The postcranial anatomy of Kembawacela kitchingi (Therapsida, Anomodontia) and the functional diversity of cistecephalid forelimbs

Cistecephalid dicynodonts were one of the first amniote clades to evolve a highly specialized ecomorphology. They share a suite of postcranial characteristics indicative of fossorial lifestyle, reminiscent of modern subterranean mammals. These characters include a tall scapulocoracoid with well-defined glenoid; a robust humerus with expanded processes, significant long-axis, diaphyseal torsion, and well-developed humeral head; an elongate olecranon process of the ulna; broad manus with spatulate unguals; and interlocking vertical zygapophyses in the vertebral column. Yet within this broad morphotype, cistecephalids exhibit distinct variation. Here we describe the postcranial morphology of a specimen of Kembawacela kitchingi from the Luangwa Basin, Zambia, which highlights this variation compared with other cistecephalids. Kembawacela possesses a less specialized version of the cistecephalid body plan, with a dorsoventrally shorter scapulocoracoid; a humerus with less long-axis torsion, a less defined humeral head, and a shorter deltopectoral crest; and a shorter olecranon process of the ulna. However, Kembawacela also exhibits features not shared by other cistecephalids, including a distinctive pinna-like process on the posterior border of the humeral shaft and a prominent facet on the trochlea of the ulna for the capitulum of the humerus. A morphometric analysis of forelimb proportions in 26 extant fossorial amniotes and seven anomodonts underscores the similarity of cistecephalids to extant fossors, but also the considerable variation among cistecephalids. Our results are consistent with a hypothesis of mosaic adaptation to a substrate-moving, fossorial lifestyle in cistecephalids, making the clade a particularly interesting case study of the evolution of extreme ecomorphotypes in early amniotes.