This 1985 book examines the origin of the present diversity of marine invertebrate animals. A brief review of the early stages in the history of life discusses the time-scale of the relevant geological periods alongside corresponding events in the evolutionary sequence. These views of the early history of life are then matched against the fossil record and conjectures drawn from the living fauna, enabling the author to attempt an overview of the early diversification of marine animal life. Transitions to the succeeding assemblages of shellbearing fossils in Palaeozoic rocks are discussed and a number of stratigraphic adjustments are suggested for the period in which evolutionary events had their greatest impact on oceans and marine rock strata. The need for an interdisciplinary approach to early evolution is emphasized.