Metatitan primus 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 : 206-220

Metatitan primus Granger and Gregory, 1943

HOLOTYPE: AMNH 26101 , a partial skull with intact left side, left C–M3, partial mandible with complete incisor row, canines, and left p2–m3.

TYPE LOCALITY:?Ulan Gochu Formation, Chimney Butte, North Mesa, Shara Murun Region, Inner Mongolia, China.

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

REFERRED SPECIMEN: (From the same locality as the holotype) AMNH 26102, a partial mandible with left c and m1–m3.

DIAGNOSIS: Metatitan primus is a large brontothere with small closely spaced frontonasal horns that are positioned high above the orbits. The nasal process and horns are elevated to the peak of a tall frontonasal process that rises anterodorsally from above the orbits at an angle greater than 45 °. The nasal incision is deep and extends to the anterior margin of the M1. The orbit is positioned over the posterior portion of M2 and the anterior portion of M3. The elevated nasal process is horizontal, relatively broad, not strongly rounded anteriorly, and with lateral walls that are deep proximally and shallow distally. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a bulbous and incompletely saddle-shaped cranium. The posterior end of the cranium is extremely widened. The parasagittal ridges are not prominent and they do not constrict the dorsal surface posteriorly. There is a small dome on the dorsal surface of the skull. The zygomatic blades are nearly straight and they extend nearly to the posteriormost end of the skull where they form a 90 ° angle with the lateral zygomatic wing of the squamosal. The external auditory pseudomeatus enters the skull in a mediolateral direction and is ventrally constricted. Finally, a broad postzygomatic process is present.

Dentally, Metatitan primus is characterized by a complex P1, a distinct P2 metacone, and hypocones on P2–P4 that are positioned close to the protocones and not well separat- ed from them. Upper and lower postcanine diastemata are absent. The molars of M. primus 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 small anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower dentition of Metatitan includes three very small incisors that form a nearly straight row. The i1 and i2 are short and wedge-shaped, while the i3 is somewhat more conical. A metaconid is present on the p3 and p4. The p2 trigonid is not much longer than the talonid. The lower molars have shallow basins and the m3 is elongate.

Metatitan relictus shares with M. primus and M. khaitshinus the unique combination of the following traits: horns and a nasal process elevated to the peak of a superorbital frontonasal process, posteriorly positioned orbits, an extremely widened skull, anteroposteriorly shortened basicranium, and lack of postcanine diastemata. M. primus differs from these species most obviously by its remarkably bulbous cranium.

DESCRIPTION

SKULL: The holotype of Metatitan primus (AMNH 26101) is complete on its left side and does not appear to be significantly distorted. The right side of the skull is severely weathered and essentially featureless (fig. 99). The facial area of M. primus is similar to that of Aktautitan hippopotamopus Mihlbachler et al. (2004a) and other species of Metatitan . The small horns are positioned closely together and are highly elevated above the orbits. The protuberances rest on a tall superorbital pillar that rises from above the orbits at an angle steeper than 45 °. The nasal process is elevated to the peak of this structure. Consequently, the nasal cavity is dorsoventrally deep, and the posterior margin of the nasal incision rises much higher than the orbits. The nasal and frontal bones are completely co-ossified, so it is not possible to determine the exact position of the frontonasal suture. However, the morphology of the face of M. primus is so similar to A. hippopotamopus , M. relictus , and M. khaitshinus (where the frontonasal contact is visible) there is little doubt that this large process is composed of the nasal bone anteroventrally and the frontal bone posterodorsally. The frontonasal protuberances are small, rounded, and slightly rugose at the surface and are perched at the lateral edges of the peak of the singular frontonasal process.

The nasal incision extends posteriorly to the anterior margin of the M1. The orbit is positioned over the posterior portion of M2 and the anterior portion of M3. The anterolateral root of M2 and the posterolateral root of M1 are positioned below the anterior rim of the orbit. The nasal process of AMNH 26101 extends nearly horizontally from the peak of the frontonasal process, although the flat dorsal surface is angled slightly downward. The nasal process is broad and slightly shorter than the premaxillomaxillary rostrum. The lateral walls of the nasal process are dorsoventrally deep and thin. The lateral walls are deepest proximally. The lateral walls extend to the end of the nasal process, but they shallow distally. The anterior edge of the nasal process is slightly turned downward. From the dorsal view the anterior edge of the nasal looks rounded and the nasal process appears to taper distally, but these are artifacts of how the specimen has been distorted. Direct inspection of the specimen reveals that the nasal process was not tapered distally, and the anterior margin was nearly flat with a distinct median notch.

