Category Archives: Geobiology, palaeontology, and evolution

The full ‘Snowball Earth’ model for episodes in the Neoproterozoic that left glaciogenic sediments at near-equatorial palaeolatitudes implies that the oceans were frozen over globally. An objection to that is the likelihood that all photosynthetic activity would have been shut down leading to near catastrophe for all life forms of the time except those based on chemoautotrophic metabolism, as around hydrothermal vents. Antarctica has around 140 lakes that have been frozen over for at least hundreds of thousands if not millions of years, the best known being Lake Vostok, deep within the continent, that Russian scientists are on the verge of tapping after drilling through more than 3 km of glacial ice. Who knows what they might find? Far less extreme, but also having perennial ice cover, is Lake Untersee close to the coast in East Antarctica. Its summer ice cover is 3 m thick and it is presumed to have remained icebound through previous interglacials, although it is fed by meltwater from a nearby glacier in summer. It is not filled with fresh water, however, having a pH up to 12.1, around that of household bleach. It also has very high oxygen content, in fact supersaturated at 50% more than the solubility expected at 0°C. Lake Untersee would be expected to have little life, being an extremely hostile environment. Nonetheless, it does boast a biome and sufficient light gets through the ice cover to support microbial mats of photosynthesising blue-green bacteria (Andersen, D.T. et al. 2011. Discovery of large conical stromatolites in Lake Untersee, Antarctica. Geobiology, v. 9, p. 280–293). As well as perhaps helping elevate the oxygen levels in the lake water, these organisms have secreted stromatolite-like cones, pinnacles and mounds, but not ones made of carbonate. Although the water contains plenty of calcium ions, there is insufficient carbon as CO₃ or HCO₃ ions for calcite to be precipitated. The carbon-poor nature of the water seems to confirm its long-term isolation from the atmosphere. Instead, the stromatolites are made of laminated clay, maybe derived by exceedingly slow breakdown of feldspars that would also yield calcium and hydroxyl ions to explain the waters peculiar chemistry. The different shapes of stromatolites are linked to different cyanobacterial communities, which may help explain morphological variations among fossil stromatolites.

The lead author is from the SETI Institute in California, and presumably visited Lake Untersee in the cause of exobiology, as reported in other commentaries on the paper. However, the peculiarities of the lake and its life seem to be just that, with little relevance to frigid sedimentation in the distant past apart from a possible explanation for varying shapes of fossil stromatolites. Nor is the lake sterilised by virtue of perennial ice cover. Being fed by glacial melting it has received rock flour that has broken down to clays, and that implies meltwater carries other materials from the ice cap. Even Antarctica is not isolated from wind-blown dust, so cyanobacteria may have been introduced by sturdy, wind-borne spores being incorporated in the ice cap, eventually to end up in Lake Untersee. It seems that the lead author actually dived in the lake, which puts the fears of contamination by careful drilling into Lake Vostok into perspective. How such an environment links to notions of life elsewhere in the universe is hard to see. The truly fascinating thing about home-grown cyanobacteria is that early variants may well have cuddled up with other simple cells for mutual wellbeing to become the chloroplasts of eucaryan photosynthesising autotrophs, on which most metazoan life on Earth now depends.

Visit: www.astrobiology.com/news/viewnews.html?id=1515

Related articles

• Antarctic Microbes Live Life to the Extreme (scientificamerican.com)
• Antarctic Lake Hides Bizarre Ecosystem (wired.com)

A distinctive Ediacaran fossil. Image via Wikipedia

The biota dominated by large, indistinct and generally flabby creatures named together with the eponymous Period (635-542 Ma) from their type occurrence in late Neoproterozoic sediments of the Ediacara Hills of South Australia is made up of imprints of a strange bunch of organisms – bags; discs; donut-shapes and the enigmatic quilted organisms that likely subsisted by osmotically drawing nutrients from ocean water through their skins – together with others that have forms suggestive of extant groups – cnidarians; bilaterian embryos; mollusc-like and segmented forms.  The Avalon fauna of Newfoundland, discovered after those of Charnwood Forest, UK and the Ediacara Hills, added other life forms, including the fractal-like rangeomorphs from earlier (~579 Ma) times in the Ediacaran. Recently, the oldest known (630-551 Ma) members of the Ediacaran biota were presented (Yuan, X. et al. 2011. An early Ediacaran assemblage of macroscopic and morphologically differentiated eukaryotes. Nature, v. 470 , p. 390-393). Unlike the better known organisms that were preserved against all odds in quite coarse sand- and siltstones, the host rocks in South China are a more familiar lagerstätten of black shales in which fossils take the form of carbonaceous films. These preserve considerable detail and are unlike the later Ediacaran organisms. Many resemble marine algae (seaweeds), some very like kelp, in their living positions and probably represent quite sunlit seabed habitats(the authors also suggest that some rare forms may be bilaterian worms and cnidarians). Dating of this Lantian assemblage stems from several ash beds and correlation of C-isotope anomalies with other Ediacaran sections.

