Embolotherium andrewsi 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 : 243-251

Embolotherium andrewsi Osborn, 1929b

HOLOTYPE: AMNH 26001 , a skull missing the premaxillomaxillary rostrum and right zygomatic arch, with fragmentary right M2– M3 and left M3.

TYPE LOCALITY: ‘‘Ulan Gochu’’, Urtyn Obo, ‘‘middle white’’ or ‘‘gray’’ beds, 125 feet below Baron Sog unconformity, Shara Murun Region, Inner Mongolia, China. (Ulan Gochu [used in quotes] denotes uncertain correlation with the type Ulan Gochu beds at Baron Sog and refers to a faunal zone rather than a formation. [ Radinsky, 1964])

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

SYNONYMS: Embolotherium ultimum Granger and Gregory, 1943 ; Embolotherium ergilense Dashzeveg, 1975 .

REFERRED SPECIMENS: (From the ‘‘ Ulan Gochu’ ’ fanual zone [sensu Radinsky, 1964] of the Shara Murun Region, Urtyn Obo and East Mesa , Inner Mongolia ) AMNH 20352 , an occipital fragment ; AMNH 26011 , a left mandibular ramus and symphysis with right p3 and p4 (partial), left p3–p4 (partial), and m1–m3 ; AMNH 26000 (specimen lost, but photos exist in vertebrate paleontology archives at the AMNH), a skull with right P1– M3 and left P2–M3 ; AMNH 26003 , a skull with right P2–M3 and left C–M3 ; AMNH 26006 , a left ramus with p2–m3 ; AMNH 26007 , a partial mandible with partial right ramus and complete left ramus with right p2– p4, left p2 (partial), and p3–m3 ; AMNH 26008 , a right ramus with p2–m3 ; AMNH 26009 , a skull with right I1–M3, left I2 (?), P1–M3, and a mandible with right i1, i2–i3 (roots), c–m3, left i2, i2–c (roots), and p2– m3 ; AMNH 26010 , a crushed skull missing the frontonasal process with right I3–M3 and left C–M3 ; IVPP V11959 , a skull with right P2–P4, M2–M3, left M2–M3, and a mandible with right i1?, p2–m3 and left i1–i3, and p2–m3 ; PIN 2200-1, an edentulous skull; PIN 2200-2, a mandible with right and left m3; (from the Baron Sog Formation at Baron Sog Mesa, Inner Mongolia ) AMNH 21604 (holotype of Embolotherium ultimum ), a crushed basicranium with right M2 (partial) and M3 ; AMNH 22114 : left maxillary fragment with P3–P4, M1 (partial); (from the Ergilin Dzo Formation , Outer Mongolia) National Museum in Ulan-Bator, item 10 (holotype of Embolotherium ergilense ), a skull missing the premaxillae and most of the maxillary dentition .

DIAGNOSIS: Embolotherium andrewsi is a very large brontothere with a tall and nearly vertical process of bone (‘‘ram’’) that originates above the orbits and appears to be composed of the frontal and nasal bones. At the peak of the ram is a rugose transverse crest that forms the distal margin of nasal channel that runs along the anteroventral surface of the ram. The nasal incision extends posteriorly to the anterior margin of P4. The orbits are positioned above the M2 and the posterior portion of M1. A horizontal nasal process is absent. The premaxillomaxillary rostrum is small and short, it slightly thickens posteriorly, and is not enclosed by bone dorsally. A pair of tall, thin bony ridges is seen on the dorsal surface of the rostrum. Other cranial characteristics include a deeply saddle-shaped cranium, an extremely robust occiput, very prominent parasagittal ridges that overhang the sides of the cranium and do not constrict the dorsal surface of the skull posteriorly, and weakly curved 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 behind M3. Large ventral sphenoidal fossae are absent.

Dentally, Embolotherium andrewsi has three very small incisors that form a semicircular row. The canines are irregular and they are not always fully erupted. There is a distinct P2 metacone. Hypocones are present on P2–P4 and strongly connected to the protocone by lingual crests. The molars of E. andrewsi 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. andrewsi includes three small incisors that form an arch anterior to the canines. Metaconids are 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 m3 is elongate.

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

DESCRIPTION

SKULL: The holotype of Embolotherium andrewsi (AMNH 26001) is a very large skull that is missing the left zygomatic arch, the rostrum, and most of the dentition (fig. 117). Except for the missing portions, the skull is well preserved and undistorted. A few areas are filled in with plaster where the skull surface is not preserved. In particular, the dorsal surface of the skull is made up of numerous fragments with small plaster-filled gaps between them, although more intact specimens show a similar morphology. More complete skulls of E. andrewsi include AMNH 26003 (fig. 118b, c, and fig. 119a), AMNH 26009 (fig. 118a, and see Mihlbachler et al., 2004a: fig. 19k, l), and IVPP V11959 (see Wang [2000] for figure). (Note that the published photo of AMNH 26003 in Osborn [1929b: fig. 5] is unreliable because the

26003, (C) dorsal view of AMNH 26003.

specimen was actually photographed while it was wrapped with plaster).

