Fossil Record of the Origin of Baupläne and Its Implications

Abstract

Animal Baupläne and Unterbaupläne appear abruptly in the fossil record, mostly developing early in Phanerozoic time; metazoan radiations near the Precambrian/Cambrian boundary may have produced scores of phylum-level taxa, and during Cambrian and Ordovician time, hundreds of class-level taxa. Ancestors and intermediates are unknown or are conjectural as fossils. About one in ten invertebrate species is a known fossil, and sampling of the Cambrian and Ordovician faunas appears at least average for the Phanerozoic, so that many species representing ancestral and intermediate lineages should have been discovered if they could be fossilized as easily as the average species which is found. Transformations between Baupläne most likely occurred in small localized populations which evolved rapidly, perhaps the restructuring of a partitioned genome via regulatory changes, commonly creating heterochronies, can account for the large-scale but rapid morphological changes required. Virtual restriction of Bauplan origin to the early Phanerozoic may be owing to the open adaptive space and to simpler metazoan genomes at that time.