EVOLUTION AND DEVELOPMENT AT THE ORIGIN OF A PHYLUM (Invited Presentation)

The developmental and evolutionary mechanics at the origin of phyla are vital in understanding the nature of the Cambrian Explosion and subsequent Phanerozoic diversifications. The rapid appearance and distinctiveness of phyla have led to testable hypotheses regarding the variability of higher-order characters that are vital in establishing body plans. The hierarchical nature of features predicts that characters with a high burden (i.e. a high volume of contingent characters) are integral in establishing body regions that characterize higher taxonomic groups and are retained through time. This phenotypic model has been expanded with new discoveries in gene regulatory networks (GRNs) forming a genetic framework for body plan evolution. This model predicts that more hierarchically nested GRN subcircuits, termed “kernels”, underlie constraint in body plan evolution and are relatively impervious to evolutionary change. Unfortunately, gene regulatory networks can only be studied in extant taxa 500 million years removed from the origin of a phylum such that direct testing is restricted to the phenotypic expression of these networks within the fossil record.

We constructed a character suite that encompasses morphological features found within Early Paleozoic echinoderms and characterized a nearly comprehensive sample of Cambrian and Ordovician genera. This character suite contains a hierarchical structure such that characters are interdependent and have variable levels of burden from character contingencies. We combined the morphological data with an independently constructed phylogenetic hypothesis based on published analyses to map morphological character evolution through the Early Paleozoic. We found no relationship between character burden and the number of character transitions or reversals, which counters the predicted model. These results may suggest that complex characters that establish class or subphylum-level body plans are modular and can disappear and reappear through time, though their evolution is not heirarchical. To further explore this pattern, a theoretical morphospace was constructed using bauplan-level characters to explore early echinoderm evolution. The theoretical morphospace was unevenly explored suggesting that the modularity in structure is not uniform across the phylum.