the Middle and Late Cambrian period of Hunan, south China.  Nature, v. 427, p. 237-240).  They are in eggs, and therefore had yet to hatch and develop further; true embryos, from their initial development to the last stage before emerging.  They are Scalidophores, which include today the individual phylla of Priapulida, Kynorhyncha and Loricefera, all marine worm-like animals (the priapulids are the notorious, and fortunately rare, penis worms from their evocative contours).  Interestingly. the embryonic stages clearly indicate direct development from egg to adult, rather than going through the intermediary larval stage that characterises most insects and other invertebrates.  Such direct development seems to be a primitive evolutionary stage from which more complex life-histories developed later.  Penis worms are well known to grow hugely once hatched, so the search is on for a fully grown adult from the Cambrian of southern China, as well as early developmental stages of other animal groups..

See also: Budd, G.E. 2004.  Lost children of the Cambrian.  Nature, v. 427, p. 205-206.

It is uncommon to find fossilised nuts, so imagine the fervour that has greeted an actual cache of them, clearly secreted by some hoarding animal.  The Garzweiler lignite pit near Cologne in Germany has long been a treasure house for Miocene terrestrial fossils, thanks largely to the keen eyes of miners who work there.  In 1992 they came across 1800 nuts in one of the sand horizons that divides the lignite deposit.  They were in a burrow through probable dune sands.  Its dimensions give a clue to the hoarder, which was about 25 cm long and weighed in at 225 grams (Gee, C.T., Sander, P.M & Petzelberger, B.E.M. 2003. A Miocene rodent nut cache in coastal dunes of the Lower Rhine Embayment, Germany.  Palaeontology, v. 46, p. 1133-1149).  This is about the size of an extinct hamster, remains of which have been found at a similar level in the lignites.  Evidently, hamsters have always worried about their future, especially when food is likely to be scarce, but are also dim-wittedly forgetful.  The hazel-like nuts are the earliest-known example of a lost food cache (about 17 Ma), and have been suggested to represent the onset of seasonality in Europe during the late Early Miocene.

The selectivity of mass extinctions

Every mass extinction, whatever its magnitude, was selective; there always were surviving organisms, otherwise we wouldn’t be here.  However, selectivity according to the lifestyles of animals that became extinct can give important clues to the causes of extinctions.  Die-off across the ecological board strongly suggests a cause that was all encompassing, such as a major impact or geochemical stress that reached into every corner, as might occur with massive flood-basalt volcanism.  At the end of the Pliensbachian Epoch of the Early Jurassic there was a significant mass extinction.  Its victims were mainly marine organisms, especially molluscs.  Study of the disappearances of bivalve species shows that those which lived in burrows suffered more than ones inhabiting open sea floor (Aberhan, M. & Baumiller, T.K. 2003.  selective extinction among Early Jurassic bivalves: A consequence of anoxia.  Geology, v. 31, p. 1077-1080).  A likely cause is loss of oxygen from the upper layer of sea-floor sediments, but a less reducing environment immediately above the sediment surface

The massive die-off at the end of the Palaeozoic Era (251.5 Ma) has focussed attention from a variety of geoscientists for over a decade.  Theories for the cause abound, including the climatic influence of the huge Siberian continental flood basalt province, which formed around the same time, explosive release of sea-floor methane, oceanic anoxia, continental aridity and a massive belch of sulphur from the deep mantle.  There is now another candidate, asphyxiation (Weidlich, O. et al. 2003.  Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop.  Geology, v. 31, p. 961-964).  The explosion of land plants in the Carboniferous and early Permian that led to the world’s great coal deposits drove up atmospheric oxygen levels to their all-time peak.  The occurrence at that time of giant insects, whose metabolism depends on direct diffusion of oxygen, suggests levels of as high as 35%.  By the end of the Permian oxygen levels may have been as low as 15%.  One line of support for such low concentrations is the growing abundance of fungal spores in the late Permian, which the authors suggest may have been related to a decline in insect populations which consume vast amounts of plant debris.  Another is the widespread evidence of anoxic conditions in the Permian oceans, including isotopic features that support a “Strangelove” ocean at the P-T boundary.  How oxygen was removed from the atmosphere in the Carboniferous to end-Permian is hard to assess.  At levels above around 25% green vegetation catches fire easily, so large firestorms may have been characteristic of the coal-forming era.  However, that would not drop levels much below those that prevail at present.  Yet the Permian is famous for its continental red beds, the red coloration being due to iron oxide (hematite).  Perhaps the missing oxygen became locked in Fe­₂O₃ as the Earth took on a distinct reddishness as the Permian progressed.

“Archaean” ironstone pods prove to be very young

For a number of reasons, including evidence that the cell-chemistry of the most primitive bacteria includes heavy metals and sulphur, the most popular current theory for the place of life’s origin suggests ocean-floor hydrothermal vents.  This has led to a search for remains of such “black smokers” in Archaean greenstone belts.  One of the most celebrated sites is in the 3.5 Ga Barberton greenstone belt on the South Africa-Mozambique border.  Within it are bodies rich in iron oxides, known as “ironstone pods” (not banded iron formations) that show many of the characteristic features of hydrothermal processes.  As well as spurring many authors into concluding that the complex organic compounds in them indicate highly developed microbial ecosystems around early-Archaean seafloor vents, scientists have used fluids included in them to speculate on Archaean oceans, and the prevailing temperatures so long ago.  They will be dismayed by a re-appraisal of the pods by Donald Lowe of Stanford University and Gary Byerly of Louisiana State University, which casts doubt on their antiquity (Lowe, D.R. & Byerly, G.R. 2003.  Ironstone pods in the Archean Barberton greenstone belt, South Africa:  Earth’s oldest hydrothermal vents reinterpreted as Quaternary hot springs.  Geology, v. 31, p. 909-912).  These pods are composed mainly of ferric hydroxide (goethite), which survives only at low temperatures, and are full of open pore spaces that include banded goethite indicating that it formed with the pores’ present orientation,  The Barberton Archaean rocks are highly deformed and were metamorphosed at greenschist facies.  The pods cut the foliation, and goethite is seen to partly replace Archaean cherts and serpentinised ultramafic lavas.  As if these features were not sufficient to rule out the pods’ formation during Archaean times, Lowe and Byerly found one that is clearly related to a now inactive modern spring that formed terraces of botryoidal goethite.  These show clear evidence of having formed as a result of modern bacterial action; they are biofilms.  In places, modern landslide debris is cemented by goethite.  Watch out for interesting correspondence in future issues of Geology from groups who stuck out their necks too far.

