Dianotitan lunanensis ( Chow and Hu, 1959 )

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 : 308-315

Dianotitan lunanensis ( Chow and Hu, 1959)

HOLOTYPE: VM 547, a complete skull with right P1–M3 and left lateral incisor (I2), P2– M3

TYPE LOCALITY: Anjetsen, Lunan, Yunnan, China; Lower part of Upper Lunan beds ( Chow and Hu, 1959), now considered the Lumeiyi Formation ( Russell and Zhai, 1987).

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

REFERRED SPECIMEN: (From the Lumeiyi Formation) VM 573, a partial skull with very worn dentition including right P4–M1 and left M2–M3.

DIAGNOSIS: Dianotitan lunanensis is a large brontothere with small- to medium-sized ovoid frontonasal horns. The horns are positioned low on the skull, and above and slightly in front of the orbits. The lateral nasal incision extends posteriorly to a point above the anterior margin of P4. The anterior rim of the orbit is positioned directly above the anterior portion of M1. The nasal process is broad, unelevated, horizontal or slightly downturned, slightly tapered distally, strongly rounded anteriorly, and with shallow and thickened lateral walls. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, a dorsal cranial surface that is moderately constricted posteriorly by parasagittal ridges, laterally bowed zygomatic arches with large lateral swellings, an unnotched nuchal crest, the lack of a narrow emargination surrounding the posterior nares, weakly curved zygomatic arches, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae and postzygomatic processes are absent.

Dentally, Dianotitan lunanensis has two small upper incisors that form a straight row, an insignificant postcanine diastema, a morphologically complex P1, and distinct premolar hypocones on P2–P4. The molars 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 triangular anterolingual cingular cusps are present. Paraconules and metalophs are absent.

Dianotitan lunanensis shares only with Duchesneodus uintensis and Megacerops coloradensis the combination of a reduced number of upper incisors, conspicuous lateral zygomatic swellings, the absence of a horseshoe-shaped emargination around the posterior nares, and an insignificant postcanine diastema. Dianotitan lunanensis lacks the parietal dome of Duchesneodus uintensis . In comparison to Megacerops coloradensis , Dianotitan lunanensis has lower horns that are more posteriorly positioned and vertically oriented, the orbits are more posteriorly positioned, and the nasal incision is longer.

DESCRIPTION

SKULL: The holotype skull of Dianotitan lunanensis (VM 547) can be seen in figs. 152 and 153. The skull is now fixed to a steel mounting frame and it was not possible to photograph the entire ventral surface. Instead, the ventral surface of a cast from IVPP is provided (fig. 155a). The holotype skull has numerous minor cracks, particularly on its dorsal surface. However, it is nearly complete and only moderately distorted. The right frontonasal horn has been reconstructed with plaster. The skull is slightly crushed dorsoventrally. The dorsoventral flattening seems to have occurred mostly at the level of the orbit. On both sides the upper orbital rim (frontal) is lowered nearly to the level of the lower orbital rim (jugal). The holotype skull is slightly sheared as well ; the dorsal surface of the skull is shifted slightly left with respect to the ventral surface.

A second skull (VM 573) is less complete (fig. 154). The right side in particular is heavily damaged. The zygomatic arches are not preserved, the distal portions of the nasal and premaxillomaxillary rostrum are not present, and most of the occiput is missing. From the ventral view of the skull it is apparent that there has been significant lateral crushing. Moreover, the basioccipital region is artificially rotated to the right.

A definitive frontonasal suture cannot be observed on either VM 547 or VM 573, although the horns of Dianotitan lunanensis are presumably composed of the frontal and nasal bones like other horned brontotheres. The horn surfaces of VM 547 and VM 573 are somewhat roughened, but they lack heavy rugosities. The left frontonasal horn of VM 547 is massive, though short, and it is rounded in cross section. The horn of VM 573 is smaller and more elliptical in cross section. However, the horn of VM 547 is more rounded in cross section than any specimen of Protitanops or Parabrontops . The horns of Dianotitan lunanensis are vertical, positioned low on the skull, and they rest directly above and slightly in front of the orbits.

From the dorsal view of VM 547 the nasal process is wide and completely obscures a view of the premaxillomaxillary rostrum. The nasal process is tapered and the distal end is strongly rounded. The nasal bones are strongly co-ossified, although the distal ends of the nasal bones are unconnected, thus forming the deep median notch at the distal tip. From a lateral view of VM 547 the nasal process is slightly longer than the premaxillomaxillary rostrum. The lateral margins of the nasal process are thickened and strongly downfolded proximally. The lateral margins of the nasal process become shallower distally. The distal margin of the nasal process is distinctly curved downward. The nasal process of VM 573, though incomplete, appears to have been somewhat thinner and narrower.

