SECRETS OF THE HARPETIDS: PHYLOGENY, DISPARITY, AND MASS EXTINCTION RESPONSE IN THE TRILOBITE ORDER HARPETIDA

This study seeks to better understand the phylogenetic relationships within the trilobite order Harpetida, with a view towards using this group to explore the relationship between extinction intensity and disparity. The harpetid response to the Late Ordovician mass extinction is of particular interest. 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 final matrix consists of 76 discrete characters, including 69 cephalic characters, three thoracic characters, and four pygidial characters. This matrix is the first attempt of its kind to characterize the morphology of Harpetida as a whole, rather than focusing on individual harpetid genera.

Exemplar species from a broad selection of harpetid genera, along with ptychopariid and redlichiid out groups, are included in the matrix. These taxa were coded from published figures and from direct observation of specimens held in the collections of the Yale Peabody Museum of Natural History. From the matrix, a hypothetical tree of harpetid phylogenetic relationships was generated. The topology of this tree indicates support for harpetid monophyly but throws doubt onto the previous hypotheses of the internal relationships of the group. Disparity analysis of Harpetida reveals a significant decline in morphological diversity across the Late Ordovician mass extinction boundary, with slow recovery beginning in the Silurian and continuing into the Devonian.