Artificial Archaean “fossils”

Debate on the existence of the world’s oldest microfossils from the 3.5 Ga Warrawoona cherts in Western Australia (see Doubt cast on earliest bacterial fossils, April 2002 EPN) has been stoked up by the creation of similar filamentous objects in vitro by geochemists from Spain and Australia (Garcia-Ruiz, J.M. et al. 2003.  Self-assembled silica-carbonate structures and detection of ancient microfossils.  Science, v. 302, p. 1194-1197).  They did this by mixing soluble barium salts in an alkaline sodium silicate solution (pH 8.5-11) exposed to CO₂ in the atmosphere.  At high alkalinity CO₂ dissolves to enrich solutions in carbonate and bicarbonate ions.  Filaments made up of precipitated barium carbonate (witherite) and silica soon form.  They take on shapes very similar to the tiny segmented worm-like structures that in 1996 were trumpeted as fossils in a now notorious Martian meteorite, as well as those from Warrawoona that are disputed by Schopf and Brazier.  The experimenters went a step further, by immersing the filaments in a formaldehyde-phenol mixture and heating them to 125ºC.  They then became coated in brownish, kerogen-like carbonaceous material, much as the Warrawoona structures are.  Such organic coatings can also be produced by heating iron carbonate (siderite) to 300ºC in water vapour. These “test-tube” analogues of microfossils formed in plausible chemical compositions under not particularly special physical conditions.  Interestingly, the Warrawoona chert contains both baryte and iron carbonate.  Reaction to the paper was mixed!

Eucarya missing from Mesoproterozoic

Naively, I am always surprised to learn of Precambrian oilfields, even though petroleum in the vast fields of Saudi Arabia partly had its source in Neoproterozoic sediments and migrated into the overlying cover.  Provided oil has not been degraded by later biological activity, it contains chemical traces of the organisms whose original decay produced the hydrocarbons, even a breakdown product of cholesterol (cholestane) that is characteristic of the former presence of Eucarya.  In the Northern Territories of Australia, Mesoproterozoic sediments (~1430 Ma) that formed in a shallow marine basin are a target for oil exploration.  Potential reservoir rocks contain bitumen in pore spaces, but there are fluid inclusion in fractures, which host liquid oil and brines.  Organic geochemists at CSIRO, the University of Sydney and Macquarie University have analysed the oil’s molecular structure (Dutkiewicz, A. et al. 2003.  Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia.  Geology, v. 31, p. 981-984).  Mass chromatography reveals a wealth of complex organic compounds, that are biomarkers for the kinds of organisms that were buried and then thermally matured to form the oil.  These are exclusively those which point to prokaryotes, especially the cyanobacteria.  Evidence for eukaryotic organism is completely absent.  This is useful evidence in assigning a maximum age for the rise of the Eucarya that evolved into all modern complex organisms.  The earliest likely eukaryote fossil is Grypania, a glossy carbonaceous spiral, found occasionally in sediments around 1400 Ma old, although dubious finds may indicate an origin as far back as 2100 Ma.  The dominance of evidence for photosynthesising blue-green bacteria indicates that the oil-forming organisms thrived in an oxygenated, shallow environment.  So there seems every reason to believe that Eukarya would have been capable of thriving as part of the trophic pyramid, had they arisen before 1430 Ma.

Since the discovery of the buried Chicxulub impact crater off the Yucatán Peninsula, Mexico, many geologists have regarded it as the “smoking gun” for the end-Cretaceous mass extinction.  Such is the heft of K-T studies that money has been raised to drill into the crater and its overlying sediments.  That began in late 2001 at an onshore site on the flank of the structure, and results are starting to emerge.  However, research has been slow in getting underway on the crucial part of the core that goes through the boundary itself.  That section was taken from the project’s headquarters in Mexico City to the Free University of Amsterdam, by Jan Smit, one of the pioneers of K-T boundary studies.  Samples began to reach other researchers in December 2002, 6 months after the boundary section arrived in Amsterdam.  For many, this was a little too slow and suspicions have been raised.  Everyone wanted to get abstracts into the AGU/EGS/EUG bun fight in Nice in April 2003, where a conference session on Chicxulub had been scheduled.  One report presented there seems set to stun the pro-impact school.  Gerta Keller of Princeton University studied foraminifera in the samples immediately above the impact breccia – there were plenty.  She claimed that they represented a period of about 300 thousand years of sedimentation that followed the impact.  Moreover, they occurred below the level of  a thin glauconite-rich horizon, which seems to represent the K-T extinction event itself.  Not surprisingly, Keller concluded that the impact could not have caused the extinction.  Smit dismisses the allegation of “hogging” the core samples, and also suggests that the foram-rich layers represent sediment that was washed back into the crater soon after it formed.  It has always struck me as odd that whenever something startling emerges from scientific research, a sort of preciousness overwhelms supposed scientific “objectivity”.  Counter claims and new variants of ideas rapidly evolve on the periphery of the discovery.  There are reputations to be built, and defended, and of course “sexy” themes attract cash.  The initial work that led to the recognition of a global layer of mass destruction, carried out by the Alvarez father and son team in the late 1970s, was a purer form of science – driven by curiosity and little else.

Sources:  Dalton, R. 2003.  Hot tempers, hard core.  Nature, v. 425, p. 13-14.  McKie, R. 2003.  I’ve got a bone to pick with you, say feuding dinosaur experts.  The Observer, 7 September 2003, p. 22.

Gamma-ray bursts and mass extinctions

There is a Gaelic saying, which roughly translated goes: There are more ways of killing a cat than drowning it in butter.  It seems to apply to mass extinctions.  A team of astrophysicists and palaeontologists from the University of Kansas and NASA, headed by Adrian Melott of the University of Kansas, has found peculiarities in the trilobite record after the Late Ordovician mass extinction (443 Ma) that are difficult to explain by the usual culprits.  Planktonic trilobites were decimated, but those living in deeper water largely came through the extinction.  Graptolites too incurred major changes, only the monograptids surviving until the Silurian.  Many palaeontologists link the end-Ordovician extinctions to global cooling, evidenced by glacial rocks mainly in Africa.  Melott and colleagues suggest that a realistic reason for a depth-related extinction pattern could be due to intense gamma rays emitted by the collapse of a nearby giant star into a black hole.  Although most would be blocked by the Earth’s atmosphere, that would be at the expense of nitrogen oxides being created in large volumes from oxygen and nitrogen molecules.  Nitrogen dioxide, the yellow colorant in photochemical smog would prevent solar radiation reaching the surface and trigger cooling.  Also acid rain would lower the pH of surface water.  Such a process could also explain the Late Ordovician glaciation of Africa.

Source  Hecht, J. 2003.  Did a gamma-ray burst devastate life on Earth?  New Scientist, 27 September 2003, p. 17

Fossil oddities – a golfing trilobite and the ox-sized rodent

Gamblers and golfers do not like distractions, and many wear eye shades of some design or other.  So it is intriguing to learn that a Devonian trilobite, Erbenochile,  found in Morocco evolved a similar device.  Richard Fortey and Brian Chatterton, of the British Museum of Natural History and the University of Alberta, respectively, analysed the peculiar eyes of this phacopid trilobite, and found that their tops had a sort of rim.  Light shining down on the beast put the compound facets in shadow (Fortey, R. & Chatterton, B.  2003.  A Devonian trilobite with an eyeshade.  Science, v. 301, p. 1689).  Not only would this arthropod have been undistracted from its activities by goings on above, but it could also see over its back.

