Embolotherium grangeri Osborn, 1929b

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 : 251-264

Embolotherium grangeri Osborn, 1929b

HOLOTYPE: AMNH 26002 , a skull with right and left P1–M3, and three isolated upper incisors.

TYPE LOCALITY: ‘‘Ulan Gochu’’, East Mesa, ‘‘middle red’’ beds, Shara Murun Region, Inner Mongolia, China. (Ulan Gochu [used in quotes] refers to a faunal zone rather than a formation. [ Radinsky, 1964])

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

SYNONYMS: Embolotherium louksii Osborn, 1929b ; Titanodectes ingens Granger and Gregory, 1943 ; Titanodectes minor Granger and Gregory, 1943 ; Embolotherium insigne Yanovskaya, 1980 .

REFERRED SPECIMENS: (From the ‘‘Ulan Gochu’’ faunal zone [sensu Radinsky, 1964] of the Shara Murun Region, East Mesa, Inner Mongolia) AMNH 26004, the complete ventral surface of a skull with left and right P2–M3 and the distal portion of the ram; AMNH 26040, the anterior portion of a juvenile skull with right DP2–DP4, M1, left P1, DP2–DP4, M1, an associated pair of premaxillae with unerupted incisors, numerous isolated deciduous incisors, and a mandible with canines and incisors (unerupted), right and left p1, dp2–dp4, and m1; (from the ‘‘Shara Murun’’ Formation [sensu Radinsky, 1964] of the Shara Murun Region, East Mesa, Inner Mongolia) AMNH 26005 (holotype of Titanodectes ingens ), a mandible in three pieces, right ramus, left ramus, and a symphysis with right i1–m3, left i1–c, and p3–m3; AMNH 26132 (holotype of Titanodectes minor ), a partial mandible with right p3–m1, left p2–m1, and a separate symphysis fragment with right i1–c and left i2–i3; (From the Ulan Gochu Formation of the Shara Murun Region, Baron Sog Mesa, Inner Mongolia) AMNH 21610 (holotype of Embolotherium louksii ), a partial skull with erupting?M3; (from the ‘‘Baron Sog’’ Formation [sensu Radinsky, 1964] of the Shara Murun Region, East Mesa, Inner Mongolia) AMNH 26018, a mandible fragment with right p4–m3 and isolated (?)left incisors and canine; (from the Shara Murun Formation of the Shara Murun Region, Baron Sog Mesa, Inner Mongolia) AMNH 21600, a mandible with right i1–i2, p2–m3, left i2–c, and p2–m3 (from the Ergilin-Dzo Svita at Khoer Dzan, Outer Mongolia) PIN 3110-52 (holotype of Embolotherium insigne ), a partial skull with left molars. The PIN also holds a large but uncatalogued collection of Embolotherium grangeri from Khoer Dzan (to be described in a later publication), including skulls from individuals of numerous ontogenetic stages.

DIAGNOSIS: Embolotherium grangeri is a very large brontothere with a tall process of bone (ram) that rises from behind the orbits at approximately a 45 ° angle. The rugose transverse crest at the peak of the ram does not form a distal emargination for the nasal channel. The nasal incision extends above the orbits to the anterior margin of the M3. The orbits are positioned above M2. The premaxillomaxillary rostrum is long, deepens slightly proximally, and is not fully enclosed by bone dorsally. A thick U-shaped ridge of bone is seen on the dorsal surface of the premaxilla. Other cranial characteristics include a saddle-shaped cranium, an unthickened occiput, prominent parasagittal ridges that do not constrict the dorsal surface of the skull posteriorly, and weakly curved and laterally bowed zygomatic arches with massive swellings. The external auditory pseudomeatus enters the skull in a posterolateral direction and is ventrally constricted. The posterior nares are shifted posteriorly behind M3. Large ventral sphenoidal fossae are absent.

Dentally, Embolotherium grangeri has three relatively large but globular upper incisors that form a semicircular row. The canines are very small. There is a distinct P2 metacone. Hypocones are present on P2–P4 and are either weakly connected to the protocone by a lingual crest or they are completely separated from it. The molars of E. grangeri 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. Deep central molar fossae and large anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower dentition of E. grangeri includes three large semispatulate incisors that form an arch anterior to the canines. The i2 is the largest incisor. A metaconid is present on p3, p4, and occasionally on p2. The p2 trigonid is only marginally longer than the talonid. The lower molars have deep valleylike basins and the m3 is elongate.