From a lateral view it can be seen that the premaxillomaxillary rostrum is mildly curved upward. The rostrum deepens proximally. From an anterior view, the rostrum is broad, although the right side has been smashed inward. The dorsolateral margin of the rostrum (intact on the left side) angles posterolaterally and the rostral cavity is not sealed over dorsally. The maxilla and premaxilla appear to be completely fused; thus, a premaxillomaxillary suture is not discernable in this specimen. The opening of the nasal cavity is large and taller than it is broad.

The overall shape of the cranium of Metatitan primus is distinctive and somewhat bizarre. In addition to the elevated horns and nasals, the postorbital cranium is remarkably bulbous. Nonetheless, the general proportions of the facial and cranial regions are similar to those of other brontotheres, with a short face and an elongate cranium. The dorsal surface in the midsection of the skull is strongly concave and forms a deep transverse channellike depression. The dorsal surface of the posterior half of the skull is convex, thus resulting in an incompletely saddle-shaped skull. Directly behind the midcranial concavity is a centrally positioned convex dome. Curiously, this dome is similar in size and position to the dorsal dome seen in Duchesneodus uintensis . Between the dome and the occiput is a distinct pit. The dorsal surface of the skull continues to broaden posteriorly and the parasagittal ridges do not constrict the skull posteriorly. The parasagittal ridges are essentially absorbed into the swollen cranium and exist only as faint ridges running from the postorbital processes of the frontal to the occiput.

Overall, the zygomatic arch is relatively deep and thin. From a lateral view the zygomatic arch is peculiar in that it extends almost to the very end of the skull. The entire zygomatic arch is straight except for the posterior end where is it deflected upward. From the dorsal view the zygomatic arch is thin and very straight and angled posterolaterally. If the right side of the skull were intact, the zygomatic arches would have given the skull a wedge-shaped appearance. A peculiar feature of the zygomatics of Metatitan primus , shared with M. relictus , is the fact that the winged-shaped extension of the squamosal that connects the cranium to the zygomatic arch is very short and extends almost straight laterally from the side of the skull and forms an abrupt 90 ° angle with the actual zygomatic blade. M. primus also has a posterior zygomatic process similar to that of Protitan grangeri , although from a lateral view, the posterior zygomatic process is much broader than that of P. grangeri .

From a dorsal view the nuchal crest is slightly concave. From a posterior view, the dorsal margin of the occiput is slightly arched dorsally. The occiput itself is vertical and extremely broad. The dorsal half is similar in width to the ventral half and the occiput is not constricted in the middle. The surface of the occiput has distinct occipital pillars with a shallow triangular depression between them.

The ventral surface of the skull of Metatitan primus is incomplete (fig. 100a), although the following details can be discerned. The posterior nares are not preserved, however a remnant of a wide lateral emargination of the posterior nares can be seen posteromedial to the left M3. This wide emargination resembles those seen in more complete specimens of M. relictus and M. khaitshinus . The basicranium of Metatitan primus is very short. The external auditory pseudomeatus is positioned at the very end of the skull and can be seen just behind the zygomatic arch from the lateral view, forming a tube-shaped opening. Although it is not possible to measure the width of the basicranium due to the damaged right side, the posterior end of the skull of Metatitan primus is very broad and the basicranium would have been much wider the outside distance across the M3s.

UPPER DENTITION: The upper incisors of Metatitan primus are unknown, but the left canine and cheek teeth are well preserved in the holotype specimen (fig. 100). The canine is of moderate size. The P1 crown is rounded in outline, but its exact morphology is obscured by damage and wear. However, its overall shape suggests a relatively advanced P1 with a distinct paracone, metacone, and well-developed lingual heel.

The crowns of P2–P4 are nearly rectangular in outline due to the nearly parallel anterior and posterior margins and the nearly flat lingual margins. The parastyle and metastyle of P2 are nearly straight. The parastyle of P3 is barely directed labially, while the P4 parastyle is more strongly angled labially. The metastyle of P3 is straight. The P4 metastyle could have been straight or slightly labially directed, but it has been worn away by the adjacent M1. There are distinct labial ribs on the paracones of P2–P4; it is broadest on P2 and is progressively narrower on more posterior premolars. There are no mesostyles on any of the premolars.

