Saturday, 24 December 2011

The Ediacaran Fauna; not animals after all?

Most modern animal groups appear abruptly at, or very shortly after, the beginning of the Cambrian period, 542 million years ago. The Cambrian starts abruptly with a layer of small shelly fossils that are hard to assign to any group, which are then replaced abruptly by fossils belonging to more familiar groups; arthropods, molluscs, brachiopods, etc., which then persist throughout the fossil record till the present day (albeit with considerable evolutionary improvements). At the same time there are abrupt fluctuations in the isotopic ratios of Carbon, Strontium and Sulphur, which are equally hard to explain.

This is how the origin of animal life is recorded in the fossil record, something many palaeontologists have found less than satisfactory for a long time, particularly as DNA evidence suggests that the major animal groups diverged from one-another at least 200 million years before the beginning of the Cambrian (the 'DNA-clock' works by measuring the rate of mutation in non-coding, or 'junk' DNA; since this DNA does not affect the organism it is not subject to natural selection and ought to mutate at a steady rate). This has lead to a great deal of interest in any animal fossils that might pre-date the beginning of the Cambrian.

The earliest single celled fossils are about 3500 million years old, with chemical indicators of life being even older in the sedimentary record. The earliest eukaryotic cells (cells with nucleuses) are about 1000 million years old, the first fossils we can identify as belonging to identifiable animal groups are just under 542 million years old; but the intervening period is not devoid of interesting fossils.

The Ediacaran 'Fauna' appear in the fossil record about 585 million years ago and persist till the start of the Cambrian, when they vanish abruptly. The fossils are clearly of multicellular organisms with pre-defined structures (like animals) but do not closely resemble any known post-Ediacaran group, living or fossil. No Ediacaran organism seems to have produced any mineralized tissue, such as bone or shell. These fossils are rare, but have attracted a great deal of attention. The have been found in England, Nova Scotia, Mexico, Namibia, Australia, Russia and China, amongst other places.

Charnia is a typical member of the Ediacaran Fauna; it has a clear structure, has been found in many parts of the world, but does not resemble any known organism.

The period of time between 635 and 542 million years ago has been formerly named the Ediacaran. The Ediacaran Period precedes the Cambrian and follows the Cryogenian, which ran from 850 to 635 million years ago, and was marked by several severe glaciations (Ice Ages) of near global extent. This makes for a plausible (if unverifiable) story for the origin of animal life. The vast glaciations of the Cryogenian somehow caused single celled animals to form organized colonies, which grew more sophisticated during the Ediacaran, then started to produce mineralized tissues at the beginning of the Cambrian as a result of some change in seawater chemistry. Biomineralization probably started as a way of secreting unwanted chemicals, but was rapidly co-opted for other purposes, provoking a radical restructuring of the bodies of those animals that adopted it.

The 23 December edition of the journal Science contains a paper by a team lead by Therese Huldtgren of the Department of Palaeozoology at the Swedish Museum of Natural History, which examines a number of globular Early Ediacaran fossils from the Doushantuo Formation of Guizhou Province in China, that have been considered to be embryonic Ediacarans. Based upon this analysis, Huldtgren et al. conclude that the 'embryos' show patterns of cell division incompatible with the embryonic development of animals, and that the organisms that produced it cannot therefore be closely related to animals.

The Doushantou 'Embryos'.

The fossils from Doushantuo are all microscopic, and come from early in the Ediacaran Period (635-551 million years ago). In addition to the 'embryos' there are achritarchs (a form of algae), ciliates, seaweeds and what may be sponges and soft corals as well as a possible planktic larvae that has been named Vernanimalcula. Since the only known large organisms at during the period are the Ediacarans, it is fairly obvious to conclude that the embryos are those of Ediacarans. However, this is t best a guess; there is no actual evidence to link the embryos to the Ediacaran Fauna, so they may represent a part of the life-cycle of some as yet unknown organism, which co-existed with, or even predated the Ediacaran Fauna.

Based upon this there is no real evidence to suggest that the Ediacarans either were, or were not, true animals. They remain a mystery.

3 comments:

  1. It is not logical that the fairly complex animals of the Cambrium did not have precursors. The discovery of the Ediacaran fossils gives a good explanation for the later Cambrian explosion. But some of these surely did not carry further into the Cambrian period, and yes, some of them may indeed also be something very different from animals.

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