Embolotherium grangeri can be distinguished from E. andrewsi in the following ways: ram less vertical, positioned behind orbits; nasal channel of ram not distally emarginated; rostrum and incisors much larger; dorsal rostral ridge larger; occiput less massive; zygomatic arches strongly bowed laterally; parasagittal ridges less prominent; premolar hypocones more strongly separated from the protocones; M3 hypocone larger; and cingulum of M3 discontinuous around the distolingual corner of the crown.

DESCRIPTION

SKULL: The holotype of Embolotherium grangeri (AMNH 26002) is a complete and nearly undistorted cranium with only minor fragments of missing bone that have been replaced with plaster (fig. 122). The most significant damage to the holotype specimen is a large crack on the dorsal and lateral surfaces of the skull just behind the base of the ram. The bone anterior to this crack is depressed about a centimeter, suggesting that the ram has been forced downward. Howev- er, comparison of this skull with other specimens seems to confirm that the angle of the ram is essentially intact. Embolotherium grangeri has never been described or figured from the fully prepared type specimen. Unfortunately, the only previously published photos and drawings of AMNH 26002 ( Osborn, 1929a: fig. 797; 1929b: figs. 3b, 7) were made while the specimen was still in its plaster wrappings and some aspects of its shape in these older figures are misleading (relating to pieces of sediment that had been wrapped with the specimen). In addition to the holotype there is an additional adult specimen of a larger individual that consists of a ventral surface of the skull and the distal segment of the ram (AMNH 26004) (fig. 123), and three partial subadult skulls (AMNH 21610, AMNH 26040, and PIN 3110-52). E. grangeri is similar in size to E. andrewsi and shares with it a massive upturned ram. The ram of E. grangeri originates from behind the orbits, rather than from above as in E. andrewsi . The ram projects from the skull at approximately 45 °. From a lateral view the ram is nearly straight, but it is slightly curved downward. The posterodorsal surface is convex and the anteroventral margin is slightly concave. Like E. andrewsi the lateral walls of the ram are deep and thick, forming a ventral nasal channel that runs to the distal end of the ram. The lateral walls are about the same depth throughout the length of the ram although they become slightly shallower distally. Proximally, the lateral walls are shallower than those of E. andrewsi and they do not constrict the nasal channel. A thin ossified ridge (osteological marker for the cartilaginous nasal septum) runs along the anteroventral surface of the ram and extends to the distal end. As in E. andrewsi , the nasal channel and ossified marker for the nasal septum indicate that the nasal cavity would have extended to the distal end of the ram, although the actual position of the nostrils is uncertain. However, because of the more posterior position of the ram and its shallower angle, the space between the ram and the rostrum is tremendous, suggesting an enormous nasal cavity ( Mihlbachler and Solounias, 2004). The nasal channel is bifurcated distally by a wedge-shaped process that extends from the anteroventral surface at the distal end of the ram. The wedge-shaped process and distal bifurcation of the nasal channel is best seen in the ram of AMNH 26004 (fig. 123b). This structure appears as a median bulge from the dorsal view of AMNH 26002 (fig. 122b). The strong rim of bone that emarginates the distal end of the nasal channel in the ram of E. andrewsi is absent in adult specimens of E. grangeri . Instead, the nasal channel is open distally. However, like E. andrewsi , the distal surface of the ram is very rugose, particularly in AMNH 26004.

The ram of Embolotherium grangeri superficially resembles the nasal processes of other brontotheres. In particular, the lateral walls, deep ventral nasal channel, and distally widening shape resemble the nasal processes of brontotheres such as Epimanteoceras formosus , although the ram is obviously longer, more massive, and more steeply angled upward. There are no discernable sutures on any of the previously described specimens of Embolotherium grangeri , and this has led to some ambiguity in interpreting the structural makeup of the ram of E. grangeri ( Mihlbachler et al., 2004a) . Howev- er, the uncatalogued collection of E. grangeri from Khoer Dzan at PIN contains specimens that clearly reveal the ram of E. grangeri to be structurally homologous to the rams of E. andrewsi and Protembolotherium efremovi , and with the paired frontonasal horns of other brontotheres.

