NO CHANGE IN MORPHOLOGICAL DISPARITY IN LICHID TRILOBITES ACROSS THE END-ORDOVICIAN EXTINCTION

Trilobites were one of the many marine invertebrate clades to suffer from the end-Ordovician mass extinction. Taxonomic diversity decreased across all families, with some families going extinct entirely. There is some evidence that the extinction may have been selective for environmental distribution and/or life history strategy. Variance in the shape of the cranidium remained high across the clade as a whole, but changes in other aspects of morphology remain unstudied, as does any association between ecology-related extinction selectivity and morphology. We further explore the effect of this extinction on trilobite evolutionary history by focusing on the speciose (394 species in 52 genera) trilobite family, the Lichidae. Our analysis is currently based on 178 characters coded for 51 different species representing almost all lichid genera, with a few genera represented by more than one species exemplar. Characters describe the cephalon (52%), hypostome (14%), thorax (4%) and pygidium (30%) and were coded using reductive coding. Apomorphic characters were included in order to include the broadest sampling of morphology possible. In order to quantify morphological disparity, the character matrix was transformed into a dissimilarity matrix using Gower’s coefficient and then subjected to a principal coordinates analysis. Preliminary data suggests that morphospace occupation was not affected by the end-Ordovician mass extinction. Instead much more substantial changes in disparity occurred during the late Silurian and early Devonian, in particular there was a greater decrease in range of morphology and greater shift in morphospace occupation during this time. These changes are coincident with the phylogenetic history of the clade: maximum parsimony and Bayesian inference both support a deep split between two subclades, with both subclades surviving the mass extinction but only one remaining diverse into the Devonian.