Epimanteoceras formosus Granger and Gregory (1943)

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 : 133-141

Epimanteoceras formosus Granger and Gregory (1943)

HOLOTYPE: AMNH 21613 , a complete skull with complete dentition.

TYPE LOCALITY: Ulan Shireh Formation, four miles north of Tukhum Lamasery, Inner Mongolia, China.

AGE: Middle Eocene (Irdinmanhan land mammal ‘‘age’’).

SYNONYMS: Dolichorhinoides angustidens Granger and Gregory, 1943 .

REFERRED SPECIMEN: (From the Ulan Shireh Formation, eight miles north of Tukhum-in-Sumu, Inner Mongolia) AMNH 21607 (holotype of Dolichorhinoides angustidens ), a partial skull with right P1–M3, left P4 (partial), and M1–M3.

DIAGNOSIS: Epimanteoceras formosus is a large brontothere with poorly developed frontonasal horns. The frontal bone intrudes into the surface of the nasal bone, thus splitting off a lateral nasal splint from the main body of the nasal bone. Some specimens of Epimanteoceras formosus exhibit a distinct but weakly developed frontonasal protuberance. The nasal incision extends as far back as the P2. The nasal process broadens distally; it is nearly horizontal, unelevated, narrow, with thin lateral walls, and without a well-defined or strongly rounded distal margin. The orbits are positioned above the posterior portion of M2 and the anterior portion of M3. The orbits do not protrude laterally. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a dorsal cranial surface that is convex but incompletely saddle-shaped, separated parasagittal ridges, distinctly curved zygomatic arches, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Additionally, E. formosus is unique in having a very robust superorbital process and a distinct lateral temporal ridge.

Epimanteoceras formosus has subcaniniform upper incisors, a metacone on the P1, a distinct P2 metacone, and weak premolar preprotocristae. Premolar hypocones are present and connected to the protocones, but they are small and occasionally absent in some of the premolars. The molars of E. formosus have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Central molar fossae are present, however, anterolingual cingular cusps are absent. Paraconules and metalophs are absent.

The skull of Epimanteoceras formosus is morphologically intermediate between Telmatherium and Protitan . It can be most easily distinguished from the former by its greater size, more widely separated parasagittal ridges, and distinct central molar fossae. It can be distinguished from the later by its more weakly developed frontonasal protuberance, incompletely saddled-shaped cranium, and lack of a postzygomatic process. The upper premolars are significantly more molarized than either Protitan or Telmatherium .

DESCRIPTION

SKULL: Epimanteoceras formosus is a large brontothere that is most similar in size to Protitan grangeri and Protitanotherium emarginatum . This species is known from a nearly complete and undistorted holotype skull (AMNH 21613) (fig. 63). The only significantly reconstructed portion of the skull is the left posterior region. An additional partial skull, AMNH 21607, is referable to E. formosus (fig. 64). This skull is less complete, laterally compressed, and the dorsal surface is distinctly warped; however, this skull is superior to the holotype in some ways; it is ontogenetically younger than the holotype with less worn cheek teeth and more distinct cranial sutures. Additionally, this specimen was less extensively prepared and some aspects of the ventral surface of the skull are less damaged and not covered with plaster.

It can be seen on AMNH 21613 and even more clearly on AMNH 21607 that the distinctive configuration of facial bones seen in some hornless brontotheres, such as Telmatherium , is shared by Epimanteoceras formosus . A large triangular process of the frontal bone splits the posterolateral portion of the nasal bone. Dorsally, the frontonasal suture recedes posteromedially, but at the midline it is acutely reoriented anteriorly. Laterally, the frontonasal suture is directed sharply backward toward the orbit. The lateral nasal splint arcs posteroventrally and contacts the lacrimal bone, thus preventing contact of the maxilla and frontal bone. The nasomaxillary suture is less distinct than the frontonasal suture (and cannot clearly be seen in the photo), but it forms a dorsally arched contact between the lateral nasal splint and the maxilla.