Not since the discovery of the Late Miocene Bullockornis in Australia (see The Ducks of Death in EPN June 2000) have Neogene palaeontologists come up with a record beater.  But now they have (Sanches-Villagra, M.R. et al. 2003.  The anatomy of the world’s largest extinct rodent.  Science, v. 301, p. 1708-1710).  The Late Miocene of Venezuela has yielded a rodent (Phoberomys), whose bones suggest that it weighed in at about 0.7 tonnes.  It is related to modern guinea pigs, and probably had much the same herbivorous habits.  Its teeth suggest that it was grazer too, and like the modern capybara (one tenth the size of Phoberomys) it lived in swamps.  Rodents now rank as the mammalian order with the greatest range of sizes.  Because the digestive systems of mammals cannot efficiently break down the high cellulose content of grasses without the aid of internal bacteria, the bigger their gut, the more efficient they are as herbivores.  So giant rodents make sense as regards their metabolism.  However, they are not as well known for galloping as many other grazers, which is why smaller rodents prefer to escape predation by diving into burrows or among boulders.  That would be difficult for a creature as big as an ox.  Swamp dwellers, like the capybara and Phoberomys, can get away with not being fleet of foot, but would not do well on open grassland.

The compiler of EPN welcomes news of odd and awesome fossils, and hopes soon to learn of mighty hamsters and their adaptation to natural treadmills.

See also:  Alexander, R.M. 2003.  A rodent as big as a buffalo.  Science, v. 301, p. 1678-1679).

The crucial step in assembling amino acids into the proteins that are central to living organisms is the formation of peptide bonds.  Amino acids are found even in meteorites and seem to form abiogenically with some ease.  Peptide bonds link simple amino acids into long chains that are the essence of complex proteins, but this does not happen spontaneously.  The bonds form in the presence of carbon monoxide, but require some kind of catalysis.  Researchers at the University of Munich, Germany have discovered that very fine-grained precipitates of iron and nickel sulphides readily perform such catalytic functions (Huber, C. et al. 2003.  A possible primordial peptide cycle.  Science, v.  301, p. 938-940).  This tallies nicely with one of the co-workers’ (Günter Wächtershäuser) hypothesis for the chemoautotrophic origin of life near sea-floor hydrothermal vents, where Fe, Ni and S are abundant, as is CO in the hot water that emanates from them.

Much has been made of ups and downs in the diversity of life from the global fossil record of the Phanerozoic, including the possibility of massive downturns in diversity related to a variety of cause for mass extinction.  However, there are many biases in what is an inevitably imperfect record of biodiversity.  There are anthropogenic influences, for a start.  Although they are becoming more adventurous, palaeontologists cut their teeth on sites close to home, and most of them live in the richer parts of the world.  Insatiable demand for fossils, but mainly of the spectacular and valuable kinds, has grown a world-wide industry of commercial fossil mining.  That may homogenise the geographic coverage of the fossil record, but it is very tempting to go for the richest troves and ignore meagre pickings.  Sedimentation is by no means guaranteed to have been constant through time, partly because of ups and downs of sea level and changes in the pace of erosion of earlier rocks.  Although Phanerozoic stratigraphy seems complete when sections from all over are pieced together, in any one place there are huge gaps of erosion or non-deposition.  It is very easy to come upon several  beds of sedimentary rock and conclude that the sequence represents a continuum in time.  Not so, as any examination of such beds forming today often reveals that intact preservation is the exception compared with erosion and reworking.  The global areas of exposed rocks that cover, say, 10 Ma chunks of Earth history is by no means constant either.  Another factor that conspires to cast doubt on the veracity of the existing fossil record is that the numbers of possible ecological niches that once existed in different tectonic environments are probably not the same.  Active oceanic arcs have few such niches, whereas tropical zones of shallow shelves have vastly more.  There are lots more uncertainties, and New Zealand palaeontologists have painstakingly tried to develop some means of allowing for them in the Tertiary record of their islands (Crampton, J.S. et al. 2003.  Estimating the rock volume bias in paleodiversity studies.  Science, v. 301, p. 358-360).  The simplest premise for estimating bias in the numbers of taxa preserved in rocks covering a particular time range is the available volume of rock from the period that can be sampled.  One approach is to see how geologists have divided up that period in terms of distinct rock formations, the other just uses estimates of the areas underlain by sedimentary rocks laid down during the period.  The first suggests that collecting should be systematically from formation to formation up a sequence, while the second implies that random grid sampling is the best approach.  The New Zealand data suggest that the area approach is most appropriate there, largely because the local rocks formed in a sedimentologically simple, active-margin environment.  Both methods seem to work in tectonically stable areas.  This is just a beginning, but is raises the issue of how much weight can be placed on existing fossil collections in pondering on both titanic and slow-but-sure episodes in the last 544 Ma.

On the same tack, attempts are underway to correct the entire fossil record from 30 thousand collections, using a similar approach to sampling bias.  John Alroy at the University of California, Santa Barbara has helped set up the Paleobiology Database (http://flatpebble.nceas.ucsb.edu/public/), following prompting by the most prolific fossil cataloguer, Jack Sepkoski, shortly before his untimely death in 1999.  The web site allows anyone to generate diversity curves, but the process is a little complicated and best tackled by experienced palaeontologists.  You can also enter information from your own collections.  Early results are conflicting.  Sepkoski’s original suggestion that diversity among marine faunas increased since the Triassic may be an artefact of the intensity of sampling which varies from age to age.  However, using just molluscs seems to confirm that at least they did indeed radiate tremendously as Sepkoski had concluded (Schiermeier, Q.  2003.  Setting the record straight.  Nature, v. 424, p. 482-483).

Origins of the vertebrates

Long before techniques were developed to investigate the genetic stuff of living organisms, and when the only known repository of primitive, soft-bodied animals was the Burgess Shale, basic anatomical analysis suggested that maybe the ancestors of vertebrates were worms, sea squirts and even echinoderms.  When the Burgess Shale fauna was re-evaluated and extended in the 1970’s by, among others, Simon Conway Morris of Cambridge University, it became clear that the fossil record was missing a great many delicate and sometimes very odd organisms.  Entirely unsuspected phylla numbered among the occupants of that famous lagerstätte (site of exceptional preservation), but little new about our own ultimate origins.