From their age, the Lantian fossils are of organisms that evolved shortly after the Marinoan (635 Ma) ‘Snowball Earth’ episode, whereas the faunas of Newfoundland and Australia followed the less prominent Gaskiers glacial epoch (582 Ma). So they represent another evolutionary surge presaged by global ice cover and massive stress for all terrestrial life. If the Lantian organisms were algae, then photosynthesising eukaryotes may have been the first large multicelled organisms. All eukaryotes – autotrophs and heterotrophs – are obliged to live in oxygenated conditions, so at least shallow water after the Marinoan glacial event must have been such, although preservation of the Lantian fossils does indicate anoxic conditions during burial. The association of evolutionary bursts with two ‘Snowball Earth’ periods ought to point palaeobiologists to the sedimentary sequences that followed the earliest such event, the Sturtian (~720 Ma), which shows similar violent swings in C-isotopes that indicate surges and declines in burial of organic matter. So far only sponge-like fossils have been found from the Cryogenian Period of the Neoproterozoic that encompasses the Sturtian and Marinoan glacial episodes (Maloof, C. & 8 others 2010. Possible animal-body fossils in pre-Marinoan limestones from South Australia. Nature Geoscience, v. 3, p. 653-659).

Related Articles

• Oldest fossils of large seaweeds, possible animals tell story about oxygen in an ancient ocean (eurekalert.org)
• Narbonne, G.M. 2011. When life got big. Nature, v. 470, p. 339-340

The uncoiled ammonite Baculites used in the study. Image via Wikipedia

Emerging in the Upper Palaeozoic and rapidly diversifying through the Mesozoic, thereby surviving over a period of 340 Ma, ammonites proved to be a stratigrapher’s dream organism as well as being the most widely collected fossils. As well as their rapid evolution of form, they were able to spread widely though the oceans in larval form, through the jet propulsion they shared with other cephalopods and because they floated when dead and drifted with currents. Much of ammonite taxonomy has centred for almost two centuries on their external for: ribs, keels, knobbles, intricacy of the sutures separating each body chamber and the previous one and whether or not their growing shell coils hid earlier parts or developed into an open spiral. These characteristics enables such a wealth of easily recognised genera and species that as zone fossils ammonites have been used to finely divide Mesozoic sediments; Jurassic ammonites locally divide the Jurassic (199 – 145 Ma) into time slices each of which represent a few hundred thousand years.

What is least familiar to non-specialists is the feeding apparatus of ammonites and what they actually ate. Thanks to the use of high energy X-ray images it turns out that, unlike squid, octopuses and the similar looking modern Nautilus, some Cretaceous ammonites would not have been able to rip apart large prey (Kruta, I et al. 2011. The role of ammonites in the Mesozoic marine food web revealed by jaw preservation. Science, v. 331, p. 70-72). Instead of a large beak-like process the ammonites studied sported a rasp-like radula, similar to that used on lettuce by the slug. The radula is armed with tiny but quite fearsome looking barbs, suitable for grating but not gnawing. The analysed ammonites may probably have eaten plankton. Indeed, one specimen turned out to have fragments of its last meal lodged in its radula; an isopod and a small gastropod. That diet tallies with the likely habitat of some ammonites; they were probably able to change their buoyancy by manipulating the gas and water content of their abandoned earlier body chambers to move up and down in the upper ocean. However, such was the stratigraphic duration, global spread and diversification of the ammonites, further studies of this kind would be needed to verify general plankton feeding. However, such a diet may well explain the conundrum of the total extinction of ammonites at the end of the Cretaceous while the superficially similar nautiloids survived and live today. The Cretaceous-Palaeocene (K-P formerly K-T) mass extinction devastated plankton, while larger marine organisms lived on to serve as nautiloids prey.

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• Ammonites’ last meal: New light on past marine food chains (eurekalert.org)

