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.

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 transverse processes of the pterygoids have a very enlarged distal end giving them a palmate morphology in ventral view as in Anteosaurus and Titanophoneus.

[2][3][1] 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.

[5][6] 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.

[7] 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.

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.

[12] 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.

[18][19] 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.

[3] Below, the cladogram of the anteosaurs presented by Kammerer in 2011 :[3] Anteosaurus magnificus Titanophoneus adamanteus Titanophoneus potens Sinophoneus yumenensis Syodon biarmicum Australosyodon nyaphuli Notosyodon gusevi Archaeosyodon praeventor Microsyodon orlovi Tapinocaninus pamelae Estemmenosuchus mirabilis Biarmosuchus tener In describing the new Brazilian anteosaur Pampaphoneus, Cisneros et al. presented another cladogram confirming the recognition of the clades Anteosaurinae and Syodontinae.

[21] Biarmosuchus tener Estemmenosuchus uralensis Ulemosaurus svijagensis Tapinocaninus pamelae Archaeosyodon praeventor Sinophoneus yumenensis Titanophoneus adamanteus Titanophoneus potens Anteosaurus magnificus Pampaphoneus biccai Notosyodon gusevi Syodon biarmicum Australosyodon nyaphuli One of the four cladograms of Cisneros et al. published in the Supporting Information of the same article, and including Microsyodon.

[21] Biarmosuchus Estemmenosuchus Ulemosaurus Tapinocaninus Archaeosyodon Sinophoneus Anteosaurus T. potens T. adamanteus Pampaphoneus Notosyodon Syodon Microsyodon Australosyodon