Small preprotocristae are seen on P2 and P3 but not on P4. The lingual sides of P2–P4 have distinct protocones and hypocones that are separated to varying degrees. On P2 the hypocone is smaller than the protocone, although the enamel of the hypocone is worn or broken off, thus giving it a slightly smaller apparent size. On P3 the hypocone is smaller and lower than the protocone and these cusps are positioned very closely together and are almost completely conjoined into a single ovoid cusp. Finally, the P4 hypocone is smaller and shorter than that of the protocone and sits on the shallow posterior slope of the protocone. Labial premolar cingula are distinct but thin. The lingual cingulum of P2 is broken off, but the lingual cingula of P3 and P4 are thick and continuous.

The upper molars of Metatitan primus are elongate, although the M1 has been significantly shortened by interstitial wear. The upper 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 (e.g., M3). The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Distinct shallow central molar fossae and small anterolingual cingular cusps are present. All evidence of paraconules and metalophs is lost. There is no hypocone on M3, although there is a distinct peak of enamel on the cingulum of the distolingual corner of M3. Labial molar cingula are thin but distinct, while lingual molar cingula are absent.

MANDIBLE AND LOWER DENTITION:

The holotype mandible consists of a symphysis and a left ramus that is missing the coronoid process and mandibular condyle (fig. 101). The inferior margin of the symphysis is angled about 45 °. From a dorsal view the symphysis is broad and extends to the anterior margin of the p4. The three incisors are very small and form a nearly straight row that is positioned only slightly anterior to the canines. The crown of i2 (most complete on the right side) is the largest incisor especially in crown height and labiolingual width. The apices of the i1 and i2 are worn off, but these incisors appear to have had very short, wedge-shaped crowns. The i3 is more rounded in outline, with a short conular crown. The i1 and i2 have a distinct but thin lingual cingulid. The lingual cingulid is faint on i3. There are no diastemata between the incisors or canines. The p1 is missing, but the p1 alveolus fills the gap between the canine and p2, indicating the absence of a postcanine diastema.

Overall, the p2–p4 are rather broad. The p2 trigonid is only marginally longer and is distinctly narrower than the talonid. The trigonids of p3 and p4 are both shorter and narrower that the talonids. The very anterior end of the of p2 is not preserved and the morphology of this area is obscure, but it seems that the paralophid is strongly curved lingually, thus creating a relatively broad lingual-trigonid notch. The p2 protolophid arches lingually nearly 90 °. The p2 protoconid is positioned labially, and the protolophid and cristid obliqua intersect at a point

(C) left p2–p4, (D) lingual view of incisors and canines, (E) labial view of incisors and canines.

lingual from the protoconid. This configuration is essentially molariform. Finally, there is a deep labial notch on the p2 between the trigonid and talonid that is directed anterolingually. In more posterior cheek teeth this labial notch is directed more lingually.

The paralophids and metalophids of p3 and p4 arch fully lingually, thus creating nearly molariform trigonid basins. Both of these premolars have a large lingually positioned metaconid. The morphology of the p2 is also consistent with a metaconid, although the crown is too damaged to determine if a metaconid was present on that tooth. The talonids of p2–p4 have well-developed cristids obliqua and hypolophids with molariform talonid basins. These structures are significantly broader in p3 and p4. Labial premolar cingulids are weak and lingual premolar cingulids are absent.

The lower molars of Metatitan primus have relatively thin lingual enamel and shallow trigonid and talonid basins. The m3 is elongate. There are no lingual cingulids. Labial molar cingulids are thin, but they are distinct and continuous around the paraconids and metaconids. A thin beaded cingulid can be seen tracing around the hypoconulid of the m3.

Metatitan relictus Granger and Gregory, 1943

HOLOTYPE: AMNH 26391 , a skull heavily reconstructed with plaster, with right I2– M3, left I2–C, P2–M3, and a mandible with right i2–c, p3–m3, left i1–i3, and p4–m3.

TYPE LOCALITY: ‘‘Houldjin’’ beds, one mile west of Camp Margetts, Inner Mongolia, China.

