Archaeohippus, GIDLEY, 1906

Maguire, Kaitlin Clare & Schmitz, Joshua X. Samuels and Mark D., 2022, The fauna and chronostratigraphy of the middle Miocene Mascall type area, John Day Basin, Oregon, USA, PaleoBios 35, pp. 1-51 : 24

ARCHAEOHIPPUS GIDLEY, 1906

Archaeohippus ultimus COPE, 1886

Holotype — From Cottonwood Creek : “nearly complete superior dentition with palate and sides of skull to the middle of the orbits and top of skull to above the infraorbital foramen” Cope (1886), AMNH FM 8174 .

Referred specimens —From Cottonwood Creek: “part of ramus with roots of one premolar and part of another” Downs (1956), USNM 18746; single lower premolar, USNM 3909. From CIT 113: lower premolar, CIT 424. From unknown locality from the “Condon Collection”: lower premolars, YPM 14258, 14259. From -3059: lower right molar, UCMP 31987; lower left molar, UCMP 26643; left dp4, UCMP 41195. From -903: right M3 and left P3, UCMP 1689; partial dentary with p3 through p4, UCMP 1700. From JDNM-4:right dentary fragment with m2 and m3, JODA 7010; right dentary fragment with p4 through m2, JODA 7368; right lower molar, JODA 14141; right lower molar, JODA 14620; left M2, JODA 16555.

Occurrence —CIT 113, Cottonwood Creek, UCMP -3059, UCMP -903, JDNM-4.

Remarks —All specimens are described in Downs (1956) except UCMP 41195 and JODA specimens. Measurements for JODA material are included in Table 4. The lower molars have the metaconid and metastylid separate and a well-developed hypoconulid. YPM 14258 and 14259 are listed from Mascall Ranch in the YPM database and Downs (1956) states they are most likely from the Mascall Formation. Hunt and Stepleton (2004) note the occurrence of Archaeohippus sp. from the Upper John Day Formation, in the late Arikareean age Johnson Canyon Member and the Hemingfordian age Rose Creek Member strata south of Kimberly, OR. Combined these records indicate the presence of the genus in Oregon from the earliest Miocene through the middle Miocene.