Hypsamynodon

Hypsamynodon is an extinct genus of amynodont that lived in East Asia during the Late Eocene. A single species is recognized, H. progressus, which lived in Mongolia during the Ergilian land mammal age. H. progressus is known only from isolated teeth, and is largely characterized by its hypsodont (high-crowned) cheek teeth.

Hypsamynodon has been suggested to be a synonym of the genus Cadurcotherium, which shares many of its dental features. The fragmentary fossils make comparisons difficult, and Cadurcotherium is not otherwise known from either Mongolia or the Eocene.

Hypsamynodon progressus was described by Vera Gromova in 1954, based on a few isolated upper molar teeth^[2][3] found in the Late Eocene Ergilin Dzo Formation of Mongolia.^[4] Gromova characterized Hypsamynodon by its hypsodont (high-crowned) cheek teeth, and the "near confluence of the anterior rib and parastyle on the molars".^[3]

In the 1980s, William P. Wall considered synonymizing Hypsamynodon with Cadurcotherium.^[2] The hypsodont teeth and the near confluence of the anterior rib and parastyle are features shared by Cadurcotherium. Another feature shared with Cadurcotherium, not mentioned by Gromova in her description of H. progressus but visible in her figures of the teeth, is a greatly reduced metaloph (one of the ridges of the tooth) on the third upper molar.^[3] Wall ultimately preferred not to synonymize Hypsamynodon with Cadurcotherium, awaiting better fossil material.^[3] Cadurcotherium is otherwise known mainly from Europe, though fossils have also been found in Pakistan and Kazakhstan.^[2] With the possible exception of Hypsamynodon, all known Cadurcotherium fossils are restricted to the Oligocene and there is no record of the genus in the Eocene.^[2]

In 1971, Elizaveta Belyaeva referred the species "Amynodon" tuskabakensis to Hypsamynodon, designating it as Hypsamynodon tuskabakensis.^[5] "A". tuskabakensis is known from a single isolated third upper molar from the Early Oligocene Kusto Formation in eastern Kazakhstan.^[2] In 2017, Averianov et al. could identify "A". tuskabakensis as a species of Cadurcotherium, based on shared derived characteristics of the teeth, especially the presence of a posteriorly directed protoloph, parallel to the ectoloph.^[2]

Assessments on Hypsamynodon have varied since the 1980s. In 1996, Spencer G. Lucas and Robert J. Emry treated Hypsamynodon as a synonym of Cadurcotherium.^[5] Wang et al. (2009) treated Hypsamynodon as a valid genus,^[6] Averianov et al. (2017) treated it as a "problematic taxon" but did not designate it as a synonym of Cadurcotherium,^[2] and Bai et al. (2020) treated it as a valid genus.^[1] In a 2022 review of the Ergilin Dzo fauna, Tsubamoto et al. listed H. progressus as Cadurcotherium progressus.^[4]

Hypsamynodon is mainly characterized by its hypsodont cheek teeth.^[6]