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

REFERRED SPECIMENS: (From the ‘‘Houldjin’’ beds near Camp Margetts, Inner Mongolia) AMNH 26395, a skull missing the right horn, nasal, and occiput, with right I1, I3, C, M1–M3, left I1–I3, and canine (fragmentary); AMNH 26396, fragments of a facial portion of a cranium, palate, and basicranium with right P2–M (damaged); AMNH 26397, a partial skull with right I1– C, P4–M3, left I3, P4 (fragmentary), and M2–M3; AMNH 26398, a complete ventral surface of a skull with nasal, horn fragment, parietal fragment, right I1–M3, left I2–M1 (partial), and M2–M3; AMNH 26399, a complete left side of a skull with right C, P1–P4, left C (partial), and P4–M3; AMNH 26406, a left premaxilla and maxilla fragment with I3–M2; AMNH 26402, a mandibular symphysis and right ramus with right i1–m3 and left i2–c; AMNH 26404, a mandibular symphysis with complete incisor row, canines, right p2, and left p2–p3; AMNH 26405, a fragmentary mandibular symphysis and left partial ramus with right left canine, and a right partial ramus with p4–m1, and m2 (partial); AMNH 26407, a right mandibular ramus with c, and p2–m3; AMNH 26420, a left partial ramus with p3–m2, and m3 (partial); AMNH 26427, a partial mandible with right i3–c, p4–m3, and left c–p4; AMNH 26429, a right partial ramus with p4 (partial) and m1–m3.

DIAGNOSIS: Metatitan relictus is a large brontothere with small frontonasal horns that are positioned high above the orbits. The nasal process and horns are elevated to the peak of a tall frontonasal process that rises from the orbits at about 45 °. The nasal incision is dorsoventrally deep and extends to the anterior margin of the M2. The orbit is positioned over the posterior portion of M2 and the anterior portion of M3. The elevated nasal process is horizontal, relatively broad, not strongly rounded distally, and with lateral walls that are deep proximally and shallow distally. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include an incompletely saddle-shaped cranium. The posterior end of the cranium is extremely widened. The parasagittal ridges are prominent, but they do not constrict the dorsal surface posteriorly. The zygomatic blades are nearly straight and they extend nearly to the posteriormost end of the skull where they form a 90 ° angle with the lateral zygomatic wing of the squamosal. The external auditory pseudomeatus enters the skull in a mediolateral direction and it is ventrally constricted. A broad postzygomatic process is present. The emargination of the posterior nares is wide, and the posterior nares are completely behind the M3. Large ventral sphenoidal fossae are present.

Dentally, Metatitan relictus is characterized by three small incisors that form a nearly straight row, a globular I1 and I2, and a subcaniniform I3. The P1 is complex and the P2 has a distinct metacone. Premolar hypocones are uncommon, but there is usually a lingual crest extending posteriorly from the premolar protocones. Upper and lower postcanine diastemata are absent. The molars of M. relictus 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 small anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower dentition of Metatitan relictus includes three very small incisors that form nearly a straight row. The i1 and i2 are short and wedge-shaped, while the i3 is somewhat more conical. Metaconids are present on p3, p4, and occasionally on p2. The p2 trigonid is not much longer than the talonid. The lower molars have shallow basins and the m3 is elongate.

Metatitan relictus shares with M. primus and M. khaitshinus the unique combination of the following traits: horns and a nasal process elevated to the peak of a superorbital frontonasal process, posteriorly positioned orbits, an extremely widened skull, anteroposteriorly shortened basicranium, and lack of postcanine diastemata. M. relictus differs from M. khaitshinus in having more posteriorly positioned posterior nares, small lower incisors forming a straight row, and a narrow labial notch on p2. M. relictus is most easily differentiated from M. primus by the extremely swollen appearance of the cranium of the latter.

DESCRIPTION

SKULL: The badly damaged but nearly complete holotype skull of Metatitan relictus (AMNH 26391) is missing large portions, although the ventral half is essentially complete and undistorted (fig. 102). None of the numerous additional skulls of M. relictus is complete (fig. 103). Those figured are AMNH 26399 (fig. 103a), AMNH 26395 (fig. 103b, c), AMNH 26397 (fig. 103d) and a part of AMNH 26398 (fig. 103e).

