EVOLUTIONARY AND BIOGEOGRAPHIC PATTERNS OF TRILOBITES DURING THE END ORDOVICIAN MASS EXTINCTION EVENT

The end Ordovician mass extinction event is believed to have been caused by a geologically brief, sudden onset glacial period that interrupted a period of extreme greenhouse conditions. The cause of this icehouse is a matter of contention, but recent a recent work proposes that a nearby gamma-ray burst could have affected the Earth's atmospheric chemistry and pushed the climate from a greenhouse into an unstable icehouse. Survivorship patterns of trilobites and their larval forms appear to agree with this theory. In order to further explore the Ordovician extinction, I conducted three individual paleontological studies to test macroevolutionary and biogeographic patterns of trilobites across the extinction. The first study is a phylogenic and biogeographic analysis of the family Homalonotidae Chapman 1890, the second is a similar analysis of the subfamily Deiphoninae Reed 1913, and the third is a GIS study of species ranges of the subfamily Deiphoninae

Evolutionary Patterns of Trilobites Across the End Ordovician Mass Extinction

The end Ordovician mass extinction is the second largest extinction event in the history or life and it is classically interpreted as being caused by a sudden and unstable icehouse during otherwise greenhouse conditions. The extinction occurred in two pulses, with a brief rise of a recovery fauna (Hirnantia fauna) between pulses. The extinction patterns of trilobites are studied in this thesis in order to better understand selectivity of the extinction event, as well to understand the effect of the extinction of the evolution of various trilobite groups. To study these patterns two phylogenetic analyses of sphaerexochines and ceraurids were generated and the overall data was combined with two older analyses of homalonotids and deiphonines from my Masters Thesis. Speciation and extinction rates were estimated in the deiphonines and sphaerexochines (two closely related cheirurid clades) to understand the different patterns of extinction and survivorship in greater detail. Then, utilizing the phylogenetic analyses, the end Ordovician is reinterpreted as a large scale analog of Vrba's Relay Model, referred to herein as the Cladal Turnover Model