Synchrotron scanning sheds new light on Lumkuia fuzzi (Therapsida, Cynodontia) from the Middle Triassic of South Africa and its phylogenetic placement

Abstract

Lumkuia fuzzi is a small non-mammalian cynodont from the Middle Triassic of South Africa. It has traditionally been phylogenetically identified as a basal Probainognathia, but some studies place it more basally as the sister-group to Eucynodontia. Lumkuia is known from a single specimen comprising a skull and partial skeleton. Only the skull has been thoroughly described. Here we use synchrotron X-ray computed tomography to describe the dental series and postcranial skeleton of Lumkuia. The shoulder girdle, forearm, manual phalanges, partial vertebral column, and ribs are described. To assess the effect of these new observations on the phylogenetic position of Lumkuia, the scoring of this taxon is updated and new characters are added to two previous analyses, one supporting Lumkuia as a Probainognathia and the other in which the taxon is usually retrieved as a basal Eucynodontia. Our phylogenetic analyses all unambiguously support the position of Lumkuia as a basal Probainognathia. The most commonly retrieved synapomorphies shared by Lumkuia and the probainognathians are cranial (absence of an ectopterygoid and pineal foramen), postcranial (the procoracoid does not contribute to the glenoid fossa), and palaeoneurological (reduction of the external nasal ramus of the maxillary canal, presence of a cochlear canal) characters. The degree of skeletal ossification and stages of dental replacement and wear support that the specimen is a subadult. The forelimbs were adapted to sprawling and occasional scratch-digging, thus suggesting a facultative fossorial ecology for Lumkuia.