Embolotherium andrewsi is one of the largest brontotheres, rivaled in size only by E. grangeri , Megacerops coloradensis (sensu Mihlbachler et al., 2004b) , and possibly Gnathotitan berkeyi . The most conspicuous feature of E. andrewsi is the large battering ram– like structure (henceforth referred to as the ‘‘ram’’) that has the superficial appearance of an enlarged and upturned nasal process. From the lateral view the ram originates just above the orbits. It curves posteriorly as it rises from the skull so that at its peak it is nearly vertical. The rams of the referred skulls tend to be less vertical than the holotype. The anteroventral surface of the ram is convex while the posterodorsal surface is concave. The ram broadens distally. A deep nasal channel runs along the anteroventral surface of the ram. The nasal channel is bordered laterally by deep and thick walls that line the side of the ram. Toward the proximal end of the ram the lateral sides are deep and constrict the nasal channel. At the distal end of the ram a thick rim of bone forms a distinct distal border for the nasal channel. The nasal channel broadens distally and is bifurcated slightly at the distal end by a short wedge of bone below the distal margin. The inner surface of the nasal channel is bisected its entire length by a thin ridge of bone that runs along the midline; this appears to represent the osteological marker for the cartilaginous nasal septum that extended to the distal peak of the ram ( Mihlbachler and Solounias, 2004).

Reconstructions of Embolotherium andrewsi in Osborn (1929a, 1929b) depict the ram as a hornlike process, with the nostrils positioned very low in a normal rhinolike position just above the premaxilla. However, the deep channel on the anteroventral surface of the ram and the ossified marker for the nasal septum indicate that the nasal cavity extended to the peak of the ram. Wang (2000) depicted the nostrils as elevated to the peak of this structure although the true position of the nostrils is uncertain.

The distal margin of the ram forms a broad transverse crest that is marked by a coarse rugosity on the anterior and dorsal surfaces of the distal margin. The upper surface of the distal margin of the ram is slightly convex from the anterior view. From the dorsal view the distal margin is thinner medially and thicker laterally. Closer inspection of the peak of the ram reveals that in most specimens a weak but distinct groove on the posterolateral side of the distal rim continues down the side of the ram for several centimeters. The groove cannot be traced farther down the ram beyond a few centimeters from its peak; however, this groove resembles the frontonasal contact seen on the frontonasal crest of Nasamplus progressus . If this groove does represent a sutural remnant, then it indicates that the frontal bone rises to the peak of the ram and that the ram is actually a frontonasal process, with the posterodorsal surface formed from the frontal bone and the anteroventral surface from the nasal bone. This frontonasal process is structurally similar to those of Aktautitan , Metatitan , and Nasamplus , although it is much taller and more massive, as are those of Protembolotherium efremovi and E. grangeri . The actual nasal process (which normally extends horizontally from the peak of the frontonasal process) appears to have been lost in Embolotherium andrewsi .

Due to the large frontonasal ram the nasal incision of Embolotherium andrewsi is dorsoventrally very deep. In specimens such as AMNH 26003 and AMNH 26009, which have the premaxillomaxillary rostrum complete, the nasal incision extends posteriorly to the anterior margin of P4. The posterior margin of the nasal incision is anterior to the orbit, and unlike E. grangeri the nasal incision does not extend over the orbit. The orbits of E. andrewsi are positioned directly above the M2 and the posterior portion of M1. The anterolateral root of M1 is positioned directly below the anterior rim of the orbit.

The premaxillomaxillary rostrum is complete in AMNH 26003 and AMNH 26009, although more details can be seen in the latter. From a lateral view the rostrum of AMNH 26003 is strongly curved upward, but in AMNH 26009 the rostrum is straighter. Premaxillomaxillary sutures are not discernable on any specimen. As a whole, the premaxillomaxillary process is short, shallow, and narrow. It is small in comparison to those of Protembolotherium efremovi and Embolotherium grangeri . The rostrum deepens slightly posteriorly. The premaxillary symphysis is not ossified and the premaxillae are separated by a small median gap. On the dorsal surface of the rostrum of AMNH 26009 are two pairs of thin, roughened ridges of bone. The smaller pair is at the distal end of the rostrum and the larger pair is positioned at about the midpoint of the rostrum. The medial surfaces of these ridges are flat. These ridges of bone probably served as osteological supports for the tall, cartilaginous nasal septum that would have extended to the peak of the ram, although they are not as thickened and rugose as the equivalent dorsal rostral ridges seen on E. grangeri .

