TERMINAL ADDITION, THE CAMBRIAN RADIATION AND THE PHANEROZOIC EVOLUTION OF BILATERIAN FORM

We argue that terminal addition, the process of addition of serial elements in a posterior subterminal growth zone during animal development, was the basal condition in Bilateria, and that modification of terminal addition was an important component of the rapid Cambrian evolution of novel bilaterian morphology. We categorize the often-convergent modifications of terminal addition from the presumed ancestral condition. Our focus on terminal addition and its modification highlights trends in the history of animal evolution evident in the fossil record. These trends appear to be the product of departure from the initial terminal addition state, as is evident in evolutionary patterns within fossil groups such as trilobites, but is also more generally related to shifts in types of morphologic change through the early Phanerozoic. Our argument is contingent on dates of metazoan divergence that are roughly convergent with the first appearance of metazoan fossils in the latest Proterozoic and Cambrian, as well as on an inference of homology of terminal addition across bilaterian Metazoa. We discuss the premises regarding the Cambrin radiation and the basis for the assessment that terminal addition is ancestral. We then explore the explanatory value of our interpretiation considering individual lineages and patterns of faunal origination in the fossil record.