HYRACODONTIDAE COPE, 1879

Bai, Bin, Wang, Yuan-Qing, Li, Qian, Wang, Hai-Bing, Mao, Fang-Yuan, Gong, Yan-Xin & Meng, Jin, 2018, Biostratigraphy and Diversity of Paleogene Perissodactyls from the Erlian Basin of Inner Mongolia, China, American Museum Novitates 2018 (3914), pp. 1-60 : 9-10

HYRACODONTIDAE COPE, 1879

Triplopus Cope, 1880

Two species of Triplopus have been reported from the Erlian Basin. Triplopus ? proficiens was assigned initially to Caenolophus by Matthew and Granger (1925b). The holotype of the species (AMNH FM 20141), a mandible with p1–m3, was found in the Irdin Manha Formation at Irdin Manha. More complete material of T.? proficien s is known from the Ulan Shireh bed in Shara Murun region, and it is smaller with a straight distal border of m3 as compared with the Irdin Manha specimens ( Radinsky, 1967). Radinsky (1967) further referred two mandibles (AMNH FM 26674, 26675) and a maxilla (AMNH FM 26673) from the “Irdin Manha beds” at Camp Margetts and Huheboerhe, respectively, to T.? proficiens . If the so-called “Irdin Manha beds” bearing T.? proficiens are indeed the Arshanto Formation, this species extends into the earlier Arshantan.

Triplopus ? progressus is known from the Shara Murun Formation at Ula Usu ( Radinsky, 1967), and was assigned initially to Caenolophus by Matthew and Granger (1925b). The species is known from a maxilla with M1–3 (AMNH FM 20298), m2–3 (AMNH FM 20309), and a M3 (AMNH FM 81872). Although T.? progressus is smaller than T.? proficiens , it is more advanced than the latter in having a reduced M3 metacone and parastyle ( Radinsky, 1967).

Teilhardia Matthew and Granger, 1926

Only one species of Teilhardia , T. pretiosa , is known from the “lower red beds” at the base of the Shara Murun Formation at Ula Usu ( Matthew and Granger, 1926). The species is known from a right mandible with p2–m3 (AMNH FM 20299), and the “lower red beds” were later called the Tukhum Formation ( Berkey and Morris, 1927). However, whether T. pretiosa was found in the Tukhum Formation is controversial because some typical Sharamurunian taxa, such as Rhodopagus ? minimus , Deperetella cristata , Triplopus ? progressus , Caenolophus promissus , and Pterodon hyaenoides , were also recorded with the same field number used for T. pretiosa (field no. 191) ( Wang et al., 2012). Radinsky (1967) suggested its affinity with amynodontids based on its relatively shorter premolars relative to the molars, proposing that it may be ancestral to Caenolophus promissus from the overlying Shara Murun Formation.

Ulania Qi, 1990a

Qi (1990a) reported Ulania wilsoni from the grayish-green sandstone of the lower part of the “Ulan Gochu Formation” at Erden Obo, and the horizon is probably equivalent to a part of the “Lower White” or the “Middle White” ( Qi, 1990b; Wang et al., 2012). The species is known from what is thought to be parts of the same individual IVPP V 8922, including a maxilla, mandibles, carpals, and phalanges ( Qi, 1990a). However, Qiu and Wang (2007) doubted the association of the specimens, and referred the maxilla to Caenolophus .

Ardynia Matthew and Granger, 1923b

Two species of Ardynia have been reported from the Erlian Basin: A. praecox and A. kazachstanensis ( Radinsky, 1967) . Ardynia praecox is known from AMNH FM 26039, which is composed of a skull, a mandible, and a few postcranial elements. The specimens are known from the “Ulan Gochu beds” at Erden Obo, and the field number (field no. 747) indicates their stratigraphic horizon as the “Middle Gray layer” ( Bai et al., 2018b). Ardynia kazachstanensis is known from a mandible (AMNH FM 26183) from the “Baron Sog beds” at Nom Khong Obo, and the horizon is equivalent to the “Upper White” ( Radinsky, 1967; Wang, 2003).

Proeggysodon Bai and Wang, 2012

Only one species of Proeggysodon , P. qiui , was unearthed from the upper part of the “Middle White” at Erden Obo ( Bai and Wang, 2012). The species is known from a mandible with incisors, canine, and cheek teeth (IVPP V 18099 ), and the species is considered ancestral to the European Oligocene Eggysodon .