RAGING AGAINST THE INEVITABLE: THE GRADUAL DECLINE OF PROETIDE TRILOBITES INTERRUPTED BY REDIVERSIFICATION

Three common outcomes of mass depletion events are: 1) total extinction; 2) radiation during post-event rebounds, or, 3) relict status leading to subsequent extinction (e.g., Dead Clades Walking [DCW]). Proetide trilobites offer a potential clade for assessing these models because of their quick proliferation in the wake of a mass extinction and environment turnover during the Late Devonian followed by a prolonged decline. However, Devonian proetides also offer some twists on the usual scenarios because trilobites as a whole began to decline in the Ordovician and continued largely uninterrupted into the Permian. The one possible interruption is a possible “comeback” by proetides and other trilobites in the Devonian. I use a series of Bayesian phylogenetic analyses to simultaneously assess both relationships among end-Silurian to the early Carboniferous members of the Proetidae, and a series of diversification models including: 1) no diversification shifts; 2) prolonged decline consistent with the Ordovician-Permian trend; 3) Devonian rebound followed by pulsed extinction and Carboniferous rebound; 4) Devonian rebound followed by prolonged (Givetian-Fammenian) decline, then rebound. Characters from thirty species within family Proetidae were coded across North America, Europe, and China. Preliminary results indicate proetide trilobites following diversification model 4: a steady decline with a minor rebound in the late Devonian/early Carboniferous. Thus, although trilobites as a whole and proetides in particular were a declining clade during the Middle Paleozoic, they had not yet become relicts, either: there were still some circumstances under which could still diversify. The phylogenetic hypotheses obtained from this study will be used in future studies to analyze the change in biogeographic diversity patterns of trilobites in the Devonian in correspondence with other marine groups such as corals and brachiopods to present a global picture of diversity changes across a mass extinction.