Geobiology, palaeontology, and evolution

New drill core penetrates the Mohorovičić Discontinuity (the ‘Moho’)

Published on June 5, 2023June 5, 2023 by zooks777Leave a comment

In 1909 Croatian geophysicist Andrija Mohorovičić examined seismograms of a shallow earthquake that shook the area around Zagreb. To his surprise the by-then familiar time sequence of P-waves followed by the slower S-waves appeared twice on seismic records up to 800 km away. The only explanation that he could come up with was that the first arrivals had travelled directly through the crust to the detector whereas the second set must have followed a longer path: it had travelled downwards to be refracted to reach the surface when it met rocks denser than those at the surface. His analysis revealed a sharp boundary between the Earth’s crust and its mantle at a depth of about 54 km below what was then Yugoslavia. Later workers confirmed this discovery and honoured its discoverer by naming it the Mohorovičić Discontinuity. Difficulty with pronouncing his name resulted in a geological nickname: ‘the Moho’. It can be detected everywhere: at 20 to 90 km beneath the continental surface and 5 to 10 km beneath the ocean floor, thus distinguishing between continental and oceanic crust.

In the late 1950s accelerating geological and oceanographic research that would culminate in the theory of plate tectonics turned its focus on drilling down to the Moho in much the same way as a lust for space travel spawned getting to the Moon. The difference was that the proposers of what became known as the Mohole Project were members of what amounted to a geoscientific glee club (The American Miscellaneous Society), which included a member of the well-financed US National Science Foundation’s Earth Science Panel. The idea emerged shortly after the Soviet Union had launched the Sputnik satellite and rumours emerged that it was proposing deep drilling into the continental crust beneath the Kola Peninsula. The Mohole’s initial target was the 3.9 km deep floor of the Caribbean off Guadalupe in Mexico and required advanced methods of stabilisation for a new oceanographic ship that was to host the drilling rig.

Huge (tens of metres high) pillars or ‘chimneys’ of carbonates formed by the Lost City hydrothermal vent near the mid-Atlantic ridge (Credit: ETH Zurich)

The Mohole was spudded in 1961, but the deepest of five holes reached only 200 m beneath the sea floor. It recovered Miocene sediments and a few metres of basalt. Deep water drilling was somewhat more complicated than expected and about US$ 57 million was spent fruitlessly. The project was disbanded in 1966 with considerable acrimony and schadenfreude. Nonetheless, the Mohole fiasco made technical advances and did demonstrate the feasibility of offshore drilling. The petroleum industry benefitted and so did oceanography with the globe-spanning deep-sea drilling of ocean floor sediments. The sediment cores produced the 200 million-year exquisitely detailed record of climate change and vast amounts of geochemical data from the basaltic oceanic crust. In 2005 JOIDES (the Joint Oceanographic Institutions for Deep Earth Sampling) had another crack at the Moho. That venture centred on the intersection of the Mid-Atlantic Ridge and the Atlantis Fracture Zone close to the ‘Lost City’ hydrothermal vent. The area around the vent is the site of a huge low-angled extensional fault that has partly dragged the basaltic ocean crust off the mantle beneath causing it to bulge. This provided an excellent opportunity to drill through the Moho. All went well, but 54 days of drilling yielded 1.4 km of basalt but nothing resembling mantle rock. So, again, the Moho had thwarted Science (and research economics). But finally it is beginning to reveal it secrets (see: Voosen, P. 2023. Ocean drillers exhume a bounty of mantle rocks. Science, v. 380 (News) p. 876-877; DOI: 10.1126/science.adi9899

The area around the ‘Lost City’ vent was originally chosen for drilling to examine the chemical processes going on there. Hydrogen emitted by serpentinisation of mantle rocks can combine with carbon monoxide in hydrothermal fluids to create a wide variety of organic compounds, which could be the initial building blocks for the origin of life. As part of the International Ocean Drilling Programme JOIDES decided to launch IODP Expedition 399 to re-examine the area around ‘Lost City’ in more detail. The expedition first tried to continue drilling the 2005 hole, but failed yet again. Finally a new drill site aimed at penetrating the extensional detachment. Within a few days the drill bit punched into mantle rocks and over a 6-week period the expedition had recovered a kilometre of core. The technical accounts for each week of drilling give a flavour of what it must be like to be a part of such a ship-borne expedition as well as describing what emerged in the drill core. It seems like a bit of a jumble, dominated by the mineral olivine– the principal characteristic of the ultramafic mantle – almost pure in the rock dunite and mixed with pyroxenes in various kinds of peridotite. There are also coarse-grained rocks that contain plagioclase feldspar, which cut through the ultramafic materials – gabbros, troctolites and norites. They are relics of intrusive basaltic magmas that did not make it to the seabed. The samples are variably altered by interaction with watery hydrothermal fluids, with lots of serpentine, talc and even asbestos: the drilling presented a health hazard for a few days. The rocks have been metamorphosed under pressure-temperature conditions of greenschist to amphibolite facies and subject to ductile deformation, probably because of the effect of extensional deformation. Whatever, there is plenty of material to be analysed, including for signs of microbial activity. So, the dreams of a 1950s academic drinking fraternity (they were all men!) have finally been realised. But since those pre-plate-tectonic times many geologists have seen and collected much the same, even putting their index fingers on the Moho itself in the time-honoured fashion. Intricate 3-D geology in ophiolite complexes such as that in Oman, provide such opportunities at the much lower cost of air travel, Land Cruiser hire and camping. Indeed what we know of the structure of the oceanic lithosphere – pillow lavas, sheeted dyke complexes, gabbro cumulates and serpentinised ultramafic mantle – has come from such bodies thrust onto continental crust at ancient plate margins. So, why the celebration in this case? They are the first samples of mantle from young oceanic lithosphere; the rocks of ophiolites may not have formed at mid-ocean ridges. These should give clues to the long-term magmatism that has created the vast abyssal basins that the mantle eventually reabsorbs by subduction. Then, of course, there is the link to biogenic processes at constructive margins that underpinned the return to the active hydrothermal venting at ‘Lost City’.

Pterosaurs had feathers and fur

Published on December 27, 2018May 13, 2019 by zooks777Leave a comment

Pterosaurs, which include the pterodactyls and pteranodons, were the first vertebrates to achieve proper, flapping flight. In the popular imagination they are regarded as ‘flying dinosaurs’, whereas the anatomy of the two groups is significantly different. The first of them appeared in the Upper Triassic around 235 Ma ago, at roughly the same time as the earliest known dinosaurs. The anatomical differences make it difficult to decide on a common ancestry for the two. But detailed analysis of pterosaur anatomy suggests that they share enough features with dinosaurs, crocodiles and birds for all four groups to have descended from ancestral archosaurs that were living in the early Triassic, and they survived the mass extinction at the end of that Period. Birds, on the other hand, first appear in the fossil record during the Upper Jurassic 70 Ma later than pterosaurs. They are now widely regarded as descendants of early theropod dinosaurs, which are known commonly to have had fur and feathers.

Pterosaurs leapt into the public imagination in the final chapter of Sir Arthur Conan Doyle’s Lost World with a clatter of ‘dry, leathery wings’ as Professor George Challenger’s captive pterodactyl from northern Brazil’s isolated Roraima tepui plateau made its successful bid for escape from a Zoological Institute meeting in Queens Hall. Yet, far from being leathery, pterosaurs turned out, in the late 1990’s, to have carried filamentous pycnofibres akin to mammalian hair. Widespread reports in the world press during the week before Christmas in 2018 hailed a further development that may have rescued pterosaurs from Conan Doyle’s 1912 description before it sprang from its perch:

It was malicious, horrible, with two small red eyes as bright as points of burning coal. Its long, savage mouth, which it held half-open, was full of a double row of sharp-like teeth. Its shoulders were humped, and round them was draped what appeared to be a faded grey shawl. It was the devil of our childhood in person.

Two specimens from the Middle to Upper Jurassic Yanliiao lagerstätte in China show far more (Yang, Z. and 8 others 2018. Pterosaur integumentary structures with complex feather-like branching. Nature Ecology & Evolution, v. 3, p. 24-30; DOI: 10.1038/s41559-018-0728-7). Their pycnofibres show branching tufts, similar to those found in some theropods dinosaurs, including tyrannosaurs. They also resemble mammalian underfur fibres, whose air-trapping properties provide efficient thermal insulation. Both body and wings of these pterosaurs are furry, which the authors suggest may also have helped reduce drag during flight, while those around the mouth may have had a sensory function similar to those carried by some living birds. Moreover, some of the filaments contain black and red pigments.

407458aa.2 — Artist’s impression of a Jurassic anurognathid pterosaur from China (Credit: Yang et al 2018; Fig. 4)

Pterosaurs may have independently developed fur and feathers; a case of parallel evolution in response to similar evolutionary pressures facing dinosaurs, birds and mammals. Alternatively, they may have had a deep evolutionary origin in the common ancestors of all these animal groups as far back as the Upper Carboniferous and Lower Permian.

Related articles: Nature Editorial 2018. Fur and fossils. Nature, v. 564, p. 301-302; DOI: 10.1038/d41586-018-07800-4; King, A. 2018. Pterosaurs sported feathers, claim scientists (The Scientist); Conniff, R. 2018. Pterosaurs just keep getting weirder (Scientific American); New discovery pushes origin of feathers back by 70 million years (Science Daily)

Oceanic hydrothermal vents and the origin of life

Published on November 9, 2018 by zooks777Leave a comment

A range of indirect evidence has been used to suggest that life originated deep in the oceans around hydrothermal vents, such as signs of early organic matter in association with Archaean pillow lavas. One particularly persuasive observation is that a number of proteins and other cell chemicals are constructed around metal sulfide groups. Such sulfides are common around hydrothermal ‘smokers’ associated with oceanic rift systems. Moreover, Fischer-Tropsch reactions between carbon monoxide and hydrogen produce quite complex hydrocarbon molecules under laboratory conditions. Such hydrogenation of a carbon-bearing gas requires a catalyst, a commonly used one being chromium oxide (see Abiotic formation of hydrocarbons by oceanic hydrothermal circulation May 2004). It also turns out that fluids emitted by sea-floor hydrothermal systems are sometimes rich in free hydrogen, formed by the breakdown of olivine in ultramafic rocks to form hydroxylated minerals such as serpentine and talc. The fact that chromium is abundant in ultramafic rocks, in the form of its oxide chromite, elevates the possibility that Fischer-Tropsch reactions may have been a crucial part of the life-forming process on the early Earth. What is needed is evidence that such reactions do occur in natural settings.

One site on the mid-Atlantic ridge spreading centre, the Lost City vent field, operates because of serpentinisation of peridotites exposed on the ocean floor, to form carbonate-rich plumes and rocky towers; ‘white smokers’. So that is an obvious place to test the abiotic theory for the origin of life. Past analyses of the vents have yielded a whole range of organic molecules, including alkanes, formates, acetates and pyruvates, that are possible precursors for such a natural process. Revisiting Lost City with advanced analytical techniques has taken the quest a major step forward (Ménez, B. et al. 2018. Abiotic synthesis of amino acids in the recesses of the oceanic lithosphere. Nature, advance online publication; DOI: 10.1038/s41586-018-0684-z). The researchers from France and Kazakhstan focused on rock drilled from 170 m below the vent system, probably beyond the influence of surface contamination from living organisms. Using several methods they detected the nitrogen-containing amino acid tryptophan, and that alone. Had they detected other amino acids their exciting result would have been severely tempered by the possibility of surface organic contamination. The formation of tryptophan implies that its abiotic formation had to involve the reduction of elemental nitrogen (N₂) to ammonia (NH₃). Bénédicte Ménez and colleagues suggest that the iron-rich clay saponite, which is a common product of serpentine alteration at low temperatures, may have catalysed such reduction and amino-acid synthesis through Friedel–Crafts reactions. Fascinating as this discovery may be, it is just a step towards confirming life’s abiogenesis. It also permits speculation that similar evidence may be found elsewhere in the Solar System on rocky bodies, such as the moons Enceladus and Europa that orbit Saturn and Jupiter respectively. That is, if the rock base of hydrothermal systems thought to occur there can be reached.

Related article: Baross, J.A. 2018. The rocky road to biomolecules. Nature, v. 564, p. 42-43; DOI: 10.1038/d41586-018-07262-8.

Neanderthal Mum meets Denisovan Dad

Published on August 23, 2018 by zooks777Leave a comment

Two bone fragments from the Denisova Cave – the former abode of an 18^th century Russian hermit called Denis – in the Altai region of Siberia yielded ancient DNA. One matches that from previously analysed Neanderthal remains and the other a genome that could only be ascribed to a hitherto unknown ancient-human population, now known as the Denisovans. Since their discovery further analysis of both modern and ancient DNA has shown that modern humans living outside of Africa contain a few percent of DNA from both ancient-human groups. Soon after leaving Africa some of their ancestors interbred with both; indeed a 40 ka-old modern-human jaw from Romania revealed genetic evidence that the individual had a Neanderthal great-great grandparent. Their descendants spread far and wide to populate Eurasia, Australasia and the Americas. Using the ancient DNA to peer back in time suggests that Neanderthals and Denisovans diverged from a common ancestor between 470 and 380 ka, itself having split from modern-human ancestry between 770 to 550 ka. Denisovan DNA also contains evidence that its ancestry included segments that could only have come from a totally unknown hominin species. Interestingly, DNA from the Neanderthal bone fragment found at Denisova contains fragments from an anatomically modern-human.

With such riches from tiny fragments of human bones unearthed from the Denisova Cave, it is no surprise that the team led by Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has subsequently analysed others that showed signs of human proteins. The latest ‘takes the biscuit’. A fragment of limb bone from someone who was at least 13 years old yielded DNA commensurate with their having been the child of a Neanderthal mother and a Denisovan father (Slon, V. and 18 others 2018. The genome of the offspring of a Neanderthal mother and a Denisovan father. Nature, v. 560, published on-line; doi: 10.1038/s41586-018-0455-x). Their child was a girl, who has been nicknamed ‘Denny’ by the team, though ‘Denise’ might seem more appropriate. The only clues to what her father, or any Denisovan, might have looked like stem from a few teeth and a skull fragment from the cave that have yielded Denisovan DNA. The teeth are much larger and the skull fragment is thicker than those of Neanderthals, suggesting that Denisovans were distinctly bigger and more robust than even the sturdy Neanderthals.

The father came from a population related to a later Denisovan found in the cave – the first to be sequenced. This suggests long-term occupancy of the area by Denisovans. But his genome also carries traces of Neanderthal ancestry. Surprisingly, the mother is more closely related to Croatian Neanderthals, rather than to an earlier Neanderthal found in the cave. Neanderthals were clearly capable of migrating between Europe and eastern Eurasia; more than 5000 km in this case. Even though very few archaic humans have been genetically sequenced it is beginning to look as if genetic mixing between diverse hominin groups in the last half million years was common, when they actually met. A custom of marrying outside a closely related group (exogamy) has been popular throughout recorded history; indeed it makes sound genetic sense. With the tiny human population density during the Late Pleistocene, it may then have been cause for mutual celebration. As documented in Chapters 2 and 3 of David Reich’s Who We Are and How We Got Here (Oxford University Press, 2018) human origins since about 470 ka until the present chart a history of episodic migrations and genetic mixing that certainly makes nonsense of earlier ideas of ‘racial purity’ and casts doubt even on the term ‘species’ as regards members of the genus Homo.

If we are ever to discover who the Denisovans were and what they looked like, the evidence is likely to come from East Asia at latitudes where climate favours preservation of DNA. Advanced sequencing equipment and techniques are now operational in China, where suspected Denisovan remains have been found

See also: Warren, M. 2018. First ancient-human hybrid. Nature, v. 560, p. 417-418; doi: 10.1038/d41586-018-06004-0); Sample, I. 2018. Offspring of Neanderthal and Denisovan identified for first time. The Guardian (22 August 2918).

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

A hint of life on Mars

Published on June 20, 2018 by zooks777Leave a comment

We can be certain that life was around on Planet Earth around 3.5 billion years ago, if not before, because unmetamorphosed sedimentary rocks of that age from Western Australia in which stromatolites occur contain a black to brownish, structureless material known as kerogen. The material is a hodgepodge of organic compounds that form during the breakdown of proteins and carbohydrates in living matter. It is the source material for petroleum compounds when kerogen-rich rocks are heated during burial. The vast bulk of organic compounds preserved on Earth are in the form of ancient kerogen, whose mass exceeds that of the living biosphere by about 10 thousand times. A good sign that it does represent ancient life lies in sedimentary kerogen’s depletion in ‘heavy’ ¹³C compared with ¹²C (negative values of δ¹³C), because in metabolising carbon dioxide living cells preferentially use the lighter of these two isotopes. Conceivably, ¹³C can be removed from inorganic carbon by metamorphic processes, so low values of δ¹³C in metasediments from West Greenland might be organically derived or, equally, they might not.

At the time of writing, geoscientists specialising in Martian matters had become excited by some results from the geochemical system aboard the surviving functional NASA rover. Curiosity has slowly been making its way up Mount Sharp at the centre of Gale Crater near to Mars’s equator. Analysis of high-resolution images taken from orbit suggest that the rocks forming the mountain are sediments. the lowest and oldest strata are suspected to have been deposited in a crater lake when conditions were warmer and wetter on Mars, about 3 billion years ago. Curiosity was equipped with a drill to penetrate and sample sediment unaffected by ultraviolet radiation that long ago would have destroyed any hydrocarbons exposed at the surface. In late 2016, before the rover had reached the lake sediments, the drill’s controller broke down. Since then, Curiosity had moved on to younger, less promising sediments. More than a year later mission engineers fixed the problem and the rover backtracked to try again. Heating the resulting samples to almost 900°C yields any volatile components as a gas to a mass spectrometer, results from which give clues to the molecules released.

