Extinct genus of cynodonts
Vetusodon is an
Discovery and naming
Vetusodon is known from four specimens, all of which have been collected in South Africa from rocks belonging to the Daptocephalus Assemblage Zone. The holotype, BP/1/7971, consists of a well-preserved skull missing the lower jaw. It was discovered in 2010 in a mudstone bed in the province of KwaZulu-Natal by a team led by the palaeontologist Bruce S. Rubidge. The referred specimen CGP GHG141 was found in 1985 by Gideon Groenewald in the Thaba Nchu Mountain of the Free State province, and like the holotype, it consists of a partial skull missing the lower jaw. The specimen SAM-PK-K10596 is the least complete of the specimens, consisting only of a snout. It was found in 1958 or 1959 by the South African palaeontologist Alfred W. Crompton. The specimen SAM-PK-K10702 is the most complete of the four specimens, and comprises a complete skull with an intact lower jaw. It was discovered in 2009 or 2010 by Derik Wolvaardt at the Ripplemead Farm, close to the town of Nieu-Bethesda, Eastern Cape province.[1]
Vetusodon elikhulu was first described in 2019 by the palaeontologists Fernando Abdala, Leandro C. Gaetano, Roger M. H. Smith and Bruce S. Rubidge. Its
generic name is derived from the
Latin word
vetus, meaning "old", and the
Greek word
οδοντος (
odontos), meaning "tooth". The
specific name elikhulu means "large" in the
Zulu language, and alludes to the animal's size.
[1]Description
Vetusodon is the largest known Permian cynodont, based on its skull length of around 18 centimetres (7.1 in). The temporal region (area behind the
eye sockets) made up around 60 percent of the skull length.
[1]The jaws were quite robust, and the snout was unusually broad compared to other cynodonts of its time. Between the
dentary bones, whereas the other (postdentary) bones were reduced in size to an extent not otherwise seen in cynodonts this primitive; as a result, the dentary stretched nearly as far back as the jaw joint. The
symphysis (joint) between the two halves of the lower jaw was quite tall, creating a
chin-like structure. Abdala
et al. (2019) interpreted the symphysis as being fused, but a 2020 study by Huttenlocker and Sidor reinterpreted it as unfused.
[2][3] The back portion of the dentary had a projection called the
coronoid process, which extended as high as the upper half of the eye sockets. The coronoid process fit quite closely to the
postorbital bar (the bony structure that divided the eye socket from the
temporal fenestra).
[1]The teeth were conical, single-rooted and had no
postcanine teeth. The upper postcanines varied in number between specimens, ranging from 7 pairs in the holotype, to 11 in the specimen SAM-PK-K10596. Unlike most cynodonts, which have postcanines with multiple
cusps, the postcanines of
Vetusodon only bore one cusp each.
[1]The outer rims of the eye sockets (orbits) were formed by the
pineal foramen at the top of the skull, between the temporal fenestrae. Running from the top of the orbits towards the pineal foramen, and bordering the temporal fenestrae, were two crests formed by the postorbital bones. Behind the pineal foramen there was a longitudinal ridge known as the
sagittal crest, which was unusually short in
Vetusodon. Behind the sagittal crest, two occipital crests, formed by the
tabular and squamosal bones, stretched backwards diagonally and joined the rear ends of the zygomatic arches.
[1]The
plesiomorphy (ancestral trait) shared with other primitive cynodonts.
[1][3]Classification
When Vetusodon was first described in 2019, it was included in a
phylogenetic analysis to test its relationships to other cynodonts. It was found to be a derived member of the clade
Epicynodontia, being more closely related to
Eucynodontia than several
Triassic genera such as
Thrinaxodon. As
Thrinaxodon and its relatives had relatively well-developed secondary palates, this would imply that the incomplete secondary palate of
Vetusodon was secondarily reduced from a more complete one.
[1][3] However, a 2020 study by Huttenlocker and Sidor,
[2] followed by a 2021 study by Pusch and colleagues, recovered
Vetusodon in a more early-diverging position within Epicynodontia, as the sister taxon of the Permian genus
Cynosaurus. If this placement is correct, the incomplete secondary palate of
Vetusodon may be an ancestral trait inherited from earlier cynodonts.
[3]Below are two cladograms from Abdala et al. (2019) and Pusch et al. (2021), which illustrate the varying phylogenetic placements of Vetusodon:
| Abdala et al. (2019)[1] |
Pusch et al. (2021)[3]
|
|
|
|
Palaeobiology
The large incisors and canines and small, simple postcanines of Vetusodon imply that the front teeth played a larger role than the postcanines in the processing of food. In this respect it closely resembled the
synapsids that existed at the time, but differed from most other cynodonts, whose postcanines are generally the most complex and important teeth. The placement of the coronoid process of the dentary close to the postorbital bar may be evidence of a well-developed
temporalis muscle, which would have allowed for a powerful bite by the front teeth. The skull shape and dentition of
Vetusodon were in general similar to those of the therocephalian
Moschorhinus, which it coexisted with. As therocephalians were only distantly related to
Vetusodon, this is thought to be an instance of
convergent evolution, where different groups that inhabit the same
ecological niche evolve similar traits.
[1]Palaeoenvironment
Vetusodon belongs to the Daptocephalus Assemblage Zone, a biozone that is part of the Beaufort Group, which itself is part of the larger Karoo Supergroup. Vetusodon belongs to a subzone of the Daptocephalus AZ known as the Lystrosaurus maccaigi-Moschorhinus Subzone.[4]
In addition to Vetusodon, several other
References
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