[From Vendian to Cambrian: the beginning of morphological disparity of modern Metazoan phyla]

Abstract

Continuity of the transition from Precambrian to the Phanerozoic metazoan fauna at the phylum level is analyzed. The discrete traces of feeding on bacterial mats by Dickinsonia and similar organisms are explained by extracorporeal digestion, characteristic ofa placozoan level of organization, as in the extant Trichoplax. The absence of a morphologically developed anterior end of the body, of food-gathering appendages of any kind, and of appendages responsible for movement suggest that these animals were at this level of organization. Probably, an expanded placozoan level of organization can be assumed for most vendian animals. Against this background, new characters emerged in the Cambrian to be included in the body plan of extant animal phyla. The relationships between the morphological archetype (body plan) and morphogenetic archetype are considered. It is shown that major features of a morphogenetic archetype could be retained from the time the taxon was formed.