Vertebrate patterns of taxonomic and ecological diversity and recovery from the End-Permian Mass Extinction: two novel test cases from southern Pangea

Abstract

Mass extinctions are among the most important events in the history of life on Earth and understanding their effects on life and the structure of biotic systems is restricted to the fossil record. Comparatively few studies have investigated the effects of mass extinctions on terrestrial ecosystems, where the preservation of fossils is sparser than in the seas. The End-Permian Mass Extinction is the largest mass extinction in Earth history, wiping out approximately 80% of marine species, with a comparable effect on land. For tetrapod vertebrates the most complete record of their evolution from the middle Permian, across the Permo-Triassic mass extinction boundary and into the Triassic comes primarily from a single place: the Karoo Basin of South Africa. Therefore, what is known about the development and collapse of complex tetrapod ecosystems in the Permian, and the subsequent rebuilding of new systems in the wake of the mass extinction, is geographically restricted. This Dissertation addresses relatively understudied records of tetrapod evolution and adds geographic complexity to our understanding of mass extinction and recovery. Chapter 1 describes the vertebrate fauna of the Middle Triassic Ntawere Formation of Zambia. The vertebrate assemblage of the Ntawere is one of a handful that can be tied to the Karoo biostratigraphically, and therefore acts as a comparison point in the post-extinction world. I bring together the historical finds of the Ntawere as well as add a number of new occurrences found during field expeditions in 2009, 2011 and 2014, including some of the earliest crocodile and dinosaur-line archosaurs. I recognize a lower and an upper assemblage within the Ntawere. Using new faunal lists I suggest a new biostratigraphic scheme centered on dicynodont and cynodont therapsid taxa shared by fossil assemblages across southern Africa and South America, but to the exclusion of the Karoo Basin. In Chapters 2 and 3 I use a synthesized database of all tetrapod fossils discovered from middle Permian through Middle Triassic rocks in three basins in Zambia and Tanzania to understand the geographic similarity of tetrapod assemblages across the mass extinction boundary. In terms of taxonomic richness, evenness, the relative abundances of higher clades (e.g., Dicynodontia), ecological guild richness, and the relative abundances of ecological guilds I find support for a largely homogeneous fauna spread across southern Pangea during the late Permian. This fauna is fragmented during the mass extinction event and by the Middle Triassic there are high levels of dissimilarity across southern Pangea. In terms of recovery from the mass extinction I find no support for a taxonomic return to Permian levels, but I find variable signals for an ecological recovery. However, the composition of Middle Triassic ecosystems differs significantly from the late Permian, indicating that though a new stability may have been reached, ecosystem structure was itself fundamentally reset by the mass extinction event.