Stenocybidae

''Sinophoneus'' is an extinct genus of carnivorous dinocephalian therapsid belonging to the family Anteosauridae. It lived 272 to 270 million years ago at the beginning of the Middle Permian (Lower Roadian) in what is now the Gansu Province in northern China. It is known by a skull of an adult individual (the holotype GMV1601), as well as by many skulls of juvenile specimens. The latter were first considered as belonging to a different animal, named ''Stenocybus'', before being reinterpreted as immature ''Sinophoneus''. ''Sinophoneus'' shows a combination of characters present in other anteosaurs. Its bulbous profile snout and external nostrils located in front of the canine are reminiscent of the basal anteosaur ''Archaeosyodon'', while its massive transerse pterygoids processes with enlarged distal ends are more similar to the more derived anteosaurs ''Anteosaurus'' and ''Titanophoneus''. First phylogenetic analyzes identified ''Sinophoneus'' as the most basal Anteosaurinae. A more recent analysis positioned it outside the Anteosaurinae and Syodontinae subclades, and recovers it as the most basal Anteosauridae.

Description

The holotype of ''Sinophoneus'' is a somewhat dorsoventrally deformed skull measuring 32 cm in length from the tip of the snout to the occipital condyle and about 35 cm in total length to the posterior border of the squamosal. The upper part of the orbits is crushed, giving them an oval shape. They had to be originally circular. Most of the temporal arches are missing, as is the dentition of the upper jaws. The lower jaw is not preserved. The skull is mainly characterized by the presence of a median ridge starting between the eye sockets (where it is poorly developed) and extending to the tip of the snout (where it is more robust) which it protrudes beyond in dorsal view. The nostrils are smaller than in other anteosaurs. As in ''Archaeosyodon'', the nostrils are entirely located in front of the upper canine whereas in other anteosaurs they are mainly located above or behind the canine. The snout profile is bulbous as in ''Archaeosyodon''. The transverse processes of the pterygoids have a very enlarged distal end giving them a palmate morphology in ventral view as in ''Anteosaurus'' and ''Titanophoneus''.

Synonymy with Stenocybus

In 1997, Cheng & Li described the small dinocephalian ''Stenocybus acidentatus'', from a complete skull (IGCAGS V 361) and jaw remains from a second specimen, found in the same site as ''Sinophoneus''. Although in their article, Cheng and Li did not compare ''Stenocybus'' directly with ''Sinophoneus'', they consider the first sufficiently different from Anteosauridae to place it in a new family, the Stenocybusidae. The name of this clade was later correctly modified in Stenocybidae by the Russian paleontologist Mikhail Feodosievich Ivakhnenko. The latter was also the first researcher to suggest that ''Stenocybus'' could be the juvenile form of ''Sinophoneus''. In 2011, Christian Kammerer also considered ''Stenocybus'' as a young ''Sinophoneus'', the proportions of the first (absence of pachyostosis, tall and narrow skull, relatively large orbits, and smaller temporal fenestra) being typical characters of juvenile therapids. In addition, the immature skull of ''Stenocybus'' already shows a beginning of the median ridge of the snout characteristic of ''Sinophoneus'', as well as the same bulbous profile of the snout as in the latter. As a result, Kammerer regards ''Stenocybus acidentatus'' as a junior synonym of ''Sinophoneus yumenensis''. A conclusion shared by Liu and Li from new specimens mentioned in a short note published the same year. In 2014, Jiang and Ji described two new specimens of ''Stenocybus'': a snout with articulated jaws, and a right dentary with an almost complete dentition, belonging to two individuals slightly larger than the holotype. Although providing new information on the anatomy of the animal, such as the number of teeth on the mandible, Jiang and Ji believe that the phylogenetic position of ''Stenocybus'' remains difficult to confirm, and that the synonymy of ''Stenocybus'' with ''Sinophoneus'' proposed by Kammerer, Liu and Li must be proved by the discovery of more complete specimens. However, Jiang and Ji's wishes had been realize a few months earlier. In 2013 Liu described the seven new skulls of ''Sinophoneus'' (including one associated with postcranial remains) that had only been mentioned in the short note of 2011. These skulls all belong to specimens smaller than the holotype, but they represent various ontogenetic stages. They confirm that the skulls of ''Stenocybus'' actually represent several juvenile stages of ''Sinophoneus''.

Ontogeny

The ontogenetic series of the genus ''Sinophoneus'' is incomplete with an unrepresented size range between the known juvenile skulls measuring between 12 and 18 cm in length and the only known adult skull measuring 32 cm. However, this ontogenetic series makes it possible to identify the important morphological changes between the juveniles and the adults of this genus. The skull of the juvenile is relatively tall and narrow, and pachyostosis is absent. In the adult, the skull becomes much wider and lower, notably the bones of the palate which widen considerably, and a pachyostosis is observed on the bones surrounding the orbits. The medial ridge on the skull roof is weak in the smallest known specimen, but a pair of bosses is present on the midline of the inter-orbital region. This ridge extends from the pineal foramen to a point slightly beyond the orbit in the juvenile. It is more pronounced in adults where it extends to the end of the snout that ends prominently. The nostrils and orbits increase only slightly in size during growth. In contrast, the dimensions of the temporal fenestrae increase dramatically. They are triangular in shape and much smaller than orbit in the smaller specimens, have a size close to that of the orbits in somewhat larger specimens, and are much larger than the orbit in the adult. The latter were therefore equipped with a mandibular adductor musculature much more developed than that of the juveniles, and could inflict proportionally much more powerful bites. These differences indicate that in ''Sinophoneus'', juveniles and adults certainly occupied distinct ecological niches and did not covet the same preys. This pattern is also known in the South African genus ''Anteosaurus''.

Geographic and stratigraphic distribution

All fossils of ''Sinophoneus'' come from a single locality, the Dashankou site, near Yumen City, in western Gansu Province, China. Stratigraphically, they belong to the Qingtoushan Formation (which before 2012 was called the Xidagou Formation), whose sediments are of fluvial origin. No radiometric dating is known for the Qingtoushan Formation, but Liu et al. assigns it a lower Roadian age (lowermost Middle Permian), thus partially filling the Olson Gap. The latter corresponds to a period of time with a poor fossil record, located between the lower Permian, dominated by the pelycosaurs, and the mid-middle Permian (Wordian), where the therapids take the first place in terrestrial ecosystems. The Roadian age attributed to the Qingtoushan Formation is an estimate derived from the degree of evolution of the Dashankou fauna. This fauna is composed both of animals belonging to ancient lineages, rather typical of Upper Carboniferous and Lower Permian, such as the temnospondyl dissorophoid ''Anakamacops'' and the Bolosauridae ''Belebey'', and also of more derived forms, rather typical of the middle and upper Permian , such as the anthracosaurs Chroniosuchidae '' Ingentidens'' and '' Phratochronis'', the moradisaurine '' Gansurhinus'', and several therapsids. The latter are, however, exclusively represented by phylogenetically basal taxa. These are ''Raranimus'', the most basal therapid known to date, and ''Biseridens'', the most basal anomodont. The new classification of ''Sinophoneus'', identified by Liu as the most basal Anteosauridae, is an additional argument in favor of a Roadian age for the Dashankou fauna. Moreover, the latter was correlated with the Russian '' Parabradysaurus silantjevi'' assemblage zone of Roadian age. Coinciding with its basal phylogenetic position, the Lower Roadian age of ''Sinophoneus'' makes it one of the oldest known anteosaurs to date, together with the Russian genus ''Microsyodon'' being of comparable age.

Classification and phylogeny

In 2011 Christian Kammerer published the first phylogenetic analysis including all anteosaurs. This one recognized the monophyly of Anteosauridae, which contain two main clades: Syodontinae and Anteosaurinae. ''Sinophoneus'' is identified as the most basal Anteosaurinae and is the sister group of an unresolved trichotomy including ''Titanophoneus potens'', ''T. adamanteus'' and ''Anteosaurus''.

Below, the cladogram of the anteosaurs presented by Kammerer in 2011 :

In describing the new Brazilian anteosaur ''Pampaphoneus'', Cisneros et al. presented another cladogram confirming the recognition of the clades Anteosaurinae and Syodontinae. ''Sinophoneus'' occupies the same position there as in the cladogram proposed by Kammerer.

The cladogram of Cisneros et al. published in the main paper and excluding the genus ''Microsyodon''.

One of the four cladograms of Cisneros et al. published in the Supporting Information of the same article, and including ''Microsyodon''.

In 2013, Jun Lui presented a new cladogram in which ''Sinophoneus'' is reinterpreted as the most basal anteosauridae and is thus excluded from Anteosaurinae.

The cladogram published by Jun Liu in 2013 :

References

{{Taxonbar, from=Q60223

Anteosaurs
Prehistoric therapsid genera
Permian synapsids of Asia
Guadalupian synapsids
Fossil taxa described in 1996
Guadalupian genus first appearances
Guadalupian genus extinctions