Explosions in the Garden of Ediacara

Some of the biggest misunderstandings about the evolution of life on earth surround the "Cambrian Explosion," the popular impression often being that complex multicellular life sprung up out of nowhere in an instant. While it does appear that there was an "explosion" and that new body plans identifying early representatives of various phyla evolved rapidly, the Cambrian extinctions are usually ignored, and so too are the strange creatures that lived during the Ediacaran. The Ediacaran Period preceded the Cambrian, and while its exact span has been difficult to determine, it has been estimated to have lasted from 635 to 542 million years ago. During this time creatures just as strange (if not stranger) than those found in the famous Cambrian deposits of the Burgess Shale and Chenjiang appeared, and a new paper in Science by Shen et al. has attempted to explain the diversity of these older forms.

Before going into what the new paper says, it should be noted that many of the interpretations of fossils from the Ediacaran and Cambrian are still disputed. Taxonomic affinities are often changed, and in some cases it's difficult to determine whether a fossil is a trace fossil or represents a soft-bodied organism in its entirety. The fossil Dickinsonia is perhaps the best example of this taxonomic shuffling, the fossil being interpreted as an annelid, a cnidarian, a coral, etc. Another Ediacaran fossil, Spriggina, appears to be a good candidate for a trilobite relative but it is difficult to tell exactly what it is, it too being interpreted in various ways. Yorgia, Pteridinium, Onega, Ediacaria, Marywadea, Arkarua, and Charnia also represent problematic Ediacaran fossils, some of which may be synonymous with each other, so it has been difficult to interpret what these creatures actually were and what they might have been doing in the Pre-Cambrian environments they inhabited. For this reason it is often better to focus on body plan rather than on taxonomic diversity, although many fossils simply do not make it easy for those who study them.

The new paper attempts to use the fossils known from three Ediacaran assemblages (Avalon in Newfoundland, the White Sea in Russia & Australia, and the Nama in North America & Namibia) in order to make determinations about the occupied morphospace of late Ediacaran organisms. Morphospace represents the possible form, shape, or structure of organisms, and it seems that despite varying levels of diversity the authors were able to interpret the Ediacaran fossils as occupying nearly all the different niches that you might expect to find in marine habitats despite the varying depths of the assemblages. Given that much of the morphospace in each environment was occupied, the authors suggest that there was an Ediacaran "explosion" (perhaps best represented by the Avalon assemblage), but for some reason new morphospace was not opened up. The fossils from these sites seem to represent a "failed experiment" (in the words of the authors), a diversification of body forms that ultimately does not appear to have took hold and given rise to a larger radiation. While not stated explicitly, the study may suggest that life on earth could have taken a different path during the Ediacaran, but for reasons unknown it did not, early members of many modern phyla not appearing until later. Still, there are some problems with our current understanding of fossils from both the Cambrian and the Ediacaran.

As I stated before, one of the major issues in studying Ediacaran and Cambrian life is figuring out whether the fossils represent a member of a new phylum or an early member of an established phylum, taxonomic shuffling being quite common and many fossils remaining enigmatic. Even between the two periods its difficult to make connections, which may suggest that there really was an earlier radiation of forms that did not carry over into the Cambrian or that we have not yet found the crucial transitions between fossils that might be good candidates to show evolution across the temporal boundary. At this point some body parts are going to be inferred and what some fossils represent is going to be open to some amount of interpretation, and while the Ediacaran and Cambrian fossils are often exquisitely preserved they're still quite rare. Let's use Dickinsonia as an example; if it's interpreted as an annelid it could have crawled along the ocean bottom, moved through the sediment just below it, or possibly even swam through the water. If it's interpreted as a cnidarian attached to the bottom, however, it probably did not move around very much and would have lived a life very different from that envisioned in the worm interpretation. Indeed, the interpretations of these fossils are very contentious, and even the exceedingly useful book On the Origin of Phyla cannot provide the reader with definitive answers for many fossils.

Personally, I like the idea of an Ediacaran explosion where new body plans were tried out but ultimately did not take hold, but personal preference doesn't mean that the hypothesis is accurate. If the disconnect between "full" Ediacaran habitats and a late Cambrian radiation of different forms is real then that is definitely something to get excited about, but for now it's probably best if we tread lightly. Further research is definitely needed, and (with luck) further fossil discoveries might help resolve some of the enigmatic taxa that have puzzled scientists for so long. Still, I think the paper is a move in the right direction as far as putting more effort into what was happening before the Cambrian, although I wouldn't expect "Ediacara" to become a household word anytime soon.

[Hat-tip and many thanks to Greg Laden for passing along the paper.]

References;

Shen, Bing, Lin Dong, Shuhai Xiao, Micha Kowalewski. (2008) The Avalon Explosion: Evolution of Ediacara Morphospace. Science 4 January 2008:Vol. 319. (5859) , pp. 81 - 84.