The entire dorsal surface of the skull of Embolotherium andrewsi is deeply saddle-shaped. The parasagittal ridges are very prominent and greatly overhang the sides of the cranium. The extent to which the parasagittal ridges overhang the skull is most pronounced in the holotype (AMNH 26001) and not quite as pronounced in other specimens. From a dorsal view the parasagittal ridges are widely separated throughout their length, although they barely constrict the dorsal surface posteriorly.

The zygomatic arches are massive. From the dorsal view the zygomatic arches are essentially straight and parallel except for a large swelling at the junction of the jugal and squamosal bones. Among specimens of E. andrewsi the size of the swelling is variable. They are largest in AMNH 26001 and somewhat smaller in AMNH 26009 and AMNH 26010.

From the lateral view the jugal and squamosal portions of the zygomatic arch have similar depths. There is a small ventral flange below the massive swelling. The jugal portion of the zygomatic is essentially horizontal, although the squamosal portion is angled slightly upward, thus giving the zygomatics a weak curvature.

The occiput of Embolotherium andrewsi is far more massive than that of E. grangeri and Protembolotherium efremovi and rivals the occiputs seen in the largest specimens of Megacerops (sensu Mihlbachler et al., 2004b) . From a dorsal view the nuchal crest is thick and rugose. It is also concave and very deeply notched medially. From a lateral view the occiput is strongly titled backward. From a posterior view the dorsal margin of the occiput is notched medially. The dorsal portion of the occiput is wider than the ventral portion. The lateral wings of the massive nuchal crest are supported by massive bony pillars and the center of the occiput is deeply recessed in the middle.

The ventral surface of AMNH 26003 is the most complete and best preserved among the available specimens (fig. 119a). The posterior nares of Embolotherium andrewsi are positioned completely behind the M3s in all specimens. Direct inspection of the specimens reveals a faint ridge of bone arching around the anterior and lateral margins of the posterior nares. This ridge indicates a horseshoe-shaped emargination similar to that seen, for instance, in Metatitan relictus , although in E. andrewsi this emargination is wider. The posterior narial canal itself is rather short in comparison to most other brontotheres. The vomer, which bisects the posterior narial canal, can be seen in AMNH 26003. The posterior narial canal seems to extend slightly into the sphenoid, but large ventral sphenoidal pits like those of Protitan or Metatitan are distinctly absent. The basicranium is not greatly widened or anteroposteriorly abbreviated like that of Metatitan . The configuration of the basicranial foramina is typical with a widely separated foramen ovale and foramen lacerum. The mastoid process is shorter than the postglenoid process and it curves anteroventrally, thus contacting the postglenoid process and forming a tube-shaped external auditory pseudomeatus. The external auditory pseudomeatus also extends into the basicranium in a posteromedial direction, a condition shared with E. grangeri and Protembolotherium , and a few other more primitive taxa such as Rhinotitan kaiseni , Dolichorhinus , and Sphenocoelus .

UPPER DENTITION: The teeth of the holotype (AMNH 26001) were not recovered except for some of the molars, but even these are fragmented. Therefore, the description of the dentition is based primarily on referred specimens, AMNH 26003 (fig. 119a–c), AMNH 26010 (fig. 119d), and AMNH 26009 (fig. 120).

The number of incisors is unreduced (three pairs). There are six intact small incisor alveoli (not shown) in AMNH 26010. The complete left incisor row is preserved in AMNH 26009. The incisors of Embolotherium andrewsi are very small, are separated by large gaps, project straight down, and form a semicircular row anterior to the canines. Because of the narrow premaxillomaxillary rostrum the diameter of the anterior tooth row is less than the width across the P1s. In AMNH 26009 a fragment of enamel is impacted into the left side of the palate medial to the diastema between I3 and P1. This is presumably a vestige of the canine. In AMNH 26003 and 26010 the canines are present, but they are only slightly larger than the incisors, and they appear to be only partially erupted, even though these specimens are clearly adults. The canine crowns have a peculiar cup-shape. The tendency for the canines to be small, highly irregular, and incompletely unerupted suggests that these teeth are truly vestigial.

In AMNH 26010 and AMNH 26003 there are very short diastemata between the I3 and canine, and between the canine and P1. Most of the available specimens of Embolotherium andrewsi exhibit heavy cheek-tooth wear. AMNH 26003 and AMNH 26010 have the least worn cheek teeth. The P1 is much smaller than the other premolars. The P1 crown is nearly as wide as it is long, but further details of its morphology are obliterated by wear. The remaining premolars are nearly rectangular in outline, although the anterior margins of the premolars are concave. The parastyle of the P2 is directed anteriorly, although the parastyles of P3–P4 are oriented anterolabially. The P2–P4 metastyles are heavily worn, but they appear to have been directed slightly lingually or they were straight. Very small labial paracone ribs are present on P2–P4. These ribs become progressively narrower in consecutively posterior premolars. None of the premolars has a mesostyle. Labial premolar cingula are generally very faint or absent.