The lateral nasal incision of VM 547 is very shallow; this is partly due to the dorsoventral crushing. However, the nasal bone is more or less above the orbit, indicating that the nasal bone was not highly elevated high above the orbits as in Diplacodon elatus . The posterior margin of the lateral nasal incision is positioned above the anterior margin of P4. The orbits are positioned directly above the posterior portion of M1 and the anterior portion of M2. The anterior orbital bony rim is positioned directly above the anterior portion of M1.

The rostrum of Dianotitan lunanensis is relatively unspecialized. From a lateral view of VM 547 the premaxillomaxillary rostrum curves upward and it lacks a dorsal bony covering. From a lateral view of VM 574 the rostrum deepens proximally as the dorsolateral surface of the rostrum strongly slopes posterodorsally and rises to about the midlevel of the orbit. The premaxilla is short and does not extend anterior to the canines. The premaxilla and maxilla are completely fused and a premaxillomaxillary suture cannot be seen.

The dorsal surface of the skull of Dianotitan lunanensis is entirely concave (saddle-shaped). The parasagittal ridges, running from the postorbital process of the frontal to the nuchal crest, are prominent in VM 547 and VM 573. The parasagittal ridges remain widely separated throughout their length and do not merge to form a sagittal crest. At their closest point the parasagittal ridges only marginally constrict the dorsal surface of the cranium. The dorsal surface of the cranium of VM 547 is notably wider than that of VM 573.

From a lateral view of VM 547 the zygomatic arches are deep, although they originate from a shallow lower orbital rim. The jugal portion of the zygomatic is relatively straight and horizontal, but the squamosal portion rises posteriorly, giving the zygomatic arch a pronounced curvature. From a dorsal view of VM 547 the zygomatic arches are strongly bowed laterally. The most conspicuous feature of the zygomatic arch is the thickened lateral swelling in the center of the arch at the junction of the jugal and squamosal bones.

From a dorsal view of VM 547 the nuchal crest is very thick, rugose, and concave. It lacks the distinct mesial notch seen in the much thinner nuchal crest of Protitanops . The nuchal crest of VM 573 appears to have been much thinner. From lateral views the occiputs of both specimens are mildly tilted backward. From the posterior view of VM 537 the occiput is semirectangular. The occiput is wider than it is tall and it is mildly hourglass-shaped due to a distinctly waisted middle. A pair of thick bony occipital pillars rise from above the foramen magnum to the dorsolateral corners of the occiput. The nuchal crest is slightly arched dorsally. The occiput of Dianotitan lunanensis closely resembles those of Duchesneodus uintensis and Megacerops coloradensis . These species show a degree of intraspecific variability in occiput robustness that resembles the variation in nuchal crest thickness exhibited by VM 547 and VM 573.

The anterior margin of the posterior nares of VM 547 is positioned slightly behind M2. The lateral and anterior margins of the posterior nares of most brontotheres are rimmed by a depressed horseshoe-shaped emargination. This trait is diminished in Dianotitan lunanensis as well as Duchesneodus uintensis and Megacerops coloradensis . The posterior nares form an elongate canal that is bisected by the vomer. Ventral sphenoidal fossae are absent. The configuration of the basicranial foramina, including the widely separated foramen ovale and foramen lacerum, is typical of brontotheres. The external auditory pseudomeatus in VM 547 is tube-shaped and enclosed ventrally by an anteroventrally curving mastoid process that contacts the posterior surface of the postglenoid process. The auditory canal enters the skull at a mediolateral angle.

UPPER DENTITION: The dentition of VM 547 is nearly complete (fig. 155) and only moderately worn. Both median incisors are missing. On the actual specimen of VM 547 only the left lateral incisor is seen, although the cast of the same specimen at IVPP has both left and right lateral incisors. The other specimen, VM 573, retains only very badly worn molars.

Though the incisor row is incomplete, the intact alveoli indicate only two pairs of upper incisors. The incisor alveoli are positioned close together except for the distinct median notch on the anterior surface of the premaxilla. The lateral incisors have tiny globular crowns. The medial incisor alveoli indicate that the medial incisors were of a similar size. The incisor row is straight and positioned slightly in front of the canine alveoli. A short diastema persists between the lateral incisor alveolus and canine alveolus. The canines are not preserved, although the diameter of the alveoli (15.2 mm) indicates rather small canines. A very short diastema, less than a centimeter in length, separates the canine and anterior premolar. This postcanine diastema is essentially insignificant in comparison to the longer postcanine diastemata of Parabrontops , Eubrontotherium , Protitanops , and Notiotitanops . On the other hand, Duchesneodus and Megacerops have either no postcanine diastema or a very short insignificant gap similar to that seen in VM 547.

