Relating mass extinctions to the effects of impacts by comets or asteroids is now a major industry, and a great number of geologists who sneered at early suggestions of extraterrestrial influences over evolution are finding ever new ways to cook and eat their headgear. Oddly, however, many of those who bore the brunt of such mean-spirited, and somewhat premature scorn still cling to the safe old K-T event. Soon all the thin K-T boundary material will have been consumed by these cautious, if meticulous scientists. Thankfully, some have ventured to seek evidence for other catastrophes that came out of the blue. In comparison with the end-Permian extinction, the K-T event is a mere bagatelle. However, attaching it to an extraterrestrial cause has proved difficult. It has attracted as many opponents of impact theories as “whizz-bang” aficionados, with much talk of the effects of sea-level changes, volcanism, ocean anoxia and climate shift. They may be in for a big surprise.
The Permian-Triassic boundary in Meishan, China is at first sight a nondescript sequence of shallow marine strata, albeit complete. The last occurrence of Permian marine genera there, with typical signs of mass extinction, coincides with a 20-fold increase in nickel concentrations. Closer examination reveals other brusque geochemical and mineralogical anomalies, including magnetic grains of iron-silicon-nickel alloy, but no iridium anomaly (the popular target for detecting asteroidal impact horizons) or examples of shocked quartz and feldspar (Kaiho, K. et al. 2001. End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle. Geology, v. 29, p. 815-818). Most significant is a sudden drop in 34S due to a large increase in the amount of isotopically light sulphur in the environment. Kaiho et al. attribute this to vast emission of sulphur from the mantle. A coincident fall in the 87Sr/86Sr ratio could also result from entry into the oceans of lots of mantle-derived strontium.
The P-Tr boundary also coincides with the time of eruption of the largest continental flood-basalt province, the Siberian Traps. No doubt other scientists will seek to account for the chemical anomalies at Meishan as distant effects of the Siberian volcanism alone, as they have for the K-T boundary anomalies because of their coincidence with Deccan volcanism. The authors prefer to suggest a causal link between impact and massive volcanism.
Surviving the Archaean with a UV jacket
Earth’s dominance, for at least the last half billion years or so, by oxygen-dependent and oxygen producing life forms stems from the evolution of photosynthetic organisms whose cell metabolism involves breaking the strong bonds in water molecules with solar energy. Chemo-autotrophic life that exploits other energy sources has been consigned to niches that are very much narrower than they were at the biosphere’s outset. The earliest primary producers using oxygenic photosynthesis were the cyanobacteria – arguably the predecessors of modern plants’ chloroplasts, in Lyn Margulis’ endosymbiotic model for the origin if the Eucarya. Carbon isotopes from the early Archaean do suggest their presence close to the start of recordable geological history, and at around 3.5 Ga the first known stromatolites were almost certainly secreted by blue-green bacteria (See Carbonates and biofilms, Earth Pages August 2001).
To thrive and colonise ocean surface waters, the shallows and perhaps even the continental surface – their water-splitting, solar powered metabolism opened up those opportunities – cyanobacteria, more than any other prokaryotes, had to resist massive damage from ultraviolet radiation. Lack of atmospheric oxygen, and therefore ozone, left Earth’s surface with no shield to the most biologically damaging, short-wave UV. Despite the fact that modern “blue-greens” can survive climatic extremes from the frigidity of Antarctica’s Dry Valleys to superheated water in hot springs, as regards UV damage they are wimpish. This is partly due to its bleaching effect on the light-harvesting pigment on which chlorophyll depends. Cyanobacteria cells do have some biochemical protection against radiation damage, but it is of no avail when bathed in the “hardest” UV likely to have characterized Archaean surface environments.
A widely held view is that “blue-greens” survived and prospered because of another function common to many single-celled organisms; their tendency to promote nucleation of inorganic compounds outside their cell walls. Stromatolites themselves are good examples of the production of biofilms, being made of minute laminae of carbonates, whose secretion helps cyanobacteria avoid calcium stress. In modern hot springs that contain dissolved silica, these organisms often help generate sinters made of silica. A team from the University of Leeds (Phoenix, V.R. et al. 2001. Role of biomineralization as an ultraviolet shield: Implications for Archaean life. Geology, v. 29, p. 823-826) has performed controlled experiments on living cyanobacteria from Icelandic hot springs to check their defences against short-wave UV. With a biofilm screen (in the experiment they used wafers made from associated iron-silica sinter, as well as colonies with a biofilm) the organisms easily survived and continued to photosynthesize. Exposed “naked” they succumbed after only a few days exposure. It seems that traces of iron incorporated in the films dramatically enhance the UV-screening, without reducing photosynthesis. Archaean iron-rich cherts are massively abundant in banded iron formations, and the first definite remains of cyanobacterial cells come from such silica-rich material. However, the ubiquitous stromatolites in limestones of early Precambrian times are the main signs of life. It remains for the UV-screening properties of carbonate biofilms to be assessed.
New phyllum from Chinese Cambrian
Incompleteness of the fossil record is partly a result of the bias towards organisms with hard parts and against soft tissue, during sedimentary processes. For preservation of soft-bodied animals, together with that of intricate parts of the usual fossils, palaeontologists look to site where preservation is exceptionally good – lagersttätten. An example is the Solenhöfen Limestone, famous for Archaeopterix. Mudstones formed under highly reducing conditions, which excluded bacteria that complete oxidize flesh, provide similar opportunities. Work through the last two decades by Simon Conway Morris of the University of Cambridge has resulted in working and interpretative methods that permit extremely detailed analysis of physiologies, beginning with the most famous lagersttätte, the Middle Cambrian Burgess Shale of British Columbia. Conway Morris and others unearthed beasts so strange that they had little choice other than to erect new Linnaean Classes and Phylla to classify them. Equally as important, such sites help fill in the details of early members of those which survive today, including the elusive penis worms.
Conway Morris has been part of a team based at the Northwest University in Xi’an China, which has discovered lagersttätten in the Lower Cambrian, closer in time to the explosive development and radiation of animals at the end of the Precambrian. Once again, unsuspected novelty has turned up (Shu, D.-G. et al. 2001. Primitive deuterostomes from the Chenjiang lagersttätte (Lower Cambrian, China). Nature, v. 414, p. 419-424). Along with excellent examples of agnathan fish and many familiar soft-bodied animals, the prize in this case are remains that warrant a new, extinct Phyllum, the Vetulicolia. The organisms are small but complex, with two main body chambers that reveal mouth, innards and gill slits. The last helps place them within the deuterostomes; an “umbrella” that groups chordates (sea squirts and vertebrates) and echinoderms (they have lost such slits, but are genetically closer to chordates than any other group). Critical to the evolutionary significance of the vetulicolians is a groove that floors what is interpreted as the anterior part of their alimentary canals. Such a groove characterizes the pharynx of chordates, where it serves as “gutter” for various glands – the endostyle, also involved with iodine in metabolism. If the vetulicolian groove is an endostyle, then they are chordates. However, lacking an axial stiffening rod (notochord of the chordates in general, and vertebral column in vertebrates) they must be primitive. Occurring with true vertebrates, in the form of jawless fish, the vetulicolians are a relic of some earlier stage in vertebrate evolution. Shu et al. take the cautious view that they are early deuterstomes from which echinoderms and chordates emerged – close to the fundamental division among animals into deuterostomes and protostomes.
(See also: Gee, H. 2001. On being vetulicolian. Nature, v. 414, p. 407-408)