Qufutitan zhoui Wang and Wang, 1997

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 : 106-113

Qufutitan zhoui Wang and Wang, 1997

HOLOTYPE: IVPP V8067 , the anterior portion of a skull with right I2, C, P2–M2, M3 (partial), and left I3–M1, M2 (partial).

TYPE LOCALITY: Dong Huangzhuang, Qufu City, Shandong Province; Huangzhuang Formation, China.

AGE: Late middle Eocene (Sharamurunian land mammal ‘‘age’’ ( Wang and Wang, 1997).

DIAGNOSIS: Qufutitan zhoui is the largest known brontothere that lacks conspicuous frontonasal protuberances. The frontal bone forms an anteriorly projecting triangular process that overlaps the nasal bone, thus splitting off a lateral nasal splint from the main body of the nasal. The lateral nasal incision does not extend posterior to the anterior margin of the P1. The nasal process slightly broadens distally; it is nearly horizontal, unelevated, narrow, with thin lateral walls, and without a well-defined or strongly rounded distal edge. The orbits are positioned directly above the posterior portion of M1 and the M2. The orbits do not project laterally. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. The premaxillary symphysis is vertical with a prominent dorsal ridge and a convex anterior margin. The dorsal surface of the skull is broad and nearly flat above the orbits, and the skull was probably not saddle-shaped.

Dentally, Qufutitan zhoui has small, globular upper incisors that form an arched incisor row. Premolar characteristics include a distinct metacone and hypocone on the P1, a distinct P2 metaconid, and weak premolar preprotocristae. Weak premolar hypocones are present on P2, P3, and P4. The lingual margins of the premolar metacones are strongly angled anterolingually. The molars of Qufutitan zhoui 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. Central molar fossae, anterolingual cingular cusps, paraconules, and metalophs are absent.

Qufutitan zhoui closely resembles Telmatherium validus , Metatelmatherium ultimum , Wickia brevirhinus , and Epimanteoceras formosus , but it retains the following unique combination of characters: a long face with a relatively short nasal incision, a broad and flat forehead, a complex P1, small premolar hypocones, and the lack of central fossae and anterolingual cingular cusps on the upper molars. Moreover, Q. zhoui is the only brontothere that lacks conspicuous frontonasal protuberance but at the same time possesses small globular upper incisors and a vertical premaxillary symphysis.

DESCRIPTION

SKULL: The holotype of Qufutitan zhoui (IVPP V8067) consists of the anterior portion of a very large skull (figs. 46–48). The antorbital region is essentially complete and in good condition, although it suffers from numerous cracks and a minor amount of distortion. The portion of the skull above the orbits is crushed downward, a large transverse crack runs across the proximal portion of the nasal process, and the nasal process appears to be artificially deflected downward. Additionally, the left nasal bone is pressed downward.

Judging by the holotype, Qufutitan zhoui is the largest brontothere to lack conspicuous frontonasal swellings. In terms of the overall shape of the skull, Q. zhoui closely resembles Telmatherium validus , Metatelmatherium ultimum , Wickia brevirhinus , and Epimanteoceras formosus . Q. zhoui is significantly larger than Telmatherium validus , Metatelmatherium ultimum , and Wickia brevirhinus , but Epimanteoceras formosus is of a similar size.

A portion of the large anteriorly projecting triangular protrusion of bone on the left dorsal surface of IVPP V8067 is partly an artifact of damage to the fossil. However, the right margin of the triangular protrusion on IVPP V8067 is a crack. The actual frontonasal sutures of IVPP V8067 can be distinguished from cracks upon close inspection (fig. 46). From the dorsal view of the skull, a triangular process of the frontal bone overrides the nasal bone. From the left lateral view, an arched splint of the nasal bone can be seen running between the maxilla and the overriding frontal process, thus maintaining contact with the lacrimal.

The lateral antorbital surface of the maxilla of IVPP V8067 forms a shallow facial concavity. The relative distance between the orbit and lateral nasal incision is greater than that of comparable brontotheres, and that distance contibutes to a longer face in Qufutitan zhoui . The nasal incision of Qufutitan is similar in length to Telmatherium validus and Epimanteoceras formosus . However, the faces of these taxa are relatively shorter and the nasal incisions of Telmatherium and Epimanteoceras extend to a point above the P2. However, the lateral nasal incision of Qufutitan zhoui does not extend posteriorly beyond the anterior margin of P1.

