Geobiology, Palaeontology and Evolution

New hope for very old molecular phylogeny …

May 2008

Although DNA has been obtained from a number of fossils, including Neanderthals, its complexity more or less rules out any being preserved in a useful state beyond a few hundred thousand years ago. However, information about molecular relatedness also emerges from protein sequences, albeit with less chance of detailed comparisons. Collagen from bone is a potential resource for palaeobiologists, and fossils as old as the Jurassic Period have provided useable sequences. Prime targets are large extinct animals, as the greater the mass of a bone, the better the chance that it preserves some. Two irresistible beasts are the American mastodon (Mammut americanum) and T. rex (Organ, C.L. et al. 2008. Molecular phylogenetics of mastodon and Tyrannosaurus rex. Science, v. 320, p. 499). Unsurprisingly, the research group from Harvard, Boston and North Carolina, found that a Pleistocene mastodon contains proteins closely similar to those of African elephants. The T. rex, however, has a passably close relationship to the ancestral chicken, the South Asian Red Junglefowl (Gallus gallus) and the ostrich (Struthio camelus).

In fact, both connections were expected by the team, for their research set out to show that it is possible to extract intact parts of protein sequences from fossil bones. The matches confirm their hopes, and seem set to launch attempts at resolving evolutionary relationships among vertebrates that hitherto have depended on morphology alone.

Life perked up by repeated impacts

March 2008

Following the blazes of publicity since the early 1980s about the demise of the dinosaurs at the K/T boundary it is easy to regard objects the size of mountains that fall out of the sky as bad news for life. That is despite the fact that, bar the Chicxulub impact structure that exactly matches the timing of the end-Cretaceous mass extinction, no other significant and rapid drop in the diversity of life has been found to be associated with an extraterrestrial impact. Whatever their cause, mass extinction events sometimes seem to be followed by bursts in biodiversity, presumably as the survivors eventually find lots of new opportunities and diversity to occupy them. One exception is the end-Ordovician mass extinction that was also preceded by a tripling in the number of families, which the extinction rudely interrupted. This has often been seen as a somewhat delayed exploitation of all the advantages and competitive opportunities conferred by the appearance of hard parts at the start of the Cambrian. But remarkable finds in the limestone-rich Ordovician of Scandinavia suggest an unexpected connection with meteorite bombardment (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53).

The most usual measure of diversity used by stratigraphic palaeontologists is the number of families at a particular time, and the overall tripling in the Middle to Upper Ordovician is notable. However, if specimens of individual groups, such as brachiopods, are collected from the Scandinavian limestones on a bed by bed basis, increased diversity at the species level is even more dramatic. There are sudden doublings or triplings over periods of what can be no more than a few hundreds of ka, especially around 470 Ma ago. In the 1960s potassium-argon dating of chondritic meteorite collections revealed a cluster of reheating ages between 500 and 450 Ma (Upper Cambrian to Upper Ordovician); about 20% of all meteorites fall into this age-cluster, and most show evidence of having been shocked as well as heated up. This seems to signify a major collision or series of collisions in the Asteroid Belt around the early Palaeozoic. More reliable and precise 40Ar-39Ar dating narrows this event to a period between 463 and 477 Ma in the Middle Ordovician. In 2001, Birger Schmitz of the University of Lund reported, with others, more than 50 sizeable chondritic meteorites in the Middle Ordovician limestones of Sweden. Schmitz and his Damnish, US and Chinese colleagues in the new paper give plots of brachiopod species and also the abundance of chromite grains of meteoritic origin in Middle Ordovician limestones from Sweden and China. Two sharp jumps in brachiopod species numbers are  preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains, so the link seems to be real. Yet what can have produced such a counter-intuitive result? One possibility is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).

An old bat from Wyoming

March 2008

The Lower Eocene Green River Formation of Wyoming is dominated by fine-grained lake sediments, mainly made of laminated limy mudstones. Many layers constitute superb lagerstätten teeming with remains of delicate organisms. As well as much else, The Green River Formation is noted for its early bats, which suddenly appear in the fossil record with all the prerequisites for flight. The cover of the 14 February 2008 issue of Nature depicts a perfect specimen showing the four elongated ‘fingers’ that supported its wing membrane, and a long tail, which few modern bats have, except in atrophied form to support the rear part of the wing. In many respects it has a transitional structure between non-flying mammals and later bats, but would definitely have been a good flyer or rather flutterer-glider.

Not only is the fossil spectacularly well-preserved, detail of its head morphology helps resolve the issue of whether echolocation preceded flight (Simmons, .B. et al. 2008. Primitive Early Eocene at from Wyoming and the evolution of flight and echolocation. Nature, v. 451, p. 818-821). Other, slightly later fossil bats from the Green River Formation probably did echolocate, as evidenced by their stomach contents, and enlarged larynx and cochlea for transmitting and receiving the now typical high pitched squeaks of many bats. Onychonychteris doesn’t have such characteristics, so it seems as if echolocation did not evolve before flight, thereby resolving one of Darwin’s vexations about the universality of natural selection. Prior to the discovery by Simmons et al. many bat-oriented evolutionists speculated that echolocation evolved among small arboreal mammals so that they could detect passing insects. A habit of leaping to grab the prey in turn selected for an ability to glide from a strategic perch, for quite obvious reasons. Success further encouraged the evolution of powered flight. Yet no other living mammals have echolocation, probably because it is a highly energy-intensive habit. However, the muscles used by a flying mammal serve also to make squeaking a ‘cost-free’ bonus. So, the findings in Onychonychteris seem to resolve the matter nicely.
See also: Speakman, J. 2008. A first for bats. Nature, v. 451, p. 774-775.

Last common ancestor of all the primates was a flying lemur

January 2008

Vertebrate palaeontologists sometimes become precious after a career peering at old bones, especially when they are as remarkably tiny as those of most Mesozoic mammals – and most of those fossils are teeth. Some defend to death the notion that primates descend from tree-shrews, while others foam at the mouth at the mere suggestion of the ur-shrew. ‘A key feature in primate evolution is reduction of the snout’, is axiomatic to yet others. Again, geneticists have provided extreme selection pressures that will either cause vertebrate palaeontologists rapidly to evolve or to become extinct.

Analysis of living primate genomes produces a phylogeny that links all primates with a group that has been said to be ‘the sort of animals that defy taxonomic categorization, confuse one’s sense of aesthetics, and seem to largely fall under the umbrella of “weird.” ‘ (Janecka, J.E and 7 others 2007. Molecular and genomic data identify the closest living relatives of primates. Science, v. 318, p. 792-794). These are the colugos, or flying lemurs that include the wonderfully named sugar glider.

Planet of the beetles

January 2008

More than 20% of the known diversity of life on Earth is made up by the order Coleoptera, which includes several hundred thousand species. Although that huge number is largely thanks to beetle collectors, Charles Darwin having been a particularly voracious one, it is difficult believe that any other order or even class of multicelled organisms will prove to be as diverse. Yet there is only a sparse fossil record of these ubiquitous creepy-crawlies. The earliest known beetle fossils date back to the Lower Permian, and the Triassic saw their radiation into wood-eating, predatory and fungus-eating clades – from morphological similarities with living beetles. Their modern diversity depends on the vast range of ecological niches that beetles can fill, many of which are environmentally so subtle that only the beetles exploiting them show that the niches exist at all. Like all organisms the evolution of the beetles has been within the interconnectedness of the whole Earth system, and it through the linkages that such subtlety has emerged and evolved. One of the best known is the sensitivity of different beetle species to small climatic changes, which has allowed their growing use for charting climate change on land: they are far better proxies for temperature than are the foraminifera of the oceans.

