Pathology in a Permian parareptile: congenital malformation of sacral vertebrae

Abstract

AbstractPareiasaurs were a group of herbivorous reptiles that lived during the middle to late Permian (265–252 Ma). About 20 species are currently recognized from a fossil record spanning Europe, Asia, South America, and Africa. All pareiasaurs have a peculiar vertebral anatomy characterized by ‘swollen’ neural arches. Some species grew to large body size (e.g., 600 kg) and evolved up to five sacral vertebrae, but the detailed anatomy of the sacral region is poorly documented. Vertebral elements referable to the derived pareiasaur subclade Velosauria were recently recovered from the upper Madumabisa Mudstone Formation (Luangwa Basin) of Zambia and are described here. The specimen exhibits an abnormal morphology: the neural arches of the second, third, and fourth sacral vertebrae share a single, elongate centrum that is fused anteriorly with a ‘normal’ first sacral vertebra, and articulates posteriorly with a slightly disarticulated first caudal vertebra. We suggest two distinct pathologies are responsible for this morphology: (1) spondyloarthropathy (re‐ossified intervertebral joint) and (2) congenital block vertebra. The processes of development of neural arches and centra, while evolutionarily independent in origin, have been found to be conserved in all amniotes. While paleopathological morphologies can provide insight into environmental and disease‐based etiologies, pathological congenital malformations are particularly rare in fossil record and have the potential to be informative for understanding the evolution of vertebral development. The anatomy described here sheds light on pareiasaurian sacral morphology and provides insight into the pathology in one of the earliest clades of large‐bodied reptiles near the base of the amniote tree.