Phylogenetic and Biogeographic Analysis of Ordovician Homalonotid Trilobites

Cladistic parsimony analysis of the trilobite family Homalonotidae Chapman 1980 produced a hypothesis of relatedness for the group. The family consists of three monophyletic subfamilies, one containing Trimerus Green 1832, Platycoryphe Foerste 1919, and Brongniartella Reed 1918; one containing Plaesiacomia Hawle and Corda 1847 and Colpocoryphe Novák in Perer 1918; and one containing Eohomalonotus Reed 1918 and Calymenella Bergeron 1890. All genera are monophyletic, except Brongniartella, which is paraphyletic; as it was originally defined it “gives rise” to Trimerus and Platycoryphe. A modified Brooks Parsimony Analysis using the phylogentic hypothesis illuminates patterns of biogeography, in particular, vicariance and geodispersal of homalonotids, during the late Ordovician. The analysis yields three major conclusions about homalonotid biogeography: homalonotids originated in Gondwana; Avalonia and Laurentia were close enough during the late Ordovician to exchange taxa, especially when sea level rose sufficiently; and long distance dispersal events occurred between Armorica and Florida, and also between Arabia and a joined Laurentia-Avalonia.