Being only sparingly preserved in rocks, how beetles evolved has long been a mystery, considering their overwhelming presence on the planet. Yet again, the rapid rise of molecular phylogeny, including means of timing when mutations took place, is starting to supplant the skills of the traditional palaeontologist (Hunt, T. and 15 others 2007. A comprehensive phylogeny of beetles reveals the evolutionary origins of a superradiation. Science, v. 318, p. 1913-1916). Toby Hunt of London’s Natural History Museum and colleagues from the UK, Czech Republic, USA, Germany and Spain have combined their own RNA sequencing with existing databases of 1880 species from all the beetle suborders, series and superfamilies, 80% of families and 60% of subfamilies, to represent more than 95% of all described species. This establishes a phylogenetic tree for the lineages that they analysed, details of which will excite the coleopterist sororities and fraternities. The general picture, however, presents a more a broadly fascinating surprise. Because a vast number of beetles are associated with plants and fungi, it might seem inevitable that their evolution has parallels with that of plants, especially their explosive diversification once the angiosperms  (flowering plants) appeared. The molecular dating clearly shows that is not the case. While the angiosperms emerged in the Cretaceous Period, more than 100 living beetle lineages appeared earlier in the geological record. Unlike the Vertebrata, which diversified after mass extinctions (including the primates), the fundamental beetle lineages were clearly good survivors that were capable of their own diversification whenever opportunities arose. I think we might grow to worry about that…

Mammal evolution makeover

January 2008

The Cenozoic has been the Era of mammals, and their diversification is the largest recorded adaptive radiation. However, the Linnean names of many mammal clades from the Mesozoic end in ..dont, i.e. they have been defined in terms of their teeth and not much else.  Most fossil mammals from the Mesozoic are small and fragile and only survive as teeth and jaw fragments. As a result most of the course of early mammal evolution has been a bit uncertain, to say the least. The view until recently has been that early mammalian evolution was a step-by-step affair in which key innovations accumulated in an orderly manner.  However, even on the basis of teeth, developing taxonomic approaches have proved able to reveal that considerably more complicated things happened (Luo, Z-X. 2007. Transformation and diversification in early mammal evolution. Nature, v. 450, p. 1011-1019). For a start, it turns out that mammals, despite their scanty remains, were almost as diverse during the Mesozoic as the dinosaurs that are often said to have driven early mammals underground or into the night (310 mammal to about 550 dinosaur genera). The potential for analysis stems from an explosive growth in fossil discoveries: from 116 genera in 1979 to the present 310, and a 200-fold increase in well-preserved specimens. Clearly, mammal-oriented palaeobiologists have been hard at work.

Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh crams most of the developments into a 6-page review, from which it is possible to learn a great deal, albeit needing quite a firm grasp of cladistic terminology. One of the highlights is how evolution of the mammals before 65 Ma involved repeated evolutionary convergence, i.e. the end products of evolutionary bursts often looked superficially similar. That tendency carried over into the Cenozoic on a grander scale. One example is that of adaptations for burrowing to produce mole-like end products, even some with semi-aquatic habits. Many of the rapid diversifications ended in extinction of the lineage, but all seem to indicate a great deal of ‘experimentation’ with a range of original forms that channelled towards similar functions. The outcome was a vigorous occupation of potential ecological niches in which mammals clearly had the advantage over reptiles, possibly because of their physiologically greater adaptability, partly stemming from warm-bloodedness.

Permian shark bites fish-biting amphibian

January 2008

It is worth queuing to await the appearance of the 22 January 2008 issue of the Proceedings of the Royal Society B: Biological Sciences. It contains unique evidence of predator-prey relations and the food chain in the Lower Permian Zechstein Sea (Kriwet, J. et al. 2008. First direct evidence of a vertebrate three-level trophic chain in the fossil record. Proceedings of the Royal Society B: Biological Sciences, v. 275, p. 181-186). The object for your amazement is a shark whose gut contains two amphibians. The last meal of one of the amphibians was a small fish.

The paper promises to be reminiscent of the final part of the Monty Python Fish Slap Dance sketch, which can be viewed at http://www.youtube.com/watch?v=d1xfp6Xeu0c&feature=related

Feared dinosaur probably feathered

November 2007

The really scary dinosaurs in Jurassic Park were the velociraptors; they were smart and speedy. But it turns out that they were probably also well covered with feathers (Turner, A.H. et al. 2007. Feather quill knobs in the dinosaur Velociraptor. Science, v. 317, p. 1721). A specimen of V. mongolensis from Mongolia has well preserved forelimb bones that show traces of regularly spaced knobs. They look very like quill knobs on the wing bone of a living turkey vulture. On the velociraptor they were clearly not for flying, but may have been for thermal insulation, perhaps sexual display or to serve some aerodynamic purpose while the beast was running.

Robot shows how fishes walked out of water

May 2007

Now and again palaeontologists relax with toys; they build models. Perhaps the most famous were flying pterodactyl gliders that featured in a BBC natural history programme. The presenter, David Attenborough, was most impressed. At a society dinner shortly after filming them, ‘Whispering Dave’ was asked by a formidable old lady what he had been doing lately. ‘Well, I was flying pterodactyls last week’, he replied, hoping to impress. ‘Yes’, said the dowager, in an instant, ‘They’re so graceful, aren’t they’. The same could not be said about the amphibians of the late Devonian and early Carboniferous, and the earlier lobe-finned fishes that managed to struggle onto land to give rise to all terrestrial vertebrates. While it is demonstrably easy for any land vertebrate to swim if needs be, the opposite would seem to be a problem. To explore the locomotive transition Swiss and French engineers and fossil aficionados built a robot, not entirely unlike a lobe-finned fish but almost so. It has a simple spinal-chord neural circuit designed to swim. Then they let the beast loose near a beach (Ijspeert, A. J. et al. 2007. From swimming to walking with a salamander robot driven by a spinal chord model. Science, v. 315, p. 1416-1420), and it did walk off. So, invasion of the land by vertebrates was not necessarily too difficult. It is almost as if they were predestined to clamber out and eventually reach for the stars…

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Ancient protein

May 2007

Mass spectrometry is often associated just with radiometric dating and stable isotope techniques by geoscientists. However, it is a prime tool in separating biological compounds according to their molecular mass. Both approaches have steadily improved in their detecting and discriminating power. To my surprise, at least, proteins have been found by mass spectrometry in bones of the late-Cretaceous fright-icon, and in those of one of the more recent monsters of the American West, the mastodon Mammut americanum (Schweitzer, M.H. et al. 2007. Analyses of soft tissue from Tyrannosaurus rex suggest the presence of protein. Science, v. 316, p. 277-280; Asara, J.M. et al. 2007. Protein sequences from mastodon and Tyrannosaurus rex revealed by mass spectrometry. Science, v. 316, p. 280-285).

There is no need to worry about Late Cretaceous Park scenarios, but every reason to expect that tiny quantities of proteins preserved in bones may help establish phylogenetic relationships among long-extinct creatures so that ideas of evolution based purely on skeletal forms can be tested and amplified.

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Ediacaran fauna reviewed

May 2007

The to-and-fro debate over millimetre-sized spherules in the ~580 Ma Doushantuo lagerstätten in China – giant bacteria versus bilaterian embryos – has overshadowed the far more important ‘megafauna’ of the latest Precambrian. Thankfully, a timely review has restored the balance (O’Donoghue, J. 2007. Life’s long fuse. New Scientist, v. 194 (14 April 2007), p. 34-38). That the period before 552 Ma was not devoid of metazoan animals, puzzled over since Darwin’s day, emerged with a schoolboy’s 1957 discovery of the ‘sea pen’ Charnia masoni contained by Neoproterozic sediments in suburban Leicester’s Bradgate Park. A decade earlier, similar fossils had been found in abundance in the eponymous Edicara Hills of South Australia, but their host rocks had been misjudged as Cambrian in age.

