BIOGEOGRAPHIC AND PHYLOGENETIC PATTERNS OF TWO GROUPS OF CHEIRURID TRILOBITES DURING THE END ORDOVICIAN MASS EXTINCTION

The end Ordovician extinction was caused by a brief, sudden onset glaciation that occurred during otherwise greenhouse conditions. Although the cause of the extinction is generally agreed upon, the effect that individual biogeographic patterns of specific clades may have played in clade survival during the extinction has not been fully investigated. To this end, we used PAUP* 4.0 to generate phylogenies for two cheirurid trilobite groups that survive the extinction event, the Deiphoninae and Sphaerexochinae. The deiphonine analysis is a revision of a previous analysis and contains twenty-one ingroup taxa classically assigned to the genera Deiphon, Sphaerocoryphe, Onycopyge, and Hemisphaerocoryphe. The sphaerexochine analysis includes taxa classically assigned to the genera Sphaexochus and Kawina. A recent review of Baltic sphaerexochine material suggests changes to the current taxonomy of several species within the group; particularly the removal of Ramsköld's form B. Sphaerexochus latifrons pygidium because this second form is likely the result of effacement due to taphonomic processes. Cladograms were used, in conjunction with modified Brooks Parsimony Analysis, to investigate biogeographic patterns. The revised deiphonine analysis suggests that the group likely originated in Baltica and Laurentia, two areas close enough in the late Ordovician to exchange taxa, potentially via sea level rise and fall. Also, long-distance dispersal occurred between Northwest Laurentia and Australia, with Australia possibly acting as a refugia for the group during the extinction event. The sphaerexochine analysis contains taxa from all over the world. The genus Sphaerexochus is of particular interest for studying the role of biogeography during the extinction because the genus is widely distributed and persists from the middle Ordovician into the middle Silurian; the group's large biogeographic range may have facilitated its survival. Also, the extinction may have caused an evolutionary bottleneck within Sphaerexochus because the cephalic morphologies of Silurian species are very conserved, whereas Ordovician species have varied cephalic morphologies.