Equinae Gray, 1821

Smith, Kent S., Czaplewski, Nicholas J. & Coombs, Margery C., 2022, Ungulates of the middle Miocene Monarch Mill Formation, Churchill County, Nevada, USA, Acta Palaeontologica Polonica 67 (1), pp. 239-256 : 247-249

Subfamily Equinae Gray, 1821

Stratigraphic and geographic range.—Early Arikareean to Recent ( MacFadden 1998); Eurasia, Africa, and North America (in the Miocene).

Equinae gen. et sp. indet.

Fig. 7 .

Material.—UCMP 141537, middle phalanx; UCMP 141698, astragalus; UCMP 141837-01, astragalus; UCMP 141837-02, isolated tooth fragments; UCMP 141899-01, proximal part of humerus; UCMP 141899-02, magnum; UCMP 141899-03 metatarsals II or IV; UCMP 141921, distal part of tibia. From localities UCMP V70138, V70142, V70143, V70145, V70148, Eastgate, Churchill County, Nevada, USA; Monarch Mill Formation, early Barstovian, middle Miocene.

Remarks.—The elements listed above were identified by Donald E. Savage in the 1950s as belonging to “ Merychippus ” sp. Traditionally, Merychippus included a paraphyletic/polyphyletic assemblage of subhypsodont to hypsodont horses ( Hulbert and MacFadden 1991; MacFadden 1998) that are now placed in several different genera. The fossil horse material from the Eastgate LF does not provide sufficient information to allow taxonomic identification below the subfamily level.

Family Chalicotheriidae Gill, 1872

Subfamily Schizotheriinae Holland and Peterson, 1914

Genus Moropus Marsh, 1877

Type species: Moropus oregonensis ( Leidy, 1873) , by Coombs et al. (2001), Sutton Mountain, Oregon, USA; upper John Day Formation, late Arikareean.

Moropus merriami Holland and Peterson, 1914 Fig. 8 .

Material.— UCMP 80398, olecranon process of ulna, distal metacarpal II, partial lunate; UCMP 141896, duplex = fused proximal and middle phalanges of digit II manus; UCMP 141898, duplex of digit II manus; UCMP 141917, proximal end of proximal phalanx; UCMP 141946, proximal end and shaft of humerus; UCMP 141947-01, metatarsal III; UCMP 141947-02, metatarsal IV; UCMP 141947-03, metatarsal II; UCMP 141948-01, metatarsal III; UCMP 141948- 02, metatarsal IV; UCMP 141948--03, metatarsal II; UCMP 141954-01, ungual phalanx; UCMP 141954-02, proximal phalanx; UCMP 141960, left metacarpal IV; UCMP 141978- 01, complete tibia; UCMP 141978-02, partial radius; UCMP 141978-03, ulna shaft; UCMP 141978-04, distal fused radius and ulna; UCMP 141839, distal end of right scapula. Additional skeletal elements of M. merriami occur in the UCMP collections from Eastgate and will be presented in a separate study by Margery Coombs. Chalicothere material has been collected from at least seven UCMP Eastgate vertebrate localities: V70141, V70142, V70143, V70144, V70146, V74103, V67245. Most of the fossil materials are from locality V74103, informally called the Chalicothere Quarry or Burmester Chalicothere Quarry in UCMP field records. Although given separate catalog numbers, some of the elements collected from V74103 articulate and probably belonged to one individual.

Remarks.— Holland and Peterson (1914) named Moropus merriami for UCMP material collected from Virgin Valley and the High Rock Canyon area in northwestern Nevada and figured by Merriam (1911) as “ Moropus (?) sp.”. They named syntypes that included an astragalus, a calcaneum, fused proximal and middle phalanges (a duplex) of digit II of the manus, two ungual phalanges, a P4, and an m2. Because the syntypes come from several localities, they cannot possibly represent a single individual. Nonetheless, Coombs (2004), while presenting a revised species diagnosis, refrained from designating a lectotype on the grounds that the complete series gives a better sense of the species than any single lectotype would do. Moropus merriami was tentatively referred to either the genus Macrotherium or Chalicotherium by Matthew (1929) and others; however, it is clear on the basis of both dental and postcranial anatomy that the species belongs to the Schizotheriinae , not Chalicotheriinae. Coombs (1978, 1998, 2004) has maintained the original referral to Moropus .

The virtual lack of craniodental material is not a significant barrier in identifying the Eastgate chalicothere material. Skull remnants and teeth are rare in general among Nevada chalicotheres; the type series of M. merriami includes only a P4 and m2. On the other hand, postcranial elements, particularly podials, metapodials, and some phalanges, are very useful in species identification. For example, Coombs (2004) used the proportions and morphology of metatarsals as important aids in the identification of chalicotheres from the Lower Snake Creek LF of the Great Plains as belonging to M. merriami . The well-preserved, robust metatarsals of the Eastgate chalicothere similarly provide important evidence to support assignment to M. merriami (see Table 3). A fuller comparison awaits a more detailed description of all Nevada material belonging to this species ( MCC unpublshed data).

