The Evolutionary Significance of Cambrian Ecdysozoan Trace Fossils

Abstract: The origin of most of the animal phyla is tied to the Cambrian explosion, a rapid diversification event that took place from about 540 million years ago. This diversification is coupled with a great number of concurrent metazoan body plan innovations such as the development of the coelom, antagonistic muscles and a through-gut. While the body fossil record and molecular clocks document this apparent diversification, albeit with somewhat divergent results, there exists a third much more accurate record for the timing of life’s journey: the trace fossil record. The ecdysozoan trace fossil record first appears in the terminal Ediacaran and extends into the Cambrian. It exhibits a similar pattern of diversification to the body fossil record, pointing to a rapid ethological expansion through this interval. Here, I investigate particular ethologies of early Cambrian ecdysozoan trace fossils and frame the corresponding trace fossil morphologies in an evolutionary context. The first part of the thesis includes observations on the Cambrian representatives of the ichnogenera Cruziana and Rusophycus. I identify the links between the two morphologies and suggest that some Cambrian Cruziana are formed by the concatenation of distinct Rusophycus. This organisation might be more prevalent than previously expected in the Palaeozoic. This study is further supplemented by 3D virtual ichnology and the production of false colour depth maps produced for the first time for invertebrate trace fossils. With virtual ichnological techniques, subtle morphological characters can be readily observed and enhance the analysis of specimens. Investigating the early Cambrian ecdysozoan fossil record, I analysed Cambrian treptichnid trace fossils with cast-like impressions of their makers that can allow the assignment of their producers to at least stem-ecdysozoans. Actualistic experiments show that extant Priapulus caudatus produce similar trace fossils and therefore priapulid-like stem ecdysozoan producers are to be expected for the treptichnid burrows appearing at the Ediacaran-Cambrian GSSP. Although Rusophycus is the first trace fossil that can be firmly attributed to euarthropods, the simple ridge pattern of Monomorphichnus that appears even earlier than Rusophycus is a potential arthropod produced trace fossil. I investigate the morphologies of the earliest Monomorphichnus and address the problems with assigning these simple trace fossils to euarthropods. I also discuss the possibility that a part of its known record might be best described as tool marks rather than trace fossils.