‘Selfie’ of Curiosity rover en route to Mount Sharp. (Credit: NASA)

The Sample Analysis at Mars (SAM) team report a range of thiophenic, aromatic and aliphatic molecules of compounds of carbon, hydrogen and sulfur (Eigenbrode, J.L and 21 others 2018. Organic matter preserved in 3-billion-year-old mudtsones at Gale crater, Mars. Science, v. 360, p. 1096-1101; doi:10.1126/science.aas9185). The blend of pyrolysis products closely resembles those which form from heated terrestrial kerogens and coals, but also from pyrolysis of carbonaceous chondrite meteorites. So, the presence of Martian kerogen is not proven. But the results are so promisingly rich in hydrocarbons that another weapon in SAM’s armoury will be deployed, dissolving organic compounds directly from the drill cuttings. This may provide more convincing evidence of collagen. Yet only when samples are returned to labs on Earth will there be a chance to say one way or the other that there was once life on Mars. The results reported in Science’s 8 June issue will surely add weight to the clamour for the Mars 2020 sample-return mission to be funded. Whether or not there is life on Mars demands a great deal more investment still…

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The great Cambrian unconformity

Published on June 13, 2018 by zooks7771 Comment

My first field trip from the Geology Department at the University of Birmingham in autumn 1964 was located within hooter distance of the giant British Leyland car plant at Longbridge. It involved a rubbish-filled linear quarry behind a row of shops on the main road through south Birmingham. Not very prepossessing but it clearly exposed a white quartzite, which we were told was a beach deposit laid down by a massive marine transgression at the start of the Cambrian. An hour later we were shown an equally grim exposure of weathered volcanic rocks in the Lickey Hills; they were a sort of purple brown, and said to be Precambrian in age. Not an excellent beginning to a career, but from time to time other Cambrian quartzites sitting unconformably on Precambrian rocks entered our field curriculum: in the West Midlands, Welsh Borders and much further afield in NW Scotland, as it transpired on what had been two separate continental masses of Avalonia and Laurentia. This had possibly been a global marine transgression.

In North America, then the Laurentian continent, what John Wesley Powell dubbed the Great Unconformity in the Grand Canyon has as its counterpart to the Lickey Quartzite the thrillingly named Tonto Group of the Lower Cambrian resting on the Vishnu Schists that are more than a billion years older. Part of the Sauk Sequence, the Tonto Group is, sadly, not accompanied by the Lone Ranger Group, but the Cambrian marine transgression crops out across the continent. In fact it was a phenomenon common to all the modern continents. Global sea level rose relative to the freeboard of the continents then existing. A recent study has established the timing for the Great Unconformity in the Grand Canyon by dating detrital zircons above and below the unconformity (Karlstrom, K, et al. 2018. Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience, v. 11, p. 438-443; doi:10.1038/s41561-018-0131-7). Rather than starting at the outset of the Cambria at 542 Ma, the marine transgression was a protracted affair that began around 527 Ma with flooding reaching a maximum at the end of the Cambrian.

Extensive flooding of the continents at the end of the Cambrian (credit: Ron Blakey , Colorado Plateau Geosystems)

It seems most likely that the associated global rise in sea level relative to the continents was a response to the break-up of the Rodinia supercontinent by considerable sea-floor spreading. The young ocean floor, having yet to cool to an equilibrium temperature, would have had reduced density so that the average depth of the ocean basins decreased, thereby flooding the continents. The creation of vast shallow seas across the continents has been suggested to have been a major factor in the explosive evolution of Cambrian shelly faunas, partly by expanding the range of ecological niches and partly due to increased release of calcium ions to to seawater as a result of chemical weathering.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Humans and mass extinction

Published on June 8, 2018 by zooks7771 Comment

It is often said that the biosphere is currently undergoing species losses that may rival those of the ‘Big Five’ mass extinction, with the rate of new extinctions being estimated at about 100 times the background rate during geological time. Scientifically, this is probably a dodgy assumption for palaeobiologists simply do not have the evidence to suggest what such a ‘normal’ rate might be. The fossil record is notoriously incomplete for a whole variety of reasons largely to do with both preservation and fossil collection strategies. For instance, as today, some genera may have been very common and widespread in past times, whereas others rare and restricted to small ecological niches. The record of life is prone to huge errors so that only huge, global shifts in diversity, such as mass extinctions, can be viewed with statistical rigour; and then only with caveats. For sure, the rapid demise of species today is cause for alarm and dismay, and more taxa – mainly of smaller and more restricted groups – probably have escaped identification, and will continue to do so. In the context of growing human impacts on ecosystems across the globe extinction is an increasingly emotive topic, as witness the clamour among some geoscientists for adding a new Anthropocene Epoch to the to the Stratigraphic Column. Does that require renaming the Holocene, beginning 11,700 years ago at the end of the last Ice Age, during which agriculture began? Should its start be assigned to some event during recorded history, such as the European invasion of the Americas after 1493, the beginning of the Industrial Revolution or the explosion of the first thermonuclear weapons in the 1940s and 50s? Or did humans begin significantly to affect the biosphere once their spread from Africa started after about 130 ka ago, i.e. in the late Pleistocene? That argument may well run and run: it is foremost a scientific issue, to which rules apply. A cogent example is that of the fate of megafaunas on the major continents except Antarctica as humans migrated far and wide.

The demise of the large flightless birds of Madagascar and New Zealand form a well known case as they almost certainly followed first colonisation by humans around 200 BC and 1300 CE respectively. The megafaunas of the much larger continents of Australia and the Americas have been deemed to have been more than decimated in the same way after about 65 ka and 15 ka respectively. There are no longer giant armadillos and ground sloths in South America, mammoths ceased to roam North America, and giant wombats, marsupial predators and kangaroos only remain as bones, to name but a few. It has been argued that their extinctions stemmed from the first human migrants literally eating their way through vast terrains. Yet the vast herds of Africa seem not to have been affected in the same way, until much more recently as population grew and modern projectile weapons became widely available. That has been suggested to have resulted from co-evolution of humans and megafauna over two million years, together with instinctive caution among large African beasts, whereas the ‘naivety’ of their counterparts in the Americas and Australia doomed them to extinction. Of course, it is likely that things were a great deal more complicated in every case, as argued in a review of Late Pleistocene megafaunal extinctions by Gilbert Price of the University of Queensland, and colleagues from Australia, the US and Denmark (Price, G.J. et al. 2018. Big data little help in megafauna mysteries. Nature, v. 558, p. 23-25; doi:10.1038/d41586-018-05330-7).

The gist of Price and colleagues’ critique of meta-analyses of data – 32 since 1997 – concerning allegedly human-induced extinctions is that much of the pertinent data is either low quality or poorly understood. For starters, much of the dating is questionable, either using inaccurate and outdated methods or based on inference. For instance, fossils of some alleged victim, e.g. Australian land crocodiles (Quinkana) and giant wombats (Ramsayia), have never been dated. Moreover, dates of the last known fossils are used when they may have remained extant until more recently: wooly Eurasian mammoths were long supposed not to have survived the last glacial maximum, yet recently mammoth bones from Wrangel island were found to be as young as the second millennium BCE. In 2010 spores of the fungus Sporormiella, in sediment cores, which grows only on digested plant matter in herbivore dung, was used as a proxy for the former presence or absence of large herbivore herds. Its decline in sediments after 13 ka in North America happened to coincide roughly with the start of the North American Clovis hunter culture, which was used to show that extinctions of large herbivores were linked to human predation. Yet such fungi also live on excrement of many animals both large and small, and its preservation is affected by changes in climate and water flow. To properly link declines and extinctions in human prey animals requires concrete evidence of predation, such as cut marks on identifiable bones within middens associated with human habitation, such as hearths.

When emotion, ambition and bandwagon tendencies become associated with science, objectivity sometimes gets compromised.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Mystery of upside-down dinosaurs resolved

Published on May 2, 2018 by zooks7771 Comment

Remains of ankylosaurs, a popular family of heavily armoured dinosaurs, occur in sedimentary sequences that range in age from early Jurassic to the close of the Cretaceous. Their defences seem almost impregnable, being constructed of thick, fused scales, often bearing formidable spines, which covered them completely. They bore a crude resemblance to modern armadillos apart from the fact that they were unable to roll-up defensively. In some species the rigid tail bears a large, knob of scale tissue. At up to 3 m long, were the tail to be swung it would have packed bone-cracking momentum. Interestingly, to a poorly sighted predator the club may have been mistaken for the animal’s head at the end of a long neck, so perhaps it lured a potential assailant within range of its devastating power. The largest ankylosaur, from the Cretaceous of western Canada, was the size of a small bus, up to 8m long, 1.5 m wide, standing 1.7 m high and weighing in at around 5 to 8 t. Such dimensions would have made it almost impossible to be bitten, even by the largest predatory dinosaurs, and difficult to turn over. Their teeth show that ankylosaurs were herbivorous, and their somewhat bulbous bodies almost certainly contained a massive digestion system.

