ACROSS THE SAVANNAH: A NEW PERSPECTIVE ON EDIACARAN EVOLUTION

Although molecular clock data continue to place the origins of animals in general, and bilaterians in particular, well before the Cambrian, fossil evidence indicates that animals evolved during the Ediacaran. Here we review the affinities of Ediacaran fossils and conclude that the famous problematic taxa belong to the stem-groups of various large animal groupings, although no crown-group animal phyla can be be definitively identified during this interval. We suggest a new ecological hypothesis for the origin of bilaterians that envisages the largely static Ediacarans providing a savannah-like heterogeneous benthic environment within which the mobile bilaterians lived and evolved, as documented by the early trace fossil record. Rather than being competitors with the early bilaterians, the Ediacaran taxa provided the essential co-operative ecological backdrop within which (and, indeed, from which) the bilaterians emerged. Remnants of this mutual cohabitation may have persisted well into the Cambrian and indeed beyond, although the evidence for Ediacaran-style biota in the Phanerozoic remains somewhat equivocal. Either way, the arrival of the bilaterians had a transformative effect on the evolution of other major animal groups, which is perhaps the reason that it is only in the Cambrian that (for example) crown-group sponges and cnidarians are found.