As with several major extinction events, the Permian-Triassic boundary is characterised by a major excursion in carbon isotopes of marine towards negative d13C. This is often taken to indicate a reduction in the burial of dead organic matter, perhaps because of low global biomass. US, Chinese and Canadian geoscientists have added great detail to the P-Tr carbon-isotope record from analysis of three continuous sections through carbonate-dominated sequences in an Early Triassic reef system in southern China (Payne, J.L. et al. 2004. Large perturbations of the carbon cycle during recovery from the end-Permian extinction. Science, v. 305, p. 506-509). This is no ordinary reef, for it was built by carbonate secretions by micro-organisms, either algae or bacteria. The tabulate coral reef builders of the Palaeozoic became extinct at the end of the Permian (251 Ma), and their successors, scleractinian corals, do not appear until about 10 Ma later. The Early Triassic was undoubtedly characterised by low animal diversity, before adaptive radiation could “re-stock” a devastated biosphere. The authors found a remarkable series of ups and downs in d13C within the reef carbonates, some of the negative excursions being even more severe than that just after the mass extinction. Some of the positive peaks go far beyond the d13C levels in preceding and following times, and could be due to periods of extremely high burial of organic matter. But the fossil record shows that such burial probably involved a restricted number of taxa, so perhaps there were huge “blooms” among a few groups that filled vacant ecological niches only to collapse. As suddenly as this see-sawing of the carbon cycle had begun, at about 246 Ma it settled to a more or less constant level, just after the start of the Middle Triassic. There are two reasonable explanations for the fluctuations. One is that biotic recovery from the mass extinction was set back three of four times by further environmental upheavals, thereby dashing diversification. The other is that the fluctuations reflect instability in the simple ecosystems of the Early Triassic and their control on carbon burial.
Calcium in the ocean and the Cambrian Explosion
If ever there was a geoscientific topic that would “run and run”, it would be explaining why creatures with hard parts just popped into being 542 Ma ago. Physiologically, members at the phylum level of the Cambrian fauna have little in common apart from hard parts made from calcium compounds, either carbonate or phosphate. Calcium carbonate was secreted as stromatolites by blue-green bacteria as far back as the Archaean, but not in an organised form linked to their function. In the very latest Precambrian, the Ediacaran, there are tiny shell-like bits and pieces in its very uppermost strata (the “small shelly fauna”) but they suggest no obvious function and no association with any of the various soft-bodied metazoans that define that Period. The Cambrian Explosion has no rudimentary precursor. Because calcium is an element with a very narrow tolerance in cells, from the level needed for viable function (it has a “messenger” function) to that at which it is fatally toxic, and it is a common element in all environments, adoption of calciferous hard parts seems very likely to have a risen as a means of avoiding toxicity, without any other role. Once established in large animals, hard parts provide a means of and a defence against predation, so losing the ability to secrete hard parts would be an evolutionary risky strategy; once established it cannot be lost except when substituted by other effective defences or mealtime tackle. There were times in the Precambrian record when calcium compounds exceeded their solubility, and they are marked by inorganically precipitated crystalline forms in sediments. The early Archaean was one such period, but if levels of Fe-2 are high in water those solubilities are enhanced. Therein lies a link between Archaean and Palaeoproterozoic stromatolites, banded iron formations and the oxidation potential of seawater. In fact precipitation of BIFs seems to link nicely with the abundance of stromatolites, because the production of oxygen by blue-green bacteria would locally have consumed electrons to oxidise soluble Fe-2 to Fe-3 that has insoluble oxides and hydroxides. This connection returned several times in the Neoproterozoic, oddly at the times of so-called “Snowball Earth” episodes, first noted by Preston Cloud. Could the last of these have triggered adoption of calcium secretion by the early metazoan animals? That is hard to judge, because it preceded the Cambrian by several tens of million years. Geochemists from the US Geological Survey, the State University of New York and the US Oak Ridge National Laboratory have taken a cunning route to shedding some light on the biggest of all palaeontological mysteries (Brennan, S.T. et al. 2004. Seawater chemistry and the advent of biocalcification. Geology, v. 32, p. 473-476). They sought crystals of evaporitic halite that spanned the Precambrian-Cambrian boundary, and which usually contain fluid inclusion containing samples of the brine from which they formed, hopefully seawater. So far, they have two sets of suitable halites that can be assigned to a marine environment, from Siberia and the Oman, and their measurements of calcium concentrations are very precise. The first is dated around 515 Ma the other set from 544 Ma. Two sample points are not enough to prove a role for elevated calcium levels in the ocean, but the results are encouraging. Calcium concentrations (with suitable corrections for changes during evaporation of restricted seas) jumped by a factor of 3 from the very end of Precambrian to Cambrian times. Over the same period, it is thought that global sea-floor spreading rates were much higher than at present, and there is also strontium-isotope evidence for an increase in ocean-floor hydrothermal activity that adds elements derived from oceanic basalts to seawater. That, however post-dates the start of the Cambrian by about 15 Ma. With a CO2-rich atmosphere and elevated continental weathering calcium is likely to have been supplied from the continents. Whatever, the results fit with models based on variation of continental and oceanic additions to seawater with changing spreading rates (Hardie, L.A. 2003. Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas. Geology, v. 31, p. 785-788). Hardie suggested that calcium in seawater fell to very low levels during the Neoproterozoic from an unprecedented high at its outset at 1000 Ma. That is a time when metazoans were probably not around, while the period when they appear in the later Neoproterozoic record was one of calcium-poor conditions. Large animals may have evolved when there was little danger of calcium shock, only to face it once they were well established. Then would have had to rid their cells of it very efficiently. Studies of fluid inclusions from marine precipitates seem likely to grow following Brennan et al.’s important discovery, though suitable samples are likely to be few and far between. One important role they need to play is verifying Hardie’s model for secular variation in seawater chemistry, which depends on difficult interpretations of rates of sea-floor spreading and continental erosion.