The nasal incision of Embolotherium grangeri is very deep. The posterior margin of the nasal incision extends above the orbit and is directly above the anterior margin of M3. The orbit is positioned above M2, with the anterolateral root of M2 and posterolateral root of M1 situated below the anterior rim of the orbit.

The dorsal margin of the rostrum is long and deepens slightly posteriorly. The dorsal margin is nearly horizontal and does not rise above the midpoint of the orbit. The premaxillae are completely fused at the symphysis and the premaxillomaxillary sutures are not discernable. The premaxillary symphysis is nearly vertical. Above the symphysis is a tall and thick U-shaped ridge of bone. This ridge is thicker and more rugose than the anterodorsal rostral ridge seen in Embolotherium andrewsi . The sidewalls of the rostrum are parallel and constrict the dorsal opening of the premaxillomaxillary chamber to form an elongate narrow channel along the dorsal rostral surface.

The dorsal surface of the cranium is deeply saddle-shaped. The parasagittal ridges are widely separated throughout their length and do not significantly constrict the dorsal surface posteriorly. The parasagittal ridges are less prominent than those of Embolotherium andrewsi and only slightly overhang the sides of the cranium. The zygomatic arches of E. grangeri resemble those of E. andrewsi in having massive swellings at the junction of the jugal and squamosal. From a lateral view, the jugal portion of the zygomatic is horizontal and it is not as deep as that of E. andrewsi . The squamosal portion of the zygomatic is deeper and slightly sloped upward, giving the zygomatic arch a weak curvature. There is a small ventral flange below the large zygomatic swelling. From the dorsal view the zygomatic arch is thinner than that of E. andrewsi except for the large central swelling. Additionally, the zygomatics are more strongly bowed laterally.

From a lateral view the posterior end of the cranium is similar to that of Embolotherium andrewsi . The dorsal surface is strongly curved upward and the occiput is strongly tilted backward. However, the occiput and nuchal crest of E. grangeri are neither as massive nor as rugose as those of E. andrewsi . From the dorsal view of the skull the nuchal crest is concave, but it is not as deeply notched as that of E. andrewsi , and the nuchal crest is comparatively thin. From the posterior view the occiput is nearly square. The dorsal margin of the occiput is essentially flat. The dorsal portion of the occiput is as wide as the posterior portion and the occiput is not constricted in the middle. The bony pillars on the surface of the occiput are not as massive as those of E. andrewsi and the central depression of the occiput is not as deep.

Like Embolotherium andrewsi the posterior nares of E. grangeri are positioned completely behind M3 (fig. 124a). Paired ventral sphenoidal fossae are absent. The proportions of the basicranium are essentially normal. The transverse width across the mastoid processes is less than the transverse distance across the M3s. The configuration of the basicranial foramina are typical, with a widely spaced foramen ovale and foramen lacerum. Like in E. andrewsi the external auditory pseudomeatus enters the skull at a posteromedial angle. The mastoid process is shorter than the postglenoid process; it curves anteroventrally, contacts the postglenoid process, and forms a tube-shaped external auditory pseudomeatus.

UPPER DENTITION: The description of the upper dentition of E. grangeri is based primarily on the holotype, AMNH 26002 (fig. 124), although important taxonomic information on the dental formula and morphology of the upper teeth of E. grangeri is provided by a juvenile specimen (AMNH 26040) that is discussed further below.

Although no incisors or canines are preserved within the holotype skull (AMNH 26002), the intact alveoli indicate an unreduced dental formula (3-1-4-3). The incisor and canine alveoli form a semicircular arch. The diameter of the anterior tooth row arch (measured transversely across the canines) is similar to the width across the P1s, so that the canines are positioned anteriorly to the P1s rather than anterolingually as in E. andrewsi . A small diastema is present between the I3 alveolus and canine. The sizes of the incisor alveoli indicate that the incisors were significantly larger than those of Embolotherium andrewsi .

