One of the nice things about going on holiday for a reasonable period of time, without computer, e-mail, cellphone etc, is that you can settle down for a bit of serious reading. In a fairly full 7 days I still managed to complete 2 books & start another. One was detective fiction (PD James rocks!). The other was Martin Brasier’s book Darwin’s Lost World.
I found this particular volume both irritating & entralling, although the fascination won out. (The irritation was caused by some – to me, anyway – rather laboured metaphors.) One of the things I particularly enjoyed was that Brasier led me to think again about things we tend to take for granted. For example, the Burgess Shale and Ediacaran fossils are over 500 million years old, & yet show much better preservation of partially or wholly soft-bodied organisms than many more recent fossil specimens. Why should this be? (I’m embarrassed to say I’d never really thought about this…) And – did the ancient jellyfish that ballooned their way through the Pre-Cambrian oceans sting? (A topical question, given that the stinger nets were up in Port Douglas & anyone swimming outside them was regarded as dicing with a very unpleasant death by box jellyfish.)
On the Ediacaran & Burgess fossils, & other similarly well-preserved ancient sites – the ‘standard’ answer has always been that the fossils formed in a particular, narrow, set of circumstances. For example, the fossils of the Burgess Lagerstätte were thought to have formed when the animals were engulfed in a mudslide & carried deep into anoxic waters. With no available oxygen decomposition was very slow, & the fine silts entombing the corpses captured their soff tissues in exquisite detail over the long, slow process of fossilisation. But for the earlier, Pre-Cambrian soft-bodied fauna, Brasier suggests there’s another factor involved – a lack of worms (in the sense of ‘worm-like invertebrates, many of which actively burrow into the substrate’).
Take modern earthworms.Darwin devoted his last book to them, detailing how earthworms process soil & dead vegetation, rapidly turning over the soil, producing humus, and aerating everything into the bargain. Many modern marine worms do much the same, leaving characteristic burrows & galleries in the mud, & trace fossils formed from such burrows are found in Cambrian rocks & provide some of the first evidence of eukaryote behaviour. The burrows of those early worms would have helped to oxygenate previously ‘dead’ seafloor sediments, so that organisms falling to the sea floor would be more likely to decay rather than enter the process of fossilisation. And the worms would have processed the dead bodies, or the soft bits anyway, meaning that the odds of any non-skeletal body parts making it through to fossilisation were much reduced. So Brasier suggests that one of the reasons why the soft-bodied Ediacaran fossils (& others even earlier) is that there were no worms around to churn things up. How different the marine ecosystems must have been, way back then!
And the jellyfish? In this case, Brasier’s saying that it would be worthwhile to stop visualising the past in terms of the present. Modern-day jellyfish do indeed sting. Like all cnidarians they possess specialised cells called cnidocytes, that contain nematocyts: the structures that do the actual stinging. These structures enable jellyfish, sea anemones & their relatives to catch their prey – and their prey are generally ‘higher’ animals: arthropods, fish & the like. Yet these animals evolved more recently; whatever those Pre-Cambrian jellyfish were consuming, it was nothing like what modern jellies eat. Brasier makes the point that [j]ellyfish are not primitive beasts. They have co-evolved to prosper in a post-Ediacaran world. In other words, the stinging nature of modern cnidarians has evolved in response to the changing nature of potential prey, and the ecological niche of Pre-Cambrian cnidarians was very probably quite different from that which they occupy today.
I’d never really thought about that before…
M.Brasier (2009) Darwin’s Lost World: the hidden history of animal life pub. Oxford University Press.