Ancestral animal?
The significant feature of the first appearance of widespread, large fossils during the Cambrian Explosion about 542 Ma ago was really the adoption of hard parts by most of the existing (and some now extinct) phylla of animals. The preceding Neoproterozoic Ediacaran Period witnessed lots of large life forms, but preserved them only as imprints; they were soft bodied. Superficially, the outset of the Cambrian appears to marked the simultaneous emergences of the rough blueprints of all subsequent animals. In reality, this was probably not a faunal explosion, but one of biochemical processes, wherein many phylla turned the fundamental cell process of excreting excess calcium as carbonate and phosphate to generating functional parts of their bodies. Why that happened explosively is still a mystery. Looking for the origin of animals requires going further back in geological time, and an element of luck as regards exceptional preservation of soft tissue. The other way is using a molecular clock approach to the genetic differences among modern phylla, but that is fraught with uncertainties and gives a very large time range (possibly 1500 to 600 Ma) in which to find tangible evidence. The maximum limit is around 2200 Ma, when oxygen became significant in the atmosphere and the upper ocean – the prime condition for eukaryote life. A rather dull carbonaceous fossil, with a spiral form and thought to be the first known multicelled eukaryote (Grypania) appears in the record about 1500 Ma ago, but what it was is unclear. The best place to look for ancestral animals is in known repositories of well preserved organisms. One such lagerstät is the Doushanto Formation in SW China. This goes back to the last “Snowball Earth” event at 600 Ma, and has been heavily mined for primitive life forms. Chinese palaeontologists, teamed up with others from the USA have indeed found something intriguing (Chan, J-Y. et al. 2004. Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian. Science, v. 305, p. 218-222). Only about 0.2 mm across, 10 specimens seems to show microscopic signs of all the basic elements of many members of the Animal Kingdom: bilateral symmetry, a mouth and gut, skin tissue and possible sensory organs. The layers from which they were extracted are between 580 to 600 Ma, well before the Cambrian Explosion. However, micropalaeontologists in general subscribe to the “once bitten, twice shy” outlook, especially following controversies over even earlier evidence for small organisms and those purported to occur in Martian meteorites, which are as likely to be results of inorganic mineralisation as fossils. Various mineral crusts and films, formed inorganically, can mimic organic structures. The one feature that persuades Chen and colleagues is that the same features show up in all the specimens, and they are all the same size. That is highly unlikely from some inorganic process.
Source: Stokstad, E. 2004. Controversial fossil could shed light on early animals’ blueprint. Science, v. 304, p. 1425.
The case of the stranded, tiny mammoths
It does seem likely that our ancestors ate all the mammoths (Mammuthus primigenius), a species that had wandered over the northern tundras bordering the Northern Hemisphere ice sheets through several glacial-interglacial periods. But some of them did escape to survive into the Holocene. They were stranded on high-latitude islands off NE Siberia and Alaska as sea levels rose. The last of them died on Wrangel island about 4 thousand years ago. A common tendency in small populations of large mammals that are restricted to islands is that they become smaller and smaller with each generation. This happened to the stranded mammoths of the Bering Straits islands, remains of which are often dwarfs. (Guthrie, R.D. 2004. Radiocarbon evidence of mid-Holocene mammoths stranded on an Alaskan Bering Sea island. Nature, v. 429, p. 746-749). St Paul Island is now only 91 km2 in area, too small to support even tiny, woolly elephants, but it was probably much larger when sea-level rise first isolated it from the vast Bering steppe across which mammoths roamed. It was that isolation about 13 thousand years ago that probably helped the stranded mammoth population avoid the hunters who colonised the Americas, until 7 900 when the last mammoth there died. The even later population on much larger Wrangel Island fell to human colonisation, but there are no signs of human intervention on St Paul. The earlier extinction there was probably a result of shrinking browse as sea level steadily rose., when St Paul would have been 5 to 10 times larger than it is now.
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