The lingual features of P2 always include a protocone and a slightly smaller hypocone. Preprotocristae are absent or so weak that they are not readily discernable. In AMNH 26010 the hypocone of P2 is slightly smaller than the protocone and completely disconnected from it and there is no trace of a lingual crest. However, on AMNH 26003 the protocone and hypocone are connected by a lingual crest. In AMNH 26010, the P3 protocone and hypocone of P2 are more closely positioned and are fully connected by a wide, tall lingual crest. The crest absorbs the cusps to such a great degree that they are barely discernable as distinct cusps. In other specimens, such as AMNH 26003, the paracone and hypocone of the P3 are more widely separated and more distinct, but a tall lingual crest always connects them. In the P4 of AMNH 26010 the protocone and hypocone are distinct cusps, they are positioned closely together, the hypocone is slightly smaller than the protocone, and it is not connected to it by a lingual crest. In other specimens, such as AMNH 26003, a distinct P4 connecting crest is always present. The texture of the surface of the enamel on the lingual side of the premolars crowns varies from highly crenulated to smooth. The lingual premolar cingulum varies from continuous around the lingual side of the protocone to slightly discontinuous.

One specimen, AMNH 22114, a maxillary fragment with an unworn P3–P4, has a small crest of enamel that stretches from the posterior slope of the hypocone to the posterior cingulum. This crest is anomalous and does not appear on any other Embolotherium andrewsi specimen.

The upper molars of Embolotherium andrewsi show typical brontotheriine traits, including tall, lingually angled ectolophs (the ectolophs of E. andrewsi are among the tallest among brontotheres), 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. There is always a hypocone on the M3 of E. andrewsi although it varies in morphology from a functional cusp, though smaller and shorter than the paracone (e.g., AMNH 26003), to a series of two or three tiny cusps (e.g., AMNH 26010). Labial molar cingula are weak while lingual molar cingula are absent. The cingulum of the M3 continuously wraps around the distolingual corner of the crown in all specimens of E. andrewsi .

MANDIBLE AND LOWER DENTITION: Two Embolotherium andrewsi skulls are associated with mandibles, AMNH 26009 (fig. 121e, f) and IVPP V11959 (see Wang, 2000). Based on comparison with these mandibles several other mandibles are referable to E. andrewsi , including AMNH 26008, a specimen with lightly worn cheek teeth (fig. 121a–d). The horizontal ramus of AMNH 26008 is shallow but it deepens posteriorly. The inferior margin of the symphysis is angled slightly less than 45 °. Overall, the ascending ramus is very short and the coronoid process is short and moderately curved backward. The symphysis, most nearly intact in AMNH 26009, is broad and extends to the m1 trigonid, but in other specimens it extends only to the p4 metaconid (e.g., AMNH 26008).

The incisors of AMNH 26009 are small and form a semicircular arch anterior to the canines. Only the crowns of the i1s are preserved, but they are so worn that they cannot readily be described although they appear to have been somewhat semispatulate. The less worn incisors of IVPP V11959 also suggest a semispatulate shape. The lower canines are fully erupted (unlike the upper canines which sometimes do not fully erupt) and they are not much larger than the lower incisors. The canine crown is blunt. All of the incisors, the canine, and the p1 are separated by very short diastemata. The anterior tooth row forms a broad semicircular arch. In other words, the canines are anterior to p1 and they are not shifted lingually as are the upper canines.

AMNH 26009 includes a small p1 although it is too worn to describe. No other specimen of E. andrewsi has a p1 preserved with it. The description of the remaining cheek teeth (p2–m3) is based on the more nearly pristine lower dentitions of AMNH 26006, AMNH 26007, and AMNH 26008. The trigonid of the p2 is only slightly longer than the talonid, but the talonids of p3 and p4 are somewhat longer than the trigonids. The talonids of p2–p4 are all wider than the trigonids. The paralophid and protolophid of the p2 are angled somewhat lingually, thus creating a narrow but long lingual trigonid notch. The trigonids of p3 and p4 are essentially molariform with fully lingually arched paralophids and protolophids and a deep and broad lingual notch. A small lingual bulge in the p2 at the junction of the protolophid and cristid obliqua of AMNH 26008 can be interpreted as a metaconid, although other specimens lack a distinct p2 metaconid. Large, lingually positioned metaconids can be found on the p3 and p4. The p2–p4 talonids of E. andrewsi have well-developed cristids obliqua and hypolophids. The talonid basins of the p2– p4 progressively broaden posteriorly and have deep talonid valleys 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 are typical brontotheriine characters. However, in Embolotherium andrewsi the trigonids and talonids of the lower molars form deep valleys rather than the broad shallow basins that characterize most other brontotheres. 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 relatively thick between them.