The Paleobiology of South African Therocephalian Therapsids (Amniota, Synapsida) and the Effects of the End-Permian Extinction on Size, Growth, and Bone Microstructure
Huttenlocker, Adam Keith
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Despite their relative diversity in terrestrial Permian and Triassic rocks, the fossil record of therocephalian therapsids (Eutheriodontia) and their utility for understanding evolutionary patterns in the therapsid forerunners of mammals remains poorly understood. In this study, I investigate the extent to which body size reductions and shifts in growth patterns in Triassic therocephalians were influenced by the end-Permian mass extinction (ca. 252.3 Ma) (rather than the culmination of longer-term phylogenetic trends traceable to their Permian predecessors). Specifically, I examine whether body size reductions observed in earliest Triassic therocephalians (`Lilliput phenomena') were the product of within-lineage size reductions, differential extinctions, or rapid diversifications of new small-bodied clades. To address this question, I first review the global diversity and taxonomic composition of therocephalians from the Middle Permian through early-Middle Triassic (Chapter 1). I then present a paleobiological investigation of the Permo-Triassic therocephalian Moschorhinus as a case study on within-lineage patterns of growth and body size evolution during the end-Permian mass extinction (Chapter 2). Finally, I examine clade-wide (among-taxon) size patterns by evaluating the stratigraphic and phylogenetic components of body size evolution (Chapter 3) and the underlying influences of bone histology and growth (Chapter 4). To examine within-lineage patterns, I studied cranial sizes and limb bone histology in Permian and Triassic specimens of Moschorhinus, the largest therapsid predator found both before and after the end-Permian mass extinction. Triassic specimens were found to have significantly decreased basal skull lengths compared to Permian specimens. Histological analysis indicated that variations in body size were associated with differences in subadult growth rate and duration (traits that are highly variable in environmentally stressed extant reptile species). Small Triassic individuals tended to display limb bones with fewer growth marks and more richly vascularized bone tissues than similarly sized Permian individuals, with an abundance of radially-oriented vascular canals, corroborating the hypothesis that conditions of the earliest Triassic favored rapid growth to a minimum body size requirement in Moschorhinus and, consequently, shortened developmental times. Broader-scale `Lilliput-type' patterns were examined in a large sample of therocephalians and compared with that of their sister clade, Cynodontia, in both geologic and phylogenetic contexts. Using a museum collections-based approach, I evaluated temporal and phylogenetic distributions of body size in Permo-Triassic eutheriodonts by time series analysis, rank order correlations, and phylogenetic model fitting. Results supported significant size reductions in earliest Triassic eutheriodonts, but suggested a pattern that was underscored largely by Brownian processes and constructive selectivity (a more general tendency to evolve smaller body sizes as in background intervals). Geologically brief size reductions were likely accomplished by the ecological removal of large-bodied species without rapid originations of new small-bodied clades or shifts from long-term evolutionary patterns. Finally, a survey of growth patterns and histomorphology in limb bones of Karoo therocephalians indicated that long-term changes in bone tissue vascularization (and thus growth) correlated with evolutionary changes in body size (e.g., smaller-bodied descendants tended to have less vascularized bone tissues than their larger-bodied ancestors). Results support a synergistic model of size reductions for Triassic therocephalians, influenced both by within-lineage heterochronic shifts in survivor taxa (e.g., Moschorhinus) and cladistically inferred survival of small-bodied taxa with short growth durations (e.g., baurioids). These findings mirror the multi-causal Lilliput patterns described in marine faunas, but contrast with skeletochronologic studies that suggest slowed, prolonged shell secretion in marine benthos. Subjecting new histologic data to phylogenetic comparative methods, as in these therocephalians, will improve our understanding of the generality of growth and size shifts in Lilliput faunas and interplay between macroevolution and extinction during this and other major geologic transitions.
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