The mystery lies in the fact that most ankylosaur fossils are found lying on their backs. Early dinosaur aficionados suggested a tendency for the lumbering beasts to tumble down slopes and become stranded on their backs, so to die miserably. Such clumsiness is hardly a positive characteristic of evolutionary fitness for such a long-lived group, and, besides, the sedimentary formations in which they are found indicate very gentle slopes. So, were they flipped by dextrous predators, as imagined in some early films purporting to look to the distant past? Probably not, for most well preserved fossils show no sign of bites or gnawing. From a study of 36 late-Cretaceous ankylosaurs from Alberta four Canadian and US palaeontologists (Mallon, J.C. et al. 2018. A “bloat-and-float” taphonomic model best explains the upside-down preservation of ankylosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 497, p. 117-127; doi:10.1016/j.palaeo.2018.02.010 support the idea of their carcases or even living animals having been picked up by flood waters when their high centre of gravity would have flipped them upside down. Bloating through decay might then allow them to be transported large distances. Unsurprisingly, their conclusions rest on model simulations.

When dinosaurs roamed the Western Isles

Published on April 3, 2018 by zooks7778 Comments

The Isle of Skye off the northwest coast of Scotland is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser known Monster, the dread Each Uisge (Water Horse). Yet evidence is emerging for the former presence in the Hebrides of other, more tangible monsters.

Skye’s great volcanic edifice rests on Mesozoic sedimentary rocks including shallow-water muddy limestones of the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age. For dinosaur specialists this is of the time when meat-eating theropods and herbivorous sauropods began growing to colossal sizes. Yet the Bathonian is notable for its global paucity in well exposed terrestrial and near-shore sedimentary sequences. Easily accessible, the Skye Bathonian sequence is much visited and has yielded a rich, though generally fragmentary fauna. A group of recent visiting palaeontologists from the University of Edinburgh, the Chinese Academy of Sciences and Skye’s Staffin Museum have discovered an extensive tract of wave-cut platform on the east shore of the Trotternish Peninsula where lagoonal carbonate muds were trampled by several dinosaurs that left around 50 tracks (dePolo, P.E. et al. 2018. A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, online; doi:10.1144/sjg2017-016).

Dinosaur foot prints from Skye. Left example of a sauropod rear-foot print; right theropod. (credit dePolo, P.E. et al. 2018, modified from Figs 8 and 9a)

Some are of medium-sized sauropods (either Parabrontopodus or Breviparopus – both names for footprints rather than any genus of dinosaur) whose crudely elephant-like footprints are up to 0.5 m across (the largest, from Western Australia, are about 1.7 m across). Although there are fragmentary dinosaur bones from the same strata, assigning the footprint to a known species is not possible. However, foot size can be used to estimate how high the creatures’ hips stood (2 to 2.5 m): hefty beasts but not the true giants of later times A variety of three-toed, clawed, somewhat bird-like, footprints also occur. They are assigned to probably bipedal carnivores or theropods. Variation in foot size suggests a range of hip-height from about 0.9 to 2 metres, so these carnivores would have been pretty formidable.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Hadean potentially fertile for life

Published on February 20, 2018 by zooks777Leave a comment

The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km²) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose... — Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.

The rise of the eukaryotes

Published on December 14, 2017 by zooks777Leave a comment

You and I, and all the living things that we can easily see belong to the most recently evolved of the three great domains of life, the Eukarya. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the Bacteria and the Archaea. Their genetic material floats around in their cell’s fluid, while ours resides mainly in the eukaryote cell’s nucleus with a bit in various organelles known as mitochondria and the chloroplasts of plant cells. Unlike the chicken and egg question, that concerning which came first, prokaryotes or eukaryotes, is answered by DNA. Eukaryote DNA contains a lot from prokaryotes, but the converse does not hold. That contrast posed the question of how eukaryotes arose from the two earlier, simpler forms of life, the answer to which Lynn Margulis suggested to be a whole series of symbiotic relationships among various prokaryotes that shared a host cell; her hypothesis of endosymbiosis. Now, the vast majority of eukaryotes depend on free oxygen for their metabolism, so when the first of them arose boils down to the period of geological history following the Great Oxidation Event around 2.4 billion years ago.

Molecular-clock estimates based on the range of variation in the genomes of a wide range of eukaryotes suggest it took place sometime between 1000 and 2000 Ma. A better means of homing in on a date for the Last Eukaryote Common Ancestor (LECA – as opposed to that of the first organism LUCA) would be that of the earliest fossil to show eukaryote affinities. Grypania from 1.85 Ga, a sort of whorl-like fossil, is a good candidate and is widely thought to be the earliest of our kind but lacks signs of actual cells. More convincing fossils – known generically as acritarchs – from times between 1.5 and 1.0 Ga look like primitive fungi, red algae and slime moulds. A comprehensive review of the microfossils of the Palaeoproterozoic (2.5 to 1.6 Ga) includes both prokaryotes and probable early eukaryotes (Javaux, E.J. & Lepot, K. 2017. The Paleoproterozoic fossil record: Implications for the evolution of the biosphere during Earth’s middle-age. Earth Science Reviews, v. 176, p. 68-86; doi: 10.1016/j.earscirev.2017.10.0001). Yet, despite rapidly accumulating evidence, especially from rocks in China, the picture remains one of monotony; for instance Grypania spans the best part of half a billion years. Bacteria and Archaea cannot be distinguished easily in the absence of preserved DNA. Despite evidence for oxygen in the oceans and atmosphere, apart from a few shallow-water oxygenated examples the chemistry of Palaeoproterozoic marine sediments is dominated by mineralogical outcomes of reducing chemistry. Many chemical isotopic environmental proxies ‘flat-line’ to the extent that the early Proterozoic is sometimes referred to as the ‘boring billion’, yet our ultimate precursors were part of the marine ecosystem. That is, unless one accepts the possibility that that fossils labelled ‘eukaryote’ are colonial prokaryotes – evidence for cell nuclei is sparse. Endosymbiosis, although an attractive model for eukaryote origins, is not proven. The reason for lingering scepticism is that there are only a tiny number of modern examples of prokaryote cells ending up inside those of other prokaryotes.

Whatever, chemical biomarkers in sediments older than about 720 Ma indicate that prokaryotes were the only notable primary producers in the oceans until the Neoproterozoic. Microscopic fossils that are inescapably eukaryotes in the form of amoeba suddenly emerge around that time. This development from the lingering marginality of early eukaryotes to thriving ecosystems that they dominated thereafter is a puzzle seeking a plausible explanation. It coincides with the onset of the Snowball Earth glaciations of the Cryogenian Period (850 to 635 Ma) and a rise in atmospheric and presumably oceanic oxygen. Then macroscopic eukaryotes ‘bloomed’ into distinctively different forms in the Ediacaran Period (635 to 541 Ma) and thereafter. Before the Cryogenian we can perhaps regard eukaryan life and the endosymbiosis that may have given rise to it as a series of ecological experiments repeatedly knocked-back by chemical conditions and competition with the vastly more abundant prokaryotes.

Banded iron formations (BIFs) reviewed

Published on December 13, 2017 by zooks777Leave a comment

During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent.

This image shows a 2.1 billion years old rock ... — 2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)

This article can now be read in full at Earth-logs in the Sedimentology and stratigraphy archive for 2017

Shock and Er … wait a minute

Published on October 6, 2017 by zooks777Leave a comment

Chicxulub2 — Enhanced gravity map of the Chicxulub crater (credit: Wikipedia)

Michael Rampino has produced a new book (Rampino, M.R. 2017. Cataclysms: A New Geology for the Twenty-First Century. Columbia University Press; New York). As the title subtly hints, Rampino is interested in mass extinctions and impacts; indeed quite a lot more, as he lays out a hypothesis that major terrestrial upheavals may stem from gravitational changes during the Solar System’s progress around the Milky Way galaxy. Astronomers reckon that this 250 Ma orbit involves wobbling through the galactic plane and possibly varying distributions of mass – stars, gas, dust and maybe dark matter – in a 33 Ma cycle. Changing gravitational forces affecting the Solar System may possibly fling small objects such as comets and asteroids towards the Earth on a regular basis. In the 1980s and 90s Rampino and others linked mass extinctions, flood-basalt outpourings and cratering events, with a 27 Ma periodicity. So the books isn’t entirely new, though it reads pretty well.

Such ideas have been around for decades, but it all kicked off in 1980 when Luis and Walter Alvarez and co-workers published their findings of iridium anomalies at the K-Pg boundary and suggested that this could only have arisen from a major asteroid impact. Since it coincided with the mass extinction of dinosaurs and much else besides at the end of the Cretaceous it could hardly be ignored. Indeed their chance discovery launched quite a bandwagon. The iridium-rich layer also included glass spherules, shocked mineral grains, soot and other carbon molecules –nano-scale diamonds, nanotubes and fullerenes whose structure is akin to a geodesic dome – and other geochemical anomalies. Because the Chicxulub crater off the Yucatán Peninsula of Mexico is exactly the right age and big enough to warrant a role in the K-Pg extinction, these lines of evidence have been widely adopted as the forensic smoking gun for other impacts. In the last 37 years every extinction event horizon has been scrutinized to seek such an extraterrestrial connection, with some success, except for exactly coincident big craters.