Although chalicothere fossils are numerous at a few unusual sites (most famously Moropus elatus Marsh, 1877 , at the Agate Spring Quarries and Morava Ranch Quarry in Nebraska; see Coombs and Coombs 1997), it is more typical to find them sporadically and as isolated elements.

Classifying these elements is often an exercise in comparing teeth from one place with footbones from another, with less than conclusive results. Fortunately, the substantial number of chalicothere elements from Nevada allow more complete and definitive comparisons. In particular, the articulating hind foot elements of M. merriami from Eastgate ( UCMP locality V74103) help to clarify the proportions of bones belonging to a single animal. Metatarsals III and IV are of subequal length in UCMP 141947 and UCMP 141948 ( Fig. 6 ; Table 2), with metatarsal IV slightly longer. Similar metatarsal IV/metatarsal III proportions occur in M. elatus ( Coombs 1978: table 5), while in the North American dome-skulled chalicothere Tylocephalonyx skinneri Coombs, 1979 , metatarsal III is clearly the longest metatarsal ( Coombs 1979: table 7).

Stratigraphic and geographic range.—Late Hemingfordian to early Barstovian ( Coombs 1998, 2004) = early middle Miocene. The earliest known occurrence of M. merriami is in the Massacre Lake local fauna. This fauna is also important for the presence of Zygolophodon , one of the earliest proboscidean occurrences in North America ( Morea, 1981; Tedford et al. 2004; Prothero et al. 2008). Other clearly identifiable remains of M. merriami are early Barstovian in age, The known geographic range includes Nevada and northwest Nebraska ( USA). North of Eastgate (Churchill County, Nevada), M. merriami occurs at UCMP localities in Washoe and Humboldt counties in the Massacre Lake fauna, Virgin Valley fauna, Fly Canyon fauna, and Smoky Creek fauna, as well as in the Yellow Hills fauna, which was excavated in the High Rock Caldera by field parties from Sierra College ( Bromm and Hilton 2011; Hilton and Hausback 2015).

Family Rhinocerotidae Gray, 1821

Subfamily Aceratheriinae Dollo, 1885

Genus Teleoceras Hatcher, 1894

Type species: Teleoceras major Hatcher, 1894 ; Loup Fork Beds , Nebraska, USA; early–late Barstovian .

Stratigraphic and geographic range.—Early Hemingfordian through late Blancan, middle Miocene to early Pleistocene ( Prothero 1998; Tedford et al. 2004) in NALMA chrostratigraphic scheme. One of the earliest known occurrences of Teleoceras is in Colorado, which is considered early Hemingfordian in age (Martin Canyon Quarry A), whereas one of the latest occurrences is considered late Blancan in age (Broadwater LF in Nebraska). USA: California ( Mt. Eden LF); Nevada (Panaca LF, Eastgate LF); Arizona (Keams Canyon LF, Jeddito LF) ; New Mexico (Nambe Fauna, Skull Ridge Fauna, Pojoaque Fauna, Round Mountain Fauna, and San Juan Fauna); Colorado (Wray localities); Nebraska (Burge and Penny Creek faunas, Sand Canyon, Broadwater LF, Snake Creek Formation); Kansas (Sawrock LF); Texas ( Lapara Creek fauna, Coffee Creek LF, Clarendon LF); Oklahoma (Optima LF); Florida (Palmetto Fauna) ; South Dakota (Ash Hollow Formation).

Figures

Fig. 7. Articulating distal tibia and astragalus and isolated magnum elements of Equinae gen. et sp. indet. from localities UCMP V70145 (A, B) and UCMP V70142 (C), Eastgate Churchill County, Nevada, USA; early Barstovian, middle Miocene.A. UCMP 141921, distal tibia in anterior view. B. UCMP 141837-01, astragalus in anterior view. C. UCMP 141899-02, magnum in dorsal view.

Fig. 8.Articulating right and left hind feet elements of chalicotheriid perissodactyl Moropus merriami Holland and Peterson, 1914, from locality UCMP V74103, Eastgate, Churchill County, Nevada, USA; early Barstovian middle Miocene. A. UCMP 141948-02, -01, -03, right metatarsals IV–II. B. UCMP 141947-03, -01, -02, left metatarsals II–IV).

Fig. 6. Dental terminology for palaeomerycid artiodactyl Barbouromeryx trigonocorneus (Barbour and Schultz, 1934), from Eastgate, Churchill County, Nevada, USA; early Barstovian, middle Miocene. A. UCMP 141518 from UCMP V70138; right dentary fragment with m1 fragment and m2–m3 in occlusal (A1) and labial (A2) views. B. UCMP 141669 from UCMP V70140; left dentary fragment with p3–m3 in occlusal view (only p3–p4 figured).