Thrinaxodon liorhinus
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Thrinaxodon Temporal range: Early Triassic, 251–247 Ma |
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| Fossil in National Museum of Natural History | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Order: | Therapsida |
| Suborder: | Cynodontia |
| Family: | †Thrinaxodontidae |
| Genus: |
†Thrinaxodon Seeley, 1894 |
| Type species | |
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T. liorhinus Seeley, 1894 |
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Thrinaxodon is an extinct genus of cynodont, most commonly regarded by its species T. liorhinus which lived in what are now South Africa and Antarctica. Thrinaxodon has been dated between the Permian-Triassic boundary and the mid-Triassic. Its survival of the extinction may have been due to its burrowing habits.
Similar to other synapsids, Thrinaxodon adopted a semi-sprawling posture, an intermediary form between the sprawling position of pelycosaurs (not unlike current Crocodylia) and the more upright posture present in current mammals.Thrinaxodon is prevalent in the fossil record in part because it was one of the few carnivores of its time, and was of a larger size than similar cynodont carnivores.
Thrinaxodon was a small synapsid roughly the size of a fox and possibly covered in hair. The dentition suggests that it was a carnivore, focusing its diet mostly on insects, small herbivores and invertebrates. Their unique secondary palate successfully separated the nasal passages from the rest of the mouth, allowing the Thrinaxodon to continue mastication without interrupting to breathe, an adaptation important for digestion.
The nasals of Thrinaxodon are pitted with a large number of foramina, giving the impression that this synapsid had whiskers. 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.
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 shaped. 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.
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