The horns and nasal process of the holotype are not preserved, but this area can be described from other specimens. The facial area of Metatitan relictus is similar to that of M. primus and M. khaitshinus . The small horns are highly elevated above the orbits and rest on a tall frontonasal process that rises from above the orbits at approximately an angle of 45 °. The horizontal nasal process is elevated to the peak of this tall process. A distinct line representing the frontonasal contact can be seen from the lateral views of AMNH 26399 and AMNH 26395. The frontonasal process is composed of the nasal bone anteroventrally and the frontal bone posterodorsally; the peak of the horn is formed by the frontal bone. The frontonasal process of M. relictus is somewhat shorter and less steeply angled than those of M. primus and M. khaitshinus ; consequently, the nasal chamber is not as deep. Nonetheless, the posterior margin of the nasal incision is dorsoventrally relatively deep in comparison to brontotheres whose nasal processes are not elevated (e.g., Protitan ). In specimens where the face is complete (AMNH 26399), the posterior margin of the nasal incision rises higher than the orbits.

The horns are variable in size, although they are always small. For instance, in AMNH 26399 they do not rise higher than the dorsal surface of the nasal process. In AMNH 26395 the horns are larger with more significant relief. Those with relief form small, rounded knobs that typically have rugose surfaces. The horns vary in their orientation ; the majority of specimens have horns that do not project laterally, however, one specimen, AMNH 26396 , has horns that project almost completely laterally .

The nasal incision of the holotype (AMNH 26391) extended at least to P4, but the nasal incision is incomplete. Other specimens with complete faces indicate that the nasal incision extended behind P4. In AMNH 26399 the nasal incision extends to the anterior margin of M2. The position of the orbit is the same as that of Metatitan primus where the posterior part of M2 and the anterior part of M3 are directly beneath the orbit with the anterior lateral root of M2 positioned below the anterior rim of the orbit.

The nasal process, most complete in AMNH 26395 and AMNH 26399, does not differ substantially from that of Metatitan primus . The nasal process tends to be shorter than the premaxillomaxillary rostrum. It projects horizontally from the skull from its elevated position, although the flat dorsal surface tends to be angled downward. The lateral margins of the nasal process are unthickened and they are deep proximally and shallow anteriorly. From the dorsal views the nasal processes of these specimens are not tapered or flared distally. The anterior edge of the nasal process is turned downward slightly and from the dorsal view it is nearly flat, although some, such as AMNH 26395, show a deep medial notch at the distal end.

The premaxillomaxillary rostrum deepens posteriorly and its dorsal margin is steeply sloped posterodorsally. A premaxillomaxillary suture is not visible in any specimen of Metatitan relictus . The rostrum is relatively wide. The dorsolateral margins of the rostrum are laterally divergent behind the premaxillary symphysis and the rostral cavity is not enclosed by bone dorsally.

The cranium of Metatitan relictus is not extremely swollen like that of M. primus . The dorsal surface of the holotype (AMNH 26391) is incomplete, but a deep concavity runs transversely just behind the orbits. Other skulls of M. relictus are also deeply concave midcranially just behind the orbits. The dorsal surface of the posterior part of the cranium is slightly convex, thus the skull of M. relictus is incompletely saddle-shaped. M. relictus lacks the central parietal dome seen in M. primus . The parasagittal ridges of M. relictus are prominent in contrast to M. primus and overhang the sides of the cranium somewhat. The dorsal view of AMNH 26395 best illustrates the very broad dorsal surface of the cranium.

The jugal portion of the zygomatic arch is shallow in comparison to the much deeper squamosal portion. From a lateral view the zygomatic blade is straight, although it is deflected upward at the posteriormost end, where there is a tall posterior zygomatic process. From the dorsal view of AMNH 26391 the zygomatic blades are thin and straight, and they diverge posterolaterally, creating a wedge-shaped skull. The zygomatic blade of AMNH 26395 is strongly bowed inward, but this seems to be an artifact of distortion. The wedge-shape of the zygomatic arch resembles Metatitan primus and M. khaitshinus where the lateral wings of the squamosals are positioned at the posteriormost end of the skull and they project laterally to form an abrupt 90 ° angle with the zygomatic blades.

