Trace fossils and the Cambrian explosion

Abstract

In two recent contributions to TREE[1xThomas, A.R.L. Trends Ecol. Evol. 1998; 13: 129Abstract | Full Text | Full Text PDF | PubMedSee all References, 2xThomas, A.R.L. Trends Ecol. Evol. 1998; 13: 322Abstract | Full Text | Full Text PDF | PubMedSee all References], Thomas has argued that trace fossils (trails, burrows, etc.) support deep Precambrian divergence of metazoans, and concludes that the Cambrian explosion is a taphonomic event of the appearance of hard parts. Speaking from experience, he calls upon `Ediacaran' sites in Sweden said to have beautifully preserved the Precambrian sea floor with an `astonishing abundance of trace fossils'. This would be indeed compelling evidence—except that there are no Ediacaran sites in Sweden. Reference to the occurrence of Psammocorallia, however, shows that Thomas is thinking of the Mickwitzia sandstone[3xJensen, S. Fossils & Strata. 1997; 42: 1–110See all References[3]. This unit is actually late Early Cambrian. In fact, there are no convincing trace fossils anywhere in the world older than 600 Ma (Ref. [4xSee all References[4]) and all moderately diverse terminal Proterozoic trace fossil assemblages may be younger than 550 Ma (Ref. [5xGrotzinger, J.P. et al. Science. 1995; 270: 598–604CrossrefSee all References[5]).Trace fossils do not suffer from the same taphonomic peculiarities that complicate arguments about the nature of the soft-bodied Ediacara-type fossils. Unless contrived ad hoc hypotheses are to be invoked, it may thus be safely assumed that their appearance marks the appearance of macroscopic bilaterians.Additionally we would like to comment on the frequently-aired hypothesis of cryptic evolution of miniature Precambrian metazoans[6xFortey, R.A., Briggs, D.E.K., and Wills, M.A. BioEssays. 1997; 19: 429–434CrossrefSee all References, 7xCooper, A. and Fortey, R. Trends Ecol. Evol. 1998; 13: 151–156Abstract | Full Text | Full Text PDF | PubMed | Scopus (119)See all References]. In this scheme the Cambrian explosion is seen as little more than an increase in size and, in some groups, the acquisition of hard parts, in already well-separated lineages. However, such miniature stem-group animals would have faced extremely different ecological conditions to macroscopic animals, and it is difficult to see how many of the shared morphological features of the modern phyla—such as blood vascular systems[8xRuppert, E.E. and Carle, K.J. Zoomorphology. 1983; 103: 193–208Crossref | Scopus (54)See all References[8]and body cavities—could have been functionally useful in such a setting. Either these features were acquired in an avalanche of convergence in the separate lineages, or—much more likely—characters that are used to define eumetazoan phyla appeared in the latest Vendian–Cambrian and in most cases even later. This in turn would imply that the cladogenesis leading to the modern phyla is indeed accurately reflected by the increase in diversity of trace and body fossils from the latest Vendian to the Cambrian, a gradual process taking at least 20 million years. We conclude that `accurate' analysis of the fossil evidence strongly suggests the Cambrian explosion is a reality representing the origins of first stem-group and then crown-group members of the Bilateria.