The Ediacaran fauna of floppy animals is everywhere in sediments of suitable type deposited after the last ‘Snowball Earth’ event. Fossils are so abundant that they dominate some of the exposures. Yet they are mere imprints, often found in sandstones, but some are big: up to 4 m. Many of them are quite bizarre-looking, especially those in rocks older than 560 Ma. Despite some palaeontologists having been inclined to shoe-horn all Ediacaran animals into phyla that are living today, a consensus is emerging that some of them were failed evolutionary ‘experiments’, which left no issue. These are the forms found in deepwater sediments, some of which are known as rangeomorphs (because they all resemble the first to be discovered, frond-like Rangea). Post 560 Ma examples from shallow-water sediments do bear some comparison with later phyla of the Phanerozoic and present.

Painstaking searches for better-preserved animals in 2004 turned up rangeomorphs preserved in fine-grained sediments from Newfoundland. These enigmatic fossils revealed an astonishing feature. Their large-scale frondiness was built in a fractal way from fronds of ever decreasing scale. With no apparent orifices, these creatures probably absorbed dissolved organic matter directly from deep seawater.

Once animal hard parts evolved, those that were turned to biting spelled the end for the Ediacaran fauna, and burrowing that began the Cambrian destroyed traces of most of Ediacarans that may have survived the Cambrian explosion. Certainly there are none now, but the selection pressures of the ‘arms versus armour’ competition of the Phanerozoic would certainly have driven some to evolve new life styles, while others disappeared totally.

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Fossil embryos debunked?

March 2007

Late Precambrian (~580 Ma) lagerstätten in China have become quite famous for their supposed fossil embryos of bilaterian animals (see Age range of early fossil treasure trove in March 2005 issue of EPN) that often occur in large numbers. The fossils are substantial, ranging up to a millimetre in size. Under the microscope they look convincingly like embryos in the process of cell division. The stir caused by several publications on the Doushantuo embryos may turn out to have been premature and embarrassing (Bailey, J.V. 2007. Evidence for giant sulphur bacteria in Neoproterozoic phosphorites. Nature, v. 445, p. 198-201). Sulfur-oxidising bacteria are the largest known single-celled organisms, up to 0.5 mm in diameter. When their cells divide, the clones often adhere to form larger structures that look very like the putative fossil embryos. Such bacteria also bloom in environments that are highly reducing and rich in phosphorus. The Doushantuo sites of special preservation are phosphorites.

In his late career, the Lapworth Professor of Geology (1932-1949) at Birmingham University, L.J. Wills was immensely attracted to fossil eurypterids (sea scorpions), the largest-ever arthropods. If the weather was kindly, he often worked at a microscope outdoors on his patio. On a notable occasion Wills announced that he had discovered eurypterid eggs in a specimen from the Silurian of Lesmahagow in Scotland. However, once it was pointed out politely that his ‘eggs' bore a close resemblance to rose pollen, he freely admitted his mistake.

Coincidentally, Bailey's sceptical view was countered by publication of yet more material from Doushantuo (Xiao, S. et al. 2007. Rare helical spheroidal fossils from the Doushantuo lagerstätte: Ediacaran animal embryos come of age? Geology, v. 35, p. 115-118). Some looking like minuscule tennis balls with arrays of pores and others with spirally twisted entrances to bodily orifices, the exquisitely preserved and imaged fossils shown by the Chinese and US team bear no resemblance whatever to giant bacteria, being far more complex. They may well be embryos, but look just as convincing as tiny metazoan animal adults to a lay person. Either way, there is no clue to their affinities for either the ignorant or the specialist.

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Microbial carbonate secretion

March 2007

Biological deposition of carbonates, mainly of calcium, has a vital role in the carbon cycle and therefore in helping to regulate climate by drawing down atmospheric carbon dioxide. It is easy to see the secretion of hard parts by metazoan animals and plants as the main way in which this happens, Phanerozoic limestones and the sediments of the deep ocean floor being full of their remains. Single-celled Bacteria and Archaea can have much the same effect, and before 542 Ma they were the dominant creators of carbonate rocks in the geological record. The carbonates produced by microbes are fine grained, and take the form of biofilms that are sometimes finely banded, to produce, for instance, stromatolites that date back to around 3.5 Ga old. Undoubtedly a proportion of all Phanerozoic carbonate deposition was microbial in origin too. Because metabolic processes are disrupted by excessive calcium within cells—it seems that needle-like calcium carbonate develops—yet calcium is abundant in most natural waters, all organisms need means of regulating calcium concentration, both in the cell and in its immediate surroundings. It is possible that the sudden evolution of abundant, calcium-rich hard parts by animals during the Cambrian Explosion stemmed from this necessity, setting off the ‘arms versus armour' that has been a major selective pressure during the Phanerozoic. So, how do microbes secrete carbonates?

There are two views: as a direct result of cell metabolism; by secondary means that involve cells helping to nucleate carbonate precipitation. The first would take place on the surface of cells, the other at some distance. A problem with direct precipitation at the cell wall is ‘entombment' of the cell. Experiments that induce carbonate secretion can help resolve the issue (Aloisi, G. et al. 2006. Nucleation of calcium carbonate on bacterial nanoglobules. Geology, v. 34, p. 1017-1020). The German and French authors used a bacterium that reduces sulfate to sulfide and is known to precipitate carbonates. It lives in highly saline lagoons whose waters are supersaturated with calcium and carbonate ions. Without the bacteria, sterile water with such a composition does not spontaneously precipitate carbonates, but as soon as a culture is introduced, they begin to appear. Under high magnification, the precipitates show up as spherical globules associated with organic compounds excreted by the cells, rather than at the cell surface. The micro-environment around the cells is therefore depleted in dissolved calcium ions and its alkalinity is lowered, to the advantage of the bacteria.

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Our relationship with sea urchins

January 2007

With their five-fold symmetry, echinoderms seem to be dubious candidates for having shared an ancestor with vertebrates, but that has long been suspected. Like chordates, their embryonic development reveals that they are deuterostomes, with bilateral symmetry (a five-fold symmetry also involves mirrored morphology). The earliest fossil echinoderms also show what seem to be gill slits. The construction of a sea urchin's genome is not only a means of testing that relatedness but extremely useful in studying the development of organisms in general, for which sea urchins have been a favourite object of research. So, it is not surprising that 22 pages of Science is devoted to preliminary discussion of the first echinoid genome (10 November 2006 issue of Science, v. 314, p. 938-962). It definitely confirms the link, there being many genes that are also essential to vertebrates, while genes typical of crustacea, molluscs and some worms are missing. A fascinating feature is the large number of genes for proteins that are involved in sensory perception, despite the fact that echinoids are not obviously sighted or able to smell. Similarly, the sea urchin's immune system is far more complex that that of vertebrates, yet several groups of genes involved in it seem to have no practical function, whereas they are central to specialised immune cells in vertebrates. It seems that aspects of the genetic make-up of the deuterostome common ancestor were harnessed in different ways by vertebrates and echinoderms, specifically in the immune and sensory systems. Even a gene central to mammalian brain development is present in the sea urchin, despite its lack of any obvious brain.

As well as opening up masses of work for biologists, the sea urchin genome should encourage a focus on the earliest echinoderms and related organisms, just after the Cambrian Explosion (Bottjer, D.J. et al. 2006. Palaeogenomics of echinoderms. Science, v. 314, p. 956-960). Interestingly, the five-fold symmetry that is so familiar from sea urchins, starfish and crinoids did not appear until the Late Cambrian, earlier, extinct relatives having developed the water vascular system and characteristic biomineralised plates.