The occiputs of most skulls of Metatitan relictus are poorly preserved, but all suggest a very broad occiput that is mildly tilted backward, unlike the more vertical occiputs of M. primus and M. khaitshinus . The skull of AMNH 26398 includes an unbroken fragment with the greater part of an intact nuchal crest (fig. 103e). This piece is consistent with a very wide occiput with a nuchal crest that is slightly concave from the dorsal view and dorsally arched from the posterior view. The occiput of AMNH 26397 is reasonably complete on the right side. That specimen suggests that the dorsal and ventral halves of the occiput are of a similar width and that it was not constricted in the middle. The surface does not suggest distinct occipital pillars, but this could relate to the poor condition of the surface of the occiput.

It can be seen from the ventral view of AMNH 26391 that the posterior nares are constricted by a wide horseshoe-shaped emargination (fig. 104a). The anterior rim of the posterior nares is shifted behind the M3s, so that the posterior nares are more posteriorly positioned than those of M. khaitshinus . The posterior narial canal appears to extend well into the sphenoid bone, although other aspects of the posterior narial canal are difficult to discern due to poor preservation and plaster. The basicranium is very wide and anteroposteriorly compressed. The width of the skull across the mastoid processes is much greater than the width across the M3s. A ratio calculated from the width of the basicranium of AMNH 26391 divided by the width across the M3s (ratio 5 1.63) is greater than that of any other brontothere for which this ratio can be calculated except for M. khaitshinus . The external auditory pseudomeatus, which is positioned at the posteriormost end of the cranium, is tube-shaped and it enters the skull in a mediolateral direction.

UPPER DENTITION: In the holotype (AMNH 26391) half of the incisors (right I1, left I1, I2) are missing and have been sculpted with plaster (fig. 104a). However, the skull of AMNH 26398 includes a complete set of right upper incisors (fig. 104e, f). The incisors are positioned slightly anterior to the canines and form a nearly straight row. The number of incisors (three pairs) is unreduced, although they are very small and vestigial in appearance. The I1 and I2 have an amorphous globular appearance. The I3 is larger and has a taller, more conical crown. The apex is blunt, but this could relate to wear. There is a distinct lingual cingulum on the I3, but cingula are absent on I1 and I2. The canines of Metatitan relictus tend to be rather small. There seems to be a short precanine diastema in most specimens, but postcanine diastemata are absent. In AMNH 26406, a young individual, the erupting canine is crowded by I3 and P1 (fig. 104d). There is no postcanine diastema, not even when P1 and P2 are missing. For instance, on the left side of AMNH 26391, P1 and its alveolus are missing and P2 is closely pressed to the canine root, indicating that some remodeling of the left maxillary must have occurred in this individual subsequent to the loss of the left P1.

In addition to the relatively worn premolars of the holotype ( AMNH 26391 ) (fig. 104c), the nearly unworn premolars of AMNH 26406 are shown in close-up (fig. 104d). The crown morphology of P1 is obliterated in AMNH 26391 . In AMNH 26406 the P1 is nearly round in outline. It has a large paracone, a distinct but smaller metacone, and a small posteriorly shifted lingual heel. The P 2 and P3 of M. relictus are slightly oblique due to a weakly distolingually angled anterior margin. P4, on the other hand, is more nearly rectangular, with parallel anterior and posterior margins. The parastyle of P2 is straight, but those of P3 and P4 are deflected anterolabially. The metastyle of P2 is straight, while those of P3 and P4 are slightly deflected posterolabially. The P 2 ectoloph is essentially straight. Small labial paracone ribs can be seen on P2– P4 ; these become smaller in more posterior premolars. Mesostyles are absent on all premolars, although there is a distinct labial bulge at the base of the crown near the P4 metacone of AMNH 26391 .

