The latest-Permian mass extinction was the greatest biotic crisis of the Phanerozoic. The extinction decimated both marine and terrestrial communities, and changed the evolutionary trajectory of multicellular life on the planet. The unique nature of the extinction’s aftermath has prompted attention from paleontologists seeking to understand the timing and pattern of the Triassic recovery. With this dissertation I have sought to shed additional light on the terrestrial side of the extinction by examining different patterns by which its survivors responded to the extinction.
Temnospondyl amphibians were one of the few tetrapod clades that were able to take advantage of the extinction to expand their diversity. In Chapter 1 I examine the relationship between taxonomic and ecological diversity of temnospondyls across the Permian-Triassic (P-Tr) boundary in the Karoo Basin of South Africa. Ecomorphological diversity, as implied by differences in cranial shape, was incorporated into the study by the use of a landmark-based geometric morphometric analysis. Both taxonomic diversity and cranial disparity were low during the Permian and increased across the Permian-Triassic boundary. Taxonomic diversity was stable through the Triassic, but disparity showed subsequent increases during the Olenekian and Anisian. Temnospondyls were restricted in size immediately following the extinction, but size range fully rebounded by the Olenekian. Tests of phylogenetic signal demonstrate that cranial shape was heavily influenced by phylogenetic relatedness, and the observed increases in disparity may be partly the result of decreases in the net relatedness of coeval Karoo stereospondylomorph temnospondyls in younger faunas. The increase in community-level taxonomic diversity for temnospondyls in the Karoo following the latest-Permian mass extinction was likely facilitated by an influx of distantly related and ecologically distinct species from other parts of Pangea.
In Chapter 2, I discuss the merits of different potential methods for quantifying rates of dispersal within clades. I then apply some of these methods to two very different scenarios, first the dispersal of crocodylians across oceanic barriers during the Late Cretaceous and Cenozoic, and second the dispersal of different groups of tetrapods across Pangea during the Permo-Triassic interval. For crocodylians, because they were dispersing across substantial geographic obstacles, I opted for a direct measurement approach utilizing the optimization of discrete dispersals onto phylogenies. I examined the history of crocodylian biogeography using both parsimony and maximum likelihood on three distinct topologies with several different methods for estimating branch lengths. Across all analyses, members of the clade Alligatoroidea consistently dispersed across oceanic barriers less frequently than did non-alligatoroids. This is consistent with the hypothesis that the greater degree of salt tolerance observed in extant crocodyloids and gavialoids played a role in shaping crocodylian biogeography. The phylogenetic and temporal distribution of high dispersal rates points to an acquisition of greater salt tolerance early in the history of Crocodyloidea and Gavialoidea, potentially near the base of Longirostres if the combined evidence topology is correct. Patterns observed for changes in dispersal rate within individual clades can be largely attributed to changes in global climate and continental configuration over their history.
The greater geographic ambiguity represented by the Permo-Triassic continental configuration makes a direct measurement approach inappropriate. For this study I instead opted for a proxy measurement approach, using the phylogenetic clustering of taxa within a community, measured using the Net Relatedness Index. I examined temporal changes in the phylogenetic clustering of five major tetrapod clades that span the Permian-Triassic boundary (Stereospondylomorpha, Parareptilia, Neodiapsida, Anomodontia, and Eutheriodontia) in order to examine patterns of extinction and origination through time, as well as rates of geographic dispersal. Some clades (Stereospondylomorpha, Parareptilia, and Neodiapsida) show evidence of phylogenetically selective extinction across the boundary, but this is not a universal pattern. Only one clade, Stereospondylomorpha, shows an unambiguous increase in dispersal rate following the mass extinction event. Other clades either show no change in dispersal rate, or have results that are mixed, depending on the parameters used in the analysis. These results show that stereospondylomorph temnospondyls were dispersing between geographical regions at increased rates during the Early Triassic, and this may explain much of their apparent increase in diversity following the latest-Permian mass extinction.
In Chapter 3, I perform a comparison between the timing of the Triassic recovery with that following the Cretaceous-Paleogene (K-Pg) mass extinction. Three terrestrial fossil-bearing successions were examined, the Lower Triassic Beaufort Group in South Africa and Cis-Ural succession in Russia, and the Paleocene faunas of the American northern Great Plains. A comparison of generic diversity of tetrapods through time for the post-extinction intervals reveals a temporal disparity between the length of terrestrial recovery after the latest-Permian and K-Pg extinctions. Both Permo-Triassic successions show a period of low taxonomic richness (4-5Myr) after the extinction event, followed by an eventual rise in richness. The North American K-Pg succession shows a different pattern, with an immediate rise in richness culminating in a plateau shortly after the extinction (1-3 Myr). This disparity in recovery times may result from prolonged deleterious environmental conditions following the P-Tr events, although several important differences exist between these sequential fossil assemblages that may be affecting the apparent speed of recovery.