We hear often of the Cambrian explosion – the period around 540 million years ago wherein many animal groups first appear in the fossil record – but, contrary to the impression that you might get from certain creationist sources, there was life before the Cambrian. For example the Ediacaran, the Period immediately prior to the explosion, is known for its enigmatic biota, including the disk-shaped Aspidella.
The nature of the Ediacaran biota is of importance: molecular studies – i.e. genetic comparisons – of living animals show that they actually diverged some time before the Cambrian, two periods back in the Cryogenian. If the common ancestors of all living animals lived in the Cryogenian, therefore, then there must also have been animals in the Ediacaran. Some evidence that this is true, therefore, would be quite nice.
But it’s not that simple. All known Ediacaran organisms are soft bodied, meaning that they leave only rare trace fossils (impressions of various kinds) that are difficult to interpret with certainty, but they are generally thought to be multicellular marine organisms. However, a paper published (pdf) around the start of the year contended that Ediacaran fossils from South Australia, which would include Aspidella, were in fact deposited on land and may even be more likely to be “lichens and other microbial colonies of biological soil crust, rather than marine animals, or protists.”
That brings us to a more recent paper, Evidence for Cnidaria-like behavior in ca. 560 Ma Ediacaran Aspidella (closed access, no pdf available). Cnidaria is a broad group of animals which includes corals, jellyfish and sea anemones: the behaviour alluded to is that of an organism living of the sea floor slowly adjusting its position to ensure that it doesn’t get buried by accumulating sediment, which leaves behind what they refer to as “equilibrium traces.” This shows in the minds of the authors that Aspidella did live in a marine environment and was an animal. The ICR’s Brian Thomas, in What Were the First Animals Like?, accepts this conclusion but draws one of his own from it. He opens:
Are Ediacaran “fossils” actually remains of ancient living things, or did simple natural processes generate fossil look-alikes? Correctly identifying these tracks (or traces) matters significantly to those who insist these Ediacaran rocks—which secularists believe to be over 550 million years old—came from a time when Earth’s earliest animal life first appeared. Are these scientists looking at fossils made by the supposed ancestor common to all animals?