Precambrian Metazoans: The Problems of Preservation, Systematics and Evolution

The unique appearance of Vendian metazoans in the Precambrian fossil record is controlled not only by taphonomic and ecological factors, but also by the level of morphological and physiological organization of these animals. The peculiar nature of these factors means that the Vendian represents an important stage in metazoan evolution. This notion is supported by the Bauplan analysis of Vendian forms, many of which have a bodyplan (architectonics) that is most unusual in comparison with animals from later periods. This new information allows a revision of the systematics of Precambrian metazoans at a high taxonomic level. The new classes Cyclozoa, Inordozoa, and Trilobozoa are recognized among Vendian Coelenterata. The phylum Proarticulata, with classes Dipleurozoa and Vendiamorpha, represents the most primitive Vendian Bilateria. These, the oldest faunas on Earth, provide indication of the earliest modes of metazoan evolution that have not been part of the theoretical predictions of neontologists, and they also serve as a tool to check certain phylogenetic models.