Vertebrates, echinoderms, sea squirts and a diverse collection of worm-like animals have one thing in common, though apparently very little else.  The first opening to emerge during embryonic development is the anus, whereas in the rest of the animals (protostomes) it becomes a mouth.  So, in the “supergroup” to which we belong, mouths appear at a later developmental stage; hence the sack-name “deuterostome”.  This oddly dichotomous embryonic unfolding points to a very early division among the animals, that might only be unveiled by discovery of even earlier lagerstätten than the Late Cambrian Burgess Shale.  So far, no such source of palaeontological richness has been discovered in late Precambrian sedimentary rocks – crude “molecular clock” approaches to genetic divergence suggest that a great deal went on before the Cambrian Explosion at 544 Ma.  However, the fossil-rich Cambrian of China does push back the record of delicate animals almost to that time.  The recently discovered lagerstätte of Chengjiang is about 530 Ma old, and, as Conway Morris and his Chinese colleagues have discovered, it is rich in fossil deuterostomes.  One group, the vetulicolians, bears a remarkable resemblance to what the pioneer vertebrate palaeontologist, Alfred Romer, suggested as a probable vertebrate ancestor – something with a front end bearing gill slits and a long, segmented tail.  The Chengjiang deposit also contains jawless fish, together with unique “almost fish” called yunnanozoans that may be intermediate links between vetulicolians and fish.  Similarly, there are intriguing hints that vetulicolians evolved towards the most primitive echinoderms, with bilateral symmetry rather than the fivefold form that emerged later.  Clearly, the Chengjiang fauna was extremely diverse and therefore had a long evolutionary history.  Since even more delicate, entirely soft-bodied Ediacaran animals were preserved as imprints in sandstones from the Late Neoproterozoic, it is maybe only a matter of time before low-energy lagerstätten are found from that time.  There are abundant undeformed mudstones from that period throughout the world, but only painstaking rock splitting will find such treasures, unlike the large, “trip-over” Ediacaran trace fossils.

Source:  Conway Morris, S.  2003.  Once we were worms.  New Scientist, 2 August 2003, p. 34-37.

The very beginning of the Cambrian is associated in every geologist’s mind with the explosive appearance and diversification of animals with hard parts.  Why this dramatic introduction to the modern biological world occurred is one of the great questions in evolution.  Some connection with the effects of “Snowball Earth” events in the late Neoproterozoic was thrown into doubt by evidence that it had little effect on micro-organisms (see Microbes showed no sign of change following a “Snowball Earth” in May 2003 EPN).  Exactly at the boundary there is a marked fall in the abundance of carbon-13, and this negative d¹³C excursion is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age.  One of the places that it occurs is in Oman, reported previously in EPN (A possible fuse for the Cambrian Explosion, January 2003).  The paper describing the evidence from Oman that the carbon-isotope excursion relates to a mass extinction is now out (Amthor, J.E. and 6 others 2003.  Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman.  Geology, v. 31, p. 431-434)  The disappearance of the distinctive eukaryote fossils coincides exactly with the carbon anomaly.  Luckily, so too does a volcanic ash horizon from which zircons provide a very precise U-Pb age of 542±0.3 Ma.  This matches less precise dates for the anomaly from Siberia and Namibia, and seems likely to become accepted as the definitive age for the start of the Phanerozoic.

“Snowball Earth” and evolutionary diversification: Australians speak out

By comparison with the vast amounts of Australian diamictites that span a range of Neoproterozoic ages, the sites elsewhere, from which evidence in support of the “Snowball Earth” hypothesis and possible effects on evolution have been drawn, are puny.  Besides that, the Late Precambrian of Australia has the best record of biological change, including the type locality for the Ediacaran fauna that presaged the Cambrian Explosion.  Although somewhat less hasty than the flurry of papers on the “Snowball” hypothesis, since 1998, the appearance of published data from the “Red Continent” is sure to push the debate decisively one way or another.  Palaeontologists from the Geological Survey of Western Australia, Macquarie University and Mineral Resources Tasmania have just unveiled details of acritarchs from late-Neoproterozoic sediments that overlie the Marinoan (~600 Ma) glaciogenic rocks in South Australia (Grey, K. et al. 2003.  Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?  Geology, v. 31, p. 459-462).  Acritarchs are spore-like fossils, that probably represent encysting algae.  Their rapid diversification makes them useful biostratigraphic indicators from the Late Precambrian to the present.  Grey et al. Found that the same assemblage of acritarchs occur before the Marinoan glaciogenic strata and after the succeeding “cap” carbonate.  They are part of a group that can be traced back to the Mesoproterozoic  However, higher in the sequence that they examined there is a distinctive layer of debris that contains evidence of impact-induced shock.  This can be correlated with little doubt to the 90 km Acraman structure in South Australia, which formed at 580 Ma with an energy likely to have had a major influence on life.  Sure enough, in the strata above this ejecta layer a completely new type of acritarch group appears and diversifies rapidly, while the pre-impact groups simply disappear.  Clearly, the Acraman impact is implicated in this sudden biological change; an extinction followed by rapid diversification.  Acritarchs are thought to represent the phytoplanktonic base of the Neoproterozoic food chain.  Immediately above the strata in which the post-impact acritarchs diversified lie sandstones that contain the famous Ediacara fauna of the first large, soft bodied animals.  The Marinoan “Snowball” event seems disconnected from this evolutionary leap.

Discussing what actually killed off around 95% of all species 251 Ma ago has become the perennial mass-extinction topic, now that the K-T boundary event is more or less done and dusted, bar a little murmuring over the Deccan Trap.  Michael Benton of the University of Bristol has summarised the current state of play for the Permian-Triassic (P-Tr) event (Benton, M. 2003.  Wipeout.  New Scientist, 26 April 2003, p. 38-41).  Despite many attempts to link an impact to the annihilation – such evidence as there is (see Buckyballs and the end-Palaeozoic extinction, EPN March 2001) has not been reproduced by independent analysis of the material.  Weighty evidence comes instead for an Earth-induced event, from the coincidence of the monstrous Siberian Traps with the 100 thousand years or less that the extinction occupied, and from complete sequences across the P-Tr boundary in a Japanese ophiolite and a shallow marine section at  Meishan in South China.  As well as an intricate series of faunal changes, the Meishan sequence has now provided a complete record of oxygen and carbon isotopes that span the boundary event.  The oxygen data suggest a 6ºC rise in global temperature at exactly the stratigraphic level of the extinction and of a massive lurch towards light carbon.  Such a high proportion of ¹²C occurs at the boundary that it cannot have been induced by sterilisation of the oceans, which may well have happened as a result of the extinctions.  Nor can even the huge belch of mantle CO₂ emitted by Siberian continental flood basalts.  The two combined only account for 40% of the carbon-isotope excursion.  Release of methane from long-term storage as gas hydrate on the Permian sea floor is the only conceivable candidate.  So it looks as if a runaway “greenhouse”, plus toxic gas and maybe acid rain put paid to most living things.  Such a wiping out left lifeless oxygen-poor oceans – originally dubbed “Strangelove oceans” by Ken Hsu after the eponymous insane doctor.  Triassic times did not see explosive reoccupation of abandoned niches, recovery taking up to 50 Ma from a tiny population of not very diverse organism.  Benton has written a book on the P-Tr event (When Life Nearly Died.  Thames & Hudson), and that is likely to be a rattling read. New Scientist maintains it’s irritating habit of never referring to sources in its articles, so to go further, you will have to buy the book.

