Epimanteoceras amplus Yanovskaya, 1976

Mihlbachler, Matthew C., 2008, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla), Bulletin of the American Museum of Natural History 311 (1), pp. 1-475 : 355-359

Epimanteoceras amplus Yanovskaya, 1976

HOLOTYPE: PIN 3109-41, a dorsoventrally crushed skull missing the frontonasal region with right and left P1–M3. An associated partial mandible was also described by Yanovskaya (1976), but I have been unable to examine it.

AGE: Late Eocene (Ulangochuian land mammal ‘‘age’’).

TYPE LOCALITY: Ergilin Dzo (lower part), Dornogobi Province, Outer Mongolia.

DETERMINATION: Nomen dubium. This species could be a synonym of Nasamplus progressus .

DESCRIPTION

SKULL: The holotype skull (PIN 3109-41) is somewhat crushed dorsoventrally and missing the frontonasal portion (fig. 183). From the ventral view, the shape of the skull is relatively intact, although the left jugal and squamosal bones have become detached on the zygomatic arch. A large crack runs through the midline of the skull. Because the frontonasal portion of the skull is missing, it cannot be determined whether this specimen possessed an enlarged ram similar to these brontotheres or whether it had a more normal frontonasal configuration. The orbit is positioned directly above M2, with the anterior rim of the orbit positioned approximately over the anterolateral root of M1. The rostrum is relatively long and shallow with a small ridge on its dorsal surface similar to Embolotherium andrewsi (e.g., AMNH 26009; see fig. 118a). It narrows considerably anterior to the cheek teeth. The entire dorsal surface of the skull appears to have been concave (saddle-shaped), and although it does not seem to have been as deeply concave as that of Embolotherium this relates in part to the dorsoventral taphonomic flattening. The parasagittal ridges remain widely separated throughout their length, but they do not greatly overhang the sides of the skull as in E. andrewsi . From a lateral view, the zygomatic arches are weakly curved. From the ventral view, the jugal process of the zygomatic arch extends posterolaterally (although the left side is distorted), giving the zygomatics a laterally bowed appearance. The degree to which the zygomatics are laterally bowed most closely resembles Protembolotherium efremovi . They are not extremely bowed out as in E. grangeri , nor are they parallel as in E. andrewsi . The zygomatics of PIN 3109-41 lack the conspicuous lateral bulges that are seen in all specimens of Embolotherium and Protembolotherium . From a lateral view, the posterior end of the skull most closely resembles P. efremovi , with a robust but relatively short and moderately swept-back occiput. The occiput is not nearly as deep as those of Embolotherium , and not nearly as massive as that of E. andrewsi .

The anterior margin of the posterior nares is positioned near the posterior margin of M3; this position is similar to Protembolotherium efremovi and more anterior than that of Embolotherium . Other characteristics of the ventral surface of PIN 3109-41 are indistinguishable from Protembolotherium and Embolotherium . The anterior and lateral sides of the posterior narial opening are rimmed by a wide U-shaped emargination. The ridge demarcating this emargination is faint, but it can be seen when examining the actual specimen. This emargination is widest on the anterior margin and tapers along the lateral margins. The opening of the posterior nares is rather short and does not extend up onto the sphenoid. The basicranium itself is rather narrow; the total width of the basicranium at the position of the mastoid processes does not exceed the distance across the right and left M3s. The configuration of the basicranial foramina is typical. The mastoid process is shorter than the postglenoid process and it curves anteroventrally, forming a tube-shaped external auditory pseudomeatus. The external auditory pseudomeatus extends into the basicranium in a strongly posteromedial direction.

UPPER DENTITION: The incisors and canines are not preserved, although their alveoli are preserved, indicating an unreduced dental formula: 3-1-4-3. The incisor and canine alveoli indicate that these elements were rather small, but by no means reduced to a completely vestigial state. The alveolar border of the rostrum arches anterior to the canines. There is a distinct postcanine diastema.