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Oxygen and the explosion of large, soft animals

January 2007

The newly defined Ediacaran Period of the latest Precambrian takes its importance from the first appearance of large animals, albeit ones without hard parts. Eukaryotes emerged before 2.1 Ga, when their oldest fossil (Grypania, probably an alga) appears. Until the Ediacaran, animals were too small to appear as recognisable fossils. All eukaryotes depend on oxygen being available, and the larger they are the more they need. A widely held explanation from the dramatic appearance of the Ediacaran fauna—over a mere metre or so around 575 Ma in each of the occurrences—is that it followed a significant boost to oxygen concentrations in both the atmosphere and the oceans. Two of the most productive Ediacaran sequences, in Newfoundland and the Oman, have now provided evidence for the pacing of such an oxygen build up (Canfield, D.E. et al. 2006. Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life. Science, v. 314 doi: 10.1126/science.1135013; Fife, D.A. et al.2006. Oxidation of the Ediacaran Ocean. Nature, v. 444, p. 744-747).

In Newfoundland and Oman Ediacaran animals appear shortly after evidence for glaciation, regarded by many as the last of the Neoproterozoic ‘Snowball Earth' events: the Gaskiers glaciation around 580 Ma. Don Canfield of the University of Southern Denmark and colleagues from the UK and Canada used variations in the proportions of geochemically and biologically active iron minerals (such as sulfides) to those that are largely inert (e.g. hydroxides) in sedimentary rocks to estimate the influence of oxygen. They found a big change following the Gaskiers glaciation to around the same proportions as throughout Phanerozoic rocks. The US group, led by Fike, focused on the sulfur and carbon isotopes in sediments that succeed the earlier Marinoan glaciation (~635 Ma). Changes in the proportions of sulfates and sulfides in seawater affect sulfur isotope data, sulfates indicating oxidising conditions. Carbon isotopes preserved in organic carbon reflect periods when dissolved organic carbon has been oxidised. They reveal three distinct jumps in oxygen levels in the ocean water that covered Oman at the time. It was during the second and third stages (580 to 550, and <550 Ma) that the Ediacaran faunas emerged globally. The first two increases began soon after glacial episodes, and maybe there was a connection between global cooling and conditions that favoured an increase in oxygen—that would have to reflect increases in both production of oxygen by photosynthesis and burial of biologically reduced carbon; i.e. an effect on phytoplankton. Interestingly, it was during the last of the oxygen boosts that small, indeterminate shell-secreting organisms appear in the fossil record: the harbingers of the Cambrian Explosion.

See also: Kerr, R.A. 2006. A shot of oxygen to unleash the evolution of animals. Science, v. 314, p. 1529.

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Global warming, sour gas and mass extinctions

January 2007

Several mass extinctions show links in time with massive outpourings of flood basalts. The best known is the connection between the Siberian Traps and the end-Permian extinction that probably put paid to 90 % of species, both marine and on land. But, what was the kill mechanism? Since all volcanoes emit carbon dioxide, flood basalt events would have had a major effect on climatic warming, and also there would have been a decrease in the pH of rain and surface seawater. One of the most powerful tools for charting ups and downs in the biosphere uses the isotopic composition of carbon in sediments. Extinctions at the ends of the Permian and the Triassic Periods are associated with repeated fluctuations in ¹³C that suggest a series of extinction events that culminated in the final catastrophes (Ward, P.D. 2006. Impact from the deep. Scientific American, v. 295, October 2006 issue, p. 42-49).

Ward links the carbon-isotope evidence to signs that in both events the oxygen content of ocean water fluctuated dramatically: it periodically became anoxic. Biomarkers in sediments leading up to both events show that photosynthetic green sulfur bacteria bloomed periodically. These organisms do not produce oxygen, but use oxidation of hydrogen sulfide gas to sulfur as an energy source, and cannot survive in oxygenated water. Their abundance demands large-scale production of H₂S in the oceans, by other anaerobic organisms, such as sulfate-reducing bacteria. Where deep anoxic waters were able to upwell to the surface, they would have released huge amounts of this toxic gas to the atmosphere. That, for Ward, would have been a major killing mechanism for both plants and land animals.

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Gliding mammal of the Jurassic

January 2007

Non-palaeontologists have grown used to regarding mammals before the Cenozoic as tiny retiring beasts that only came out at night, avoiding dinosaurian predators. Well, as far back as the Middle Jurassic, there were some that could glide to snatch insects (Meng, J. et al. 2006. A Mesozoic gliding mammal from northeastern China . Nature, v. 444, p. 889-893). Being found in yet another Chinese lagerstätte, Volaticotherium has well-preserved signs of fur and the characteristic skin flap linking all four feet of modern flying squirrels and sugar gliders, without which it would just be another sharp-toothed little mammal. It is 70 Ma older than the earliest, previously known flying mammal, and may have taken to the air before birds did.

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Fossil bee: the right place and the right time

November 2006

Amber from a mine in Myanmar generates a steady income from sales to palaeoentomologists, each bead of the lithified resin being a possible lagerstätte in its own right. Two scientists at Oregon State University and Cornell were fortunate enough to find a small, Early Cretaceous bee that is so well-preserved as to show even the leg hairs on which bees carry pollen (Poinar, G.O. & Danforth, B.N. 2006. A fossil bee from Early Cretaceous Burmese amber. Science, v. 314, p. 614). Indeed, the hairs carry several grains of pollen. This is the oldest known bee by more than 35 Ma, and it coincides with the start of the explosive radiation of flowering plants.

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Near-pristine traces of life before Earth’s surface became oxidising

August 2006

Only around 2.2 Ga did the atmosphere contain sufficient oxygen to oxidise iron(II) to iron(III) and leave its trace in red soils and terrestrial sedimentary rocks. That opened the way for the emergence and evolution of the Eucaryan domain of organisms, most of which depend on oxygen. For their predecessors, the prokaryote Bacteria and Archaea, oxygen would have been intensely toxic, especially for those which used anoxygenic forms of metabolism. Almost certainly oxygen was released for more than a billion years before the Great Oxidation Event, by blue-green bacteria, only to be mopped up by oxidation of abundant iron(II) ions dissolved in sea water. Getting an idea of the diversity of pre-2.2 Ga life is possible by examining the organic chemicals produced when they decayed under anoxic conditions, i.e. from oil and kerogen. Unfortunately, the great age of their host rocks has resulted in many Precambrian sediments having been heated and metamorphosed, so that different biomarkers break down into less distinctive compounds. There are, however, sediments that may have remained more or less unaffected, and one sequence in the Canadian Shield has yielded astonishing results (Dutkiewicz, A. et al . 2006. Biomarkers from Huronian oil-bearing inclusions: An uncontaminated record of life before the Great Oxidation Event. Geology , v. 34 , p. 437-440).

The sediments are conglomerates rich in uranium, having been deposited under reducing conditions that helps precipitate uranium from solution, and have been mined extensively in the Elliot Lake area of Ontario. Oil seems to have entered fluid inclusions in quartz that cemented the conglomerates, shortly after the conglomerates were deposited at about 2.45 Ga. The oil contains a host of complex organic compounds that have never been degraded by heating. Some can be linked to blue-green bacteria, which undoubtedly created of oxygen continuously. That they gave rise locally to favourable conditions for oxygen-using organisms is clear from other biomarkers. Those are steranes that are derived by breakdown of sterols, which in turn are only known to be created by the enzymes exclusive to Eucaryan metabolism. Steranes have been found in even older sediments, but they were back shales that could easily have been contaminated by much younger organic materials seeping through the host rock. Oil in fluid inclusion within diagenetic minerals is far less likely to have been contaminated, so the Elliot Lake samples define a minimum age for the emergence of the Eucarya far earlier than the appearance of actual microfossils that show the distinctive cell nucleus that defines the domain Eukarya.