The K-Pg event is the only one that shows a clear temporal connection with a small mountain falling out of the sky, most of the others seeming to link with flood basalt events and their roughly cyclical frequency – but hence Rampino’s Shiva hypothesis that impacts may have caused the launch of mantle plumes from the core-mantle boundary. Others have used the ‘smoking gun’ components to link lesser events to a cosmic cause, the most notorious being the 2007 connection to the extinction of the North American Pleistocene megafauna and the start of the Younger Dryas return of glacial conditions. Since 1980 alternative mechanisms for producing most of the impact-connected materials have been demonstrated. It emerged in 2011 that nano-diamonds and fullerenes may form in a candle flame and their global distribution could be due to forest fires. And now it seems that shocked mineral grains can form during a lightning strike (Chen, J. et al. 2017. Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters, v. 44, p. 8757-8768; doi:10.1002/2017GL073843). Shocked or not, quartz and feldspar grains are resistant enough to be redistributed into sediments. Although platinum-group metals, such as iridium, are likely to be mainly locked away in Earth’s core, some volcanic exhalations and many flood basalts – especially those with high titanium contents – significantly are enriched in them. So even the Alvarez’s evidence for a K-Pg impact has an alternative explanation. Rampino is to be credited for acknowledging that in his book.

An awful lot of ideas about rare yet dreadful events in the biosphere depend, like many criminal cases, on the ‘weight of evidence’ and defy absolute proof. The evidence generally permits alternatives, such the cunning Verneshot hypothesis for the extinction-flood basalt connection supported by one of the founders of plate tectonics, W. Jason Morgan. As regards The K-Pg extinction, it is certain that a very large mass did fall on Chicxulub at the time of the mass extinction, whereas the Deccan flood basalts span a million years or so either side. But the jury is out on whether either or both did the deed. For other events of this scale and larger ones the money is on internal origins. As for Rampino’s galactic hypothesis, the statistics are decidedly dodgy, but chasing down more forensics is definitely on the cards.

English: From source; an animation showing the... — Animation showing the Chicxulub Crater impact. ( credit: University of Arizona, Space Imagery Center)

Ancient footprints

Published on September 8, 2017 by zooks777Leave a comment

To see traces of where our forebears walked, such as the famous Australopithecus afarensis trackway at Laetoli in Tanzania, the footprints of Neanderthal children in 350 ka old Italian volcanic ash (The first volcanologists? Earth Pages March 2003) or even those of Mesolithic families in estuarine mud is about as heart stopping as it gets for a geologist. But imagine the astonishment of members of a multinational team working on Miocene shore-line sediments on Crete when they came upon a bedding surface covered with what are almost certainly the footprints of another bipedal animal from 5.7 Ma ago (Gierliński, G.D. et al. 2017. Possible hominin footprints from the late Miocene (c. 5.7 Ma) of Crete? Proceedings of the Geologists’ Association, online; https://doi.org/10.1016/j.pgeola.2017.07.006). Trackways preserve a few moments in time, however old they are and the chances of their being preserved are very small, yet they can supply information that is lost from even the best preserved fossil, such as gait, weight, speed and so forth.

Untitled-4 — Track bearing surface; (b) two footprints in 5.7 Ma old Miocene sediments at Trachilos, Crete (credit: Gierliński, G.D. et al. 2017; Figures 2 and 8)

The tracks clearly indicate that whatever left them was bipedal and lacked claws, and closely resemble those attributed to A. afarensis at Laetoli in a 3.7 Ma old volcanic ash. What they do not resemble closely are those of non-hominin modern primates, such as chimpanzees. They are diminutive compared with adult modern human prints, being about 12.5 cm long (equivalent to a UK child’ shoe size 4 – US size 4.5, EU 20) and about a third to half the size of those at Laetoli. Were they around the age of those at Laetoli or younger there seems little doubt that they would be widely interpreted as being of hominin origin. But being from an island in the Mediterranean as well as far from sites in Africa that have yielded Miocene hominins (Ardipithecus kadabba from Ethiopia, Orrorin from Kenya and Sahelanthropus from Chad), such an interpretation is bound to create controversy. Somewhat less controversial might be to regard them as having been created by a late-Miocene primate that convergently evolved a hominin-like upright gait and foot. Being preserved in what seem to be coastal marine sediments, there is probably little chance of body fossils being preserved in the exposed horizon. Since foot bones are so fragile, even if a primate fossil is discovered in the late Miocene of Crete the chances of resolving the issue are pretty remote. Yet fossil primate specialists will undoubtedly beat a well-trodden path to the Trachilos site near Kissamos on Crete

Wildfires and climate at the K-Pg boundary

Published on September 4, 2017 by zooks7772 Comments

It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO₂ and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO₂emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here

The late-Ordovician mass extinction: volcanic connections

Published on July 27, 2017 by zooks7771 Comment

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

Untitled-1 — Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO₂. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO₂ into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

508-Million-Year-Old Sea Monster Had 50 Legs and Giant Claws

Earliest hydrothermal vents and evidence for life

Published on March 3, 2017 by zooks777Leave a comment

That seawater circulates through the axial regions of rifts associated with sea-floor spreading has been known since well before the acceptance of plate tectonics. The idea stems from the discovery in 1949 of brines with a temperature of 60°C on the central floor of the Red Sea, which in the early 60s turned out to be anomalously metal-rich as well. Advanced submersibles that can withstand the high pressures at great depth a decade later produced images of swirling clouds of sediment from large sea-floor springs, first on the Galapagos rift and subsequently on many others. The first shots were of dark, turbulent clouds, prompting the term ‘black smoker’ for such hydrothermal vents and it turns out that others produce light-coloured clouds – ‘white smokers’. Sampling revealed that the sediments in black smokers were in fact fine-grained precipitates of metallic sulfides, whereas those forming white smokers were sulfates, carbonates and oxides of barium calcium and silicon also precipitated from solute-rich brines produced by partial dissolution of ocean floor through which they had passes.

Excitement grew when hydrothermal vents were shown to have complex animal ecosystems completely new to science. A variety of chemical evidence, most importantly the common presence of proteins and other cell chemicals built around metal sulfide groups in most living organisms, prompted the idea that hydrothermal vents may have hosted the origins of life on Earth. Many fossil vent systems also contain fossils; macrofossils in the Phanerozoic and microbial ones from the Precambrian. But tangible signs of life, in the form of mats ascribed to bacteria or archaea holding together fine-grained sediments, go back no further than 3830 Ma in the Isua area of SW Greenland. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen. Signs of deep sea hydrothermal activity are common in any geological terrain containing basalt lavas with the characteristic pillows indicating extrusion beneath water. So to trace life’s origins all that is needed to trigger the interest of palaeobiologists are the oldest known pillow lavas. Until quite recently, that meant the Isua volcano-sedimentary association, but heating, high pressures and very strong deformation affected those rocks when they were metamorphosed half a billion years after they were formed; a cause for skepticism by some geoscientists.

The primacy of Isua metavolcanic rocks has been challenged by more extensive metamorphosed basalts in the Nuvvuagittuk area in Quebec on the east side of Hudson Bay, Canada. They contain hydrothermal ironstones associated with pillowed basalts that are cut by more silica-rich intrusive igneous rocks dated between 3750 and 3775 Ma. That might place the age of basalt volcanism and the hydrothermal systems in the same ball park as those of Isua, but intriguingly the basalts’ ¹⁴⁶Sm-¹⁴²Nd systematics suggest a possible age of magma separation from the mantle of 4280 Ma (this age is currently disputed as it clashes with U-Pb dates for zircon grains extracted from the metabasalts around the same as the age at Isua). Nonetheless, some parts of the Nuvvuagittuk sequence are barely deformed and show only low-grade metamorphism, and they contain iron- and silicon-rich hot spring deposits (Dodd, M.S. et al. 2017. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, v. 543, p. 60-64; doi:10.1038/nature21377). As at Isua, the ironstones contain graphite whose carbon isotope proportions have an ambiguous sign of having formed by living or abiotic processes. It is the light deformation and low metamorphism of the rocks that gives them an edge as regards being hosts to tangible signs of life. Extremely delicate rosettes and blades of calcium carbonate and phosphate, likely formed during deposition, remain intact. These signs of stasis are in direct contact with features that are almost identical to minute tubes and filaments formed in modern vents by iron-oxidising bacteria. All that is missing are clear signs of bacterial cells. Ambiguities in the dating of the basalt host rocks do not allow the authors claims that their signs of life are significantly older than those at Isua, but their biotic origins are less open to question. Neither offer definitive proof of life, despite widespread claims by media science correspondents, some of whom tend metaphorically to ‘run amok ‘ when the phrase ‘ancient life’ appears; in this case attempting to link the paper with life on Mars …

You can find more on early life here

Dinosaurs in the flesh and feathers

Published on February 13, 2017 by zooks777Leave a comment

Until only a few decades ago artistic portrayals of dinosaurs had them as leathery and scaled like lizards or crocodiles, as indeed rare examples of their fossilized skin seemed to suggest. The animatronic and CGI dinosaurs of the first Jurassic Park film were scary, but brownish grey. Later films in the franchise had them mottled and sometimes in colour, but still as mainly scaled leathery monsters. Reality soon overtook imagination as more and more exquisitely preserved fossils of small species were turned up, mainly in China, that were distinctly furry, fuzzy or feathered as shown below in a Microraptor gui fossil. It is now well-established that birds arose in the Jurassic from saurischian dinosaurs, the order that also included all of the large carnivorous dinosaurs as well as the many more nimble and diminutive ones whose feathers sometimes conferred an ability to glide or fly. Even the other main order, the ornithischia noted for hugeness and herbivory, has yielded fossil skin that suggest furry or feathered pelts. Once fur and feathers had been found, the next big issue became whether or not dinosaurs may have been as gaudy as many modern birds.

