Acritohippus, KELLY, 1995

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 : 25

ACRITOHIPPUS KELLY, 1995

Acritohippus isonesus KELLY, 1995

FIG. 14

Holotype —From UCMP V 67153: skull with left I1 through 3, C, P2 through M3 and left P2 through M3, AMNH FM 8175 (holotype of Hippotherium isonesum Cope, 1889 , which was later renamed Acritohippus isonesus by Kelly (1995)).

Referred specimens —From CIT 183: skull with P2 through M3, LACM (CIT) 532. From JDNM-4: skull with P2 through M3, left I1 and right I1, I2, JODA 1316; skull with P2, dP3 through erupting M3 with broken rostrum and right zygomatic arch, JODA 1317; poorly preserved partial skull with P3 through M3, JODA 1318.

Occurrence —CIT 183, UCMP V 67153, JDNM-4*.

Description —Narrow preorbital bar, moderately deep lacrimal fossa, infraorbital foramen on floor of lacrimal fossa, shallow malar fossa with no foramen, malar fossa separated from lacrimal fossa by low, indistinct ridge. Cheek teeth have protoloph connected to protocone, no crochets, one medium pli caballin, simple fossette borders with occasional pli protoconule and pli postfossettes, open hypoconal groove, simple hypocone, considerable cement,and hypsodont tooth crown height.

Remarks —This is the larger of the two merychippine species present in the type Mascall Formation, which was identified by Downs (1956) as Merychippus seversus Cope, 1878 , in part. AMNH FM 8175 from AMNH Mascall locality (=UCMP V67153) is the type specimen for Acritohippus isonesus ( Kelly 1995) , and the original type specimen for M. isonesus (referred to as Stylonus isonesus by Kelly and Lander (1988), as Hippotherium isonesum by Cope (1889) and as H. seversum by Cope (1886)). Downs (1956) synonomized that taxon with M. seversus . Kelly (1995) distinguished Acritohippus based on a shallow malar fossa only separated from the dorsal preorbital fossa (DPOF) by a low, indistinct ridge. All of the skulls referred here have this feature, but a formal comparison, currently being undertaken by M.O. Woodburne (pers. communication), needs to be completed to confirm assignment of specimens to this taxon. There are hundreds of (>300) dental specimens collected from this taxon in the AMNH, JODA, UCMP and YPM collections, many of which Downs (1956) assigned to M. seversus , which are also being re-evaluated. Fragmentary enamel and postcranial material of a larger merychippine was recovered from upper Mascall deposits (JODA 15616, JODA 15617 and JODA 15630) demonstrating this species’ persistence into the later part of the formation and Late Barstovian.

Figures

Figure 14. Acritohippus isonesus skull, JODA 1316. A. Dorsal view. B. Lateral view. C. Ventral view. Scale bar=3cm.