The lingual features of P2 are obliterated in AMNH 26391 . Though heavily worn, the lingual features of P3 and P4 are still discernable. On both P3 and P4, there is an oval area of dentine exposed behind the protocone, suggesting that a small hypocone was present and well separated from the protocone. The lingual premolar morphologies of other specimens differ notably from that of AMNH 26391 . In AMNH 26406 there is a large P2 protocone and a slightly smaller hypocone that is positioned closely to the protocone. The hypocone is connected to the protocone by a short lingual crest that is almost as tall as the cusp apices. On the P2 of AMNH 26399 , a large ovoid cusp rests on the lingual side of the crown that could represent two conjoined cusps, but the hypocone is not distinct from the protocone. The P 3 of AMNH 26406 has a large protocone and a distinct lingual crest, but there is no hypocone. The P 4 of that specimen retains a single, large protocone with no crest or hypocone. The protocones of AMNH 26406 and AMNH 26391 are anteriorly positioned, but the protocones of P3 and P4 are sometimes more centrally positioned by comparison. Moreover, a lingual crest is variably present (e.g., AMNH 26397 ) and absent (e.g., AMNH 26396 ) on P3 and P4. Apparently, P 3 and P4 hypocones were infrequently present ; no other AMNH specimen of Metatitan relictus other than the holotype ( AMNH 26391 ) has distinct P3–P4 hypocones. A small preprotocrista is usually visible on P2, but it is exceedingly faint on P3 and absent on P4. Labial premolar cingula are distinct but thin. The lingual premolar cingula are most often continuous around the lingual sides of the crowns .

The upper molars of Metatitan relictus are elongate, although the lengths of M1 and M2 on the holotype (AMNH 26391) have been significantly shorted by interstitial wear (fig. 104b). The molars of this specimen are well worn, but along with molars of other specimens, they indicate typical brontotheriine apomorphies, 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. Distinct shallow central molar fossae and small anterolingual cingular cusps are present. All evidence of paraconules and metalophs is lost. There is no trace of a hypocone on M3, although the distolingual cingulum of the M3 is thickened and raised. Labial molar cingula are weak and lingual molar cingula are absent.

MANDIBLE AND LOWER DENTITION:

The holotype mandible (AMNH 26391) is complete except for the left condyle and a large portion of the symphysis, which has been reconstructed with plaster (fig. 105a, c) The right coronoid process is tall, narrow, and slightly curved. The ventral margin of the symphysis is slightly steeper than 45 °. (Other specimens with less plaster in the symphysis confirm that the reconstruction of AMNH 26391 is accurate). The symphysis is broad and extends posterior to the m1 metaconid.

The incisors are very small and form a nearly straight row between the canines. The i2 is somewhat larger than the other incisors. The incisor crowns of AMNH 26391 are heavily worn, but they are clearly short and have small lingual cingulids. The incisors of AMNH 26402 are less worn (fig. 105d, e). The crown of i1 is essentially featureless. The crown of i2 is somewhat larger and more wedge-shaped with a small lingual cingulid. The i3 is smaller than the i2, and it is more rounded in outline with a blunt apex. The lower canines of Metatitan relictus are variable in size, but they are generally small. There are no precanine or postcanine diastemata. In the holotype (AMNH 26391) p1 and p2 have fallen out. On the right side there is small space between the canine and the p2 alveolus, but on left side the p2 diastema is right next to alveolus of the canine. Other specimens with more complete sets of premolars lack postcanine diastema altogether, even when p1 is absent.

The more complete and less worn premolar row of AMNH 26402 is shown in close-up (fig. 105b). The p1 crown consists of a small cusp with a broad talonid heel. The trigonids of p2–p4 are similar in length to the talonids. The trigonid and talonid of p2 are of similar width, but on p3 and p4 the talonid is wider than the trigonid. The paralophid of p2 arches strongly lingually, although the lingual trigonid notch is small. The lingually positioned protolophid extends from the protoconid in a posterior direction. There is no p2 metaconid on AMNH 26402. Another specimen, AMNH 26427, exhibits a small p2 metaconid and a more lingually oriented protolophid. Like Metatitan primus , a deep and narrow labial groove extends between the p2 trigonid and talonid. This groove is directed anterolingually, while those of p3 and p4 are more lingually directed. In p3 and p4 of AMNH 26402 the paralophids and protolophids arch fully lingually, creating nearly molariform trigonid basins. The p3 and p4 also have large lingually positioned metaconids. The talonids are well developed in p2–p4 with long hypolophids, long cristids obliqua, and broad talonid basins. Labial premolar cingulids are weak and lingual premolar cingulids are absent.

The molars of Metatitan relictus are typical with shallow trigonid and talonid basins and thin lingual enamel. The m3 is very elongate. Labial molar cingulids are distinct, but they vary in thickness and are strongest on the talonid and usually discontinuous on the trigonid. Lingual molar cingulids are absent. The m3 cingulid of all specimens of M. relictus ends on the labial side and does not wrap around the hypoconulid as it does in M. primus .