Fossil of a feathered dinosaur Microraptor gui from the early Cretaceous Jiufotang Formation in China (source: Wikipedia)

One of the first palaeobiologists to become immersed in the search for colourful dinosaurs was Jakob Vinther, now of Britain’s Bristol University. In The March 2017 issue of Scientific American he summarises the progress that he and his colleagues have made (Vinther, J. 2017. The true colors of dinosaurs. Scientific American, v. 316(3), p. 42-49). On his account, the major breakthrough was Vinther’s discovery of tiny spherules in fossilised octopus ink that were identical to the granules of the pigment melanin that give the famous cephalopod ‘smoke screen’ its brownie-black colour. Melanin, or more precisely the melanosomes in which it is enclosed, is a key to coloration throughout much of the animal kingdom, especially in fur and feathers. There are two basic kinds, one conferring blackness and the other that imparts a rusty red hue, which combined with paleness due to lack of melanin together produce a gamut of greys, reds, browns oranges and yellows. Elongated melanosomes when lined up produce the phenomenon of interference fringes that yield iridescence, responsible for the bright colours of starlings, hummingbirds and some ducks when in bright light. There are other pigments, such as carotenoids (bright reds and yellows) and porphyrins (green, red and blue) that add to the gamut possible in animals, but it was melanosomes that captured Vinther’s attention because of their importance in living feather colours.

Melanosomes occur in distinctively grouped assemblages, according to actual colour, and very similar microscopic structures turned up in the first fossil bird feathers that he studied. Others had assumed that they were bacterial colonies, which had grown during decay. The breakthrough was finding a fossil bird feather in which different structures were arranged in stripes; clear signs of patterning. Vinther’s concept bears fruit in a range of furry and feathered dinosaurs. One (Anchiornis) with a black and white body and limb speckles had a bright red crest and another (Sinosauropterix) was ginger over its back with a tiger striped tail and a white underside; an example of countershaded camouflage. His team has even been able to assign different kinds of patterning to a variety of possible habitats. Given superbly preserved specimens it seems likely that dinosaur and bird coloration may be traceable back more than 200 Ma.

English: Illustration of the small theropod di... — Artist’s impression of the small theropod dinosaur Microraptor showing colours predicted by analysis of melanosomes on its feathers.(credit: Wikipedia)

Another aspect of the filmic licence of Jurassic Park was its hinging on preservation of genetic material from the Mesozoic, specifically in a parasite preserved in amber, so that the creatures could be resurrected by bio-engineering. The only relevant find is a 46 Ma old mosquito whose abdomen was blood-engorged when it was fossilised. But all that remains are high iron concentrations the organic molecule porphyrin; break-down products of haemoglobin. Given that fossil DNA can only be reassembled from millions of fragmentary strands found in fossils in digital form that corresponds to the order of AGCT nucleobases that is barely likely to be possible – the oldest full genome yet analysed is that of a 700 ka horse. However, another biological material that varies hugely among living animals, protein, has proved to be tractable, albeit in a very limited way. Frozen mammoth meat, somewhat bloody, is sometimes unearthed from Siberian permafrost, but according to one Russian mammoth expert even the best preserved is inedible.

Beyond the Pleistocene the search for fossilised proteins has been hesitant and deeply controversial, particularly in the case of that from dinosaurs, for the obvious reason of publicity suspicions. But again, it is a story of persistence and patience. Mary Schweitzer of North Carolina State University claimed in 2007 that she had found some, but was howled down by other palaeontologists on the issues of its unlikely survivability and contamination. But other researchers had pushed back the age limits. By repeating their earlier analyses with the greatest possible care Schweitzer’s team confirmed their earlier results with several strands of the protein collagen about 15 amino acids in length from an 80 Ma old duck-billed dinosaur. Moreover they were able to show a closer affinity of the partial proteins to those of modern birds than to other reptiles, tallying with tangible fossil evidence (Schroeter, E.R and 8 others 2017. Expansion for the Brachylophosaurus canadensis Collagen I Sequence and Additional Evidence of the Preservation of Cretaceous Protein. Journal of Proteome Research, v. 16, p. 920-932). The work continues for other dinosaurs and early fossil birds, with better reason for confidence and a chance of tying-down genetic relatedness. Another approach shows that collagen may still be preserved in a Jurassic (195 Ma) sauropod dinosaur’s rib (Lee, Y-C. and 9 others 2017. Evidence of preserved collagen in an Early Jurassic sauropodomorph dinosaur revealed by synchrotron FTIR microspectroscopy. Nature Communications, v. 8 doi:10.1038/ncomms14220).

See also: Service, R.F. 2017. Researchers close in on ancient dinosaur remains. Science (News in depth), v. 355, p. 441- 442.

Earliest signs of vertebrates’ ancestor?

Published on February 3, 2017 by zooks777Leave a comment

Studies of DNA among living animals suggest that our own group, the vertebrates of the phylum Chordata, originated from a common ancestor that we share with echinoderms (sea urchins, star fish, sea cucumbers etc) and one of many worm-like phyla. This superphylum comprises the deuterostomes, but it is just one of several that encompass all animals and happens to be one of the smallest in terms of the number of living species that belong to it. We deuterostomes are vastly outnumbered by arthropods, nematodes, other worm-like creatures, molluscs, the rest of the animal kingdom and, of course, single-celled organisms, plants and fungi. Yet the DNA-based Circle of Life reveals that the deuterostome ancestral spoke originated early on in animal evolution.

The ‘Circle of Life’ as compiled by Cody Hinchliffe of the University of Michigan and 21 collaborators from the USA, and partly based on Fischetti, M. 2016. The circle of life. Scientific American, v. 314 (March 2016).

The majority of animals of all kinds are blessed with a mouth separate from means of expelling waste products and can be divided into two similar halves, hence their name bilaterians. The earliest fossils judged to be of this kind date to about 580 to 600 Ma ago, in the Doushantuo Formation of southern China, all of them visible only using microscopes. A DNA-based molecular clock hints at around 900-1000 Ma for the emergence of all animal body plans known today. Now another important time marker has turned up, again in sediments showing exquisite fossil preservation from China (Han, J. et al. 2017. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature, v. 542, (online); doi: 10.1038/nature21072). The Chinese-British team of palaeontologists has found tiny, bag-like fossils preserved in phosphate, which have a mouth surrounded by folds and conical openings on either side of the body. They lived in limy muds on the sea bed now preserved as limestones at the base of the Cambrian System (541 Ma) and probably had a habit akin to worms in the most general sense. The authors sifted through 3 tonnes of rock to recover the fossils, rather than relying on a lucky hammer stroke.

Reconstruction of Saccorhytus coronarius from the lowest Cambrian of Shaanxi, China. (credit: Han et al 1917) — Reconstruction of Saccorhytus coronarius (diameter about 1 mm) from the lowest Cambrian of Shaanxi, China. (credit: Han et al 1917)

Not especially prepossessing, the fossils are said to show more affinity to deuterostomes than anything else and to be the earliest known fossil examples. Yet the world’s media pounced on them as the ‘earliest known human ancestors’, which is a bit rich as they could equally be the earliest sea urchins or may have led to several odd-looking fossils known only from the later Cambrian. It isn’t possible to say with any certainty that they lie on the path that led to chordates and thus ourselves. Of course, that would not raise headlines in newspapers of record, such as Britain’ Daily Telegraph, on the BBC News website or Fox News. The authors are much more honest, claiming only that the Saccorhytus coronarius fossils are probably deuterostomes whose affinities and later descendants are obscure. Their most important conclusion is that the cradle of our branch of animals lay in deep water muds laid down around the Precambrian-Cambrian boundary, ideal for subtly varied small, flabby creatures behaving like worms. Many more varieties are likely remain to be found in similar rocks of the late Precambrian and slightly younger Cambrian when they are studied painstakingly in microscopic detail. A start has been made, that’s all.

