Huananodon

Huananodon is an extinct genus of amynodont that lived in East Asia during the Middle or Late Eocene. Huananodon is known only from isolated fossil teeth found in southern China. Two species of Huananodon have been named, the type species H. hypsodonta from the Gongkang Formation and H. hui from the Naduo Formation.

Although little is known of Huananodon on account of the poor fossil material, its teeth are distinguishable from other amynodonts by their high tooth crowns and the high degree of molarization (becoming molar-like) of its premolars. The large size of its teeth and their unique features suggest that Huananodon was a large and specialized amynodont.

Huananodon hypsodonta and Huananodon hui were described by Y. Z. You in 1977,^[1] based on fossils from Guangxi, China.^[1][2] Both species are based mainly on isolated upper premolars.^[1][3] H. hypsodonta was based on the upper right second premolar tooth crown IVPP V.5006, found in Baise, Guanxi,^[1] in the Gongkang Formation.^[4] Additional teeth from the same site were also referred to H. hypsodonta, including additional premolars and a partial second upper molar.^[1] H. hui was based on several associated premolars and fragments of incisors and canines (IVPP V.5008), from Tiandong, Guanxi,^[1] in the Naduo Formation.^[4] You distinguished Huananodon from other amynodonts mainly by its unusually high tooth crowns, especially the height of the outer walls of the teeth.^[1]

In 1986, William P. Wall and Earl Manning argued that the type specimens of both Huananodon species were extremely poor specimens that cannot clearly be identified as belonging to amynodonts.^[3] Donald Prothero and Robert M. Schoch accepted Huananodon as a valid amynodont genus in 1989, classified as incertae sedis in the family.^[5] In 1998, Esperanza Cerdenõ considered Huananodon to be a rhinocerotid rather than an amynodont, but noted that this was doubtful and that "many Asian taxa need a full revision".^[6] Tsubamoto et al. (2000) followed Cerdenõ in provisionally listing Huananodon as a rhinocerotid.^[4]

In 2019, Bin Bai, Jin Meng, Fang-Yuan Mao, Zhao-Qun Zhang, and Yuan-Qing Wang reaffirmed Huananodon's placement in the amynodonts by pointing out that its teeth showed a pattern of premolar molarization (premolars becoming more molar-like) through separation between the protocone and the metaconule, a feature known in perissodactyls only in the amynodonts.^[7] Bai et al. also noted that the premolars are molarized to a higher degree in Huananodon than in other Asian amynodonts.^[8]

Huananodon is very poorly known. Based on the size and unique features of its teeth, Y. Z. You determined that it was a very large and specialized amynodont.^[1] You provided measurements only for the upper teeth of H. hypsodonta:^[1]

The precise ages of the Gongkang and Naduo formations are disputed. They were originally treated as either Late Eocene or Early Oligocene in age. Stratigraphic revisions of Asian mammal faunas have since shifted the Eocene–Oligocene boundary; sites formerly considered Early Oligocene are now considered Late Eocene, and sites formerly considered Late Eocene are now considered late Middle Eocene.^[9] The Gongkang and Naduo formations are variously described as Middle^[7][10] or Late^[11][12][13] Eocene in age. The Gongkang Formation overlays the Naduo Formation, but it has been suggested that the faunas of the two formations should be regarded as a single assemblage.^[4]

The Naduo Formation preserves a diverse mammal fauna.^[4] Among the perissodactyls, H. hui coexisted with the rhinoceros Guixia simplex,^[1][4] a brontothere tentatively referred to the species "Metatelmatherium" browni (which recent analyses favor as outside Metatelmatherium^[14]), the chalicotherioid Eomoropus cf. quadridentatus, the deperetellid Deperetella sp., and other species of amynodonts tentatively identified as Paramynodon sp. and Caenolophus sp.^[4] Carnivorans were represented by two species of the bear Cephalogale, the amphicyonid Guangxicyon sinoamericanus, and a amphicynodontid, perhaps Pachycynodon. Hyaenodonts were also present, perhaps belongong to the genus Propterodon, and three species of mesonychids are known (Guilestes acares, G. sp., and cf. Harpagolestes sp.). Among the artiodactyls, the Naduo Formation preserves unidentified fossils of entelodonts, peccaries, suids, and possibly choeropotamids; several ruminants are also known (Notomeryx besensis, N. major, Indomeryx cotteri, and Gobiomeryx sp.), as well as a large number of anthracotheres (Anthracothema rubricae, Anthracokeryx birmanicus, Anthracokeryx moriturus, Anthracokeryx sp., Bothriodon? chyelingensis, Heothema bellia, Heothema media, and Huananothema imparilica).^[4] Also found in the formation is the placental (order uncertain) Eodesmatodon spanios.^[4]

The Gongkong Formation preserves a smaller number of mammals. Fellow perissodactyls included the rhinoceros Guixia yougiangensis, the chalicotheres Schizotherium nabanensis and S. sp., and the paracerathere Forstercooperia sp. nov.^[4] Two carnivorans are known from the Gongkong Formation, an unidentified machairodont and a nimravid, possibly Hoplophoneus. Artiodactyls were represented by an unidentified species of the peccary Eopecarihyus, as well as a similar diversity of anthracotheres as in the Naduo Formation (Anthracokeryx kwangsiensis, A. gungkangensis, A. sp., Bothriodon? tientongensis, Heothema media, H. chengbiensis, and H. angusticalxia).^[4]