The Pillow Creatures That Time Forgot

Did the Ediacaran fauna lead to anything else, or was it a dead end?

While to a molecular biologist, the evolution of the eukaryotic cell is probably the greatest watershed event after the advent of life itself, most others would probably go with the rise of animals and plants, after about three billion years of exclusively microbial life. This event is commonly located at the base of the Cambrian, (i.e. the Cambrian explosion), which is where the fossils that Darwin and his contemporaries were familiar with began, about 540 million years ago. Darwin was puzzled by this sudden start of the fossil record, from apparently nothing, and presciently held (as he did in the case of the apparent age of the sun) that the data were faulty, and that the ancient character of life on earth would leave other traces much farther back in time.

That has indeed proved to be the case. There are signs of microbial life going back over three billion years, and whole geologies in the subsequent time dependent on its activity, such as the banded iron formations prevalent around two billion years ago that testify to the slow oxygenation of the oceans by photosynthesizing microbes. And there are also signs of animal life prior to the Cambrian, going back roughly to 600 million years ago that have turned up, after much deeper investigations of the fossil record. This immediately pre-Cambrian period is labeled the Ediacaran, for one of its fossil-bearing sites in Australia. A recent paper looked over this whole period to ask whether the evolution of proto-animals during this time was a steady process, or punctuated by mass extinction event(s). They conclude that, despite the patchy record, there is enough to say that there was a steady (if extremely slow) march of ecological diversification and specialization through the time, until the evolution of true animals in the Cambrian literally ate up all the Ediacaran fauna. 

Fossil impression of Dickinsonia, with trailing impressions that some think might be a trail from movement. Or perhaps just friends in the neighborhood.

 
For the difference between the Ediacaran fauna and that of the Cambrian is stark. The Ediacaran fauna is beautiful, but simple. There are no backbones, no sensory organs. No mouth, no limbs, no head. In developmental terms, they seem to have had only two embryological cell layers, rather than our three, which makes all the difference in terms of complexity. How they ate remains a mystery, but they are assumed to have simply osmosed nutrients from their environment, thanks to their apparently flat forms. A bit like sponges today. As they were the most complex animals at the time, (and some were large, up to 2 meters long), they may have had an easy time of it, simply plopping themselves on top of rich microbial mats, oozing with biofilms and other nutrients.

The paper provides a schematic view of the ecology at single locations, and also of longer-term evolution, from a sequence of views (i.e. fossils) obtained from different locations around the world of roughly ten million year intervals through the Ediacaran. One noticeable trend is the increasing development or prevalence of taller fern-like forms that stick up into the water over time, versus the flatter bottom-dwelling forms. This may reflect some degree of competition, perhaps after the bottom microbial mats have been over-"grazed". A second trend is towards slightly more complexity at the end of the period, with one very small form (form C (a) in the image below) even marked by shell remains, though what its animal inhabitant looked like is unknown. 

Schematic representation of putative animals observed during the Ediacaran epoch, from early, (A, ~570 MYA, Avalon assemblage), middle, (B, ~554 MYA, White River and other assemblages), and late (C, ~545 MYA, Nama assemblage). The A panel is also differentiated by successional forms from early to mature ecosystems, while the C panel is differentiated by ocean depth, from shallow to deep. The persistence of these forms is quite impressive overall, as is their common simplicity. But lurking somewhere among them are the makings of far more complicated animals.

Very few of these organisms have been linked to actual animals of later epochs, so virtually all of them seem to have been superceded by the wholly different Cambrian fauna- much of which itself remains perplexing. One remarkable study used mass-spec chemical analysis on some Dickinsonia fossils from the late Ediacaran to determine that they bore specific traces of cholesterol, marking them as probable animals, rather than overgrown protists or seaweed. But beyond that, there is little that can be said. (Note a very critical and informed review of all this from the Discovery Institute, of all places.) Their preservation is often remarkable, considering the age involved, and they clearly form the sole fauna known from pre-Cambrian times. 

But the core question of how the Cambrian (and later) animals came to be remains uncertain, at least as far as the fossil record is concerned. One relevant observation is that there is no sign of burrowing through the sediments of the Ediacaran epoch. So the appearance of more complex animals, while it surely had some kind of precedent deep in the Ediacaran, or even before, did not make itself felt in any macroscopic way then. It is evident that once the triploblastic developmental paradigm arose, out of the various geologic upheavals that occurred at the bases of both the Ediacaran and the Cambrian, its new design including mouths, eyes, spines, bones, plates, limbs, guts, and all the rest that we are now so very familiar with, utterly over-ran everything that had gone before.

Some more fine fossils from Canada, ~ 580 MYA.

• A video tour of some of the Avalon fauna.
• An excellent BBC podcast on the Ediacaran.
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