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Precambrian bonanza for palaeoembryologists

Signs of relatedness among groups of organisms often show up well during their early growth as embryos, so their fossils in very old rocks are of great use in establishing when different groups emerged (see Ancient baby penis worm hits the news in EPN February 2004 issue). A deposit containing possible embryos of deutorostomes (see Age range of early fossil treasure trove , in EPN March 2005 issue), in which the first orifice to emerge during embryonic development is the anus, is of considerable interest. Nowadays, the group contains animals with mirror symmetry (bilaterians), including the vertebrates. First reports of fossil embryos from the 580 Ma old Doushantuo Formation of southern China in 2004 drew fire from palaeontologists who preferred to believe that the smooth almost spherical objects, like the fictitious life forms in a supposedly Martian meteorite, were probably oolith-like mineral growths. Undeterred, their finders have extracted yet more from the exposures (Chen, J-Y. and 12 others 2006. Phosphatized polar lobe-forming embryos from the Precambrian of southwest China . Science , v. 312 , p. 1644-1646). They demonstrate clearly that the objects do show lobes in an early stage of development that break the embryos initial symmetry so that different kinds of tissue can develop to form adults. The find matches well with evidence from the genes of modern bilaterians that the basic branching of the Animal Kingdom occurred well before the Cambrian Explosion of shelly fossils. Since more or less all modern phyla are represented by Cambrian fossils, that is not surprising.

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Pocket sauropods

August 2006

The largest animals to roam the land were vegetarian dinosaurs of the sauropod group. The biggest reached a length of more than 30 metres, and were commensurately tall. These giants permeate our perception of Mesozoic life on the continents, along with their monster predators. Now, children made nervous by such titanic creatures (and I was definitely one of them) can be reassured that there were ones that were not so crushingly big (Sander, P.M. et al . 2006. Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur. Nature , v. 441 , p. 739-741). A near-complete skeleton of a sauropod that was only 6 metres long turned up in Lower Saxony in Germany, along with other remains suggesting individuals as small as 1.7 m. Europasaurus was first thought to be a juvenile of a much larger species, but Sander et al . developed means of microscopic bone analysis that clearly show fully mature bone growth. In the Late Jurassic central Germany was covered by sea, except for a number of large islands. The most likely explanation for such a tiny species is that it adapted to island life in much the same way as other, more recent mammals did, such as pigmy elephants and hippos on many islands in the Mediterranean and the Indonesian archipelago.

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A fish-quadruped missing link

May 2006

Rich as the fossil record is, it is terribly incomplete, for the obvious reason that the chance of preservation over fragmentation and destruction of body parts is extremely small. That is especially the case for the high-energy and oxidising land and freshwater environments. Each fossil species can easily be assumed to be a one-off, appearing, thriving for a short while and then disappearing: ripe for the assumption of divine creation, as Linnaeus assumed. Very rarely indeed, specimens emerge that fill in the many gaps needed by evolutionary theory, the most celebrated being Archaeopterix that bridged the gap between dinosaurs and birds. That transition has been enriched by a whole series of older fossils from Chinese lagerstätten that show the transition in sublime detail.

The comparative anatomy of fish and land vertebrates suggests a common ancestry, and the Devonian to Early Carboniferous terrestrial record has yielded tantalising fish with lobed fins (e.g. Eusthenopteron and Panderichthys) and almost fish-like animals with four rudimentary limbs (e.g. Acanthostega and Ichthyostega). Yet a gap remained to be filled in the apparent transition from aquatic to land-dwelling vertebrates. US palaeobiologists engaged in seeking candidates from the Late Devonian of Arctic Canada have found one that reduces any uncertainty tremendously (Daeschler, E.B et al. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, v. 440, p. 757-763. Shubin, N.H. et al. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, v. 440, p. 764-771). The fossil, prepared with lengthy and painstaking care, shows such amazing anatomical detail as to demonstrate clearly that the fin and shoulder girdle are indeed intermediate between fish and tetrapods, whereas previous candidates supporting a transition are either definitely fish or tetrapods. Tiktaalik slots nicely into the time gap too, about 2 Ma younger than the most tetrapod-like fish Panderichthys and slightly older than fish-like quadrupeds. The outcome of a deliberate search for an animal to fit the gap, Tiktaalik above all demonstrates the predictive capacity of palaeontology, which counters a common epithet flung by those bent on divine intervention and/or intelligent design. Based on this outstanding success, fossil hunters will be encouraged to sift on a stratigraphically finer scale for yet more steps in vertebrate evolution, including our own.

See also: Ahlberg, P.E. & Clack, J.A. 2006. A firm step from water to land. Nature, v. 440, p. 747-749.

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Methane, methanogens and early climate control

April 2006

Expulsion of methane from gas hydrates in shallow marine sediments has been implicated several times as the likely cause for sudden bouts of global warming, such as that at the end of the Palaeocene 55 Ma ago. The gas, produced by primitive, anaerobic prokaryotes known as methanogens, is more powerful at delaying loss of heat to space than is carbon dioxide. It is a greenhouse gas of enormous potential power, although in an oxygen-rich atmosphere it has a short life before being oxidised to CO2 and water. Methanogens themselves, which survive only in airless places, evolved very early in the Earth's history as witnessed by their genetic molecules being very different from those of other members of the Bacteria and Archaea domains. The ambiguities that prevent carbon isotopes in ancient carbonaceous rocks from being able to discriminate different metabolic processes, has led to considerable debate about when methanogens first made their appearance. That was undoubtedly long before the oceans were able to contain dissolved oxygen, which is highly toxic to anaerobic prokaryotes. Good evidence that such cells were around would be, in some way, to detect their main metabolic product, methane. The place to look would be in fluid inclusions enclosed in minerals that were definitely produced by seafloor sedimentary processes. The best candidate would be quartz in cherts precipitated from seafloor hydrothermal vents, where such organisms could obtain both the energy and the fuel to thrive. A group of Japanese geochemists have systematically looked for such fluid inclusions in a variety of Archaean cherts and they found sufficient evidence to at least give a minimum age for the presence of methane-producing bugs (Ueno, Y. et al. 2006. Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era. Nature, v. 516, p. 516-519).

The Dresser Formation (3.45-3.50 Ga) of the early Archaean of Western Australia contains abundant pillow basalts and chemogenic, silica-rich sediments. These cherts seem to have been fed by fissures through which hydrothermal fluids moved, and it is quartz from these syn-sedimentary quartz-rich dykes that revealed abundant fluid inclusions that had clearly formed as the quartz crystals grew. The inclusions contain carbon dioxide with traces of methane. Most important, the carbon in the methane is highly enriched in heavy 13C, evidently due to cell processes drawing in the lighter isotope 12C; the methane is almost certainly biological in origin. So it is possible to say both that methanogens had evolved before 3.5 Ga, and that they added methane to the Archaean atmosphere. Such a highly reduced gas would then have become a permanent constituent of the air, because oxygen had yet to be released by other organisms so that methane would be not oxidised quickly, as happens today. The discovery by Ueno et al. is important from another standpoint than the appearance of a particular kind of metabolic process.

From the time of its accretion until well into the early Precambrian, the Earth received a great deal less energy from the Sun than it does today. Solar hydrogen fusion had not then achieved the level of efficiency that it has now. Without some means of trapping heat in the atmosphere, the Earth's mean surface temperature would have been well below the freezing point of water. Without a `greenhouse' effect of some kind, the planet, well endowed with water, would have been inescapably locked inside a thick crust of ice. In some respects it would have resembled a large version of one of the Outer Planets' icy moons, such as Enceladus (see Yet another weird world later). Life would have found it difficult to emerge, if at all, at such low temperatures. Like Enceladus and other distant moons, some liquid water would have been present due to heating from the mantle and magmas, but the white surface would always have reflected away most of the Sun's heat – geothermal heat is vastly less than that of solar origin. The most recently proposed means whereby the Earth could have escaped permanent frigidity and sterility from the `weak, young Sun' is that volcanic exhalation of CO2 would eventually have developed `greenhouse' conditions. However, it would have had to reach much higher atmospheric concentrations than at present, perhaps greater than some geochemists believe to be theoretically possible. Being a much more powerful `greenhouse' gas, methane helps overcome such theoretical difficulties. Yet it can only be produced in quantity by biological processes, and that poses a conundrum, despite Ueno et al.s discovery. Without an atmosphere containing gases that could trap solar warmth since shortly after planet formation, the cold trap would have taken an early icy grip, thereby holding back the emergence of life, such as primitive methanogens. Does that therefore imply that such organisms emerged far earlier than the start of tangible geological history?