Microbes showed no sign of change following a “Snowball Earth”

The “Snowball Earth” hypothesis has suffered quite a lot since its original promotion (see: Meltdown for Snowball Earth?, February 2002 EPN; Snowball Earth hypothesis challenged, again, December 2002 EPN).  Whatever the eventual fate of the notion that the entire Earth was iced over from pole to pole, the fact that glaciers reached sea level at low latitudes at least twice in the Neoproterozoic seems to be an established fact.  Such climate swings must surely have had an effect on life, either by driving up the rates of extinction and adaptive radiation because of stress, or perhaps providing nutrients to the oceans in vast amounts that allowed the phytoplankton base of the food chain to explode (see: The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic, September 2002).  One of the first discoveries of low-latitude glaciogenic deposits was around Death Valley, California by the late Preston Cloud, who worked there during the 1960s.  So it is fitting that palaeobiologists associated with the Preston Cloud Research Laboratory at the University of California, Santa Barbara have dissected sediments within and immediately beneath the 750 Ma diamictites that Cloud interpreted as glacial in origin, to test for signs of evolutionary change (Corsetti, F.A., Awramik, S.M. & Pierce, D. 2003. A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA.  Proceedings of the National Academy of Science, v. 100, p. 4399-4404).  In cherts within carbonate units they found a surprisingly diverse range of undoubted microfossils, that are probably auto- and heterotrophic Eucarya, but no difference between pre-glacial and glacial levels, in terms of their biota.  Although this single piece of work does not prove that there was no biological change associated with a major cooling during the Neoproterozoic, it does cast doubt on the severity of its effects on life.  Most important, the study shows that well-preserved cellular material is available for study in sediments that occur with glaciogenic diamictites, and should open up a new line of research bearing on the rise of the metazoan (multi-celled) Eucarya, which appeared in large numbers shortly after the last (~600 Ma) glacial epoch.  Most if not all Neoproterozoic carbonates, whose universal presence in close stratigraphic proximity to glaciogenic strata first hinted at low-latitude frigidity, contain abundant chert nodules that are the best preserving medium for delicate and tiny cell structures.

Steadily, the remarkable fossil record in Cretaceous terrestrial sediments in China is revolutionising ideas about vertebrate evolution, particularly among small dinosaurs and early birds (see The Early Cretaceous lagerstätten of NE China in EPN March 2003).  The long-held view that birds simply emerged fully fledged and flying from the dinosaurs has had to be thoroughly amplified.  The sheer diversity, combined with intricate preservation in the Chinese sediments reveals feathering on a host of animals that are not birds, but earlier, bipedal dinosaurs.  Some may have flown, but others had feathers for some other reason.  Feathers are not a prerequisite for flying, and are so odd and complex in morphology and growth, that it has always been probable that they emerged and evolved over a long period preceding the appearance of true birds.  Now it is possible to begin dissecting that strange evolutionary divergence, and Richard Prum and Alan Brush of the Universities of Kansas and Connecticut combine information about feathers and discussion of new fossils in a superbly illustrated review in the March issue of Scientific American (Prum, R.O. & Brush, A.H. 2003.  Which came first, the feather or the bird.  Scientific American, March 2003, p. 60-69).

Squirrels and tectonics

The squirrel family (Sciuridae) is one of the most widespread groups of mammals, only Australia, the Pacific islands and Antarctica being squirrel-free.  The main reason is that squirrels are basically a primitive group among the rodents, themselves accounting for almost 50% of all living mammals species.  The earliest fossil squirrel (Douglassciurus jeffersoni) was found in Late Eocene sediments in western North America, and the family seems to have originated there.  The present wide distribution of squirrels bears witness to the many opportunities for migration in the Palaeogene, when continental masses were much less dispersed than they are today, together with changing environmental conditions that would have acted to drive migration.  In the same way as human migrations have been charted and timed using genetic sequencing and molecular clock hypotheses, this unique group has been studied in detail (Mercer, J.M. & Roth, L. 2003.  The effects of Cenozoic global change on squirrel phylogeny.  Science, v. 299, p. 1568-1572).  The general picture outlined by Mercer and Roth is that the Sciuridae migrated first across Beringea to reach Asia, then Europe and eventually Africa.  In terms of migration rates, this was fast, the earliest European squirrel (Palaeosciurus) occurring in Early Oligocene sediments – this is also the earliest representative of squirrels that bear signs of the distinctive chewing muscles whose use today delights us all.  Near identical musculature is found in the Red Squirrel and many other tree squirrels (Sciurus sp.), and their “living fossil” anatomy is borne out genetically.

As well as giving a fascinating insight into how modern genetic techniques help organise the cladistics of animals, the paper is full of information about the sheer diversity that this lowly group has achieved in about 50 Ma.  Ground squirrels, rock squirrels, marmots, and tree squirrels abound, but none are so fascinating as the flying squirrels.  Their teeth are similar to those in early Oligocene fossils, and genetic analysis suggests a common ancestry relatively early in squirrel evolution and migration.  However, fossils of flying squirrels, in the areas where they are found today (North America and Asia) appear quite late in the stratigraphic column.  The authors suggest that perhaps flying ability arose several times independently, based on a labile trait in the genes of their clade.  There is also evidence for population “bottlenecks” that preceded adaptive radiation in several area.  For instance, the entire radiation of South American squirrels seems to have stemmed from a single lineage that crossed the Isthmus of Panama shortly after it formed in Pliocene times.  African squirrels can be accounted for by just two colonisations in the Miocene, and those of Indonesian archipelago east of the Wallace Line by migration during the Late Miocene, when sea-level was at its lowest before the Pleistocene lowstands.  Most astonishing of all, is the Giant Squirrel of Borneo (Rheithosciurus), which is genetically closest to the squirrels of North America rather than its more diminutive cousins in the Sunda Shelf islands – did its ancestors move with astonishing speed, or did all related squirrels along its migration route become extinct quite rapidly?

A possible answer to the origin of the Giant Squirrel of Borneo lies in a collection made recently from a unique lagerstätten in a clay-filled pocket within laterites of northern Karnataka in India.  The discoverer, Dr P.U. Siffli of Sringeri Institute of Palaeontology, has posted provisional results on his web site (http://geocities.yahoo.com/pusiffli/squirrels.html).  The range of fossil rodents from near Sringeri is astonishing.  Among them are bones of an undoubtedly primitive squirrel of enormous dimensions – approximately the size of a large child.  Its masticatory musculature is similar to that of the North American Douglassciurus jeffersoni of Eocene age, i.e. unlike that of modern tree-squirrels.  The biggest surprise lies in the dentition of the Sringeri giant squirrel.  The typical rodent second incisors are serrated and arranged in a similar way to the shearing canines of mammalian carnivores.  Its back teeth bear close resemblance to carnivore carnassials.  As if this was not sufficient, the body cavity of the best preserved fossil contains pellets made up exclusively of bones from primitive hamsters, which abound in the lagerstätten.  In a personal communication, Pandit Unmer makes a convincing case that he has discovered the only known predatory squirrel (provisionally named Titanosciurus sringeriensis), and will soon submit his finding for peer review.  His only regret is that establishing a stratigraphic age for the laterite-bound pocket is proving to be very difficult.  Sitting atop Archaean gneisses, the laterite can be correlated with similar palaeosols that cover the 64 Ma Deccan flood basalts some 130 km to the north, yet they defy dating by palaeontological or radiometric means.  Dr Siffli would welcome offers to date the Sringeri lagerstätten (pusiffli@yahoo.com).

