THE CHANGING OF THE ECOLOGICAL GUARD AT THE EDIACARAN–CAMBRIAN TRANSITION (Invited Presentation)

The Ediacaran Period [635–539 million years ago (Ma)] archives several important events in the evolutionary history of animals. Not only do the earliest fossils of macroscopic and complex multicellular life appear in the record, but towards the Period’s conclusion, Earth’s first ‘mass extinction’ may have occurred. Coincident with the decline of the archetypal Ediacaran organisms, the terminal Ediacaran fossil record becomes increasingly populated by novel tube-dwelling organisms—collectively, the ‘cloudinomorphs’. While small, sessile, and epibenthic, the morphotypic group cloudinomorphs (including their iconic namesake Cloudina) appeared with key adaptations that may have enhanced their chances for ecological success. These adaptations included: (i) the advent of macroscopic biomineralization in the form of shelly external tubes, potentially serving as an impediment to predation; (ii) the establishment of gregarious habits that may signal the onset of metazoan ecosystem engineering behaviors; and (iii) the development of enhanced larval dispersal mechanisms versus stolon-like reproductive modes of other Ediacarans. Owing to these ecological innovations, the cloudinomorphs may have been central players in ushering a phase of fundamental ecosystem reform and increased trophic complexity. Although its cause is equivocal at present, this ecological changeover from largely sedentary Ediacara-type communities to much more dynamic syn-‘Cambrian Explosion’ ecosystems was well underway during the finale of the Ediacaran Period. The most critical task that remains is to unravel potential relationships between the organisms of the Ediacaran and those well-defined as metazoans in the Cambrian. As recently demonstrated, the cloudinomorphs are one of the few groups known to span the Ediacaran-Cambrian boundary, and thus understanding their phylogenetic position may be paramount to untangling the evolutionary and ecological relationships between two seemingly disparate biomes: Ediacaran biota and Cambrian animals.