The premolars of VM 547 lack species diagnostic characters. The left P1 is not present. The right P1 is heavily worn but the crown is rounded in outline. Brontotheres with similarly shaped P1s tend to have morphologically advanced P1s with two labial cusps, a large paracone and a smaller metacone, as well as a small lingual heel, often with a small protocone or a lingual crest. This morphology is detectible on the P1 of VM 547, although these advanced features are nearly obliterated by wear. The lingual bulge of the paracone can clearly be seen on the P1. On the occlusal surface of that tooth the smaller metacone is separated from the paracone by a short fossa that is not completely obliterated. The P1 metacone is partially worn away by interstitial wear. Finally, a long and narrow lingual heel is present on P1, although any cusps or crests that might have been present on this structure are worn away.

P2 is oblique with a strongly posterolingually angled anterior margin. However, P3 and P4 are nearly rectangular in outline with parallel anterior and posterior margins. The P2 parastyle curves slightly lingually, the P3 parastyle is straight, while the P4 parastyle is angled in a somewhat labial direction. The metastyles of these three premolars are short and essentially straight. The ectoloph of P2 is more rounded than those of P3 and P4. The paracone forms a prominent vertical labial rib on P2, P3, and P4. On the other hand, the metacone forms a prominent labial rib only on P2. The labial metacone rib is much smaller on P3 and it is not seen on P4. Mesostyles are not seen on any of the premolars.

The lingual sides of P2, P3, and P4 each possess a single oblong exposure of dentin, formed by the lingual cusps and the lingual crest that connects these cusps. The posterior bulge in the lingual exposure of dentin of P2 suggests a rather large hypocone. A heavily worn lingual crest extends anterolabially from the metacone and connects to the lingual base of the paracone. The preprotocrista, protocone, and lingual crest of P2 seem to have been fused into a single, large crest. However, P3 and P4 each possess a distinct paracone that is similar in size to the hypocone and positioned closely to it and not entirely separated from it. A distinct preprotocrista can be seen on P3. The P4 paracone swelling is significantly larger than the incompletely separated metacone swelling. The left P4 appears to retain a very short and indistinct preprotocrista, although no sign of this structure is visible on the right P4. Labial premolar cingula are weak on VM 547. The anterior and posterior cingula wrap around the lingual corners of the premolar crowns, but they do not join to form lingual cingula.

The molars of Dianotitan lunanensis exhibit a rather tall ectoloph. The vertical labial ribs of the paracone range in distinctness from faint to absent. The lingual band of ectoloph enamel is much thinner than the labial band and in comparison to the labial band it is almost entirely worn away. Nonetheless, the broad dentin exposure on the lingual sides of the paracone and metacone retain their original wedge shape. The M3 metacone, the least worn cusp on this specimen, retains a wedge-shaped lingual enamel margin. M1 and to a lesser extent M2 have been made significantly narrower by interstitial wear. However, interstitial wear between the contact of M2 and M3 is minor and the proportions of M3 are essentially intact. Because of interstitial wear portions of the anterior cingulum of M1 and M2 are obliterated. On M3 a very thin lingual cingulum can be seen passing around the anterolabial corner of the crown below the apex of the parastyle. A distinct central molar fossa can be seen on all three molars. In addition, each molar exhibits a well-developed triangular anterolingual cingular cusp. Paraconules are not seen on any of the molars. Each molar exhibits a distinct hypocone, although the M3 hypocone is slightly smaller than those of M1 and M2. The M1 hypocone is heavily worn. A short metalophlike ridge extends from the M2 and M3 hypocones to the lingual base of the metacone. A similar loph is visible on M3 although it passes anterior to the lingual base of the metacone. Labial cingula are very weak and lingual cingula are essentially absent on all of the molars.

REMARKS

Dianotitan lunanensis ( Chow and Hu, 1959) is based on a skull (VM 547) reported from ‘‘Anjetsen, Lunan, Yunnan; Lower part of Upper Lunan beds’’ ( Chow and Hu, 1959), now considered the Lumeiyi Formation ( Russell and Zhai, 1987). The age of the Lumeiyi Formation is only crudely resolved; the known fauna contains several elements of the Irdinmanhan fauna and some from the Sharamurunian fauna ( Russell and Zhai, 1987), thus suggesting a middle Eocene age ( McKenna and Bell, 1997). Although Chow and Hu (1959) mention only the holotype specimen of Dianotitan lunanensis , another less complete specimen (VM573) is referable to this species.

