Scaphohippus intermontanus, PAGNAC, 2006

Stoneburg, Brittney E., Mcdonald, Andrew T., Dooley Jr., Alton C., Scott, Eric & Hohman, Charlotte J. H., 2021, New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA, PaleoBios 38, pp. 1-10 : 3-6

SCAPHOHIPPUS INTERMONTANUS PAGNAC, 2006

FIGS. 3, 4

Referred specimens —WSC 8922 partial right dentary, p4–m3; WSC 8933 upper right DP3; WSC 8934 upper left M2; WSC 9618 highly worn upper left DP2; WSC 9967 lower left p4.

Occurrence —Cajon Pass, San Bernardino National Forest, San Bernardino County, California. Exact locality data are on file at the U.S. Forest Service and WSC. Subdivision Tcv5, Cajon Valley Formation ( Woodburne and Golz 1972, Morton and Miller 2003); middle Miocene, late Hemingfordian–middle Barstovian NALMA; Tcv5 (unit 5) spans approximately 16.5–14 Ma ( Liu 1990).

Description —WSC 8933 and WSC 8934 exhibit fewer than ten plications and moderately oval-shaped isolated protocones, supporting referral to the genus Scaphohippus ( Pagnac 2006) . Furthermore, metric data (Supplemental Material 1) indicate that the occlusal surfaces of all referred teeth are consistent in size with Scaphohippus rather than the larger penecontemporaneous Acritohippus ( Kelly 1995) , with its larger occlusal surface ( Pagnac 2006: figure 3). Additional cranial and dental diagnostic features described by Pagnac (2006) cannot be assessed at this time.

Having demonstrated that our sample is best referred to Scaphohippus , the question of which species it is becomes paramount. Scaphohippus intermontanus and S. sumani were originally referred to the genus Merychippus by Merriam (1915) before being referred to the new genus Scaphohippus by Pagnac (2006), who defined the two species by differences in tooth morphology and temporal and geographic range. In terms of range, S. intermontanus is restricted to the Barstow Formation of Southern California, stratigraphically located in the informal faunal subdivisions Barstow Fauna and Second Division Fauna ( Pagnac 2006). Scaphohippus sumani is assigned a much larger range; found throughout California, Nebraska, New Mexico, and Colorado, it is also found in the Barstow Formation, in the Barstow Fauna and Second Division ( Pagnac 2006). This results in the two species overlapping stratigraphically and geographically.

According to previous descriptions, S. intermontanus dentition exhibits “higher crown height and simpler enamel complexity” ( Pagnac 2006: p. 49) than S. sumani . Merriam (1915) in part used complexity of plications (“enamel bordering fossettes,” the phrase then used by Merriam to describe plications) in order to distinguish S. sumani from S. intermontanus . Pagnac (2006) quantified this, stating that the average number of plications in S. intermontanus is two, with a maximum of four; for S. sumani , the average number is three with a maximum of seven. However, complex inner plications can be found in both S. sumani and S. intermontanus , depending on tooth position and wear state ( Figs. 3, 4). For example, in the right tooth row of UCMP 316891, assigned to S. intermontanus , P4 displays five plications, as in the holotype of S. sumani (UCMP 21422; Fig. 4A, C). In the holotype of S. intermontanus (UCMP 21400), P4 has four plications. Given the considerable overlap in the number of plications observed between the two species, we regard the difference between four and five plications as having no taxonomic significance. Although this feature might be a species-level difference, it might also result from other sources of variation, such as stratigraphic placement of specimens, population variation, and taphonomic artifacts.

Specimens of both species display an isolated protocone on the upper molars that connects to the protoconule with increased wear; however, only a 5–10% difference in wear (with an upper limit of 30% of wear) is used to characterize this particular morphological difference between the dentitions of the two species ( Pagnac 2006). It is possible that other factors, such as stratigraphic, ecological, or population differences could account for this minor dissimilarity in the appearance of certain occlusal surface features, rather than it being a defining feature of two separate taxa.

These observations from both species lead us to conclude that previously proposed differences in enamel complexity and wear states are not sufficient to distinguish between the two taxa. We refer all specimens of S. intermontanus to S. sumani , which has taxonomic priority ( Merriam 1915).