EXAMINING PATTERNS OF TAXONOMIC AND PHYLOGENETIC EXTINCTION SELECTIVITY IN PALEOZOIC TEREBRATULIDE BRACHIOPODS

Using phylogenetic hypotheses to investigate extinction dynamics is a relatively unexplored field of study in paleobiology; methods to do so are not self-evident, and empirical examples are few. We attempt such an analysis here with Terebratulida, the most diverse and abundant clade of extant brachiopods that survived (and thrived following) a series of mass extinction episodes over its 400-million-year history. We investigated patterns of taxonomic selectivity in pre-Jurassic terebratulides and, not surprisingly, found them to vary greatly among taxonomic levels at the end-Frasnian and end-Permian extinction episodes. Two superfamilies, three families, but only one genus (possibly two additional genera) survive the end-Frasnian episode; two superfamilies, three families, but no single genus, survive the end-Permian. The need for a lineage-based approach to the study of these extinction episodes is obviously critical; detailed phylogenetic analysis is necessary. Despite rich morphologic, stratigraphic, and biogeographic information available, the terebratulide sister group is not agreed upon, and our understanding of phylogenetic relationships among long- and short-looped terebratulides is still unclear.

We assembled a list of 84 morphological characters, examined 24 Paleozoic and Triassic terebratulide genera (as familial exemplars, using the classification in Lee et al., in press), and analyzed their phylogenetic relationships using PAUP 4.0b10. Thirteen atrypide, athyridide, and rhynchonellide genera were used as outgroups. Our preliminary analyses: confirm results of earlier studies that retzioid athyridides appear to be most closely related to terebratulides; agree well with the stratigraphic ordering of taxa; do not provide strong support for the monophyly of named Paleozoic terebratulide superfamilies; and do not reveal a striking phylogenetic bias to extinction selectivity at either the end-Frasnian or end-Permian extinction episodes. We are currently analyzing relationships of species and genera within each of these pre-Jurassic families to further clarify phylogenetic relationships among terebratulide brachiopods and to investigate extinction dynamics at finer taxonomic and phylogenetic scales.