DEATH DEFYING MORPHOLOGIES: EXTINCTION IN AND DISPARITY IN THE ORDER HARPETIDA

This study seeks to better understand phylogenetic relationships within the trilobite order Harpetida, with an eventual view towards using this group to explore the relationship between extinction intensity and disparity, particularly in the harpetid response to the end-Ordovician mass extinction.

A discrete morphological character matrix was created from the formal descriptions of harpetids in the published trilobite literature, and refined using first-hand observations of harpetid fossils. The finished matrix consists of 100 discrete characters, including 84 cephalic characters, 7 thoracic characters, and 9 pygidial characters. This matrix is the first to attempt of its kind to characterize the morphology of Harpetida as a whole, rather than focusing on individual harpetid genera.

Exemplar species from each harpetid genus, along with a ptychopariid outgroup, are included in the matrix, coded according to published figures and direct observation of specimens held in the collections of the Peabody Museum. The matrix was used to generate a hypothetical tree of harpetid phylogenetic relationships, the topology of which indicates support for harpetid monophyly but throws doubt on the previous hypotheses of the internal relationships of the group. It is important to establish an accurate phylogeny of harpetid trilobites if we are to draw conclusions about the evolutionary history of this group.