The orbit of Qufutitan zhou i is positioned directly above the posterior portion of M1 and M2, while the anterior orbital rim is situated above the anterior root of M1. This orbital position is slightly more anterior than that of Epimanteoceras formosus , Metatelmatherium ultimum , and Wickia brevirhinus , but similar to Telmatherium validus .

The moderate downward angle of the nasal bone of IVPP V8067 is probably a taphonomic artifact and, originally, it would have extended more horizontally from the skull. The nasal process is slightly longer than the premaxillomaxillary rostrum. The sides of the nasal process form deep and thin lateral walls that extend to the distal end of the nasal process. The nasal bones are incompletely co-ossified. The left and right nasal bones have become detached at the midline and the left side has been artificially depressed. From the dorsal view it can be seen that the nasal process is narrower than the premaxillomaxillary rostrum. The nasal process is of nearly constant width throughout its length, although it broadens slightly near the distal end. The anterior margin of the nasal process is mostly broken away although intact remnants of the distal edge remain on the anterolateral corners, indicating a thin and roughened distal nasal margin. The slight distal flaring of the nasal process resembles that in Epimanteoceras formosus in particular. However, the overall the morphology of the nasal process of Qufutitan zhoui does not differ significantly from that of Telmatherium , Metatelmatherium , or Wickia .

Some aspects of the rostrum of Qufutitan zhoui are unspecialized and typical. For instance, from the lateral view of IVPP V8067, the rostrum deepens posteriorly; the dorsolateral margin slopes posterodorsally and rises to about the midlevel of the orbit. The rostrum lacks the highly specialized dorsal bony cover seen in Dolichorhinus and Metarhinus . Despite these regularities, the rostrum of Q. zhoui has several peculiar features that distinctly differ from most other brontotheres. The elongate premaxillomaxillary rostrum of Q. zhoui contributes to the appearance of the relatively long face. Although there is a slightly upward curvature to the rostra of most brontotheres, the rostrum of IVPP V8067 seems to curve slightly downward. The premaxillomaxillary suture can be seen on the left side of the specimen, running along the dorsolateral border of the rostrum. The premaxilla does not contact the nasal bone; however, the nasal process of the premaxilla is long, slender, and extends nearly to the posterior notch of the lateral nasal incision. The premaxillary symphysis of Q. zhoui is nearly vertical, thus giving the incisors a nearly vertical orientation. In lateral profile the anterior margin of the symphysis is concave. Finally, there is a prominent U-shaped ridge of bone surrounding the notch on the dorsal surface of the premaxillary symphysis that is formed by the posterolaterally divergent premaxillary nasal processes. In contrast to Qufutitan zhoui , the premaxillae of nearly all other brontotheres have a more posterodorsally reclined premaxillary symphysis with more anteriorly angled incisors, a more convex or flat anterodorsal surface, and a less prominent (or absent) dorsal ridge.

Little can be said about the postorbital portion of the skull of Qufutitan zhoui ; however, the skull does not appear to have been saddle-shaped. The forehead is realtively broad and flat. In this respect Q. zhoui resembles both Telmatherium and Epimanteoceras . The foreheads of Metatelmatherium and Wickia are narrower and more transversely arched.

From the ventral view of IVPP V8067 the anterior rim of the posterior nares is positioned just anterior to the M3. There is a relatively wide emargination along the anterior margin that is marked by a thin ridge of bone. The lateral margins of the posterior nares are not preserved; however, the emargination probably continued around the lateral margins of the posterior nares, thus forming a horseshoe-shaped emargination similar to that seen in most other brontotheres.

UPPER DENTITION: The holotype of Qufutitan zhoui (IVPP V8067) retains a nearly complete set of upper dentition (figs. 48, 49, 50). Only two incisors are preserved, although the alveoli are reasonably well preserved and indicate a dental formula of 3-1-4-3. The incisors form an arched row anterior to the canines. A small median notch on the alveolar surface of the premaxilla suggests a minor diastema between the central incisors. The right I2 and left I3 are in a minimal state of wear. Both incisors are similar in size and are relatively small and globular in shape. There are no incisor cingula. The remaining unworn surfaces of the enamel crowns are crenulated. Each incisor bears one or more irregularly shaped and irregularly positioned enamel protuberances. The relatively small globular incisors, a condition similar to the incisors of more advanced horned brontotheres (e.g., Duchesneodus , Dianotitan ), are unusual for a hornless brontothere. There is a short diastema between the I3–C and a longer postcanine diastema. The canines of IVPP V8067 are very large and posteriorly curved.

