Bone histology of varanopids (Synapsida) from Richards Spur, Oklahoma, sheds light on growth patterns and lifestyle in early terrestrial colonizers.

Varanopids were a group of small to medium-sized synapsids whose fossil record spans the Carboniferous through middle Permian. Although their phylogenetic relationships have received some interest in recent years, little is known about other aspects of their palaeobiology, including their skeletal growth, allometry and habitat preference. Here, we describe varanopid long bone histology based on a sample of well-preserved femora from the lower Permian Richards Spur fissure fill locality, Comanche County, Oklahoma, USA. The sample includes five femora from at least two varanopid taxa-Mycterosaurus and the large varanodontine Varanops brevirostris-and four additional mycterosaurine femora not diagnosed to genus. Prior work on femoral bone compactness provided a baseline to make lifestyle inferences and evaluate whether varanopids were ancestrally terrestrial. Moreover, the large availability of specimens spanning different sizes made possible an assessment of size-related ontogenetic histovariability. All specimens revealed moderately dense cortical bone tissues composed of sparsely vascularized parallel-fibred and lamellar bone with radially arranged rows of longitudinal canals (mostly simple), and many preserved regularly spaced growth marks (annuli and lines of arrested growth) as in modern varanids. We show that bone histology has the potential to explain how ballast was shed and the skeleton lightened for terrestrial mobility in ancestral synapsids and their basal amniote kin, as well as how adjustments in postnatal growth influenced the evolution of larger body sizes in the terrestrial frontier. This article is part of the theme issue 'Vertebrate palaeophysiology'.