In AMNH 21613 the overlapping frontal process forms a small but distinct protuberance above and directly anterior to the orbit. This frontonasal protuberance is not as well developed as those of Protitan species, but it is more distinctly more developed than that of Telmatherium in which the triangular frontal process is flush with the dorsal surface of the skull. In addition to the frontal nasal protuberance, AMNH 21613 has a very thick and prominent superorbital process that overhangs the orbits. This superorbital process is thickened with a roughened surface that is semicontinuous with that of the frontonasal protuberance. In AMNH 21607, the triangular process does not form a distinct frontonasal protuberance and it is smooth and nearly flush with the surface of the skull. Likewise, the superorbital processes seem less laterally prominent but this could be attributed to the fact that the skull is laterally compressed, and/or that the specimens is ontogenetically younger than the holotype skull.

The maxilla forms a shallow preorbital concavity. The face is only moderately constricted by the nasal incision, which extends to a point above the anterior margin of P2. The orbit is positioned over the anterior portion of M3 and the posterior portion of M2. The anterolateral root of M2 and posterolateral root of M1 rest directly below the anterior rim of the orbit.

The nasal process is the same length as the premaxillomaxillary rostrum. The nasal process of AMNH 21613 is nearly straight and the dorsal surface of the nasal process is slightly convex from the lateral view. The nasal process projects anteriorly from the skull in a slightly upward direction. In contrast, the nasal process of AMNH 21607 has a slight downward curvature. This subtle difference in nasal orientation may relate to taphonomic distortion in the latter specimen. The sides of the nasal process form deep and thin vertical walls that extend to the distal end of the nasal process. From the dorsal view it can be seen that the nasal bones are poorly ossified. The nasal process is narrower than the premaxillomaxillary rostrum, although the nasal process significantly broadens distally. The distal edge of the nasal process is thin, roughened, and nearly flat from a dorsal view. However, the anterior view reveals that the anterior edge is turned downward. The mesiodistal corners of each nasal bone form a single downturned distal tip.

The premaxillomaxillary rostrum is robust and from a lateral view it appears to deepen in a posterior direction. The dorsolateral surface of the rostrum rises posteriorly at a shallow angle so that the posterior notch of the nasal incision is level with the upper rim of the orbit. The premaxillomaxillary suture is distinct in AMNH 21613. The nasal processes of the premaxillae are truncated anterior to the posterior notch of the nasal incision and do not contact the nasal bone. The premaxillary symphysis of AMNH 21613 is not ossified. From the anterior view the lateral margins of the rostrum are posterolaterally divergent and the rostral cavity is open dorsally and continuous with the nasal cavity.

The dorsal surface of the skull of Epimanteoceras formosus forms a shallow concavity. However, the skull is incompletely saddle-shaped; the dorsal surface is flat or slightly convex posteriorly. From a dorsal view the parasagittal ridges remain separate throughout their length and only moderately constrict the dorsal surface posteriorly. The parasagittal ridges are prominent and they can be seen standing out in relief from the posterior view of AMNH 21613.

The zygomatic arches are rather thin. From a dorsal view the jugal portion of the zygomatic is straight and strongly angled posterolaterally. From a lateral view the jugal portion is shallow and horizontal, while the squamosal portion of the zygomatic arch is deeper and rises posteriorly at a shallow angle, thus giving the zygomatic arch a distinct curvature. A small ventral flange can be seen below the jugal-squamosal contact, but this flange is not nearly as conspicuous as that of Metatelmatherium . A postzygomatic process, as seen in Protitan , is not present. One peculiar aspect of the skull of Epimanteoceras formosus is the presence of a prominent temporal ridge that extends anteroposteriorly on the lateral surface of the skull behind the zygomatic arch.

The occipital region is the least well-preserved portion of the holotype specimen. From the right side it can be seen that the occiput is strongly tilted backward. From a dorsal view the nuchal crest is mildly concave. From the posterior view the dorsal margin is weakly dorsally arched. The upper half of the occiput is somewhat narrower than the lower half and it appears to be somewhat constricted in the middle. One can discern a weak occipital pillar on the right posterior surface of the occiput although few other details of the occiput are preserved.