Gaia: the ultimate frontier

April 2006

That life plays a role in surface geological processes is self-evident. Death and the burial of dead organic matter feed back to climate by removing carbon from the atmosphere and hydrosphere, thereby reducing the `greenhouse' effect and increasing the oxidation potential of the outer Earth – a discovery of the late 20th century. James Lovelock's Gaia hypothesis proposes that life's influence as a means of balancing conditions for its own continuity is a primary factor behind the behaviour of our home world, although a great many geoscientists doubt that bold generalisation. It seems to many that the influence of both deep mantle processes and extraterrestrial forces not only provided the conditions for planetary evolution, both inside and at the surface, but created the conditions for life's emergence and its survival. Life has been pushed to the brink of complete extinction several times by both truly primary parameters. Yet Gaia is still a persuasive idea, or at least a metaphorical itch that must be scratched from time to time. Perhaps the boldest attempt at pushing Lovelock's notions to the limit appears in a recent essay (Rosing, M.T. et al. 2006. The rise of continents – An essay on the geologic consequences of photosynthesis. Palaeogeography, Palaeoclimatology, Palaeoecology v. 232, p. 99-113).

Assuming that carbon-isotope evidence from the oldest known sediments (3.8 Ga, West Greenland) that life selectively took up light 12C is valid, there seems to be a remarkable coincidence between the origin of life on Earth and the oldest known continental rocks (4.0 Ga, northern Canada). Rosing et al. suggest that this is no coincidence, but the result of the effect of living organisms on magmatism at subduction zones, most particularly on the mineralogy of old oceanic lithosphere that descends there. Their essay starts by emphasizing that modern photosynthesis contributes three times more energy to surface processes than does heat flow from the mantle, and that energy must accomplish a commensurately significant amount of mainly geochemical work, some of which occurs in basalts of the ocean floor as they spread from constructive margins. Continental crust is widely accepted to form as a result of hydrous fluids rising above subduction zones to cause different conditions for melting of the overriding mantle wedge than those for partial melting of mantle rock beneath mid-ocean ridges and oceanic islands. Multistage fractionation processes that operate on basaltic magmas formed by this wedge melting result in separation of residual magmas that are sufficiently enriched in silica and other elements to crystallize as, broadly speaking, granitic rocks. Since they cannot be metamorphosed to a form that exceeds the density of the mantle, such rocks cannot be subducted, unless debris shed from them mixes as sediment with subducting oceanic lithosphere. So continents become more or less permanently growing edifices on the face of the Earth. The central questions that Rosing et al. focus upon are: why did continents not form from the outset of the Earth's evolution, once tectonics and oceans had stabilized, and why the coincidence? Their answer to both is that life played a fundamental role in increasing the amount of water that ends up in old, cold oceanic crust, thereby helping the peculiarities of wedge melting to become established. Essentially they appeal to life's ability to transform energy of different sources, for example heat from the mantle and the energy carried by electromagnetic radiation, and transmit it through biogeochemical cycles from its source to the lithosphere. Specifically, they speculate that this life-mediated energy transfer accelerated the conversion of dry minerals in basalt to water-rich clays. In turn, that had its effect on subduction-zone geochemistry.

Rosing et al.'s seems to have a willful flaw: they focus on the incorporation of solar energy into the Earth system by photosynthesis from the time when continental materials first appeared in substantial bulk, between 3.8 and 4.0 Ga. So far there is a mere shred of evidence from ambiguous carbon isotope studies that photosynthesising organisms were around before about 3.4 to 3.5 Ga. There is no trace of such shallow-water organisms as stromatolites until that time. Nor is there any significant sign of where one end product of photosynthesis, oxygen, must have been secreted away by reaction with dissolved iron(II) – banded iron formations only become prominent in the later Archaean. Whatever organic activity might alter ocean-floor basalts, it is hardly likely to have used photosynthesis, unless the early oceans were shallow enough (200-300 m) to pass light to their floor. The key to alteration of anyhydrous minerals in basalt to form clays is the availability of hydrogen ions (products of oxidation) to donate electrons through hydration reactions, and they are available from a great many processes other than living ones. Then, of course, there is the key issue of whether any influence – direct or indirect – by photosynthesis can be seen on modern ocean-floor geochemical processes. Since it doesn't go on down there, whereas a great many oxidation reactions that produce hydrogen ions do, makes the hypothesis impossible to test. In fact it is not a hypothesis but speculation, and it has a great deal of company from other ideas to explain the missing 600-800 Ma of Earth's evolution. Most of those centre on the mechanics of slab-pull force, the pace of sea-floor spreading and the angle of likely subduction during geothermally much hotter times. Oddly, the third author, Norman Sleep, introduced a great deal of basic theory behind these other explanations. This is one of two articles from March 2006, whose time of publication – close to 1 April – may give a clue to its weight. It is interesting seasonal reading, and everyone should look forward to further debate. However, like the magnificent Verneshot hypothesis (See Mass extinctions and internal catastrophes in June 2004 issue of EPN), it may die in a deafening silence.

There is one final, obvious point about the coincidence from which Rosing et al. begin: since all rock older than about 170 Ma resides in the continental crust, it would be difficult to find signs of life that date from a time before the oldest of that crust formed.

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Faster recovery after mass extinctions

March 2006

Mass extinctions have been the principal time markers in the Phanerozoic stratigraphic column since 19th century palaeontologists recognised sudden changeovers in the fossil record. Two close the Palaeozoic and Mesozoic Eras, two more end Periods (Ordovician and Triassic) and others mark Stage boundaries. Greatest focus has been on the magnitudes of each extinction, greatly assisted by the statistics compiled by the late Jack Sepkoski. The adaptive radiations that filled abandoned niches and restored and, in most cases, expanded diversity are equally interesting. Such recoveries from depleted stocks of organisms have been of immense influence over biological evolution. Resulting from chance events, as far as the Earth's biota are concerned, the families and species that arose would not otherwise have appeared: the most powerful blow to any notion that biological advances are in any way pre-ordained.

Until recently, it seemed that each recovery was an extremely protracted affair. Over 5 to 10 million years seemed to be the case for aftermaths of the largest extinctions. To a marked extent, analysing recoveries from the fossil record is not so easy as tying the great declines in diversity to a time. It is a matter of working out the rate at which new genera arose or originated through speciation, and that is affected by geographic biases in the fossil record. They arise from less collecting in remote areas and variations in the volume of exposed strata in others. Correcting the biases is possible to some extent, but that still leaves the challenge of statistical analysis. From an extraordinary expansion of analytical expertise, which extends to economists' methods of understanding stock market trends and the flair of physicists, a very different story of restocking seems about to emerge. A technique called vector autoregression applied to faunal diversification corrected for biases suggests that recoveries were very much faster than previously thought, in fact almost immediate by comparison with the time-precision of the stratigraphic column (Lu, P.J. Motohiro Yogo, M and Marshall, C.R., 2006. Phanerozoic marine biodiversity dynamics in light of the incompleteness of the fossil record.

Proceedings of the National Academy of Sciences, v. 103, p. 2736-2739). See also: Kerr, R.A. 2006. Revised numbers quicken the pace of rebound from mass extinctions. Science, v. 311, p. 931.

