Whatever happened at the Triassic-Jurassic boundary (around 200 Ma ago), the palaeontological shifts then coincided with eruptions of flood basalts of the Central Atlantic Province and the start of Atlantic opening (see And now, the Tr-J boundary, Earth Pages May 2002). Although questioned as a mass extinction event, the boundary contains extremely high proportions of fern spores, that may signify the land being cloaked by rapidly spreading ferns after it had been wiped clean of other vegetation. New evidence suggesting the influence of an impact at the time emerges from a geochemical study of the fern-rich boundary layer (Olsen, P.E. and 9 others 2002. Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary. Science, v. 296, p. 1305-1307), which revealed anomalously high levels of iridium. High iridium is only one pointer to possible extraterrestrial influences, and the clinching factor of shocked mineral grains has yet to be shown convincingly.
The novel feature of the paper by Paul Olsen of the Lamont-Doherty Earth Observatory and colleagues from the USA, Canada, Italy and Austria is how they used trace fossils to reach a remarkable conclusion. They combed eastern US terrestrial sediments either side of the boundary for reptilian foot prints. They tracked time using evidence for climate change paced by Milankovich cycles. Their records of 10 thousand sets of tracks show a decline in non-dinosaur footprints, and a jump in the proportion left by dinosaurs from 20 to 50% of the total, as the boundary is crossed. Those of some Triassic reptiles that had survived for 20 Ma end abruptly at the boundary. It seems that, whatever the boundary event was, early dinosaurs were able to adapt to change better than evolutionarily more primitive reptiles, so that they could speciate rapidly when their Triassic companions bit the dust. Dinosaur evolution seems to have been similar to that of the mammalian adaptive radiation that followed the K-T extinction event.
Gigantic claims for “geogenomics”
Fossils and their stratigraphic ages no longer offer the only clues to biological evolution, now that is possible to judge the degree of relatedness between living organisms from sequences of genes and proteins that their cells contain. The molecularly inferred family trees of modern animals, plants and micro-organisms help scientists to visualize the relative antiquities of the sharing of a common ancestor by different pairs of a living group. By assuming constant rates for genetic mutation and protein evolution, some palaeobiologists have asserted that they are able to assign absolute ages to evolutionary divergences. If that were so, then it would be possible to correlate evolutionary milestones with transformations brought on by geological and climatic upheavals, and also with other past changes in the biosphere. Good examples would be linking fossil and genetic changes in ruminant mammals to the rise of grasses, or the rise and divergence of corals following the end-Permian mass extinction. The inter-linkage between palaeontology and genomics is in its infancy. That it promises a great deal by way of insights, as well as possible bloomers, is nicely brought out by a recent review (Benner, S.A. et al. 2002. Planetary biology – paleontological, geological and molecular histories of life. Science, v. 296, p. 864-868). Whether charting the “planetary proteome” will become “a civilization-wide enterprise”, as Steven Benner and his colleagues predict, is something that I would not care to comment on during the 2002 World Cup. As Bill Shankly once observed, some things are far more important than matters of life and death.
Too much iron, too little phosphorus delayed an oxygen-rich atmosphere
The age of the earliest blue-green bacteria hinges on the imagination of some palaeobiologists and how well they can focus a microscope (Doubt cast on earliest bacterial fossils, Earth Pages April 2002). Without doubt, it was blue-greens that first began breaking the chemical equilibrium of water to release free oxygen to the environment, yet it was some 2½ billion years after the Earth had formed that atmospheric oxygen had a tangible effect on the Earth’s bare surface. In rocks around 2.2 to 2.0 Ga old geologists find the first evidence for that in soils that are rich in oxidized Fe-3. For iron to lose an electron and change from soluble Fe-2 to Fe-3, whose oxides and hydroxides are highly insoluble, demands the abundant presence of an electron acceptor, or oxidizing agent. The most likely of these in the atmosphere and hydrosphere is oxygen. However, there are sedimentary rocks that form vast repositories of Fe-3 and oxygen that predate the first well-accepted oxygen-rich atmosphere. They are known as banded iron formations or BIFs, whose minuscule layering seems to signify that they formed as precipitates from water, when dissolved Fe-2 met a source of oxygen to produce hematite – Fe2O3 – and goethite – Fe(OH)3 BIFs signify deep ocean water devoid of oxygen, to enable soluble Fe-2 to circulate abundantly, yet a sizeable supply of oxygen where they were precipitated. Since only organic photosynthesis is capable of breaking the powerful bond in water, some kind of photosynthetic bacteria are implicated in the formation of BIFs. Whether or not palaeobiologists and geochemists can demonstrate evidence for the first appearance of such bacteria, BIFs more or less prove their existence, in the absence of any other plausible means of formation.
Until recently, the huge delay in the Earth’s surface environments becoming oxygenated has been ascribed to the mopping up of any biogenic oxygen by its reaction with a vast excess of dissolved Fe-2. However, once blue-green bacteria evolved photosynthesis, their chemical trick of splitting water molecules to provide hydrogen for processes at the cell level should have meant that they would have spread like wildfire across the ocean surface. In that respect they are unique among bacteria, most of which exploit very narrow ecological niches. Oxygen should have quickly come to dominate both oceans and atmosphere. That is, unless there was some check on the living ocean biomass. It turns out that BIFs may contain the answer, for they are rich in phosphates, adsorbed onto the surfaces of their iron minerals (Bjerrum, C.J. and Canfield, D.E. 2002. Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides. Nature, v. 417, p. 159-162). Phosphorus is vital in any organism, being an essential component of nucleic acids and phospoholipids. By working out the partition coefficient between water and iron oxide, and estimating the production rate of BIFs before 1.8 Ga when their production ceased, Bjerrum and Canfield conclude that phosphorus was an order of magnitude less abundant in sea water until then. Such a deficiency in a vital nutrient would have limited the scope of blue-greens, and the rate at which they produced oxygen.
Just why the Fe-P checks and balances on oxygen production collapsed around 2.2 to1.8 Ga is something of a mystery. One possibility is that the iron concentration in sea water fell, perhaps as sea-floor spreading waned from its high early rates; basalt magma provides the main input of iron through ocean-floor hydrothermal activity. Less production of BIFs would leave more phosphorus in solution, helping greater biological productivity, whose oxygen output would eventually remove soluble iron from sea water.
See also Hayes, J.M. 2002. A lowdown on oxygen. Nature, v. 417, p. 127-128.