For more on early evolution see here and here

Amazonian forest through the last glacial maximum

Published on January 13, 2017 by zooks7772 Comments

Note: Earth-Pages will be closing as of early July, but will continue in another form at Earth-logs

Accelerated evolution may occur when a small population of a species – whose genetic variability is therefore limited – becomes isolated from all other members. This is one explanation for the rise of new species, as in the Galapagos archipelago. Creation of such genetic bottlenecks encourages rapid genetic drift away from the main population. It has been suggested to explain sudden behavioural shifts in anatomically modern humans over the last hundred thousand years or so, partly through rapid and long-distance migrations and partly through a variety of environmental catastrophes, such as the huge Toba eruption around 74 ka. Another example has been proposed for the teemingly diverse flora and fauna of the Amazon Basin, particularly among its ~7500 species of butterflies, which has been ascribed to shrinkage of the Amazonian rain forest to isolated patches that became refuges from dry conditions during the last glacial maximum.

A great deal of evidence suggests that during glacial maxima global climate became considerably drier than that in interglacials, low-latitude deserts and savannah grasslands expanding at the expense of humid forest. Yet the emerging complexity of how climate change proceeds from place to place suggests that evidence such continental drying from one well-documented region, such as tropical Africa, cannot be applied to another without confirming data. Amazonia has been the subject of long-standing controversy about such ecological changes and formation of isolated forest ‘islands’ in the absence of definitive palaeoclimate data from the region itself. A multinational team has now published data on climatic humidity changes over the last 45 ka in what is now an area of dense forest but also receives lower rainfall than most of Amazonia; i.e. where rolling back forest to savannah would have been most likely to occur during the last glacial maximum (Wang, X. et al. 2017. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature, v. 541, p. 204-207; doi:10.1038/nature20787).

Their study area is tropical karst, stalagmites from one of whose caves have yielded detailed oxygen-isotope time series. Using the U/Th dating technique has given the data a time resolution of decades covering the global climatic decline into the last glacial maximum and its recovery to modern times. The relative abundance of oxygen isotopes (expressed by δ¹⁸O) in the calcium carbonate layers that make up the stalagmites is proportional to that of the rainwater that carried calcium and carbonate ions dissolved from the limestones. The rainwater δ¹⁸O itself depended on the balance between rainfall and evaporation, higher values indicating reduced precipitation. Relative proportions of carbon isotopes in the stalagmites, expressed by δ¹³C, record the balance of trees and grasses, which have different carbon-isotope signatures. Rainfall in the area did indeed fall during the run-up to the last glacial maximum, to about 60% of that at present, then to rise to ~142% in the mid-Holocene (6 ka). Yet δ¹³C in the stalagmites remained throughout comparable with those in the Holocene layers, its low values being incompatible with any marked expansion of grasses.

One important factor in converting rain forest to grass-dominated savannah is fire induced by climatic drying. Tree mortality and loss of cover accelerates drying out of the forest floor in a vicious circle towards grassland, expressed today by human influences in much of Amazonia. Fires in Amazonia must therefore have been rare during the last ice age; indeed sediment cores from the Amazon delta do not reveal any significant charcoal ‘spike’.

See also: Bush, M.B 2017. The resilience of Amazonian forests. Nature, v. 541, p. 167-168; doi:10.1038/541167a

K-T (K-Pg) boundary impact probed

Published on November 24, 2016 by zooks777Leave a comment

One of the most eagerly followed ocean-floor drilling projects has just released some results. Its target is 46 km radially away from the centre of the geophysical anomaly associated with the Chixculub impact structure just to the north of Mexico’s Yucatan Peninsula. In the case of large lunar impact craters the centre is often surrounded by a ring of peaks. Modelling suggests such features are produced by the deep penetration of immense seismic shock waves. In the first minute these excavate and fling out debris to leave a cavity penetrating deep into the crust. Within three minutes the cavity walls collapse inwards creating a rebound superficially similar to the drop flung upwards after an object is dropped in liquid. This, in turn, collapses outwards to emplace smashed and partially melted deep crustal material on top of what were once surface materials, creating a crustal inversion beneath a mountainous ring of Himalayan dimensions that surrounds a by-now shallow crater. That is the story modelled from what is known about well-studied, big craters on the Moon and Mercury. Chixculub is different because the impact was into the sea and involved debris-charged tsunamis that finally plastered the actual impact scar with sediments. The drilling was funded for several reasons, some palaeontological others relating to the testing of theories of impact processes and their products. Chixculub is probably the only intact impact crater on Earth, and the first reports of findings are in the second category (Morgan, J.V. and 37 others 2016. The formation of peak rings in large impact craters. Science, v. 354, p. 878-882; doi: 10.1126/science.aah6561).

The drill core, reaching down to about 1.3 km below the sea floor penetrates post-impact Cenozoic sediments into a 100 m thick zone of breccias containing fragments of impact melt rock, probably the infill of the central crater immediately following the first few minutes of impact. Beneath that are coarse grained granites representing the middle continental crust from original depths around 10 km. The granite is intensely fractured and riven by dykes and pods of impact melt, and contains intensely shocked grains that typify impacts that produce a transient pressure of ~60 GPa – around six hundred thousand times atmospheric pressure. From seismic reflection surveys this crustal material overlies as yet un-drilled Mesozoic sedimentary rocks. Its density is significantly less than that of unshocked granite – averaging 2.4 compared with 2.6 g cm³. So it is probably filled with microfractures and sufficiently permeable for water to have penetrated once the impact site had cooled. This poses the question, yet to be addressed in print, of whether or not this near-surface layer became colonised by microorganisms in the aftermath (Barton, P. 2016. Revealing the dynamics of a large impact. Science, v. 354, p. 836-837). That is, was the surrounding ocean sterilised at the time of the K-T (K-Pg) mass extinction?; an issue whose resolution is awaited with bated breath by the palaeobiology audience. OK; so theory about the physical process of cratering has been validated to some extent, but will later results be more interesting, outside the planetary sciences community?

Read more about impacts here and mass extinctions here .

Signs of life in some of the oldest rocks

Published on September 4, 2016 by zooks7771 Comment



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Vic McGregor (left) and Allen Nutman examine metasedimentary strata at Isua, West Greenland

For decades the record of tangible signs of life extended back to around 3.4 billion years ago, in the form of undulose, banded biofilms of calcite known as stromatolites preserved at North Pole in the Pilbara region of Western Australia. There have been attempts to use carbon-isotope data and those of other elements from older, unfossiliferous rocks to seek chemical signs of living processes that extracted carbon from the early seas. Repeatedly, claims have been made for such signatures being extracted from the 3.7 to 3.8 Ga Isua metasediments in West Greenland. But because this famous locality shows evidence of repeated metamorphism abiogenic formation of the chemical patterns cannot be ruled out. Isua has been literally crawled over since Vic McGregor of the Greenland Geological Survey became convinced in the 1960s that the metasediments could be the oldest rocks in the world, a view confirmed eventually by Stephen Moorbath and Noel Gale of Oxford University using Rb-Sr isotopic dating. There are slightly older rocks in Canada, which just break the 4 Ga barrier, but they were metamorphose at higher pressures and temperatures and are highly deformed. The Isua suprcrustals, despite deformation and metamorphism show far more diversity that geochemically can be linked to many kinds of sedimentary and volcanic rock types.

Two of the Isua addicts are Allen Nutman of the University of Wollongong, Australia and Clark Friend formerly of Oxford Brookes University, UK, who have worked together on many aspects of the Isua rocks for decades. Finally, thanks to melt-back of old snow pack, they and colleagues have found stromatolites that push the origin of life as far back as it seems possible for geoscientists to reach (Nutman, A.P. et al. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature). The trace fossils occur in a marble, formerly a limestone that retains intricate sedimentary structures, which show it to have been deposited in shallow water. The carbon and oxygen isotopes have probably been disturbed by metamorphism, and no signs of cell material remain for the same reason, but the shape is sufficiently distinct from those produced by purely sedimentary processes to suspect that they resulted from biofilm build-up. The fact that they are made of carbonates suggests that they may have been produced by cyanobacteria as modern stromatolites are.

isua strom — Stromatolite-like structures from a metasediment in the Isua area of West Greenland (credit Allen Nutman, University of Wollongong, Australia)

The age of the structures, about 3.7 Ga, is close to the end of the Late Heavy Bombardment (4. 1to 3.8 Ga) of the Solar System by errant asteroids and comets. So, if the physical evidence is what it seems to be, life emerged either very quickly after such an energetic episode or conditions at the end of the Hadean were not inimical to living processes or the prebiotic chemistry that led to them.

You can find more on early life here

Allwood, A.C. 2016. Evidence of life in Earth’s oldest rocks. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature19429

Hunting down the Tully Monster

Published on May 19, 2016 by zooks777Leave a comment

The word ‘monster’ has its origin in the Latin monere ‘to warn’ but has broadened out in its usage. It has even reverted to its origins as a verb: a highly critical, verbal attack. But I prefer ‘something about which one needs to be warned’, and the Tully Monster encapsulates that meaning. It once lived in Illinois, specifically at just a single location, Mazon Creek, where thousands of them have been seen. But should you be especially fearful of Tullimonstrum gregarium? Well, at first sight, no; it’s only about 10 cm long and apparently has no proper bones and it’s dead. The first was spotted in a coal-mine waste heap by Francis Tully in 1958, a pipefitter with an interest in Carboniferous fossils. Two years after his death in 1987, he and his monster were honoured by a bill that the Illinois State Legislature passed to make it the official State Fossil.