Dunkleosteus (10 m long) of the Late Devonian. Image by Travis S. via Flickr

Probably the greatest ecological truism is that without oxygen there would be no life forms on Earth above the level of a restricted number of prokaryotes. Since around 2.4 Ga, when free atmospheric oxygen first appeared, levels have risen to the present 21% – it was probably as high as ~30% in the Carboniferous and Cretaceous Periods. Charting the rise has been difficult and the history of oxygen is written with a very broad brush. If there had been sudden increases in the availability of oxygen in the atmosphere and oceans there ought to have been a bursts of evolutionary radiation and diversity, but often oxygen-related causality for events such as the Cambrian Explosion have been speculative, as have cases for the inverse, declines due to downturns in oxygen levels (see Oxygen depletion before P-T extinction in the November 2003 issue of EPN). Recently a proxy for the redox chemistry of the global ocean, and therefore for relative changes in atmospheric oxygen, has been developed. It is based on the abundance and isotopic composition of the element molybdenum (Mo) in sedimentary rocks: higher ⁹⁸Mo relative to ⁹⁵Mo (the d⁹⁸Mo value) signifies higher oxygen levels. Its recent use in relation to evolutionary radiations (Dahl, T.W. et al. 2010. Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of the US, v. 107, p. 17911-17915) has produced interesting results. The US-Swedish-Danish-British team analysed the Mo in euxinic (reduced) marine black shales, which concentrate the element from seawater, in the Proterozoic and Phanerozoic Eons. Increases in δ⁹⁸Mo occur at the time of the Cambrian Explosion, as expected, and also during the Devonian. The latter correlates with increasing diversification of large fishes and among early terrestrial plants, and may have been the greatest leap in the bioavailability of oxygen in Earth’s history, stemming from the ‘greening’ of the land. So far Mo-isotope data have not been obtained from Carboniferous, Permian or Cretaceous back shales, but the ratio of Mo to organic carbon content in black shales of those ages  – a less constrained proxy –  does confirm what has been suspected: highs (greater than present levels) in the Carboniferous and Cretaceous and lows during the Permian and Triassic. However, any hopes that the approach can be calibrated to actual oxygen levels seem likely to be optimistic as the controls over dissolved molybdenum supply to the oceans and its transfer to sediments are extremely complex.

Added 14 January 2011. Some of the team feature in a related article (Gill, B.C. et al. 2011. Geochemical evidence for widespread euxinia in the Later Cambrian ocean. Nature, v. 469, p. 80-83) that ticks all the geochemical boxes for the evolutionary effects of depleted oxygen; i.e. extinctions. They use new measurements of sulfur isotopes in conjunction with published carbon-isotope  and other geochemical data from a wide range of Late Cambrian sediment types and environments in six well-known sections of that age. Spikes in the relative abundance of ³⁴S match those in ¹³C along with a decrease in Mo in one section (see above), suggesting temporary increases in carbon and sulfide burial during periods of oxygen deficiency in the Late Cambrian ocean. Massive sequestration of organic carbon may have led to the extremely cold Late Cambrian climate, as described in A chilly Late Cambrian (this issue). Combined with changes in redox conditions associated with ocean anoxia this would have especially stressed animals, even on continental shelves had oxygen depleted water risen from the depths where sulfur and carbon burial were going on.

See also: Shields-Zhou, G. 2011. Toxic Cambrian oceans. Nature, v. 469, p. 42-43.

Related Articles

• Oxygen crash led to Cambrian mass extinction (newscientist.com)

Image by Cliff Beckwith via Flickr

Though it is highly likely that burial of fossils for millions of years destroys any trace of their DNA the massive bones of large creatures can preserve cell material. A near complete 67 Ma old Tyrannosaurus rex, fondly known as ‘Big Mike’ has revealed blood cells in thin sections of its bone (Schweitzer, M.H. 2010. Blood from stone. Scientific American, v. 303 (November 2010), p. 38-45). Her article also covers traces of blood vessels, and collagen of similar antiquity. The research involved positive reaction of antibodies against proteins, thereby proving the materials to be organic and not products of biomineralisation formed during the process of fossilisation. Potentially such forensic work can tease out relationships among animal groups whose fossils preserve organic materials, in a similar way to indications of the rise of prokaryote groups by biogeochemical marker molecules in carbonaceous shales. Indeed, sequences of fossil proteins from dinosaurs closely resemble that of modern birds. One of the great surprises of the late 20^th century was the growing evidence that the stem-line for birds was dinosaurian, specifically the theropod group. This is nicely summarized by another review article (O’Donoghue, J. 2010. Flight of the living dead. New Scientist, v. 208 (11 December 2010), p. 36-40) that addresses the certainty of birds’ evolution from dinosaurs; which of the fossils is bird, which feathered dinosaur and when did they separate; and why did birds survive the end-Cretaceous mass extinction while dinosaurs famously succumbed – probably a matter of breeding; its pace, that is. The two articles together suggest a fruitful way forward for palaeobiologists.

Further material about biochemical relics in fossils and methods used to detect and analyse them can be found in Hecht, J. 2011. Waking the dead. New Scientist, v. 209 (22 January 2011 issue), p. 43-45.

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.
One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were especially noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos (Bennett, K. 2010. The chaos theory of evolution. New Scientist, v. 208 (16 October 2010), p. 28-31)?

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules. Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.

One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos?

Comet impacts’ candidature for origin of life

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules.  Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.