The posterior nares of both specimens of Epimanteoceras formosus are rimmed by a Ushaped emargination (figs. 64b, 65a). The posterior narial canal is elongate, but it does not extend into the basisphenoid. The anterior margin of the posterior nares is positioned between the M3 protocones in AMNH 21613, although the exact edge is obscured by plaster that fills the posterior narial canal. In AMNH 21607 the anterior edge of the posterior nares is slightly anterior to the M3 protocones. Many details of the holotype specimen are obscured by plaster filling the posterior narial canal. However, in AMNH 21607 many of the thinner more fragile elements of the posterior narial canal can be seen (fig. 64b). A thin, raised, horizontal plate of bone extends posteriorly from the palatal margin of the posterior nares and covers the choanae to a point behind M3. A large crack can be seen running through this bony palatal extension and the posterior portion is crushed. Behind the palatal extension, the posterior narial canal is filled with sediment. The thin elongate vomer can be seen bisecting the posterior narial canal. Two thin and broken choanal pouches of bone suspended in the remaining sediment are exposed directly behind the secondary bony palate. These bony pouches most likely represent posteriorly shifted maxilloturbinates, as seen, for instance, in Dolichorhinus . Behind these choanal pouches, the sediment filling the posterior half of the right posterior naris is free of any bone fragments. The left posterior naris has been filled with white plaster and is clearly visible. This plaster filled gap fills the posteriorly shifted functional posterior nares. Thus, the functional posterior nares are posteriorly shifted in Epimanteoceras formosus , a condition closely resembling that of Dolichorhinus .

The configuration of the basicranial foramina is typical; the foramina of the alar canal, the foramen ovale, and the foramen lacerum are widely separate. The external auditory pseudomeatus is wide, ventrally unconstricted, and enters the skull in a mediolateral direction.

UPPER DENTITION: The holotype (AMNH 21613) retains a complete set of upper dentition (fig. 65) indicating an unreduced dental formula (3-1-4-3) while the referred specimen, AMNH 21607, retains a set of less worn cheek teeth (fig. 64b, c, d). The incisors of AMNH 21613 are similar in size to those of Protitan . There are small diastemata between each incisor. The incisor apices are worn, but these teeth are essentially subcaniniform, with short, pointed, and slightly lingually curved crowns, and with distinct lingual cingula. The incisors become progressively larger from I1 to I3. The incisor row forms a broad arch anterior to the canines. The canines of AMNH 21613 are heavily worn and broken though they appear to have been quite large.

The premolars of the two skulls differ in several ways and there are notable bilateral asymmetries in right and left dentitions, suggesting that the premolars may have been morphologically unstable in Epimanteoceras formosus . The P1 of AMNH 21613 is heavily worn, although it appears that a paracone and a smaller metacone were present. The P1 of AMNH 21607 is incomplete, but it is less worn. That specimen suggests a more complex P1 with a lingual heel and possibly a small protocone. The P2 of both specimens is more oblique than P3 or P4. The parastyle of AMNH 21613 is straight, although the less worn P2 of AMNH 21607 suggests that the parastyle arches slightly lingually. The parastyle of P3 on AMNH 21613 is broken, but it is straight on AMNH 21607. The p4 parastyles of both specimens are angled slightly labially. The metastyles of P2 and P3 are straight while that of P4 is angled slightly labially. Distinct labial paracone ribs can be seen in AMNH 21607; these become smaller in more posterior premolars. Finally, the metacone of the P 2 in both specimens is slightly lingually shifted.

The lingual features of the premolars (P2– P4) are higher in relief than those of Protitan and the lingual heels are relatively broader. Very faint preprotocristae can be seen on the P2–P4 of AMNH 21613, but only on the P2 of AMNH 21607. The P2s of AMNH 21613 each retain a prominent lingual crest extending posteriorly from the protocone, although there are no P2 hypocones. In contrast, hypocones on the P2s of AMNH 21607 are present and positioned very closely to the protocone. On AMNH 21613, the hypocones of the P3s are bilaterally asymmetrical; it is absent on the right P3 but on the left P3 there is a hypocone positioned close to the protocone and connected to it by a short lingual crest. The P3s of AMNH 21607 have distinct hypocones that are poorly separated from the protocones. The P4s of AMNH 21613 have distinct hypocones that are connected to the protocones by short lingual crests. Hypocones are absent on the P4s of AMNH 21607; nonetheless, the P4 crowns retain prominent lingual crests extending posteriorly from the protocones. The labial premolar cingula of both specimens are weak. On AMNH 21613 the lingual cingulum of P2 is continuous, while cingula of P3 and P4 are slightly discontinuous. On AMNH 21607 the lingual cingula of the P2–P3 are thicker and more continuous, but this final difference may relate to the fact that the premolars of this specimen are less worn.