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Is the Cambrian Explosion real evidence for an evolutionary burst?

March 2006

About 543 Ma ago, remains of organisms that secreted hard parts suddenly appear in the fossil record. Most palaeontology has focussed on such easily fossilised organisms from the Phanerozoic Eon that began at that time. Whether or not the Cambrian Explosion was a truly significant event, bar the appearance of hard parts – that is quite a mystery in itself – is highlighted by the presence of members of almost all modern animal phyla in the Early Cambrian record. Did they all suddenly explode onto the scene at its outset, or were they around well beforehand as almost completely soft-bodied creatures? Comparative molecular biology of living animals, and the concept of molecular `clocks' has for a while suggested that the origination of modern phyla was considerably earlier than the start of the Phanerozoic. Increasing the database on which such ideas can be based helps improve their precision and scope, assisted by novel methods of mathematical analysis. The 23 December 2005 issue of Science contained an analysis of more than 12 thousand amino acids involved in the genomes of members of 9 or 26 extant animal phyla (Rokas, A.. et al. 2005. Animal evolution and the molecular signature of radiations compressed in time. Science, v. 310, p. 1933-1938). Preliminary study suggests that indeed the early history of the metazoans was remarkably compressed in time, probably in the 50 million years after the ~600 Ma Snowball Earth event, and possibly within a few million years of the base of the Cambrian. However, tests of hypotheses based on such indirectly related data are notoriously difficult, and Rokas et al. have taken a bit of stick (Jermiin, L.S. et al. 2005. Is the 'Big Bang' in animal evolution real? Science, v. 310, p. 1910-1911). It seems yet more work on molecular biology of the remaining 17 phyla and a great deal of mathematical wrangling is yet to come.

Yet more on the end-Permian extinction

January 2006

Sequences that reveal the Permian-Triassic boundary continue to receive a great deal of attention, spurred by the seemingly cryptic Nature of the conditions that caused up to 90% of all living things to die. Globally, the boundary is marked by a sudden and large fall in the proportion of 13C in carbonates and sedimentary organic matter. Since the d13C anomaly follows the biotic decline, it is less likely to reflect any cause of the extinction, such as a massive methane release from destabilised gas hydrates and global warming, than an effect of whatever went on. Joint research by UK, Dutch and US organic geochemists focused on the P/Tr boundary in northern Italy, where it is dominated by shallow-marine carbonates (Sephton, M.A. et al., 2005. Catastrophic soil erosion during the end-Permian biotic crisis. Geology, v. 33, p. 941-944). They analysed the organic compounds preserved in the section, and found that the extinction zone coincides with a major increase in total organic carbon, which is dominated by large amounts of compounds (polysaccharides) that typify soils and leaf litter. They explain the anomaly as the result of a short period of rapid soil erosion from the terrestrial hinterland of the shallow Late Permian sea. Since virtually all continental crust had stabilised in the Pangaea supercontinent, tens of millions of years beforehand, such erosion was unlikely have been a result of some sudden tectonic uplift. But it might have been triggered by sudden loss of the vegetation that retards soil erosion on the continental surface. The P/Tr extinction affected both marine and terrestrial organisms, and Sephton et al recognise that their discovery of evidence for soil stripping on a grand scale reflects that unified fate. Acid rain from the massive Siberian continental flood volcanism could well have been the trigger for ill thrift of land vegetation, or maybe removal of stratospheric ozone by release of halogen (chlorine and bromine) compounds let in destructive UV radiation.

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Fig leaves over Palaeocene-Eocene boundary

December 2005

Methane-induced warming around 55 Ma ago was one of the greatest environmental upheavals of recent geological time. Pretty quickly, all the methane belched out by destabilisation of sea floor gas hydrates would have forced up atmospheric CO2 concentrations. The estimated climatic effect was astonishing: a global temperature rise of the order of 5-10°C in 10-20 thousand years. The early Eocene world would have become a steamy place, and the changes certainly tally with shifts in a range of faunas, from foraminifera to large mammals. Not many people have reported any coincident changes in plant fossils, even though a moist atmosphere charged with CO2 would have encouraged growth enormously. A reflection of the changed conditions does come from rapidly changing leaf shapes and sizes, however. One of the key sections that does reveal floral change is in terrestrial sediments preserved in the Bighorn Basin of Wyoming, USA (Wing, S.L. et al. 2005. Transient floral change and rapid global warming at the Paleoene-Eocene boundary. Science, v. 310, p. 993-996). Tied down from a dramatic change in carbon isotopes, the boundary section not only shows the rapid dominance of leaves with extended `drip tips' that allow rainwater to be shed quickly, but an influx of genera unknown from the Palaeocene below. The invasive groups are known from sediments of that age from much further south in the US, and even from Europe at the other side of the opening Atlantic Ocean. So it seems that there was a rapid northward plant colonisation over 4 to 20 degrees of latitude. The section perhaps gives a flavour of floral changes that might occur should modern anthropgenic warming go unchecked.

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Dinosaur dung, the Deccan Trap and grass

December 2005

Yes, it has to come to a pretty pass when geologists will tramp to the very base of the Deccan continental flood basalts, dig up and then finger through dinosaur crap. The temptation of a bed consisting of little other than coprolites deposited by sauropods, especially beneath the very lavas implicated by some in their demise, is huge. It isn't the first time that coprophilia has struck the vertebrate palaeontological community, for a very good reason: if dinosaurs grew so darned big what did they eat? That it included grass is a surprise for palaeobotanists, but would have been a great treat for the thunder lizard, for there is nothing more toothsome to a herbivore than a hay snack; much better than a monkey puzzle leaf. Indian and Swedish geologists hit the headlines with their discovery (Prasad, V. et al. 2005. Dinosaur coprolites and the early evolution of grasses and grazers. Science, v. 310, p. 1177-1180). The lithified dung contains unmistakable traces of silica-rich phytoliths that occur only in grasses. Some possible grass pollen has been found before in Late Cretaceous sediments, but the crown-group Poaceae, that still thrive today, had been thought to have appeared later than the Early Eocene. It now seems likely that grasses appeared first in Gondwana, being transferred to Eurasia by the collision of its wandering fragment India around 50 Ma ago – India had already begun to move independently at the time of Deccan eruptions. Genetic studies of grasses points to their origin about 80 Ma ago, so it is likely that those in the dung are among the earliest. The Indian titanosaurs that ate them were not grazers, however, because the dung is also full of remains of conifers, palms and other vegetation that would have been abundant in those times. Interestingly, mammals from palaeosols within the Deccan lava sequence have cheek teeth reminiscent of the dominant grazers of later time.

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Clay minerals and the origin of life

December 2005

J.D. Bernal, a former student of J.B.S. Haldane, had as wide a range of interests as his mentor. Though a member of the Communist Party of Great Britain at the height of its loyalty to Stalin, during World War II he was a scientific advisor to Churchill. Among his many contributions was an idea inspired by Haldane's conviction that life emerged from the inorganic world through simple chemical processes. Bernal thought in terms of a template sufficiently complex to shape early organic molecules, and clay minerals fitted that particular bill because they contain loosely bonded, yet complex passageways between the sheets of linked SiO4 tetrahedra that form the bulk of their structure. A group of geochemists from Arizona State University have experimented on the organic catalytic potential of clays by simulating conditions around sea-floor vents that may have been the haven in which terrestrial life first formed (Williams, L.B. et al. 2005. Organic molecules formed in a `primordial womb'. Geology, v. 33, p. 913-916). Their `feedstock' was dilute methanol and the clays that they chose were montmorillonite, illite and saponite, the last a member of the smectite group with high magnesium that forms by hydrothermal alteration of olivine and pyroxene in basalts. More complex hydrocarbons, with up to 20 carbon atoms per molecule, did indeed form in their experiments. The results suggest that smectite clays protect such unstable hydrocarbons from thermal decay, but no distinct life-forming molecules, such as amino acids, showed up. The products were polycyclic aromatic hydrocarbons, but it is possible that they would have formed a diverse feedstock for other processes once the hydrothermal clays were deposited in cooler conditions.

