Bone Microstructure of Pareiasaurs (Parareptilia) from the Karoo Basin, South Africa: Implications for Growth Strategies and Lifestyle Habits

Abstract

Numerous morphological studies have been carried out on pareiasaurs; yet their taxonomy and biology remain incompletely understood. Earlier works have suggested that these herbivorous parareptiles had a short juvenile period as compared to the duration of adulthood. Several studies further suggested an (semi-) aquatic lifestyle for these animals, but more recent investigations have proposed a rather terrestrial habitat. Bone paleohistology is regarded as a powerful tool to assess aspects of tetrapod paleobiology, but few studies have been conducted on pareiasaurs. The present study assesses intra and inter-specific histovariability of pareiasaurs and provides fresh insights into their paleobiology, thereby permitting a re-evaluation of earlier hypotheses. Our sample comprises various skeletal elements and several specimens covering most of the taxonomic and stratigraphic spectrum of South African pareiasaurs, including large and basal forms from the Middle Permian, as well as smaller and more derived forms from the Late Permian. Our results concerning size of elements and histological tissues show that for pareiasaurs, element size is not a good indicator of ontogenetic age, and furthermore, suggest that the specific diversity of the Middle Permian pareiasaurs may have been underestimated. The bone histology of these animals shows that they experienced a relatively rapid growth early in ontogeny. Periosteal growth later slowed down, but seems to have been protracted for several years during adulthood. Pareiasaur bone microanatomy is unusual for continental tetrapods, in having spongious stylopod diaphyses and thin compact cortices. Rigorous paleoecological interpretations are thus limited since no modern analogue exists for these animals. Anat Rec, 300:1039–1066, 2017. © 2016 Wiley Periodicals, Inc.