The molars of Epimanteoceras formosus exhibit numerous brontotheriine apomorphies, including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in those molars that are not heavily worn. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Shallow central molar fossae are present, but anterolingual cingular cusps are absent. E. formosus molars lack vestigial paraconules, and all evidence of metalophs in M1 and M2 is lost. The M3 of AMNH 21613 has a small metalophlike ridge, while AMNH 21607 has a very tiny hypocone. Labial molar cingula in AMNH 21613 are thin and beaded, while lingual molar cingula are absent.

REMARKS

Epimanteoceras formosus was first described from a single specimen (AMNH 21613), an essentially complete and remarkably undistorted skull with heavily worn teeth. Granger and Gregory (1943) remarked that E. formosus was ‘‘distinctly advanced beyond the stage of Manteoceras (5 Telmatherium ) and in the direction of Rhinotitan ’’ ( Granger and Gregory, 1943: 358). Seemingly, this remark stems from the fact that the holotype cranium of E. formosus is intermediate in size and morphology between Telmatherium validus and those of brontotheres with conspicuous frontonasal protuberances such as Rhinotitan . Although other brontothere species with the characteristic overlapping triangular frontal process, such as Telmatherium validus , have been misleadingly described as having rudimentary horns (e.g., Osborn, 1929a), E. formosus is the only brontothere known to have weakly developed frontonasal protuberances. Other brontotheres either lack a distinct frontonasal protuberance or have a more conspicuous one.

Granger and Gregory (1943) noted that another specimen, AMNH 21607, is remarkably similar to Epimanteoceras formosus , but they erected a new taxon, Dolichorhinoides angustidens , for it. It is curious that Granger and Gregory (1943) chose the name Dolichorhinoides because they state ‘‘this skull differs conspicuously from those of the American Dolichorhinus ’’ ( Granger and Gregory, 1943: 363). Mader (1998) considered both Dolichorhinoides and Dolichorhinus to be junior synonyms of Sphenocoelus . It was demonstrated earlier in this paper that Dolichorhinus is actually very distinct from Sphenocoelus (see remarks under Dolichorhinus hyognathus ), and it is argued below that the holotype of Dolichorhinoides angustidens is likewise different from both Sphenocoelus and Dolichorhinus .

The bony palatal extension, the extremely posteriorly situated maxilloturbinates, and posteriorly shifted functional posterior nares seen in AMNH 21607 resemble Dolichorhinus hyognathus . However, because these elements are extremely fragile, they cannot be investigated in the majority of brontothere specimens, and hence these characters are unknown for many species. Additionally, AMNH 21607 is inconsistent with the following aspects of Mader’s (1998) diagnosis of Sphenocoelus (which, in his concept, included Dolichorhinus ). (1) Sphenocoelus (sensu Mader, 1998) lacks hypocones on the premolars (unmolarized premolars, sensu Mader, 1998) whereas the P2–P3 of AMNH 21607 have hypocones. (2) Small, vestigial paraconules or metaconules are variably present on molars of Sphenocoelus (sensu Mader, 1998) , but they are absent on AMNH 21607. (3) The skulls of Sphenocoelus (sensu Mader, 1998) are highly elongate; however, the apparently elongate skull of AMNH 21607 is largely a product of moderate lateral crushing. (4) Sphenocoelus (sensu Mader, 1998) possesses a large infraorbital process, but AMNH 21607 lacks this character. Additionally, the triangular process of the frontal bone overlapping the nasal bone that is clearly discernable on AMNH 21607 is not seen in Sphenocoelus uintensis or Dolichorhinus hyognathus . Finally, the postorbital portions of the crania of Dolichorhinus and Sphenocoelus are arched dorsally while that of AMNH 21607 is not.

After further examination of the holotypes of Epimanteoceras formosus (AMNH 21613) and Dolichorhinoides angustidens (AMNH 21607), these specimens do not appear to vary in any way that is seemingly taxonomically significant, and these specimens almost certainly represent the same species. Variability in the size and distinctness of the frontonasal protuberance is found among nearly all species of horned brontotheres. Likewise, extensive variation in the lingual morphology of the premolars is the rule rather than an exception within brontothere species.