Three complete isolated incisors that are consistent in size with the incisor alveoli of the holotype skull were assigned the same field number (770) and catalog number ( AMNH 26002 ) (fig. 124d, e). The association of these incisors with the holotype is uncertain because the canine alveoli of the skull are filled with sediment, indicating that the incisors of the holotype must have fallen out of the skull before burial. The isolated incisors were probably found somewhere in close proximity to the skull, but they do not necessarily belong with the holotype. Nonetheless, similar incisors found in the juvenile skull ( AMNH 26040 ; see below) confirm that these isolated incisors belong to the species E. grangeri . The ovoid-globular crowns are angled lingually. One of them has a small irregular lingual cingulum. Another incisor crown has a shallow pit on the lingual side of the crown apex and the third has a completely smooth surface. Otherwise, the crowns are essentially featureless and globular, although they are abnormally large in comparison to other brontotheres with similar globular incisors. Nonetheless, the absence of regular wear facets on these teeth suggests they may have been vestigial .

The canine alveoli of AMNH 26002 suggest that the canines were not much larger than the incisors, and the juvenile specimen described below (AMNH 26040) confirms this. The postcanine diastema is similar in length to the P2. The P1s are heavily worn, but the left P1 reveals a paracone, a metacone, and a small posteriorly shifted lingual heel with a small lophlike cusp. The remaining premolars (P2–P4) are nearly as rectangular (not oblique), although the anterior margins of these premolars are concave. The parastyle of P2 is angled slightly lingually. The parastyles of P3 and P4 are directed somewhat labially. The metastyles of these teeth are worn, but appear to have been straight or slightly curved labially. There are distinct but weak labial paracone ribs on each of these premolars, although they are relatively smaller on progressively posterior premolars. A mesostyle is not present on any premolar. Preprotocristae are not seen on the premolars of AMNH 26002. There are two distinct lingual cusps on P2–P4. The P2 hypocone is about the same size as the protocone and it is barely connected to it. The hypocones of P3 and P4 are slightly smaller then the protocones. These cusps are completely separated because there are no lingual connecting crests on the premolars. The labial premolar cingula are weak while the lingual cingula of P2–P4 are weak and discontinuous around the lingual base of the paracone.

The premolars of another adult specimen, AMNH 26004, differ from those of AMNH 26002 in the following ways: the hypocone and protocone of P2 are connected by a stronger, taller lingual crest; in P3 and P4 the hypocones are smaller and connected to the protocone by a thin connecting crest; the lingual cingulum is thicker and continuous.

The upper molars of Embolotherium grangeri show typical brontotheriine characteristics, 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 molars that are not heavily worn. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Deep central molar fossae and large anterolingual cingular cusps are present. All evidence of paraconules and metalophs is lost. The M3 hypocones of E. grangeri are nearly the same in size and shape as those of the M1 and M2. Labial molar cingula are weak while lingual molar cingula are absent. The cingulum of M3 is discontinuous and does not wrap around the distolingual base of the crown of M3 as it does in Embolotherium andrewsi .

MANDIBLE AND LOWER DENTITION: None of the adult skulls of Embolotherium grangeri is associated with a mandible, although a juvenile skull, AMNH 26040, is associated with a mandible that includes a complete set of unerupted adult anterior dentition. That specimen, described further below, is an important link for identifying other mandibles of E. grangeri . Among these are AMNH 26005, AMNH 26132, and AMNH 21600 (fig. 125). The largest of these specimens, AMNH 26005, consists of three pieces, two separate mandibular rami (not shown), and a crushed fragment of the symphysis (fig. 125e). The most complete specimen is AMNH 21600, a nearly complete and uncrushed mandible that is missing some incisors and cheek teeth (fig. 125a–c). Finally, AMNH 26132 consists of the anterior portion of the mandible with a separately preserved anterior fragment of the symphysis with lightly worn incisors and canine (fig. 125f–h).

The horizontal ramus of AMNH 21600 is shallow and deepens slightly posteriorly. The ventral margin of the symphysis has a shallow angle (, 45 °). The symphysis extends to the talonid of the p4.