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Photosynthesis during a `Snowball' epoch

May 2005

In Neoproterozoic sedimentary sequences evidence for low latitude glaciation crops up at two and probably several other times; so-called `Snowball Earth' events. Opinion is divided on several aspects of these events: whether or not they truly coated the Earth in glacial ice; their influence on biological evolution; the processes that started and terminated them. From a biological standpoint, a completely ice-bound surface – both land and oceans – would have stressed organisms to the extreme. Marine life (all that there was in those times) may only have survived in a few refuges from the ice, perhaps around submarine hydrothermal vents or in ephemeral sea-ice leads and polynya. If that were so, then these frigid episodes would have created important evolutionary `bottlenecks', from which sprang several adaptive radiations: `Snowball' epochs may have determined the forms and genetic diversity of all later life, especially among the Eucarya, of which we are a part. Probable deep-ocean anoxia would have been particularly stressful for organisms that depend on oxygen.

The key to establishing whether or not Neoproterozoic frigid episodes did bring eucaryan life to the verge of extinction lies in the diversity of life during those periods. That is not an easy task as all life until just before the Cambrian Explosion was both soft-bodied and minute. One means of assessing diversity is to study biochemical remnants of cell processes preserved in reduced ocean sediments (Olcott, A.N. et al. 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science, v. 310, p. 471-474). Olcott and colleagues studied black shales from Brazil whose age is within that of a frigid episode (740-700 Ma), and which contain textural evidence for abundant sea ice and low temperatures. Recovered biochemical compounds indicate considerable diversity, with a mixture of photosynthetic blue-green bacteria and eucaryan algae, with anaerobic bacteria of several types. The results indicate open water to allow photosynthesis – although it is possible for light to penetrate several metres of sea ice – together with deeper anoxic waters. Since the samples span a section almost 100 m thick, it seems this diversity persisted for a long period. However, the most that it can establish with certainty is that thin sea ice or open water did persist at the low palaeolatitude of late-Precambrian Brazil. The Neoproterozoic record has abundant, widespread black shales, and quite possibly there are others associated with evidence for glacial events. The importance of the paper lies in showing that biomarkers can be used as effectively in the Precambrian as in the Phanerozoic, and an expansion of this approach can be expected.

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New twist for end-Permian extinctions

May 2005

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

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

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

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

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Evolutionary rhythms

April 2005

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

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

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

No graphite in Akilia apatites, no sign of life?

February 2005

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

Age range of early fossil treasure trove

February 2005

The Doushantuo Formation of southern China dates from just before the Cambrian Explosion, and has become a source of astonishing information about animals that preceded the appearance of those with hard parts. It contains fossil embryos, algae, achritarchs, and small bilaterians that are purportedly the Earth's earliest animals. Moreover the formation rests on the cap carbonates of a diamictite reckoned to represent a late Neoproterozoic glacial epoch, and provides a variable trend of carbon-isotope variation that extends up to the base of the Cambrian in southern China. Because the sequence contains a number of volcanic ash beds it is potentially dateable. Using a single-zircon U-Pb method, Daniel Condon of MIT and colleagues from the Chinese Academy of Science have established the ages of both top and base of the Doushantuo Formation with considerable precision (Condon, D. et al. 2005. U-Pb Ages from the Neoproterozoic Doushantuo Formation, China. Science Express, 24 February 2005). Sedimentation is bracketed between 635 and 550 Ma, the oldest age coinciding with that for the Ghaub tillite in Namibia. Time-calibration of the carbon-isotope record allows it to be matched with others in Namibia, Oman and Newoundland. There is one snag; within the sequence is a formation boundary that signifies non-deposition, which the authors correlate with a glacial epoch recognised in Newfoundland (the Gaskiers diamictite), citing sea-level withdrawal as the cause of non-deposition in China. The well-constrained correlation suggests a major, global increase in the burial of ¹²C that produced a marked negative excursion in ∂¹³C that spans around 90% of the Ediacaran Period that saw the rise of large soft-bodied animals shortly before the emergence of shelly faunas. The interpretation placed by the authors on this signature of burial of dead organic matter, which relates to no sign of glaciation, is that it would have elevated oxygen levels in the Late Neoproterozoic oceans. That might have increased productivity by primitive eukaryotes, and possibly opportunities for predation. The uppermost part of the Doushantuo Formation broadly coincides with the first appearance of complex trace fossils and mollusk-like bilaterians, and elsewhere there are signs of the first reef formation by weakly calcified metazoans at around that time. Clearly, it is well-dated sections such as these that may hold the key to what exactly prompted the general secretion of skeletal material; the hallmark of the 10 Ma later explosion in fossil animals.

Evidence goes against end-Permian impact

January 2005

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

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

New predators on the Mesozoic block

January 2005

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

Another large igneous province implicated in mass extinction

December 2004

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

Iron isotopes enter the Archaean life debate

December 2004

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

A volcanic role in the origin of life?

October 2004

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

Tighter link of end-Permian extinction with Siberian Traps

September 2004

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

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Ancient baby penis worm hits the news

January 2004

China is proving to be the repository of a vast wealth of well-preserved ancient faunas, thanks to several lagerstãtten, the most famous being that which hosts early ancestral birds that show links with dinosaurs. But Chinese strata with exceptional preservation also occur in Cambrian sediments, close enough to the first appearance of preservable life forms to make any out-of-the-ordinary finds especially revealing. Ten years ago many palaeontologists scoffed at reports of trilobite embryos being unearthed in southern China, yet there has been a steady flow of material that opens up what might be called "palaeoembryology". Being able to describe and analyse an entire life cycle of an organism is vital in studies of the inter-relatedness of living metazoans. The lack of data on fossil life histories to some extent thwarts attempts to place extinct animals accurately within an evolutionary scheme. Palaeontologists from the University of Bristol and Peking University have therefore put such studies on the map through finding exquisitely preserved Cambrian embryos of what is now a rare and bizarre animal group, but one thought to lie at the root of the explosive radiation of the arthropods, which includes insects (Dong, X. et al. 2004. Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature, v. 427, p. 237-240). They are in eggs, and therefore had yet to hatch and develop further; true embryos, from their initial development to the last stage before emerging. They are Scalidophores, which include today the individual phylla of Priapulida, Kynorhyncha and Loricefera, all marine worm-like animals (the priapulids are the notorious, and fortunately rare, penis worms from their evocative contours http://www.blackwellpublishing.com/chengjiang/Paraselkirkia%20jinningensis.asp). Interestingly. the embryonic stages clearly indicate direct development from egg to adult, rather than going through the intermediary larval stage that characterises most insects and other invertebrates. Such direct development seems to be a primitive evolutionary stage from which more complex life-histories developed later. Penis worms are well known to grow hugely once hatched, so the search is on for a fully grown adult from the Cambrian of southern China, as well as early developmental stages of other animal groups..

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

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Fossil hamster's food cache

December 2003

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

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The selectivity of mass extinctions

December 2003

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

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Another K-T row

October 2003

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

Sources: Dalton, R. 2003. Hot tempers, hard core. Nature, v. 425

October 2003, p. 13-14. McKie, R. 2003. I've got a bone to pick with you, say feuding dinosaur experts. The Observer, 7 September 2003, p. 22.

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Gamma-ray bursts and mass extinctions

October 2003

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

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Source

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

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

October 2003

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

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

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

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

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Setting the fossil record to rights

August 2003

Much has been made of ups