Global Taxonomic Diversity of Anomodonts (Tetrapoda, Therapsida) and the Terrestrial Rock Record Across the Permian-Triassic Boundary

Abstract

The end-Permian biotic crisis (∼252.5 Ma) represents the most severe extinction event in Earth's history. This paper investigates diversity patterns in Anomodontia, an extinct group of therapsid synapsids (‘mammal-like reptiles’), through time and in particular across this event. As herbivores and the dominant terrestrial tetrapods of their time, anomodonts play a central role in assessing the impact of the end-Permian extinction on terrestrial ecosystems. Taxonomic diversity analysis reveals that anomodonts experienced three distinct phases of diversification interrupted by the same number of extinctions, i.e. an end-Guadalupian, an end-Permian, and a mid-Triassic extinction. A positive correlation between the number of taxa and the number of formations per time interval shows that anomodont diversity is biased by the Permian-Triassic terrestrial rock record. Normalized diversity curves indicate that anomodont richness continuously declines from the Middle Permian to the Late Triassic, but also reveals all three extinction events. Taxonomic rates (origination and extinction) indicate that the end-Guadalupian and end-Permian extinctions were driven by increased rates of extinction as well as low origination rates. However, this pattern is not evident at the final decline of anomodont diversity during the Middle Triassic. Therefore, it remains unclear whether the Middle Triassic extinction represents a gradual or abrupt event that is unique to anomodonts or more common among terrestrial tetrapods. The end-Permian extinction represents the most distinct event in terms of decline in anomodont richness and turnover rates.

Geographic range did not confer resilience to extinction in terrestrial vertebrates at the end-Triassic crisis

“…Although formation counts are regarded as effective sampling proxies for terrestrial data sets 35 , redundancy between sampling proxies and diversity metrics (arising from the probable non-independence of formation and fossil content) remains problematic [36][37][38] . In practice, the level of this redundancy is likely to be minimized by the use of all terrestrial vertebrate-bearing formations 39,40 , rather than by adopting a stricter count of only those formations containing a particular group of terrestrial vertebrate fossils 31,[41][42][43] . However, standardization of geographic range data results in the removal of significant correlations between range size and fossil abundance, coupled with a weakening of the correlation between range size and total range size (that is, standardizing geographic range calculations to a constant sample size across all lineages in each time bin appears to remove putative sampling effects).…”

The origin and early radiation of dinosaurs

“…The most common approach is to measure the rates of origination and extinction of lineages over time, which gives an overall rate of diversification (e.g., Ricklefs, 2007;Fröbisch, 2008;Ruta and Benton, 2008). Other rate metrics quantify the rate of molecular sequence change (e.g., Mindell and Thacker, 1996;Lavin et al, 2005) or various aspects of continuous phenotypic change, such as body size (e.g., Garland, 1992;Gingerich, 1993;Pagel, 1998;Roopnarine, 2003;O'Meara et al, 2006).…”

Vegetation history across the Permian–Triassic boundary in Pakistan (Amb section, Salt Range)

“…therapsids ("mammal-like reptiles") were severely affected by the Permian-Triassic extinction (Kemp, 2005). Several lineages became extinct; others such as the dicynodonts were reduced and recovered in the Middle Triassic (Benton et al, 2004;Ward et al, 2005;Fröbisch, 2008). In contrast, skull morphology of the Cynodontia shows no significant change across the Permian-Triassic boundary, but changes significantly only in the late Olenekian-Anisian (Abdala and Ribeiro, 2010).…”