Chow and Hu (1959) originally assigned this species to Parabrontops . Later Chow et al. (1974) reassigned it to a new genus, Dianotitan . Dianotitan lunanensis was originally diagnosed by Chow and Hu (1959) as having well-developed horns, expanded zygomatic arches, reduced upper incisors, and a short postcanine diastema. Chow and Hu (1959) compared D. lunanensis only to Parabrontops gobiensis ( Osborn, 1925) . Chow and Hu (1959) differentiated Dianotitan lunanensis from P. gobiensis in the following ways: (1) a broader skull, (2) a reduced number of incisors, (3) smaller size, (4) a shorter nasal with a narrower anterior end, (5) smaller frontonasal horns, and (6) relatively smaller premolars. Utilizing a series of cranial indices, Chow et al (1974) also found D. lunanensis to have a more brachycephalic skull and different cheektooth proportions than P. gobiensis . However, only the first two of Chow’s and Hu’s (1959) observations and the first observation of Chow et al. (1974) are valid morphological distinctions. (1) D. lunanensis has large laterally expanded zygomatic swellings, whereas P. gobiensis does not, thus giving the skull a broader or more brachycephalic appearance. (2) Secondly, D. lunanensis retains only two upper incisors (per side), while the incisor alveoli preserved in specimens of P. gobiensis indicate three upper incisors (per side).

The remaining differences noted by Chow and Hu (1959) and Chow et al. (1974) do not have any clear taxonomic significance, but rather seem related to taphonomic distortion, dental wear, and patterns of intraspecific variability that are common among brontothere species. The two specimens of Dianotitan lunanensis are reportedly smaller than the two known skulls of Parabrontops gobiensis ; however, these differences are dubious. The overall sizes of the skulls differ by no more than a few centimeters (differences that could be attributed to some extent by taphonomic distortion). The dental dimensions of P. gobiensis and D. lunanensis typically differ by no more than a few millimeters, and in some dental measurements, specimens of D. lunanensis actually exceed those of P. gobiensis . In sum, these species seem to be about the same size, with slightly fluctuating proportions that may relate to distortion or dental wear.

The nasal bone of Dianotitan lunanensis (VM 547) is shorter and more distally tapered than that of the holotype of Parabrontops gobiensis (AMNH 20354). However, the P. gobiensis holotype skull is severely distorted and the reconstruction contains a significant amount of plaster in the nasal bone and other parts of the skull. In contrast, a less significantly distorted skull (AMNH 26020) referred to P. gobiensis has an undamaged nasal bone that is similar in appearance to that of D. lunanensis . Therefore, D. lunanensis and P. gobiensis do not appear to have differentiated nasal bones (contra Chow and Hu, 1959).

Chow and Hu (1959) stated that Dianotitan lunanensis has smaller horns than Parabrontops gobiensis , although this is misleading. Both D. lunanensis and P. gobiensis show variation in horn size. The horns of the holotype of D. lunanensis (VM 547) are intermediate in size between the two known specimens of P. gobiensis , while the smaller horns of the referred skull (VM 573) are similar in size to the holotype of P. gobiensis (AMNH 20354).

I could not confirm that Dianotitan lunanensis has relatively smaller premolars than P. gobiensis , as Chow and Hu (1959) and Chow et al. (1974) suggest. The calculation of a ratio of P2–P4 length/M1–M3 length yields a value of 0.50 for D. lunanensis (VM 547). A precise ratio cannot be calculated for the holotype of P. gobiensis due to severe distortion. However, the referred skull (AMNH 26020) yields a similar value of 0.53. However short taphonomic gaps are present between the premolars of AMNH 26020 that add about a centimeter to the premolar row length. Recalculation of the ratio after subtraction of these gaps yields a ratio of 0.48 for P. gobiensis . Thus, the premolars of D, lunanensis do not appear to be notably smaller than those of P. gobiensis (contra Chow and Hu, 1959).

The two skulls of Dianotitan lunanensis (VM 547 and VM 573) show variation in horn size, nasal thickness, and overall skull robustness; these characteristics are commonly variable within other brontothere species and probably represent sexual dimorphism. Both specimens possess horns that are positioned very low on the skull, above and slightly in front of the orbits, and with a nearly vertical horn orientation. This configuration most strongly resembles Parabrontops gobiensis . Nonetheless, D. lunanensis can be differentiated from P. gobiensis by (1) the presence of conspicuous lateral zygomatic swellings, (2) a reduced number of upper incisors, (3) an insignificant postcanine diastema, (4) and the absence of a distinct horseshoe emargination around the anterior and lateral margins of the posterior nares. Only two North American species share this combination of traits, Duchesneodus uintensis and Megacerops coloradensis . Duchesneodus uintensis exhibits an autapomorphic parietal dome that is clearly not present in Dianotitan lunanensis . Furthermore, both Duchesneodus uintensis and Megacerops coloradensis have more anteriorly situated orbits and shorter nasal incisions.