Bone microstructure of the basal anomodont Suminia getmanovi supports its arboreal lifestyle

The paleohistology of Permo-Triassic anomodonts has been extensively studied and, independent of phylogeny, body size and lifestyle, reflects a pattern of rapid growth indicated by a woven-parallel complex. Moreover, anomodonts uniformly show a relative bone cortical thickness (RBT) exceeding 30% and a medullary cavity generally filled by trabeculae. Here, we investigate the paleohistology of the basal anomodont Suminia getmanovi from the Permian of Russia, which has been hypothesized as one of the earliest arboreal tetrapods. Osteohistology and skeletal proportions reveal that our sample comprises at least two late juvenile to early subadult individuals, exhibiting well-vascularized and mostly uninterrupted woven-parallel complex or parallel-fibered tissues, suggesting relatively high growth rates, consistent with other anomodonts. However, all elements of Suminia present an open medullary cavity virtually free of bony trabeculae and a RBT lower than 18%. The microanatomy of Suminia thus differs from all other anomodonts studied so far, including its closest relative Galeops, as well as more basal synapsids that also tend to show higher RBT values and/or a medullary territory obstructed by trabeculae. Compared to extant climbers, which possess thinner bone walls and lower compactness than their terrestrial and aquatic relatives, the bone architecture of Suminia further supports its arboreal lifestyle.