The left premolar row (P1–P4) of IVPP V8067 is complete with a particularly well-preserved and minimally worn P1 and P2. However, the surfaces of the left P3 and P4 are somewhat weathered. The right P1 is missing although the remaining premolars are complete, minimally worn, and unweathered. The P1 of IVPP V8067 is substantially smaller than the other premolars, although it exhibits relatively advanced crown morphology. The P1 crown has both a well-developed ectoloph and lingual heel. The crown is nearly rectangular, although the anterior edge is strongly angled posterolingually, thus giving the tooth an oblique appearance. There is both a distinct paracone and metacone on the P1. The labial swelling of the paracone is much larger than the metacone. The parastyle is long and slightly lingually arched, while the metastyle is shorter and uncurved. The lingual side of the crown bears a well-developed and somewhat posteriorly shifted protocone, as well as a strongly developed preprotocrista that connects the protocone to the lingual base of the paracone. A much less distinct and shorter lingual crest projects straight posteriorly from the protocone. A continuous cingulum stretches from the lingual edge of the parastyle, wraps around the lingual margin of the crown, and meets the lingual edge of the metastyle.

The P2, P3, and P4 are more nearly rectangular in outline than the P1, with parallel anterior and posterior sides. Distinctly pinched paracone ribs can be seen on the labial surfaces of P2–P4. The labial swellings of the metacone are broader and rounder that the labial paracone ribs. The metacone of P2 is only slightly more lingually positioned than those of P3 and P4. The P2 parastyle projects straight anteriorly, while the P3 and P4 parastyles are strongly angled labially. The metastyles of P2 and P3 project straight posteriorly, while the P4 metastyle is angled somewhat labially. The lingual band of enamel on the ectolophs of P2, P3, and P4 is thinner than the labial band of enamel. A small but distinct preprotocrista connects the paracone and metacone on P2. P3 and P4 have progressively smaller and less distinct preprotocrista. No paraconules or metaconules can be seen on any of the premolars. The lingual margins of the metacone of P2, P3, and P4 form distinct vertical wedges that are strongly angled anterolingually.

A lingual crest of relatively low relief extends posteriorly from the protocone of the P2 and connects it to a very small hypocone that can now be identified only by a small exposure of dentine on the posterolingual corner of the crown. The lingual morphologies of the P3 and P4 are similar, although the hypocones are more distinct than that of the P2. Each of these premolars (P2–P4) has a strong continuous cingulum that wraps around the anterior, lingual, and posterior margins of the tooth. Likewise, each of these premolars has a distinct and continuous labial cingulum.

The premolars of Qufutitan zhoui distinctly differ from those of Telmatherium , Metatelmatherium , and Wickia . These taxa have simpler single-cusped P1s, and hypocones are not seen on any of the premolars. Moreover, Telmatherium occasionally has premolar paraconules and a diastema between P1 and P2. The premolars of Epimanteoceras formosus compare well with Q. zhoui in most respects. In particular, both taxa tend to have poorly developed hypocones. However, the strongly wedged and anterolingually angled margin of the premolar metacones of Q. zhoui is an autapomorphic condition.

The right molar row of IVPP V8067 is moderately worn and the posterior portion of M3 is not preserved. Typical brontotheriine molar apomorphies seen in Qufutitan zhoui include a narrow anterolabial cingulum that passes below the apex of the parastyle, a relatively tall and lingually angled ectoloph, weak labial paracone and metacone ribs, thin lingual ectoloph enamel, and wedge-shaped lingual margins of the paracone and metacone. Anterolingual cingular cusps and central molar fossae do not occur on any of the molars. Q. zhoui molars lack vestigial paraconules. Although the posterolingual corner of the M3 is lost, enough of the specimen is preserved to suggest that a hypocone either was not present or was poorly developed. Lingual molar cingula in IVPP V8067 are thin and discontinuous around the margins of the protocone.

REMARKS

Wang and Wang (1997) named a new genus and species of very large hornless brontothere, Qufutitan zhoui , based on the anterior portion of a skull (IVPP V8067). Wang and Wang (1997) diagnosed Q. zhoui by its large size, straight nasals, shallow nasal incision, weak hornlike swelling, well-developed premaxillary suture projection, upper incisors reduced with ‘‘global-shaped’’ crowns, large upper canine, developed C1– P1 diastema, nonmolariform premolars, weak hypocone on P2–P4, wide upper molars with no preconule (5 paraconule as used in this paper) or metaconule. Not all of these characters are particularly diagnostic. For instance, the molars of Qufutitan , described as wide by Wang and Wang (1997), are not different in overall proportions than other brontotheres. Moreover, the premolars of all brontotheres can roughly be described as ‘‘nonmolariform’’.