The incisors of these specimens form semicircular arches in front of the canines. The i2 is the largest incisor. The crowns of i1 and i2 are semispatulate with convex labial sides and flat or slightly concave lingual sides. The i3 crown is more conular. Lingual cingulids are very weak or absent. Labial cingulids are not seen, although faint labial cingulids can be seen in the unworn incisors of AMNH 26040 (fig. 128). The canines of AMNH 26005 and AMNH 21600 are about the same size as the incisors, no greater in diameter or height than i3. However, the canines of AMNH 26132 are taller than the incisors and they are more lingually curved. Nonetheless, the canines of all specimens can generally be described as very small. There are no diastemata between the anterior dentitions of these specimens. However, a postcanine diastema is present.

The p1 is not preserved in AMNH 21600, nor is there a p1 alveolus, although the p1 could have fallen out in life with the alveolus being lost due to bone remodeling. In AMNH 26005 the p1 is tiny in comparison to the posterior cheek teeth and it is heavily worn. The p1 of the juvenile specimen (AMNH 26040, fig. 125d) is unworn. Its crown consists of a single cusp and a talonid heel. In AMNH 21600, the p2 trigonid and talonid are similar in length, although the trigonid is slightly narrower. The trigonids of p3 and p4 are slightly shorter and narrower than their talonids. The p2 paralophid of AMNH 21600 is broken. The p2 paralophid of AMNH 26132 is strongly curved lingually. The p2 protolophid of AMNH 26100 is straight and not angled lingually, although the entire lophid is positioned lingually. The p2 protolophid of AMNH 26132 is angled lingually and the trigonid has a broad lingual notch. Additionally, AMNH 26132 has a small p2 metaconid while AMNH 26100 does not. Similar intraspecific variability in the presence and absence of a p2 metaconid is seen in Embolotherium andrewsi . The paralophids and protolophids of p3 and p4 arch lingually 90 °, forming nearly molariform trigonid valleys. The talonids of p2–p4 have well-developed cristids obliqua and hypolophids with deep valleylike talonid basins similar to those of the molars. Labial premolar cingulids are weak while lingual premolar cingulids are absent.

The thin lingual enamel of the lower molars and the elongate m3 of Embolotherium grangeri are typical brontotheriine traits. However, like E. andrewsi , the trigonids and talonids form deep valleys rather than broad shallow basins. Lingual molar cingulids are absent and the labial cingulids tend to be discontinuous around the labial bases of the trigonids, talonids, and hypoconulid heel, but the cingulids can be somewhat thick between them.

JUVENILE SKULL AND MANDIBLE: AMNH 26040, a juvenile specimen, is the only skull of Embolotherium grangeri associated with a mandible (figs. 126–128). It is a critical specimen for linking the adult skulls and mandibles described above because it includes numerous isolated deciduous anteri- or teeth and complete sets of unerupted adult incisors embedded within the premaxilla and mandible. Parts of the specimen are described in detail by Granger and Gregory (1943), although their interpretation of the anterior dentition is erroneous. Gregory ( Granger and Gregory, 1943: figs. 10, 11) worked out an arrangement of the isolated deciduous dentition of AMNH 26040; however, there are clearly errors in that arrangement. For instance, the root of the specimen referred to as a left deciduous I3 (diameter 5 8.5 mm) is too wide to fit within the left dI3 alveolus (diameter 5 7.1 mm). Any attempt to correctly arrange the deciduous incisors of AMNH 26040 at this point would be guesswork.

Other errors in Granger and Gregory’s (1943) interpretation of the unerupted adult anterior dentition of AMNH 26040 led them to mistakenly assign adult mandibles (described above) to a completely different genus, Titanodectes . The original and corrected interpretations of the dentition of the premaxillary fragment of AMNH 26040 can be seen in fig. 127. Granger and Gregory (1943) identified a ridge running along the side of the premaxillary fragment as the premaxillomaxillary suture, leading them to conclude that there were only two pairs of adult incisors. The large unerupted crowns preserved in the premaxillary fragment were interpreted as I2, I3, and the canine. The first incisor (I1) was presumed to be lost. Howev- er, all six of the unerupted adult teeth in this fragment are incisors. The canine itself can be seen in an unerupted state on the left maxillary of the main skull piece and not on the detached premaxillary fragment. The portion that was labeled as a maxillary in the original figure of Granger and Gregory (1943; seen here in fig. 127a) is actually the contact surface on the premaxilla for the maxilla. No part of the maxilla is actually attached to this fragment. Therefore, the three unerupted teeth visible in the fragment are actually I1, I2, and I3. The unerupted incisors of AMNH 26040 resemble the isolated incisors that are associated with the holotype (AMNH 26002) with large globular crowns and short lingual cingula on I2 and I3, but not on I1.

