Famoso—Dental variation in Oligocene equids


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Figure 1. Distribution of localities utilized in this study. Localities are color coded by member; localities with unknown stratigraphy are not mapped. The locations of Portland, Eugene, and John Day are marked. Oregon is highlighted in black on the map of the United States of America.


This study focuses primarily on the equid faunal assemblage of the Turtle Cove Member (30.8–25.9 Ma), which consists of ~400m of section that Albright et al. (2008) divided into 14 lithostratigraphic subunits (A–K2; Fig. 2). Included within the member are several dated tuffs and the Picture Gorge ignimbrite—a super-volcanic event related to the Yellowstone hotspot (Seligman et al., 2014). The faunas of the Turtle Cove Member are assigned to the Whitneyan and Arikareean (subages Ar1 and Ar2) North American Land Mammal Ages (Albright et al., 2008). Specimens from the lower Big Basin Member and the higher Kimberly and Haystack Valley members are also included in this study, but these samples are much smaller than those of the Turtle Cove Member.


Taxonomic background


Miohippus is a genus of relatively moderate-sized equid (~53.8 kg, M1-3 length =34–50mm) belonging to the para- phyletic subfamily “Anchitheriinae” Leidy, 1869 (MacFadden, 1986; Prothero and Shubin, 1989; MacFadden 1998). Miohippus is coeval with the smaller Mesohippus and the larger Kalobatippus, Osborn, 1915, all of which are members of “Anchitheriinae” (MacFadden, 1998). Miohippus is considered to be distinct from Mesohippus based on the presence and condition of the articular facet on the third metatarsal, which articulates with the cuboid, larger hypostyles (Fig. 3); a longer face; and a deeper facial fossa (Prothero and Shubin, 1989; MacFadden, 1998); however, these two genera are difficult to distinguish (Stirton, 1940). Species within Miohippus in the Great Plains are diagnosed primarily on the basis of tooth-row length and hypostyle condition (Prothero and Shubin, 1989). Often, multiple hypostyle conditions can be observed in the same individual (Prothero and Shubin, 1989; Fig. 3). The Great Plains species have been the subject of more intensive study than the John Day species. However, this does not mean that the currently recognized Great Plains species are more likely to be valid than the John Day species. Further character and variation analyses of the Great Plains material may reduce the number of recognized species.


There are currently 16 species of Miohippus considered


valid in North America. Of these, eight species have been named from the John Day Formation (Miohippus annectens Marsh, 1874; M. condoni [Leidy, 1870]; M. anceps [Marsh, 1874]; M. equiceps [Cope, 1878]; M. primus Osborn, 1918; M. quartus Osborn, 1918; M. acutidens [Sinclair, 1905]; and M. equinanus Osborn, 1918) (Prothero and Shubin, 1989; MacFadden, 1998). Many of the characters that diagnose these species are present in the molars and premolars. These char- acters include the anterior-posterior length and transverse width of molars and premolars, the texture of the enamel, and the morphology of the protoconules, metaconules, hypostyles, cingula, tubercal, paracone, metacone, hyperconulid, medivallum cusps, protoloph, protoconule, metaloph, ectoloph and parastyle (Osborn, 1918; Prothero and Shubin, 1989). The relative size of the M3 versus the M1-2 and the morphology of the incisors and canines have also been invoked as a diagnostic character (Osborn, 1918). The position of the orbit, dorsal preorbital fossa (DPOF) depth, lacrimal fossa depth, position of the infraorbital foramen, malar morphology, and muzzle shape have also been used to diagnose species (Osborn, 1918). The morphology of postcrania is only described in M. equinanus, and includes the metatarsals and cuboid (Osborn, 1918). The validity of many of these characters could be evaluated using geometric morphometric analyses, however before studies of that type can be undertaken it is necessary to quantify and understand the range of variation these characters display. This study helps to lay the foundation for those future studies. It is important to note that M. condoni is only known from a partial dp3 that is about the same size as M. annectens and M. anceps (Osborn, 1918). Osborn (1918) stated that Marsh’s (1874) description of Miohippus (= Anchitherium) anceps does not follow the morphology of the type specimen. Only dental characters can be compared among these eight


species, and most species do not have descriptions of cranial or postcranial morphology (Osborn, 1918; Prothero and Shubin, 1989). It is well known that dental characters vary with wear, and great care must be taken to sample taxa from medial stages of wear when comparing morphology (MacFadden, 1998).


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