I was not able to confirm that IVPP V8067 has a ‘‘weak horn-like swelling’’ (contra Wang and Wang, 1997). As noted in the above description, there is a large triangular projection of bone on the left dorsal surface of the skull that overlaps the nasal bone, although in Qufutitan zhoui the surface of the overlapping frontal process is flush or nearly flush with the dorsal surface of the skull. A number of other middle Eocene brontothere taxa are known to exhibit the same frontonasal configuration, but like Qufutitan they lack conspicuous frontonasal protuberances. These taxa include Telmatherium validus , Metarhinus fluviatilis , Sthenodectes incisivum , Wickia brevirhinus , Metatelmatherium ultimum , and Epimanteoceras formosus . Among these taxa, only Epimanteoceras formosus has a minor frontonasal swelling, and even in that species the swelling occurs in only one of the two known specimens.

Wang and Wang (1997) briefly compared Qufutitan zhoui to Metatelmatherium ; a more extensive comparison of Qufutitan zhoui to other brontotheres is warranted. As noted above, Qufutitan zhoui closely resembles Telmatherium validus , Wickia brevirhinus , Metatelmatherium ultimum , and Epimanteoceras formosus . All express the same derived frontonasal configuration, but they otherwise have relatively unspecialized skulls. Closer examination reveals that in addition to its unusually large size Qufutitan zhoui expresses a unique combination of characters that strongly differentiates it from Telmatherium , Wickia , Metatelmatherium , and Epimanteoceras ; these characters include a long face with a relatively short nasal incision, a broad and flat forehead, a complex P1, small premolar hypocones, and the lack of a central fossa and anterolingual cingular cusps on the upper molars. Additionally, the strongly posterolingually angled premolar metacones is an autapomorphic condition that is not seen in other brontotheres. However, it is the unusual morphology of the premaxilla and the small globular incisors that most conspicuously distinguishes Qufutitan zhoui from other hornless brontotheres. With the exception of Qufutitan zhoui , small globular incisors are only seen among the most advanced horned brontotheres. All hornless brontotheres other than Qufutitan zhoui have larger and more subcaniniform incisors.

Strangely, several aspects of the incisor and rostral morphology of Qufutitan zhoui resemble the giant late Eocene Asian brontothere, Embolotherium . For instance, E. andrewsi , E. grangeri , and Q. zhoui are the only brontotheres to have globular upper incisors but retain a plesiomorphically arched incisor row. Other brontotheres with globular incisors exhibit a straight incisor row. Secondly, the vertical orientation of the premaxillary symphysis of Q. zhoui , with its concave anterior margin and pronounced dorsal ridge, are conditions that are otherwise only seen in the premaxillae of E. andrewsi and E. grangeri . Aside from these similarities Q. zhoui is obviously very different from Embolotherium .

Qufutitan zhoui is known only from a partial skull. However, there are a variety of brontothere species, known only from mandibles, to which Qufutitan cannot be directly compared. Nonetheless, it is worth considering the possibility that one of these ‘‘mandible taxa’’ is synonymous with Q. zhoui . Based on the long rostrum and long postcanine diastema of Q. zhoui , the mandible probably had a relatively elongate symphysis with a rather long lower postcanine diastema and small globular or semispatulate incisors. Pollyosbornia altidens and Hyotitan thomsoni closely resemble the anticipated mandible of Q. zhoui . Both are relatively large brontotheres exhibiting an arched incisor row, a long symphysis, and a long postcanine diastema. Hyotitan thomsoni , in particular, seems a likely synonym of Q. zhoui . The lower incisors of Hyotitan are rather smallish and form a slightly arched row. Further congruence between the skull of Q. zhoui with the mandible of H. thomsoni is represented by the relatively advanced upper and lower premolar morphologies, the very large canines, and the unusually pronounced labial premolar cingula/cingulids. H. thomsoni is more or less compatible with the anticipated mandible and lower dentition of Q. zhoui and it is possible that these species are synonyms. Therefore one of these species must be considered dubious. H. thomsoni Granger and Gregory, 1943 , is the senior name. However, as a matter of convenience, I presently consider Q. zhoui to be the valid species; the character information derived from its holotype skull is superior to the more limited character data retrievable from the holotype jaw (AMNH 26401) of H. thomsoni . If further discoveries indicate that these species are indeed synonymous, Q. zhoui should be considered a junior synonym of H. thomsoni .