Granger and Gregory (1943) also mistakenly interpreted the mandible of AMNH 26040 as having two pairs of incisors when there are actually three. Removal of bone fragments from this specimen reveals additional unexposed teeth that were not known to Granger and Gregory (1943) (fig. 128). The unaltered specimen contains two pairs of unerupted but exposed crowns of large adult incisors (fig. 128b). Removal of the left i1 reveals an additional unexposed incisor directly behind the i1 and i3 (fig. 128c). Although not shown, the right i2 is also buried in bone behind the right i1 and i3. Finally, the anterior portion of the jaw containing all of the incisors was removed. The piece that was removed is that portion anterior to the large crack just in front of the p1 that can be seen from the lateral view of the specimen in figure 128a. Removal of this piece revealed two unerupted canine crowns (fig. 128d). The lower adult incisors of AMNH 26040 are very large, tall, and semispatulate with rounded occlusal edges and faint traces of labial cingulids. The canines are small and are not nearly caniniform due to their very blunt rounded tips. These incisors are distinctly different from the very small lower incisors of Embolotherium andrewsi , but the incisors and canines closely resemble those of adult mandibles of E. grangeri .

REMARKS

Osborn (1929b) described the genus, Embolotherium , and three species, E. andrewsi , E. grangeri , and E. louksii , from material collected in the Shara Murun Region of Inner Mongolia. He erected a new subfamily for this genus, Embolotheriinae . The most conspicuously diagnostic feature of Embolotherium is the massive, upturned, and elongated ‘‘battering-ram nose’’ for which this beast is named and which Osborn believed functioned in ‘‘battering, assaulting, attacking, and tossing’’ ( Osborn, 1929b: 19). The three species recognized by Osborn (1929a, 1929b) were distinguished primarily on characteristics of the ram and the rostrum. Granger and Gregory (1943) recognized the same three species, and added an additional species, Embolotherium ultimum , based on a particularly large partial cranium (AMNH 21604). Granger and Gregory (1943) mistakenly erected a new genus, Titanodectes , and two species, T. ingens and T. minor , for mandibles that turn out to belong to E. grangeri . Additional species of Embolotherium were added by Dashzeveg (1975) ( E. ergilense ) and Yanovskaya (1980) ( E. insigne ) based on skulls from the Ergilin Dzo of Mongolia. Most recently, Wang (2000) described an additional skull (IVPP V11959) of E. andrewsi .

Reanalysis of the known specimens reveals, however, that among these six species, only the first two, Embolotherium andrewsi and E. grangeri are valid. E. andrewsi was designated as the type species by Osborn (1929b). E. grangeri differs from E. andrewsi in several respects. The ram of E. andrewsi is curved upward while that of E. grangeri is straight or slightly curved downward. The ram of E. grangeri originates from the skull at a point posterior to the orbits, while that of E. andrewsi rises from above the orbits. Consequently, the nasal incision of E. grangeri is posteriorly much deeper. The large rim of bone that emarginates the distal border of the ventral nasal channel of the ram in E. andrewsi is not seen in adult specimens of E. grangeri . The rostrum of E. grangeri is longer and more robust than that of E. andrewsi with a much thicker dorsal premaxillary ridge. However, the nuchal crest and occiput of E. grangeri are far less robust. The zygomatic arches are shallower and more strongly bowed laterally. The upper and lower incisors of E. grangeri are much larger than those of E. andrewsi , and the anterior tooth row has a greater diameter. The hypocone of the upper premolars is more separated from the protocone. The M3 hypocone is taller and larger, and finally, the cingulum of the M3 is not continuous around the distolingual corner of the crown as it is in E. andrewsi .

Embolotherium ultimum (AMNH 21604) is a junior synonym of E. andrewsi . It was diagnosed primarily by its larger size. In particular, Granger and Gregory (1943) noted, ‘‘M3 of great size and relative width’’. However, the size of M3 is exaggerated by numerous wide cracks in the tooth. Moreover, AMNH 21604 shares numerous characteristics with E. andrewsi . These include massive occipital pillars, a small M3 hypocone, and a continuous cingulum wrapping around the distolingual corner of M3. The holotype of Embolotherium ergilense (National Museum in Ulan-Bator, item 10) is also morphologically congruent with E. andrewsi , although appears to be larger and more robust than other specimens.

