Thrinaxodon

Thrinaxodon is a genus of extinct cynodonts known from the Early Triassic of South Africa and Antarctica. Only one species is recognized, *Thrinaxodon liorhinus*. It is common in the fossil record and holds the distinction of being the oldest cynodont with clear evidence of burrowing. Its posture was semi-sprawling, a middle ground between the fully sprawled limbs of early tetrapods and the upright stance seen in modern mammals.

About the size of a fox, this small synapsid was likely an omnivore, based on skull shape analysis. Its hearing relied on a tympanic membrane, much like that of living mammals, as shown by studies of its ear bones and soft-tissue eardrum using finite element models.

The snout bones (nasals) are covered in many small pits (foramina). They narrow toward the front, widen farther forward, and connect directly to the frontals, prefrontals, and lacrimals, but not to the jugals or eye sockets. The upper jaw bone (maxilla) is also heavily pitted. The pattern of these pits resembles that of lizards like *Tupinambis*, and there is a single large infraorbital foramen. This suggests *Thrinaxodon* had lizard-like, non-muscular lips rather than the flexible, muscular lips of mammals. Without that foramen and the facial mobility it provides, it likely lacked whiskers.

On the skull roof, the suture between the frontals and nasals forms an arrow shape, unlike the simpler transverse line in more primitive skulls. The small prefrontals sit slightly forward and below the frontals, touching the postorbitals, frontals, nasals, and lacrimals. Farther back, the parietals lack a sagittal crest. The skull roof is narrowest just behind the nearly circular parietal foramen. The temporal crests stay separate along the skull’s length, and the temporal fenestrae contain ossified fasciae, indicating some kind of temporal muscle attachment.

The upper jaw has a secondary palate that separates the nasal passages from the mouth, letting *Thrinaxodon* breathe while holding food in its mouth. This adaptation allowed it to chew food more thoroughly, speeding digestion. The maxillae and palatines meet along the midline of the upper jaw, forming a suture that includes a posterior palatine foramen. The vomer makes up a large part of the palate roof, just above the choana (internal nostril). The pterygoid bones extend in the upper jaw and enclose small interpterygoid openings on each side of the parasphenoid’s cultriform processes. The parasphenoid and basisphenoid are fused, except at the very front and top where a slight gap remains in the trabecular attachment of the basisphenoid.

The ear region is defined by areas around the temporal fenestrae. A notable deep recess sits just in front of the fenestra ovalis, showing evidence of smooth muscle contact with the skull. This contact is interpreted as marking the tympanum, suggesting that this recess and the fenestra ovalis together outline the origin of the ear in *Thrinaxodon*. This feature is a new synapomorphy that arose in this genus and was retained in later cynodonts. The stapes had a heavy cartilage plug that fit into the sides of the fenestra ovalis, but only half of its articular end covered that opening. The rest of the pit opens into an unossified region near the cochlear recess, implying that inner ear articulation happened directly there.

The skull of *Thrinaxodon* is an important transitional fossil, showing a reduction in the number of skull bones over time. The most dramatic drop in bone count occurred between *Thrinaxodon* and *Probainognathus*—a change so large that the fossil record for that step is likely incomplete. Still, *Thrinaxodon* has fewer skull bones than its pelycosaur ancestors.

Dental data for *T. liorhinus* come from micro CT scans of many skulls ranging in length. The tooth patterns resemble those of *Morganucodon*, suggesting these patterns originated within Thrinaxodontidae and continued into early mammals. Adult *T. liorhinus* has four incisors, one canine, and six postcanines on each side of the upper jaw. The lower jaw has three incisors, one canine, and seven or eight postcanines per side. This gives a total of 44 to 46 teeth in adults. The upper incisors curve backward, are pointed at the tip, and become broader and rounder near their base in the premaxilla. The fourth upper incisor looks like a small canine in shape but sits too far forward to function as one.