Gnathotitan berkeyi ( Osborn, 1925 )

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 : 196-202

Gnathotitan berkeyi ( Osborn, 1925)

LECTOTYPE: AMNH 20106 , a mandible with right c–m3, left c, and p2–m3.

TYPE LOCALITY: Telegraph Line Camp, Irdin Manha Formation, Inner Mongolia, China.

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

REFERRED SPECIMENS: (From the Irdin Manha Formation, Inner Mongolia) AMNH 20107, a partial right mandible with i1–m1, m2 (partial); AMNH 20115, a partial juvenile mandible with unerupted incisor, p1, dp2– dp4 (all damaged), and m1; AMNH 20121, a left maxilla fragment with canine alveolus and P1–M3; AMNH 141231 (formerly part of AMNH 20106), a right maxillary fragment with C–M3.

Two other specimens, AMNH 20124 (a mandible) and AMNH 20127 (a palate with premolars and molars), were referred to Gnathotitan berkeyi by Osborn (1925). Both of these specimens were sent to the Chinese Geological Survey in 1928 and are apparently lost.

DIAGNOSIS: Gnathotitan berkeyi is a large brontothere with a semicomplex P1 (indistinct metacone and small lingual heel) and a distinct P2 metacone. Lingual crests extending from the protocones of the P2–P4 are always present. A premolar hypocone is occasionally present although it is never well separated from the protocone. The molars of G. berkeyi are elongate and 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. Distinct central molar fossae and weakly developed anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower dentition of G. berkeyi includes three large subcaniniform incisors. There is a distinct postcanine diastema, a p2 trigonid that is more than twice as long as the talonid, a small metaconid on p3, and a large metaconid on p4. The lower molars have shallow basins and the m3 is slender and very elongate.

Gnathotitan berkeyi can be distinguished from all other brontotheres by its disproportionately large and deep mandible, which exceeds the mandibles of all other brontotheres in depth and size.

DESCRIPTION

SKULL: The skull of Gnathotitan berkeyi is known only from two maxillary fragments, AMNH 20121 (fig. 94) and AMNH 141231 (fig. 95). The lower portion of the orbit is seen in AMNH 20121 and suggests that the anterior rim of the orbit was positioned over the posterior portion of M2. The dorsal margin of the maxilla is intact to a point between the P1 and P2 and indicates that the nasal incision extended at least to this point, but the true anteroposterior length of the nasal incision is unclear.

UPPER DENTITION: The upper incisors of Gnathotitan berkeyi are unknown, but the cheek-tooth rows are complete in both maxillae (figs. 94, 95). The canine of AMNH 20121 is not preserved, but the alveolus indicates a canine of very small diameter. In contrast, the canine of AMNH 141231 is large with a bulbous root. A postcanine diastema follows the canines of both specimens.

The P1 is a small ovoid tooth that consists of a large paracone, a much shorter and indistinct metacone, and a very small lingual heel with a small protocone. A short preprotocrista connects the protocone to the paracone. The size of the lingual heel is variable. On AMNH 20121 the P2 is more oblique than P3 or P4 due to the posterolingually angled anterior margin and the lingually shifted metacone. The parastyle of P2 is nearly straight, while the metastyle is angled lingually. The P3 and P4 are more rectangular because of nearly parallel anterior and posterior sides and a nearly flat lingual side. Likewise, the metacones are not as strongly shifted lingually in comparison to P2. The parastyles of P3 and P4 are directed somewhat labially, while the metastyles are nearly straight. The paracone of P2 is swelled buccally, while P3 and P4 have small but well-defined labial paracone ribs.

(A) Right (labial) view, (B) ventral view, (C) right premolars.

The lingual features of the P2–P4 have relatively high relief. On each of the molars, a very low and short preprotocrista can be seen; it is strongest on the P2 and smallest on the P4. A long lingual crest extends posteriorly from the protocone of P2. On the P3 of AMNH 20121 a tall but short lingual crest connects the protocone to a large hypoconelike swelling. The P3 is similar to P4 except that the protocone and hypocone are even more closely positioned, so that they form a single large ovoid cusp with a short lingual crest at its apex. The lingual premolar morphology of AMNH 141231 differs from that of AMNH 20121. In that specimen, the lingual sides of the P2, P3, and P4 have a single lingual cusp (protocone) and a long lingual crest extending posteriorly from the protocone. The labial premolar cingula are very weak. In AMNH 20121 the anterior and posterior premolar cingula do not join lingually. In AMNH 141231 there is a continuous lingual cingulum on P3 and P4 but not on P2.

