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Vertebrate Evolution during the Permian and Triassic
Origin of Mammals
The synapsid lineage includes modern mammals and all of their extinct relatives. This lineage is remarkably ancient, dating minimally to the Middle Pennsylvanian (~300 Ma). The earliest synapsids (such as the familiar sailback, Dimetrodon) were "reptilian" in overall cast (e.g., ectothermic physiology, simple teeth, sprawling posture, etc.), but within 100 million years nearly all of the anatomical features that characterize mammals were in place. In what sequence were these mammalian features evolved? What was the rate at which these characters were acquired? Were these features evolved once or independently in multiple groups? The bulk of my research has been devoted to gathering detailed anatomical and phylogenetic data to address these questions in a quantitative framework.
Paleobiogeography of Pangea
The largest mass extinction in Earth history occurred at the end of the Permian, roughly 251 Ma. Although the cause of this catastrophe has been difficult to pin down, its effects on terrestrial ecosystems are clearly seen in the fossil record. To date, most of the data on extinction and survivorship for this critical interval come from the Karoo Basin of South Africa. However, this restricted geographic and sedimentary area constrains the degree to which broader statements can be made about global patterns of tetrapod evolution. For this reason, I have been actively involved in fieldwork that expands the geographic coverage of Permian and Triassic localities. Recent exploration in Antarctica, Tanzania, Zambia, and Niger have produced fossils that change the way paleontologists view Pangean faunas.