Thrinaxodon is an extinct genus of cynodonts which lived in what are now South Africa and Antarctica during the Early Triassic. The genus contains a single species, T. liorhinus. Similar to other therapsids, Thrinaxodon adopted a semi-sprawling posture, an intermediary form between the sprawling position of basal tetrapods and the more upright posture present in current mammals. Thrinaxodon is prevalent in the fossil record, and one of the specimens represent the oldest known record of burrowing behavior among cynodonts.
Description Thrinaxodon was a small synapsid roughly the size of a fox. Allometric study of its skull suggests that Thrinaxodon was an omnivore. It was capable of and reliant on tympanic hearing similar to hearing of extant mammals, as evidenced by the examination of its mandibular ear bones and soft-tissue eardrum based on finite element analyses.
Skull The nasals of Thrinaxodon are pitted with a large number of foramina. The nasals narrow anteriorly and expand anteriorly and articulate directly with the frontals, pre-frontals and lacrimals; however, there is no interaction with the jugals or the orbitals. The maxilla of Thrinaxodon is also heavily pitted with foramina. The arrangement of foramina on the snout of Thrinaxodon resembles that of lizards, such as Tupinambis, and also bears a single large infraorbital foramen. As such, Thrinaxodon would have had non-muscular lips like those of lizards, not mobile, muscular ones like those of mammals. Without the infraorbital foramen and its associated facial flexibility, it is unlikely that Thrinaxodon would have had whiskers. On the skull roof of Thrinaxodon, the fronto-nasal suture represents an arrow shape instead of the general transverse process seen in more primitive skull morphologies. The prefrontals, which are slightly anterior and ventral to the frontals exhibit a very small size and come in contact with the post-orbitals, frontals, nasals and lacrimals. More posteriorly on the skull, the parietals lack a sagittal crest. The cranial roof is the narrowest just posterior to the parietal foramen, which is very nearly circular in shape. The temporal crests remain quite discrete throughout the length of the skull. The temporal fenestra have been found with ossified fasciae, giving evidence of some type of a temporal muscle attachment. The upper jaw contains a secondary palate which separates the nasal passages from the rest of the mouth, which would have given Thrinaxodon the ability to breathe uninterrupted, even if food had been kept in its mouth. This adaptation would have allowed the Thrinaxodon to mash its food to a greater extent, decreasing the amount of time necessary for digestion. The maxillae and palatines meet medially in the upper jaw developing a midline suture. The maxillopalatine suture also includes a posterior palatine foramen. The large palatal roof component of the vomer in Thrinaxodon is just dorsal to the choana, or interior nasal passages. The pterygoid bones extend in the upper jaw and enclose small interpterygoid vacuities that are present on each side of the cultriform processes of the parasphenoids. The parasphenoid and basisphenoid are fused, except for the most anterior/dorsal end of the fused bones, in which there is a slight separation in the trabecular attachment of the basisphenoid.
The otic region is defined by the regions surrounding the temporal fenestrae. Most notable is evidence of a deep recess that is just anterior to the fenestra ovalis, containing evidence of smooth muscle interactions with the skull. Such smooth muscle interactions have been interpreted to be indicative of the tympanum and give the implications that this recess, in conjunction with the fenestra ovalis, outline the origin of the ear in Thrinaxodon. This is a new synapomorphy as this physiology had arisen in Thrinaxodon and had been conserved through late Cynodontia. The stapes contained a heavy cartilage plug, which was fit into the sides of the fenestra ovalis; however, only one half of the articular end of the stapes was able to cover the fenestra ovalis. The remainder of this pit opens to an "un-ossified" region which comes somewhat close to the cochlear recess, giving one the assumption that inner ear articulation occurred directly within this region. The skull of Thrinaxodon is an important transitional fossil which supports the simplification of synapsid skulls over time. The most notable jump in bone number reduction had occurred between Thrinaxodon and Probainognathus, a change so dramatic that it is most likely that the fossil record for this particular transition is incomplete. Thrinaxodon contains fewer bones in the skull than that of its pelycosaurian ancestors.
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