The earliest indisputable traces of metazoan animals are quite literally that – the impressions of soft-bodied organisms preserved as the Ediacaran fauna of Australian and other late-Neoproterozoic sediments dated around 565 Ma.  However, the profound differences in genetic make-up of existing animal phylla, which clearly at the time of the Cambrian Explosion, have been expressed as indicators of animals’ origins more than a billion years ago.  Consequently, the discovery in 1998 of what appeared to be non-Ediacaran trace fossils in the Neoproterozoic Vindhyan Supergroup of India triggered considerable interest.  The problem with many of India’s Precambrian sediments is their lack of precise and verifiable dates.  Occurrences of the sedimentary silicate glauconite in the Vindhyan prompted use of the K-Ar method, which suggested that they were pre-1100 Ma, but that is a notoriously unreliable technique.  Part of the lower Vindhyan succession contains poorcellanites that show textural evidence for having originated at ignimbrites, and they contain zircons of volcanic origin.  Once sampled, it was only a matter of time before precise single-zircon U-Pb dates became available.  In fact, two teams published simultaneously in the February issue of Geology, and gave similar ages from different places (Ray, J.S. et al. 2002.  U-Pb zircon dating and Sr isotope systematics of the Vindhayan Supregroup, India.  Geology, v. 30, p. 131-134;  Rasmussen, B. et al, 2002. 1.6 Ga U-Pb zircon ages for the Chorhat Sandstone, lower Vindhayan, India: Possible implications for early evolution of animals.  Geology, v. 30, p. 103-106).  The first paper gave an age of 1631 Ma for strata immediately beneath the supposedly fossiliferous formation, whereas the second bracketed it between 1628 and 1600 Ma for rocks beneath and above it.

If the structures preserved in the Chorhat Sandstone do prove to be true trace fossils, there will be little doubt that animals appeared at least three time earlier than the previous fossil-based estimate, more in line with the molecular evidence.  However, the structures are disputed, and there is another oddity about the palaeontology of the Vindhyan.  Limestones that conformably overly the 1600 Ma dated horizon have been reported to contain brachiopods and “small, shelly faunas” typical of the earliest Cambrian elsewhere.  Since the limestones are only a few hundred metres higher in the Vindhyan sequence, and contain ⁸⁷Sr/⁸⁶Sr isotope ratios that are appropriate for Neoproterozoic seawater, brings their content of Cambrian fossils into doubt.  Clearly, a great deal more work is needed to resolve the significance of the Vindhyan finds, particularly establishing accurate, basin-wide stratigraphic correlation.

Are mass extinctions artefacts of sampling bias?

Evidence for mass extinctions comes from inventories of fossil species, genera and families collected from the sedimentary record.  There has always been a geographic bias in this sampling towards more accessible areas and those with the greatest number of palaeontologists, i.e. towards rich countries.  Increasing grants for expeditions to remote areas and the slow growth in numbers of specialists in less well-endowed countries does smooth out the bias.  However, because of many factors, including ups and downs in sea level and the effects of orogeny on rates at which deformed sediments have been eroded, the stratigraphic record itself does not accurately represent time with exposed rocks.

The data on which extinction records rest are those compiled by the late Jack Sepkoski, yet until recently there has been little attempt to weight them according to stratigraphic record, although much statistical re-evaluation has gone on (e.g. The “Big Five” become the “Big Three”? Earth Pages of January 2002).  This stratigraphic evaluation to some extent pulls the rug from under those who speculate on the causality of extinction (Peters, S.E. and Foote, M.  2002.  Determinants of extinction in the fossil record.  Nature, v. 416, p. 420-424).  A great many ups and downs in the fossil record do seem to depend on the amount of exposed sedimentary rock.  Widespread gaps in the sedimentary record result in spurious and abrupt ends to evolutionary lineages; pseudo-extinctions.  Although the period- and era-ending extinctions seems still to be statistically valid, those at stage boundaries are suspect.  One of the lessons to be learned is that the previous good correlation between sea-level change and extinction and origination rates is particularly suspect, as eustasy is a first-order contributor to chages in sedimentary deposition and preservation.

Doubt cast on earliest bacterial fossils

In autumn 1996 two of the most blatant hyperboles in the recent history of the Earth sciences hit the world’s headlines; two groups of scientists, one from the USA, the other British, announced their discovery of fossil life forms in meteorites reputed to have originated on Mars.  The evidence was in the form of organised structures revealed by scanning electron microscopy.  Subsequently, most biologists and palaeontologists concluded that the case was, in the manner of the third possible verdict in Scottish courts, “not proven”.  Kindly scientists regarded the hype as being prematurely optimistic.  However, critical attention focussed on the announcements because they claimed first discovery of extraterrestrial life.  If one finds a mammoth while digging a ditch, there is some cause for celebration, and the world will believe and congratulate the finder, for the mammoth is unmistakable.  That is not the case for fossilized micro-organisms.  In 1993, William Schopf of UCLA, and co-workers, announced their discovery of the oldest known fossil bacteria in 3465 Ma cherts in a greenstone belt near Marble Bar in Western Australia.  They were microscopic wisps of carbonaceous material, that a trained eye might resolve into filaments made of bacterial cells.  Since the most common living filamentous bacteria are photosynthetic cyanobacteria, that bear close resemblance to sketches of the ancient structures, Schopf and colleagues performed the palaeontological equivalent of Aristotle’s syllogism, by declaring that indeed some of the structures were blue-green bacteria.  In what was generally regarded as an anoxic Archaean world, it seemed there were organisms working to oxygenate the environment.  Various lines of evidence, such as the isotopic composition of carbon in Archaean sediments, were later claimed by others to support such an early arrival of cyanobacteria, that eventually transformed the atmosphere and the conditions for life, so that oxygen-demanding Eucarya, such as ourselves, might evolve and diversify.

There is one snag with the Marble Bar chert.  It almost certainly formed by hydrothermal activity on the Archaean ocean floor; deep and dark.  Photosynthesis using solar energy would be unlikely.  Re-examination of the putative fossil filaments, using both microscope and Raman spectroscopy (means of estimating C/H ratios from spectra excited from carbonaceous matter by a laser) has raised a minor storm.  Martin Brazier of Oxford University and colleagues from Britain and Australia question the biological origin of the structures (Brazier, M.D. et al. 2002.  Questioning the evidence for Earth’s oldest fossils.  Nature, v. 416, p. 76-81).  Amazingly, one of their observation while examining Schopf’s original material with a high powered microscope was that by racking the objective up and down to visualize the structures in 3-D, most showed to be highly irregular smears of carbonaceous stuff.  Only one position provided life-like shapes.  While Brazier et al. do not deny that life was around in the chert-forming hot spring – probably chemautotrophic prokaryotes – they are convinced that Schopf’s structures are artefacts formed by hydrothermal reworking of degraded organic molecules.  In a rejoinder, Schopf and US colleagues accept the deep-water, hydrothermal origin of the cherts and concede that none of the structures are blue-green bacterial cells, but still maintain that they are biogenic (Schopf, J.W. et al. 2002.  Laser-Raman imagery of Earth’s earliest fossils.  Nature, v. 416, p. 73-76).  The earliest undisputed fossil micro-organisms are almost 1.4 billion years younger than those of Marble Bar.  They are from cherty layers in banded iron formations, formed probably in shallow water by the combination of oxygen produced by cyanobacteria with dissolved ferrous iron.  The Archaean contains plenty of BIFs, and perhaps a search for the oldest biotas in them would give more definite results.

