INVESTIGATING EVOLUTIONARY RADIATIONS USING MORPHOMETRICS, BIOGEOGRAPHY, AND SPECIATION RATES: A CASE STUDY INVOLVING DEVONIAN TRILOBITES

Evolutionary radiations represent important macroevolutionary patterns marked by profuse taxonomic diversification of species within a clade. A geometric morphometric analysis in a phylogenetic context can provide insight into these radiations by quantitatively documenting how morphology changes at speciation events. Here we use a robust phylogenetic hypothesis in conjunction with geometric morphometrics to quantify morphological change during a classic example of evolutionary radiation in the fossil record: the diversification of the Devonian Metacryphaeus group calmoniid trilobites. Rates of speciation and extinction and biogeographic patterns have been previously characterized for the group. Rates of speciation were high during the earliest history of the group and then tail away to zero shortly thereafter; extinction levels remained moderate to low throughout. The group disappeared because new species failed to replace those that vanished; geological complexity correlated strongly with elevated rates of evolution. Specimens were photographed and digitized with landmark data points on the trilobite cephalon. Generalized least-square Procrustes superimposition was used to standardize size and orientation, and a principal components analysis was performed on the resulting warp scores. Morphometric character data were placed at the terminals and then mapped to the nodes of the tree using squared change and linear parsimony. Analyses were conducted to correlate degree of morphological change with rates of speciation and biogeographic mode: in particular, origin due to vicariance or dispersal. Results of the analysis show close statistical convergence between morphological change and patterns of speciation. These appear to be associated with waxing and waning opportunities for geological isolation facilitated by changes in the geological complexity of the region these taxa occupied. Geology appears to have mediated opportunities for speciation and concomitant morphological diversification. The relationship between evolutionary radiations, Vrba's Turnover Pulse, and ideas on the emergence of species are also explored to develop a more unified view of evolutionary radiations preserved in the fossil record.