The remaining four species are synonyms of Embolotherium grangeri . These include Embolotherium louksii , Titanodectes ingens , Titanodectes minor , and Embolotherium insigne . E. louksii (AMNH 21610) and E. insigne (PIN 3110-52) are based on partial crania. Although the teeth of AMNH 21610 are mostly unpreserved, part of an unerupted right molar (M3?) is present, indicating that this individual is a subadult. Likewise, PIN 3110-52 appears to be a young individual with a newly erupted M3. Both specimens are consistent with E. grangeri in the posteriorly positioned ram behind the orbit, its,45 ° angle of orientation, and the posteriorly deep nasal incision. Originally, E. louksii and E. insigne were distinguished from E. grangeri by the fact that the rams of their holotypes are shorter and appear straight rather than slightly curved downward. When one considers the fact that the proximal portion of the ram of AMNH 21610 consists of small bone fragments and significant amounts of plaster, this difference is not significant. Furthermore, it is hard to imagine that there were not radical ontogenetic changes in the size and the shape of the ram. Therefore, minor variation in the size and shape of the ram among subadults or young adults is undoubtedly ontogenetic and probably not taxonomically relevant. New data from the undescribed Embolotherium grangeri collection from Khoer Dzan currently housed at PIN will help resolve this problem.

Granger and Gregory (1943) erected a new genus, Titanodectes , for adult mandibles of E. grangeri because of their erroneous interpretation of the anterior dentition of Embolotherium . They described Embolotherium adults as retaining the deciduous incisors. It was thought that not only were the adult incisors reduced to two pairs, but also they remained unerupted throughout adulthood. The origin of this puzzling interpretation originates primarily from a juvenile skull and mandible (AMNH 26040) of E. grangeri (though referred to E. louksii by Granger and Gregory [1943]). This juvenile specimen (described more fully above) includes a set of deciduous incisors and a full set of six unerupted adult incisors. Granger and Gregory (1943) recognized the juvenile age of this specimen but for unknown reasons inferred that its condition characterizes adults, despite the fact that numerous adult specimens completely contradict their interpretation. All adult skulls of Embolotherium indicate three, not two, incisors. In adult skulls of E. grangeri the large incisors are fully erupted and the small deciduous incisors are not retained.

In recognizing Granger and Gregory’s (1943) error it is apparent that the mandibles previously referred to Titanodectes actually belong to Embolotherium grangeri . Although Granger and Gregory (1943) named two species of Titanodectes , T. ingens and T. minor , the holotypes of these species were distinguished only by their size differences. Embolotherium grangeri skulls (particularly those from the undescribed Khoer Dzan collection at PIN) seem to demonstrate about as much size variability as the ‘‘ Titanodectes ’’ jaws. Therefore, both Titanodectes species can be regarded as synonyms of Embolotherium grangeri .

A final note is made on the stratigraphic occurrences of Embolotherium . Most of the Embolotherium material from the AMNH expeditions was collected from the ‘‘Ulan Gochu’’ and ‘‘Shara Murun’’ faunal zones (sensu Radinsky, 1964) of the East Mesa and Urtyn Obo of the Shara Murun region. In reviewing field records of these expeditions, Radinsky (1964) found that the stratigraphic data associated with specimens from those areas was questionable due to difficulties correlating the strata with the type Ulan Gochu and Shara Murun sections at Baron Sog Mesa. However, Embolotherium materials are more definitively known from the Shara Murun Beds at Baron Sog Mesa (AMNH 21600, E. grangeri ), the Ulan Gochu of Baron Sog Mesa (AMNH 21601, E. grangeri ), as well as from the Baron Sog Formation at Baron Sog Mesa (AMNH 21604, E. andrewsi ). The occurrence of Embolotherium in all three of these formations at Baron Sog Mesa as well as their occurrence in the Ergilin Dzo of Outer Mongolia suggests a Sharamurunian-Ulangochuian age for Embolotherium .