See also:  Kerr, R.A. 2002.  Earliest signs of life just oddly shaped crud?  Science, v. 295. P. 1812-1813.

Barely a month passes without some weird fossil emerging from the widespread excavations in Early Cretaceous lacustrine sediments of north-east China.  It is probably the most productive palaeontological formation in the world, and has shed light on more than just the dinosaurian origin of birds, and rives ideas on the rise of angiosperm plants and early mammals.  As well as abundant fossils, the lagerstätten formed under low-oxygen conditions and preserves exquisite detail of soft tissue.  A review of the material and the environment in which it formed is welcomed by all palaeontologists (Zhou, Z, Barrett, P.M. & Hilton, J. 2003.  An exceptionally preserved Lower Cretaceous ecosystem.  Nature, v. 421, p. 807-814).  Zhou et al. Discuss the formation from two angles.  Scientifically their focus is on the potential for building a complete ecosystem for the area during the Early Cretaceous.  However, they also record the massive problems that result from haphazard collection by organised teams of locals and fossil dealers – incidentally the source of the infamous Archaeoraptor forgery (see “Piltdown” bird, in March 2001 issue of Earth Pages News).  Their review is also a plea for some kind of firm regulation of collection, although experience from many other lagerstätten suggests that is unlikely in the short-term.

Did terrestrial life emerge later than geochemists think?

A lot hangs on the notion that life can make it from abiogenic chemistry very quickly once a world has watery seas.  Evidence from oxygen isotopes in the oldest known terrestrial zircons suggests that liquid water was around on Earth by about 4400 Ma (see Pushing back the “vestige of a beginning” in Earth Pages News of February 2001, and The Hadean was cool June 2002).  It lies behind the search for signs of life on Mars and the fiasco surrounding the premature announcement of bacterial fossils in a meteorite reputedly from the Red Planet.  Right here, controversy has been raging over the once-living status of tiny patterns in 3500 Ma cherts from Western Australia (see Doubt cast on earliest bacterial fossils in Earth Pages News, April 2002), and on the true significance of isotopically light carbon trapped in apatite crystals in the 3800 ma Akilia metasediments of West Greenland.  Both have been claimed as signs of early, well-organised life, but the evidence is circumstantial.

Investigative journalism is very welcome in science, mainly because most scientists are either too polite, or grumble quietly in the coffee room.  Jon Copley, who teaches at Southampton University, has ventured into the field by interviewing some of the main antagonists in the “Is this a sign of life” debate (Copley, J. 2003.  Proof of life.  New Scientist, 22 February 2003, p. 28-31).  His article is most revealing, by getting down to brass tacks.  There is a lazy tendency in science to invoke William of Ockham’s “Razor”, i.e. that the simplest explanation of data is the best.  That is fine for the Old Bailey, in the manner of Roman legal argument of cui bono (who benefits?), but the natural world has a cussed tendency to pay no attention to human linear thought,  It is not a place for “elegance”, no matter how much scientists feel in awe of elegant mathematical proofs.  That it is wielded in favour of the most complex process in the universe to account for geochemical and other data is a bit odd.  Central to Copley’s sharp journalism lies something of which C-isotope specialists do not speak much.  At temperatures around 400ºC and a few hundred times atmospheric pressure can result in carbon monoxide and hydrogen combining to form hydrocarbons.  Fischer-Tropsch synthesis of hydrocarbons that fuelled Nazi Germany and South Africa under apartheid does occur in nature.  The ideal place is around deep-sea hydrothermal vents.  The reactions favour ¹²C over the heavier ¹³C and results in d¹³C just as negative as do living processes.  Isotopically light carbon in rocks that do not contain cast iron confirmation through tiny fossils, cannot be seen as proof that life existed.  Probably the oldest irrefutable fossils are of bacteria in the 1900 Ma Gunflint Chert of Ontario.  If we cannot be sure that C-isotopes help detect living processes on the early Earth, then results from missions, such as Beagle-2, to Mars could be exercises in futility.

Robert Williams (Oxford University) and João Fraústo da Silva (Technical University of Lisbon) have an unconventional, but plausible take on the conditions for life’s origin and evolution (Williams, R.J.P & Fraústo da Silva J.J.R. 2003.  Evolution was Chemically Constrained.  Journal of Theoretical Biology, v. 220, p. 323-343).  However life began, presumably as cytoplasm containing DNA, RNA and proteins within a semi-permeable wall, it was surrounded by the chemistry of whatever environment it appeared in.  The proto-cell would have drawn hydrogen ions from water, to perform the proton pumping that is essential to all living organisms, and thereby created more oxidising conditions in its immediate vicinity.  Oxidation would have generated nitrogen from ammonia, released metals from their sulphides and converted other sulphides to sulphates.  Conversely, ions in its surroundings would have been able to “leak” into the cell itself.  By creating oxidised radicals, this inward leakage would have rebounded the cell’s activity on itself, with potentially toxic consequences.  Survival depended on two things: exploiting the opportunities, such as nitrogen fixation, using oxygen and even photosynthetic chemistry; and fending off potential toxic shock.  One of the most interesting aspects is the role assumed by calcium ions.  Their presence inside a cell would have precipitated DNA, by binding to it, with fatal consequences.  The upshot, according to Williams and Fraústo da Silva, is the special role of calcium as a messenger ion, perhaps having arisen through the necessity to pump it out again.  Today, the range of calcium concentrations in cells is extremely limited; too much or too little being fatal.  Perhaps a sudden change in the calcium-ion concentration in seawater in the late Neoproterozoic was responsible for the extreme excursions in carbon isotopes that are ascribed to mass extinction and equally massive adaptive radiations.  My own stab in the dark, is that a protective response to calcium stress by metazoans at that time may explain the sudden appearance of calcium-rich hard parts, which we know as the Cambrian Explosion.  They evolved means of excreting calcium from their many cells, so creating an outer “shell” that eventually developed into “armour” or “armament”.

The delightful aspect of Williams and Fraústo da Silva’s ideas is that they break from pure genetic determinism and the dominance of pure chance in addressing the central issue in the whole of science – the complete interconnectedness of real nature.

The sudden appearance of shelly fossils between about 544 to 542 Ma is the most astonishing feature of biological evolution, especially as representatives of every modern animal phylum (and some which have vanished) appear at that time.  A means to explain this short-lived blossoming has eluded palaeontologists.  Part of the problem is that the record of the immediately preceding Neoproterozoic Era cannot resolve whether the phyla sprang up at the same time as they developed hard parts, or had been evolving as flaccid forms for much longer.  Another aspect is the difficulty in accounting for the sudden adoption of calcium carbonate and phosphate hard parts.  It seems inescapable that the issue of hard parts, which is really what the “Explosion” is all about, cannot be separated from the chemistry of seawater at the time.

