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Body size and growth patterns in the therocephalian Moschorhinus kitchingi (Therapsida: Eutheriodontia) before and after the end-Permian extinction in South Africa

Published online by Cambridge University Press:  01 March 2013

Adam K. Huttenlocker  and
Jennifer Botha-Brink
Adam K. Huttenlocker
Affiliation:
Department of Biology, University of Washington, Seattle, Washington 98195, U.S.A. E-mail: huttenla@u.washington.edu. Corresponding author
Jennifer Botha-Brink
Affiliation:
Department of Karoo Palaeontology, National Museum and Department of Zoology and Entomology, University of the Free State, Bloemfontein 9300, South Africa. E-mail: jbotha@nasmus.co.za
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Abstract

The continuous fossil record of therocephalian therapsids (Eutheriodontia) across the Permo-Triassic boundary and their differential survivorship of the end-Permian extinction offer an exceptional deep-time perspective on vertebrate life-history evolution during episodes of large-scale ecological perturbation. To examine potential impacts of the extinction on body size evolution (e.g., “Lilliput” effects) and growth patterns, we investigated cranial sizes and limb bone histology in the therocephalian Moschorhinus kitchingi both before and after the end-Permian extinction, facilitated by analysis of thin-sections of 23 limb bones from an ontogenetic sample of ten individuals across the Permo-Triassic boundary. In general, early subadult Moschorhinus displayed propodial cortices with extensive woven- and parallel-fibered bone (PFB) with dense radial and reticular vascularization and a moderately thickened bone wall with few growth marks. The outer cortex of propodials and epipodials showed a transition to PFB and lamellar bone with longitudinally oriented canals in individuals interpreted as late subadults or adults (>80% largest size). Most elements displayed several (3+) growth marks, though growth marks were more faithfully recorded in the epipodials of Permian individuals. Pearson product-moment correlation tests were performed to examine the relationship between size and robusticity on growth proxies (% cortical vascularity, mean primary osteon diameter), but variation in histomorphology could not be explained by size alone. Variation in body size may be affected by differences in juvenile growth rate and duration, which are highly variable in environmentally stressed extant reptile species. Geologic stage was a more consistent predictor of cortical vascularity. We suggest that Permian and Triassic Moschorhinus exhibited differential rates of early skeletal growth, corroborating the hypothesis that increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and, consequently, shortened developmental times.

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Articles
Information
Paleobiology , Volume 39 , Issue 2 , Spring 2013 , pp. 253 - 277
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Copyright © The Paleontological Society 

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