Artist's impression of the Carboniferous Tully Monster ( — Artist’s impression of the Carboniferous Tully Monster (*Tullimonstrum gregarium)* (credit: Sean McMahon, Yale University)

It seems to have become a ‘monster’ by stumping all previous attempts to categorise it; so much so that it long served as a warning to eager palaeontologists not to tangle with its taxonomy. That’s not surprising, because as well as bearing a passing resemblance to Captain Nemo’s submarine in Jules Verne’s 20 000 leagues Under the Sea, it has some truly astonishing features. Portholes down its sides are not the weirdest – actually they are gill openings. It has a biting apparatus at the end of an absurdly lengthy forward protuberance, that would not be unexpected if it were one of those fish from the Amazon that, you know, men really ought to be warned about. Most of us would not share a bath with it if we had been. And then, there are the eyes on the ends of a dorsal bar which would give Tullimonstrum gregarium superb stereoscopic vision to guide it unerringly to its target, lashing its efficient-looking caudal fin. The fact that it has only a single nostril is merely puzzling by comparison.

Six decades on, Victoria McCoy of Yale University (now at Leicester University, UK) and 15 undeterred colleagues have pored over more than 1200 Tully Monster fossils and seem to have cracked its affinities (McCoy, V.E. et al. 2016. The ‘Tully monster’ is a vertebrate. Nature, v. 532, p. 496-499). In fact, it’s surprising that it has remained an enigma for so long, because McCoy and colleagues have documented almost every aspect of its anatomy, available from a huge number of superbly preserved specimens – teeth, fin, muscle traces, gills, nostril, notochord, gut and so on. As well as being a vertebrate, its dreadful proboscis is very like that of the Cambrian oddity Opabinia from the Burgess Shale. A separate study by four British palaeontologists and a Texan concentrated on the eyes using electron microscopy and found ‘ultrastructural details’, including pigment cells (Clements, T. et al. 2016. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature, v. 532, p. 500-503) which unequivocally confirm that it is a vertebrate. It has all the hallmarks of being related to lampreys and hagfishs. They devour rotting, drowned corpses.

Further pounding for ideas on the Ediacaran fauna

Published on January 8, 2016 by zooks7773 Comments

About 635 Ma ago fossils of large-bodied organisms first appeared in the geological record: some quilt like, others with a crude bilateral symmetry, more looking like ‘mud-filled bags’ and ribbed discs but none that can easily be distinguished as animals, plants or colonial microorganisms. First found abundantly in the Ediacara Hills of South Australia, hence their sack-name the Ediacaran biota, it now seems that they were distributed globally in the late Neoproterozoic Era. Interpreting their metabolism is risky enough – some are reckoned to be animals that absorbed nutrients through their skin, others said to be dependent on photosynthesis – but a controversy has raged for many years over the kind of environment in which they thrived. In a detailed 2012 study of sedimentary structures petrography in the South Australian sandstones from which they were first described, Gregory Retallack of the University of Oregon inferred that some lived on land and are now found in palaeosols: they include Spriggina, Dickinsonia and Charnia that are among the most favoured candidates for being animals or some kind. Others inhabited shallow water. Anticipating fiery disputes a Nature editorial appeared in same issue in which Retallack published his paper .

Rich fossil assemblage of the Ediacaran Mistaken Point Formation, Newfoundland. (Credit: Alex Liu, Earth Sciences, University of Bristol)

Retallack has now moved on to the even more fossil-rich Ediacaran sediments of Newfoundland (Retallack, G.J 2016. Ediacaran sedimentology and paleoecology of Newfoundland reconsidered. Sedimentary Geology, v. 333, p. 15-31). Eye-wateringly detailed sequence stratigraphy of the now famous Mistaken Point locality and others suggests that the ecosystem there was an intertidal salt marsh. In detail it contains evidence for shallow-water graded bedding, signs of regular storms and perhaps tsunamis together with interbedded palaeosols and subaerial volcanic crystal tuffs whose feldspars survive intact. The palaeosols can be subdivided into several pedogenic types akin to those used to classify modern soils. Unlike the arid setting of the South Australian Ediacaran sediments, whose palaeosols show signs of freezing, the Newfoundland package indicates humid, cool-temperature climes

As in Australia, the palaeosols are rich in Ediacaran fossils, including the best known; the leaf-like Charnia and its discoidal support structure that appears in Retallack’s reconstruction of the environment in an analogous way to salt-tolerant shrubs in modern tidal flats. They occur together with encrusting fossils that bear some resemblance to modern foliose fungi or lichens. Further chuntering in the palaeontological community seems inevitable, but the sedimentological observations alone knock one hypothesis on the head: it has been said that the graded bedding common to both major Ediacaran assemblages constitutes evidence for deep marine origins from turbidity currents. But there is further compost in which controversy may thrive, in that Retallack ascribes the repeated palaeosols to glacially controlled sea-level fluctuations: the Newfoundland sequence contains two diamictites interpreted as tillite, one dated at ~583 Ma the other undated but at the top of the sequence.

A rational view of the start of human influences on Life and Geology

Published on January 2, 2016 by zooks777Leave a comment

Regular readers will know that I have strong views on attempts to burden stratigraphy with a new Epoch: the Anthropocene. The central one is that the lead-in to a putsch has as much to do with the creation of a bandwagon, to whose wheels all future geologists will be shackled, as it does to any scientific need for such a novelty. Bound up as it is with the fear that Earth may be experiencing its sixth mass extinction, the mooted Anthropocene will likely become a mere boundary marked by future stratigraphers as a Global Boundary Stratotype Section and Point or GSSP between the existing Holocene Epoch and that sequence of sedimentary strata and their fossil record that will be laid down on top of it. Or not, if humanity becomes extinct should the economically induced, dangerous modifications of our homeworld of the last few decades or centuries not be halted. Either way, it defies the stratigraphic ‘rule book’.

No one can deny that humanity’s activities are now immensely disruptive to surface geological processes. Nor is it possible to rule out such disruptive change to the biosphere in the near-future that a latter-day equivalent of the K/Pg or end-Permian events is on the cards: such confidence does not spring from the interminable succession of grand words and global inaction reiterated in December 2015 by the UN Paris Agreement on economically-induced climate change. Still, it was a bit of a relief to find that palaeontological evidence, or rather statistics derived from the fossil record in North American sedimentary rocks since the Carboniferous, emphasises that there is no need for the adoption of Anthropocene as an acceptable geological adjective.

To ecologists, extinctions are not the be all and end all of disruption of the biosphere. Major shifts in life’s richness are also recorded by the way entire ecosystems become disrupted. A classic, if small-scale, example is that way in which the ecosystem of the US Yellowstone National Park changed since the eradication by 1926 of the few hundred grey wolves that formerly preyed mainly on elk. In the 20 years since wolf reintroduction to the Park in 1995 the hugely complex but fragile Yellowstone ecosystem has showed clear signs of recovery of its pre-extirpation structure and diversity.

A consortium of mainly US ecologists, led by Kathleen Lyons of the National Museum of Natural History at the Smithsonian Institution in Washington DC, has assessed linkages between species of fossil animal and plants since the Carboniferous (S.K. Lyons and 28 others, 2015. Holocene shifts in the assembly of plant and animal communities implicate human impacts. Nature, published on-line 16 December 2015 doi:10.1038/nature16447). They found that of the 350 thousand pairs of species that occurred together at different times throughout the late Palaeozoic to the last Epoch of the Cenozoic, the Holocene, some pairs appeared or clustered together more often than might be expected from random chance. Such non-random association suggests to ecologists that the two members of such a pair somehow shared ecological resources persistently, hinting at relationships that helped stabilise their shared ecosystem. For most of post-300 Ma time an average of 64% of non-random pairs prevailed, but after 11.7 ka ago – the start of the Holocene – that dropped to 37%, suggesting a general destabilisation of many of the ecosystems being considered. This closely correlates with the first human colonisation of the Americas, the last of the habitable continents to which humans migrated. This matches the empirical evidence of early Holocene extinctions of large mammals in the Americas, which itself is analogous to the decimation of large fauna in Australasia during the late Pleistocene following human arrival from about 50 to 60 ka ago. Significant human-induced ecological impact seems to have accompanied their initial appearance everywhere. The ecological effects of animal domestication and agriculture in Eurasia and the Americas mark the Holocene particularly. In fact, in Europe the presence of Mesolithic hunter gatherers is generally inferred, in the face of very rare finds of artefacts and dwellings, from changes in pollen records from Holocene lake and wetland sediments, which show periods of tree clearance that can not be accounted for by climate change.

There is no need for Anthropocene, other than as a political device.

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