The evolution of larval morphology during the post-Paleozoic radiation of echinoids

Abstract

The post-Paleozoic radiation of echinoids entailed a rapid diversification not only of adult morphology, but also of larval morphology. The timing, order, and phylogenetic distribution of evolutionary transformations in echinopluteus larvae are reconstructed here under maximum parsimony assumptions from a large neontological data base. Many echinoid larval apomorphies apparently evolved within the Paleozoic stem lineage and were subsequently retained during much of the crown-group radiation. This suite of apomorphies includes most (and perhaps all) of the skeletal elements and some features of soft anatomy such as vibratile lobes. Other features apparently arose or were modified during the post-Paleozoic radiation. These include skeletal features such as arm-rod structure and length, and soft structures such as epaulettes and skeletal muscles. Transformational hypotheses of this kind can be supported or rejected with further neontological data, and many can potentially be tested from fossil evidence. Many post-Paleozoic transformations in echinopluteus structure may be adaptive. For example, increases in arm length and flattening of arm ectoderm may increase feeding rate and efficiency, and both types of transformation have occurred several times within the crown group. Relational hypotheses of this nature can be tested through comparative functional studies in extant echinoplutei. Parallel evolutionary losses of feeding in echinoplutei are accompanied by loss or modification of characteristic structures. This suggests that developmental constraints do not fully explain the conservation of these structures in planktotrophic echinoplutei. Comparisons of larval and adult morphology over congruent time intervals demonstrate that the origin of many orders was accompanied by a suite of synapomorphies in larval morphology that was subsequently conserved. Many details of echinopluteus morphology are therefore of taxonomic significance. Intraordinal patterns of larval diversity, however, vary considerably. That larval morphology has diversified independently of adult morphology indicates that mosaic evolution has occurred within the life cycle and suggests that echinoid larvae and adults can and do respond to selection independently. Taken together, these findings underscore the complex ways in which complex life cycles can evolve.