Close-ups of the cheek teeth can be seen in fig. 183c–d. The P1 is heavily worn, but its broad shape is consistent with those that have relatively advanced crown morphology. The P2–P4 are nearly rectangular and they have thick, beaded lingual cingula and dual lingual cusps. The hypocone is consistently smaller than the protocone and connected to the protocone by a small lingual crest. The premolars are essentially indistinguishable from those of Protembolotherium efremovi and Embolotherium andrewsi but the lingual cusps of E. grangeri tend to be more completely separated. The upper molars show typical brontotheriine apomorphies including tall and lingually angled ectolophs, very weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in the least worn molars. Deep central molar fossae can still be seen on M2 and M3, though it is worn off on M1. A small anterolingual cingular cusp can still be seen on M3, but it is damaged. The M3 hypocone is a large as those of the other molars. Labial molar cingula are very weak and lingual molar cingula are mostly absent or have been worn off.

MANDIBLE AND LOWER DENTITION: I could not find the holotype mandible of PIN 3109-41, although Yanovskaya (1980) provided a photo of it (reproduced here in fig. 184). The mandible has a fairly broad symphysis with a strongly arched incisor row that compares well with Embolotherium andrewsi . The symphysis of E. grangeri is longer and more slender. The horizontal ramus of PIN 3109-41 does not seem to deepen posteriorly as strongly as E. andrewsi . The postcanine diastema is also somewhat longer. The premolars also compare well with Embolotherium , including the broad and nearly molariform p2 talonid, in addition to the strongly molariform p3 with a distinct metaconid. The thin lingual enamel of the lower molars and the elongate m3 are typical brontotheriine characters. In Embolotherium the trigonids and talonids form deep valleys rather than the broad shallow basins that are typical of most other brontotheres. The photograph of PIN 3109-41 suggests similar deep molar valleys.

REMARKS

Yanovskaya (1976) erected Epimanteoceras amplus on a nearly complete skull and associated partial mandible from Ergilin Dzo, Mongolia (PIN 3109-41). I was able to relocate the skull but not the mandible. Yanovskaya (1980) later incorrectly synonomized this species with Protitan robustus Granger and Gregory (1943) . She also erroneously transferred Protitan robustus to the genus Epimanteoceras . At any rate, PIN 3109-41 clearly differs from all the species that Granger and Gregory (1943) originally assigned to Protitan and Epimanteoceras . PIN 3109-41 has a shorter, broader symphysis, and significantly more molarized lower premolars than the holotype mandible of Protitan robustus (AMH 20104: fig. 69). Furthermore, PIN 3109-41 differs substantially from E. formosus and P. grangeri (the senior synonym of P. robustus ) due to its widened parasagittal ridges, reduced rostrum, smallish incisors, more posteriorly positioned posterior nares, shortened posterior narial canal, narrow basicranium, tubular and strongly posteromedially angled external auditory pseudomeati, advanced P1, advanced P2–P4 with dual lingual cusps, central molar fossae, anterolingual cingular cusp, and a prominent M3 hypocone.

PIN 3109-41 belongs to a more advanced brontothere similar to Embolotherium and Protembolotherium , as suggested by the posteriorly positioned posterior nares, short posterior narial canal, steeply angled external auditory pseudomeatus, narrow basicranium, and widely separated parasagittal ridges. Nonetheless, PIN 3109-41 can be assigned to none of the species of these genera. The reduced rostrum most closely resembles Embolotherium andrewsi ; although the short- er occiput, more anterior position of the posterior nares, and less prominent parasagittal ridges are more similar to Protembolotherium efremovi . Unlike Protembolotherium and Embolotherium, PIN 3109-41 lacks lateral swellings on the zygomatic arches.

Nasamplus progressus , another late Eocene brontothere, from the ‘‘Ulan Gochu’’ faunal zone (sensu Radinsky, 1964) of Inner Mongolia, is closely related to Embolotherium and Protembolotherium , and is a possible synonym of Epimanteoceras amplus . Presently, Nasamplus progressus is known only from a cranial fragment (AMNH 26014: fig. 112) including the frontal, nasal, and part of the maxilla; therefore, only a very limited comparison is possible. The diagnostic features of Nasamplus progressus are found in the nasal and frontal elements, portions of the skull that are missing in PIN 3109-41. Therefore, Epimanteoceras amplus is a nomen dubium, but if it is found to be synonymous with Nasamplus progressus , this fossil material will provide valuable character data for an otherwise poorly known species.