Ancient Animals or Something Else Entirely?

In their report “Discoidal impressions and trace-like fossils more than 1200 million years old” (10 May, p. [1112][1]), B. Rasmussen and colleagues present evidence for what might be the oldest known animals, from the Stirling Range Formation of southwestern Australia. Thanks to the kindness of Rasmussen and Fletcher, I recently had the opportunity to study this material. I suggest that some of their conclusions may be premature. The supposed trails are clearly all of a kind, but they show three peculiar features. First, although not remarked upon by Rasmussen et al. , some ridge margins show a striking imbrication with the adjacent sediment, reminiscent of the overfolding of a flexible surface. Second, on one surface, despite the presence of discrete “trails,” an adjacent area is strongly corrugated (see left-hand side of Rasmussen et al. 's Fig. 2E). The texture and scale of this region are closely similar to the supposed trails, indicating a possible common origin. Finally, albeit a weaker argument and one already addressed by Rasmussen et al. , there is the striking parallelism of the two most prominent “trails” (Fig. 2, C and D), the ridges of which show the same sense of imbrication. These observations suggest that alternative explanations may be preferable. The most probable is that these structures result from flexible microbial mats that coated sediment surfaces and were subsequently disrupted. The associated discs (unillustrated) have only a vague resemblance to the later Ediacaran structures known from type localities in Australia and Russia. A striking feature of some of the Stirling Range discs is that the concentric rings show pronounced undulations. Despite Rasmussen et al. rejecting this possibility, interpretation of these structures as microbial colonies seems reasonable. Claims of pre-Ediacaran traces are important if they verify a deep metazoan ancestry, consistent with molecular clock estimates. Nevertheless, to date, all such claims are controversial and share two features. All differ from one another, and all are highly localized. Nor is it explained why such a biological invention that existed circa 500 million years before “Snowball Earth” failed to take off. Pre-Ediacaran metazoans ([1][2]) there may well be, but searching for images based on Phanerozoic expectations may be misleading. 1. [↵][3]1. M. Pagel 1. S. Conway Morris , in Encyclopedia of Evolution, M. Pagel, Ed. (Oxford Univ. Press, New York, 2002), vol. 1, pp. 129-133. [OpenUrl][4] # Response {#article-title-2} We welcome a discussion of the enigmatic Stirling biota. Like us, Conway Morris accepts a biological origin of the fossils and rejects the earlier identification of the discoidal forms as Ediacaran. Unlike us, however, he thinks that the trace-like fossils were probably formed by disruption of flexible microbial mats. He does not account for, or even mention, the features that constitute our evidence that the structures were made by vermiform, motile organisms: the recurrent pattern of ridge pairs with a distinct U-shaped ending. We tried in vain to find a plausible mechanism whereby such a pattern could be formed through disruption of a microbial mat. Conway Morris does not present such a mechanism; he merely lists three “peculiar” features, all given the label “strong” or “striking,” which in his opinion would favor alternatives over our trace-fossil interpretation. None of them challenges our interpretation—in fact, they are to be expected in a situation where mucus trails left by organisms in a muddy environment occasionally crowd and break up, get replaced by sand, and suffer compaction and low-grade metamorphism. On the other hand, the regular, recurrent structures that we have described in the less crowded areas are indeed peculiar, strong, and striking, and they pose a formidable challenge to the microbial-mat hypothesis. Any explanation that doesn't take them into account is not only premature—it's stillborn. Conway Morris remarks that alleged pre-Ediacaran trace fossils are all controversial, different, and localized. There is no question that pre-Ediacaran trace-like fossils are rare. If explainable as disrupted microbial mats, however, they should be all but rare in a world dominated by microbial communities unaffected by grazers. In any case, we shall never know how rare, different, and localized pre-Ediacaran traces are unless they are searched for, reported, described, and analyzed without being forced into currently accepted evolutionary scenarios. Only then will we have a reasonable chance of establishing whether and, if so, why motile multicellularity did not become a prominent theme in evolution until the Cambrian explosion. As Conway Morris seems to take us to task for not providing an explanation of why the invention of motile multicellularity failed to take off until the Cambrian explosion, we must enter the merry realm of unfettered speculation. Thus, we offer the following: A slimy young worm in the making Found a gal he considered worth taking. But she cried in despair: “The Precambrian air Is too stuffy—my neuron is aching!” [1]: /lookup/doi/10.1126/science.1070166 [2]: #ref-1 [3]: #xref-ref-1-1 View reference 1 in text [4]: {openurl}?query=rft.jtitle%253DEncyclopedia%2Bof%2BEvolution%26rft.volume%253D1%26rft.spage%253D129%26rft.atitle%253DENCYCLOPEDIA%2BOF%2BEVOLUTION%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx