101-6 Size disparity among Karoo Basin tetrapods tracks synchronous basin-wide environmental change before and after the Permo-Triassic mass extinction in residents, but not in immigrant taxa

Abstract:

Recent geochronological research in South Africa’s Karoo Basin has clarified the timing of tetrapod turnover during the Permo-Triassic mass extinction (ca. 251.9 Ma; PTME), providing a framework for understanding the extinction’s effects on terrestrial diversity and community structure. Prior analyses of body size disparity in Karoo tetrapods have demonstrated Triassic “Lilliput” phenomena–i.e., post-extinction body size reductions that have been attributed to a variety of causes–but only in a limited number of taxa (e.g., therocephalians and Lystrosaurus) and lacking detailed geospatial information. We investigated patterns of body size disparity across the PTME from a sample of 863 specimens collected from two major study sites in the vicinity of Nieu Bethesda and Bethulie and addressed the following questions: 1) Did large-bodied taxa have shorter lineage durations (and, thus, were more prone to extinction) than smaller taxa during “background” intervals? 2) Did body size disparity in Karoo tetrapods decrease with secular climate drying and warming synchronously throughout the basin? 3) How did extinction responses vary between resident and immigrant taxa? We show that lineage durations are random with respect to size and display no size-selectivity before the extinction. However, there was a slight decrease in size disparity in the Palingkloof Member with the greatest reduction occurring across the PTME at both study sites. Size disparity across the basin remained low during Triassic hyperthermal conditions and did not return to pre-extinction levels until Cynognathus Assemblage Zone times, at least 2.6 million years after the extinction. Logistic regressions indicate that large-bodied taxa had a higher probability of extinction during the PTME than small-bodied taxa. However, residents were most affected by reduced disparity, whereas immigrant taxa filled vacant niche spaces such as the large predatory gorgonopsian Inostrancevia and the archosauriform Proterosuchus in the latest Permian and earliest Triassic, respectively. These spatial patterns–particularly the roles of residents and immigrants–mirror other terrestrial mass extinctions, such as North American K-Pg mammal communities, and shed light on the “rules” of extinction and recovery during similar major biodiversity crises.

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