Granger and Gregory (1943) concluded that the only clear distinction between Epimanteoceras formosus (AMNH 21613) and Dolichorhinoides angustidens (AMNH 21607) was the more elongate molars of the later. However, the difference can be attributed to dental wear in AMNH 21613, and, less significantly, to taphonomic deformation of AMNH 21607. Wood (1938) documented extreme ontogenetic shortening of the molars of rhinoceroses. A similar phenomenon occurs among brontotheres. Brontothere upper molars are longer on the labial side of the tooth. In AMNH 21607, where the molars are less worn, they are longer on the labial side than on the lingual side. Consequently, adjacent molars contact each other on the labial side but do not contact each other on the lingual side (fig. 64c). In comparison, the dentition of AMNH 21613 is more extensively worn (fig. 65a). In the later specimen, the labial and lingual sides of M1 and M2 are nearly the same length and the adjacent molars contact each other both labially and lingually. However, the anterior and posterior enamel of the M1 and the anterior enamel of M2 are completely worn off. Clearly, the ectolophs of these teeth have been significantly shortened by interstitial wear, yet the sides of the molars are still in close contact. Apparently, the maxilla remodeled as the teeth shortened so that molars maintained contact despite having become significantly shorter on the labial side. Wear patterns in other brontothere molars exhibit the same pattern of ontogenetic tooth-row shortening. It is therefore evident that maximum lengths of the molars and even tooth-row lengths are influenced by dental wear.

In addition to differences in molar proportions due to interstitial wear, the molar proportions of AMNH 21607 have been taphonomically distorted. That specimen is laterally compressed and although the teeth appear to be minimally damaged, closer inspection reveals that the molars have been labiolingually compressed, thus exaggerating their relative length. A structural weakness exists in brontothere molars between the ectoloph and the lingual cusps. Those features contributing to the weak point include (1) increased ectoloph height without associated increase in height of the lingual side of the tooth, and (2) the appearance of the central molar fossa at the base of the ectoloph. Brontothere molars with these features are often cracked at this weak point and are susceptible to significant distortion from lateral crushing but without obvious damage to the tooth. Essentially, the tall ectoloph can be forced up over the lower lingual side of the tooth. This sort of damage is most evident in the right M3 of AMNH 21607 (fig. 64b) where the tooth has been considerably narrowed because the ectoloph has been forced over the lingual side of the tooth. The distortion is subtler on the left M3 where the shape of the tooth along the anterior margin is intact, but the posterior half of the ectoloph has been lingually displaced (fig. 64c). The damage to the M1 and M2 is less obvious, but tiny thrust faults in the enamel suggest a small amount of lingual displacement of the ectolophs.

A reliable comparison of the relative dimensions of the molars of AMNH 21613 and AMNH 21607 is clearly hampered by the extensive wear in the former and subtle distortion in the latter. However, measurements of a limited number of seemingly intact dimensions are possible. For instance, the left M3 of AMNH 21613 is less extensively worn. It is thus possible to compare M3 anteroposterior length in AMNH 21613 and 21607 if measured at the proximal base of the crown. At this location, the lengths are similar (AMNH 21613 5 70 mm; AMNH 21607 5 68 mm). Likewise, the left M3 of AMNH 21607 seems to be nearly intact along its anterior edge, thus allowing for a single comparison of the undistorted labiolingual widths of the M3s (AMNH 21613 5 58 mm; AMNH 21607 5 57 mm). Ratios calculated from the length of M3 divided by the width of the anterior margin yield nearly identical values (1.2 for AMNH 21613 and 1.19 for AMNH 21607). Therefore, it is relatively safe to conclude that whatever apparent differences exist in the dental dimensions of AMNH 21613 and 21607 that were observed by Granger and Gregory (1943) they can largely be attributed to wear and distortion. With little else to distinguish Dolichorhinoides angustidens as a valid taxon, it is here considered a junior synonym of Epimanteoceras formosus .

The only valid species of Epimanteoceras is E. formosus . Other species assigned to Epimanteoceras , including E. praecursor Yanovskaya (1953) from the upper Eocene of Kazakstan, and E. amplus Yanovskaya (1976) from the late Eocene Ergilin Dzo of Mongolia, are dubious species and are further discussed in the section dealing with nomina dubia and other problematic taxa. Another dubious taxon, ‘‘ Protitan ?’’ cingulatus , known only from a set of mandibles, is possibly a junior synonym of E. formosus .