PANKO, L.J.: How to distinguish the basal Triassic cynodonts Galesaurus and Thrinaxodon from one another using rib morphology

A peculiar feature of certain non-mammalian cynodonts is their broad, overlapping ribs. Galesaurus and Thrinaxodon have broadened ribs in both thoracic and lumbar regions, though the details of morphology vary along the trunk. In the past, Galesaurus has been reconstructed as having the same pattern of rib morphology as occurs in the better-known genus Thrinaxodon. Cynodont specimens BP/1/3911 and BP/1/4506 at the Bernard Price Institute, South Africa, reveal features in the mid-dorsal and lumbar ribs of Galesaurus that clearly distinguish them from the ribs of Thrinaxodon. Mid-dorsal ribs of Galesaurus have a caudally-projecting lobe of bone approximately as long as it is wide, whereas mid-dorsal ribs of Thrinaxodon have a projecting shelf of bone that is transversely wider than long. The lumbar ribs of Galesaurus are expanded cranio-caudally, with a roughly oval shape in lateral view. The lumbar ribs of Thrinaxodon are less elongated cranio-caudally, with a roughly square shape in lateral view. The presence of distinct variation in rib morphology between these two genera indicates that among non-mammalian cynodonts, ribs may be of use in (1) identification of skeletons when the skull is incomplete or missing, (2) phylogenetic analyses, and (3) functional interpretation of axial skeleton evolution.