The upper molars of Gnathotitan berkeyi are notably elongate. This is most evident in M2 and M3. M1 appears to have been shortened somewhat by interstitial wear. The molars show typical brontotheriine traits including tall, 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. Shallow central molar fossae are seen on each of the molars. On the M1, there is a wear facet on the anterolingual cingulum; on M2 and M3 small but distinct anterolingual cingular peaks are evident. All evidence of paraconules and metalophs is lost. The M3 hypocone varies. In AMNH 20121, the M3 hypocone is nearly as large as those of the more anterior molars and is separated from the protocone by a deep valley. However, in AMNH 141231, there are two very small hypoconelike points of enamel. Labial molar cingula are weak, and lingual molar cingula are absent.

MANDIBLE AND LOWER DENTITION:

The mandible and lower dentition of Gnathotitan berkeyi are known from the lectotype (AMNH 20106), which consists of a nearly complete mandible that is missing its incisors (fig. 96). (The left p1 is mistakenly glued onto the mandible as an incisor.) The lectotype is laterally crushed, thus its proportions are distorted, but from a lateral view the shape of the mandible seems reasonably intact. Additionally, a partial right ramus with well-preserved incisors, premolars, and a symphysis offer more details (fig. 97).

At 805 mm in length, AMNH 20106 is the largest known brontothere mandible. (The second largest brontothere mandible I have ever encountered (724 mm in length) is AMNH 1051, a specimen of Megacerops sp. (sensu Mihlbachler et al., 2004b). Despite the profoundly large mandible, Gnathotitan berkeyi was probably not the largest brontothere in terms of body mass. For instance, the length of the cheek-tooth row is exceeded by other large brontotheres (e.g., Aktautitan ). In addition to its excessive size, the mandible is unique in the great depth of the horizontal ramus. Another peculiar aspect of its shape is the distinct bulge on the inferior margin of the horizontal ramus. The bulge is present on both sides, but it is somewhat asymmetrical. On the right side it is below m2 and m3, while on the left side it is below m3 and somewhat posterior to m3. The peculiar shape of the inferior margin was considered by Osborn (1925, 1929a) to be a species-level character. However, the bulge could be a neoplasia or tumor paleopathology (Bruce Rothschild, personal commun., 2003), although a thorough paleopathological analysis has not been undertaken. In addition to the large bulge in the center of the ramus, the mandible bulges directly below the symphysis. This second characteristic is repeated in AMNH 20107, suggesting that it is probably not related to a paleopathology.

Osborn (1925) described the coronoid process of AMNH 20106 as wide, though there is so much plaster involved in the reconstruction of that part of the mandible that this observation is doubtful. The symphysis of AMNH 20106 is long and its inferior margin is steeply angled slightly more than 45 °. This great length and steep orientation of the symphysis is clearly related to the extreme depth of the ramus. However, lateral crushing has clearly exaggerated the narrow-elongate shape of the symphysis of the lectotype. The cross-sectional shape of the symphysis can be seen from the medial view of AMNH 20107. The symphysis extends to the talonid of the p3.

Although no incisors are preserved with AMNH 20106, there are clearly six alveoli that form an arched row anterior to the canines. A complete right lower incisor row is preserved in AMNH 20107. The incisors are large and positioned closely together with no diastemata. Each crown is composed of a dulled point with narrow mesial and distal margins. The i2 is the largest incisor, though barely so. It exceeds the other incisors in size primarily in crown height. The i3 is slightly more mesiodistally elongate than the other incisors, but otherwise it is similar to i1 and i2. Each incisor has a distinct lingual cingulid. There are no labial incisor cingulids in Gnathotitan berkeyi .

The canine of AMNH 20106 is very small, while the canine of AMNH 20107 is much larger. Likewise, the postcanine diastema of AMNH 20107 appears to be shorter than that of the lectotype.

