Phylogenetic and Biogeographic Analysis of Ordovician Homalonotid Trilobites

Abstract

Cladistic parsimony analysis of the trilobite family Homalonotidae Chapman 1980 produced a hypothesis of re- latedness 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 Col- pocoryphe Novak 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 particu- lar, 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 dur- ing 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.