Print this pageAbstract: Early Cambrian Shallow Subtidal Environment--Time and Place for Taxonomic Expansion
An explosive increase of metazoans in the late Precambrian/Early Cambrian led to an immediate diversification at the phyletic and class levels during the Early Cambrian. A detailed study of two intervals of the Lower Cambrian section, western Great Basin, Nevada, stratigraphically pinpoints this expansion. Just above the base of the Cambrian, the Deep Springs interval contains a simple assemblage of filamentous blue-green algae and the traces of several kinds of burrowing deposit feeders. By comparison, the younger Poleta interval (middle Early Cambrian) contains a much more diverse fauna. The simple niches of filamentous producer and burrowing deposit feeder are still occupied in the younger community, but the presence of calcareous skeletons provided alternative means o locomotion, feeding, support, and protection.
The initial response of the Early Cambrian taxa was to exploit all major adaptive zones available and to occupy all the major modes of life filled by taxa of Middle Ordovician-Early Devonian age. Several trends are evident for this Early Cambrian diversification: (1) the habit of deposit feeder/scavenger was most readily adapted to by Early Cambrian forms; (2) the habit of suspension feeder seems to have developed at a slightly later date than that of deposit feeder/scavenger; (3) the expansion of metazoans, which occurred simultaneously with the introduction of calcareous hard parts, also included organisms without hard parts.
The expansion of metazoans in the Early Cambrian indicates an explosive radiation as opposed to a gradual differentiation and evolution over a long period of time. A Precambrian ground plan for metazoan expansion, employing among other factors, a generalized coelomate form in the late Precambrian, is strongly corroborated.
AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA