Biogeographic and Phylogenic Patterns of Trilobites during the End Ordovician Mass Extinction

The end Ordovician mass extinction event was caused by a brief, sudden onset glacial period that interrupted the greenhouse conditions of the Late Ordovician. Trilobites exhibit an interesting pattern of survivability during the extinction; groups that have an inferred benthic larval stage were least affected, while groups with an inferred planktonic larval stage were more strongly affected. To investigate this pattern in a paleobiogeographic context we used PAUP 4.0 to generate cladograms for two phacopine trilobite groups that survived the extinction, the Homalonotidae (benthic larvae) and a group of cheirurids, the Deiphoninae (planktonic larvae). The homalonotid analysis considered fifteen ingroup species. The deiphonine anlaysis considered eighteen ingroup species, including several representatives of Sphaerocoryphe and Deiphon. Cladograms were well resolved, and indicated that several genera as traditionally defined were paraphyletic. Cladograms were used, in conjunction with a modified Brooks Parsimony analysis, to study biogeographic patterns. The analysis suggests the homalonotids originated in Gondwana and later geodispersed to Laurentia. Homalonotid subfamilies were endemic to specific landmasses, suggesting that these trilobites did not widely disperse. Gondwanan homalonotids went extinct at or before the glaciation, and forms derived from Laurentian/Avalonian faunas repopulated Gondwana in the early Silurian. The Deiphoninae originated either in Bohemia, Armorica, or Xinjiang. The group geodispersed to Laurentia, then to Baltica and Avalonia. Before the upper Caradoc, a long distance dispersal event occurred between northwestern Laurentia and Australia. The Deiphoninae appear to have been very widespread, which may be due to their planktonic larval stage or the close proximity of late Ordovician landmasses. Both groups diversified in Avalonia and Laurentia; landmasses separate from Gondwana with an equatorial paleolatitude. Perhaps geodispersal to Laurentia-Avalonia before the extinction contributed to these groups' survival. The equatorial region would have been buffered from the sudden cooling caused by the forming continental glaciers in Gondwana.