The left p1 is a small narrow tooth with a single cusp and a small talonid heel. The talonid of p2 is more than twice the length of the talonid but about the same width. The p3 and p4 trigonids are about the same length as their talonids, but the talonids are somewhat wider. The paralophid of p2 arches slightly lingually, thus creating a small lingual-trigonid notch. The protolophid of p2 is straight and slightly deflected lingually. There is no metaconid on the p2. The p3 paralophid and protolophid are more strongly angled lingually, creating broader lingual-trigonid notches. The p3 has a small metaconid that is positioned about equally lingually and posteriorly from the protoconid. The p4 trigonid is essentially molariform with a fully lingually arched paralophid and protolophid, a molariform trigonid basin, and a large lingually positioned metaconid. The cristids obliqua and hypolophids of p2– p4 are well developed, although on the p2 the talonid basin is small. The talonid basins of p3 and p4 are broader and nearly molariform. Labial premolar cingulids of Gnathotitan berkeyi tend to be weak, while lingual premolar cingulids are absent.

The molars of Gnathotitan berkeyi are typical, with shallow basins, thin lingual enamel, and an elongate m3. However, the m3 is among the most elongate among brontotheres with an m3 length/width ratio (2.94) that rivals Aktautitan ( Mihlbachler et al., 2004a) . Labial molar cingulids are thin and lingual molar cingulids are absent. The m3 hypoconulid has a faint cingulid tracing around it.

REMARKS

The Central Asiatic Expedition of the American Museum of Natural History recovered seven specimens (two are now lost) that represent a species of very large brontothere with elongate molars and a very deep mandible. Osborn (1925) assigned holotype status to a mandible and maxilla that had originally been given the same catalog number (AMNH 20106), although these specimens do not actually belong to a single individual (see below for clarification). Osborn (1925, 1929a) originally assigned Gnathotitan berkeyi to the genus Telmatherium . However, G. berkeyi is clearly different from any other material that Osborn (1929a) had referred to Telmatherium . Osborn’s (1925) only indication as to why he though G. berkeyi belonged to Telmatherium was that ‘‘the canines resemble those of Telmatherium and Menodus ’’ ( Osborn, 1925: 9). However, no special similarities in the canines of these taxa were actually pointed out, nor could I find any. Granger and Gregory (1943) correctly recognized this species as distinct from all other brontotheres and assigned it to its own genus.

The holotype of Gnathotitan berkeyi (AMNH 20106), as previously described by Osborn (1925; 1929a) and Granger and Gregory (1943), includes a mandible and right maxilla that were both originally assigned to AMNH 20106. However, Walter Granger’s field notes for the 1923 Central Asiatic Expedition in the AMNH vertebrate paleontology archives indicate for field number 163 (5 AMNH 20106), ‘‘association between maxilla and lower jaw questionable’’. This detail was either ignored or unnoticed by Osborn (1925, 1929a) and Granger and Gregory (1943), who considered the maxilla and mandible to be parts of a single holotype. Because the jaw and maxilla are not certainly associated (and evidence presented below further suggests they represent separate individuals), the jaw and mandible should be considered syntypes rather than a single holotype. The AMNH catalog has been modified to reflect this by assigning a new number to the maxilla (AMNH 141231), while the mandible retains the original number (AMNH 20106). Among the syntypes, the mandible (AMNH 20106) is designated as the lectotype.

Other evidence suggests that the lectotype mandible (AMNH 20106) and the maxilla (AMNH 141231) represent separate individuals of the same species. Most notably, the canine of the maxilla is much larger than the canine of the mandible. The diameter of the maxillary canine (36.4 mm) at the base of the crown is 71 % greater than that of the mandibular canine (21.2 mm). Canine size is commonly variable in brontothere species. However, the upper and lower canines of a single individual are usually of a similar size. For comparison, the upper canine of KAN N2/875 (the holotype of Aktautitan hippopotamopus ) is only 5 % larger in diameter than the lower canine ( Mihlbachler et al., 2004a). The disparity in canine size between the maxilla (AMNH 141231) and mandible (AMNH 20106) of the G. berkeyi syntypes strongly suggests that these specimens represent two individuals, possibly of different sexes. The other specimens referred to G. berkeyi also indicate canine size dimorphism. The additional mandible, AMNH 20107, has canines that are much larger (diameter 5 29 mm) than those of the lectotype mandible. Likewise, the additional maxilla, AMNH 20121, has a canine alveolus that is much smaller in diameter than that bulbous canine root of the syntype maxilla.