A new insight into what was going on was presented at the October GSA meeting in Denver by John Grotzinger and colleagues at MIT, who have been examining drill cores through the Precambrian-Cambrian boundary beneath the south Omani oil fields.  The Late Neoproterozoic basin in which the deposits began to form was a semi-enclosed basin, dominated by stromatolitic carbonates.  Seawater in it contained excess calcium and carbonate ions.  Periodically, the basin was cut off and evaporites began to form; it became hypersaline.  In the cyclical sequence the very earliest carbonate-shelled organisms (Cloudinia and Namacalathus) left fossil remains.  However, in cycles of earliest Cambrian age they simply disappear, not merely in Oman but world wide.  Moreover, rocks from which they are missing show abnormally light d¹³C, generally interpreted as a result of mass extinction.  The demise of two organisms, albeit the only ones that could have left any record, may not seem very dramatic.  But Grotzinger and colleagues suggest that a sudden extinction could mark a critical period in evolution that both reduced the population of all organisms and sterilised ecological niches for future adaptive radiation.  Interesting, but still not explaining why hard parts were adopted to become so very necessary in subsequent animal evolution.

Source:  Kerr, R.A. A trigger for the Cambrian Explosion?  Science, v. 298, p. 1547.

One of the many crises through which life passed during its evolution was the widespread appearance of oxygen.  This occurred once the release of soluble iron-2 to the oceans from sea-floor processes fell below a rate that buffered the photosynthetic generation of oxygen through the precipitation of iron-3 oxides in marine sediments.  Oxygen is life-threatening, largely through its encouraging the formation of simple compounds that are more potent oxidizers than oxygen (O₂) itself, such as O^–, H₂O₂ and HO.  In cells they can lead to genetic degeneration, progressive ageing and eventually cell death.  Free oxygen in the environment was a stealthy threat to all life forms that existed around 2200 Ma.  A possible evolutionary response that may have opened the way for the later rise of the Eucarya, and the huge diversification that permitted, is nicely summarized by Doris Abele in Nature of 7 November 2002 (Abele, D. 2002.  The radical life-giver.  Nature, v. 420, p. 27).  The main strand of her argument is that mitochondria, the energy converters in eukaryote cells, also serve to keep oxygen levels inside cells high enough for metabolism, yet low enough to minimise the formation of threatening oxidants.  Her object of study has been the noble ocean quahog, Arctica islandica (incidentally, a clam often referred to by Herman Melville in Moby Dick) which mysteriously burrows into anoxic muds for a while and drops its metabolism alarmingly.  By this habit, the quahog has achieved what middle-aged Californians yearn for; spectacular life extension to as much as 220 years.  Abele believes that this protective function of mitochondria  is deployed by the quahog, having arisen in the earliest Eucarya, after the oxygenation of the planet.  However, as Lyn Margulis observed in developing her endosymbiotic hypothesis for the emergence of eukaryotes, mitochondrial RNA is very like that of oxygen-respiring purple bacteria.  Anti-oxidant mechanisms may therefore be more ancient.  The other main defence against free radicals takes the form of a range of vitamins and other complex compounds, some of which seem to have their origins in heat-shock proteins; possibly harking back to life’s origins near deep-ocean hydrothermal vents.

In a similar vein, linked to the rise of oxygen concentrations, doubt has been cast on the role of photosynthesising cyanobacteria since the earliest times..  Most geologists hold them responsible for creating stromatolites since 3500 Ma, and also for providing an early source of oxygen that was rapidly scavenged by the precipitation of iron oxides in banded iron formations.  Carrine Blank, a palaeobiologists at Washington University in St Louis, has genetically compared cyanobacteria with a range of other living Bacteria, to asses their relatedness.  Her work suggests that the blue-greens were late additions to early life, perhaps long after the first BIFs appeared (report on the annual meeting of the Geological Society of America, in New Scientist 9 November 2002, p. 25).

The Permian to Triassic transition involved a transformation from globally cool conditions to a hothouse, as well as the largest mass extinction in the fossil record.  It also spanned a time when most continental lithosphere was clumped in the Pangaea supercontinent.  In the case of plants, it is not easy to sort the effects of climatic shifts from those due to catastrophic events, either the effects of the huge Siberian flood-basalt event (see Earth Pages August 2002, Flood basalts of Siberian Traps doubled at a stroke) or a yet to be proven impact.  Allister Rees of the University of Chicago has painstakingly organised global Permian and Triassic floral data to see if the changes were slow (climatically influenced) or sudden ( possible evidence for a catastrophic collapse),a nd if they coincide from region to region.  He found that in some regions big changes happened quickly around the P-Tr boundary, but in others the shifts were protracted and unrelated to faunal extinctions (Rees, P. McA. 2002.  Land-plant diversity and the end-Permian mass extinction.  Geology, v. 30, p. 827-830).  This clearly implies caution in the interpretation of detailed local records as signs of massive events, and also points out the need to place such records in the contexts of global climate belts and biases that result from varied degrees of biotic preservation.

Dinosaurs did urinate

News is coming in (New Scientist, 19 October 2002, p. 26) of a startling find along a dinosaur trackway in Colorado.  At the October meeting of the Society of Vertebrate Palaeontology, Katherine McCarville of the South Dakota School of Mines and Technology described a bath-sized pit preserved among sauropod footprints.  Seemingly, all the evidence points to it having been excavated by a gargantuan stream of liquid pouring from above.  Ranking as a candidate for the IgNobel Awards of 2003, this evidence for dinosaurian bladder relief may shake the theory that birds are descended from dinosaur ancestors; birds do not urinate.

Continents colonised a billion years ago

The Torridonian of NW Scotland is a thick sequence of mainly terrestrial sediments that accumulated on the Laurentian craton, between 1200 and 1000 Ma ago.  Much of the sequence evidences braided-stream deposition, with brief lacustrine episodes.  Any geologist who examines these mainly siliciclastic rocks will find abundant evidence for subaerial conditions in the form of desiccation cracks, often affecting directional current ripples.  However, it takes a keen eye and some knowledge of biofilms to spot any signs of microbial activity.  In sandstones they manifest themselves by having increased the normally very low cohesiveness of wet sand by their binding action (Prave, A.R. 2002.  Life on land in the Proterozoic: evidence from the Torridonian rocks of northwest Scotland.  Geology, v.  30, p. 811-814).  Prave analysed the shapes of desiccation polygons to show that the Torridonian sands were unusually cohesive, and recognised other features likely to have been formed by microbial crusts.  These finds add to the growing evidence for substantial terrestrial biomass, long before the “official” colonisation by land plants in the Silurian and Devonian.  Whether or not such an expansion of the biosphere added significantly to carbon burial and drawdown of atmospheric CO₂, as it did in post-Silurian times, remains to be determined from average carbon contents of quite rare Precambrian terrestrial sediments,