Protitan minor 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 : 152-157

Protitan minor Granger and Gregory, 1943

HOLOTYPE: AMNH 26416 , a skull missing the distal end of the nasal process. (Presently the canines are missing although they were originally recovered and can be seen in older photographs of the specimen).

TYPE LOCALITY: ‘‘Probably top of Irdin Manha beds’’, Camp Margetts, Inner Mongolia, China.

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

REFERRED SPECIMEN: (From 1 mile west of Camp Margetts,?Irdin Manha beds, Inner Mongolia) AMNH 26417, a left anterior quarter of a skull with I3 and P2–M3.

DIAGNOSIS: Protitan minor is an intermediate-sized brontothere with small but distinct elliptical frontonasal horns that are positioned low on the skull and slightly in front of the orbits. The nasal incision extends posteriorly to the M1 and is dorsoventrally shallow. The nasal process is slightly angled downward, unelevated, long, broad, and with thin and shallow sidewalls. The orbits are positioned above the posterior portion of M2 and the anterior portion of M3. The premaxillomaxillary rostrum thickens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, separate parasagittal ridges that strongly constrict the dorsal surface of the cranium posteriorly, postzygomatic processes, nearly straight zygomatic arches, and a ventrally unconstricted and posteromedially angled external auditory pseudomeatus.

Dentally, Protitan minor has three large subcaniniform upper incisors, a simple P1, and a distinct P2 metacone. Premolar hypocones are absent but short lingual crests are present in some premolars. 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. Very shallow central molar fossae are present. Anterolingual cingular cusps are absent. Paraconules and metalophs are absent.

Protitan minor most closely resembles Protitanotherium emarginatum and Protitan grangeri . It can be most easily distinguished from Protitanotherium by the larger and more subcaniniform incisors, postzygomatic processes, and the ventrally unconstricted external auditory pseudomeatus. Protitan minor can be distinguished from Protitan grangeri by a posteriorly deeper nasal incision, thinner nasal process, more posteriorly positioned horns, and a posteromedially angled auditory pseudomeatus.

DESCRIPTION

SKULL: AMNH 26416 is a nearly complete skull with significant amounts of plaster filling in a large portion of the occiput and parts of the frontoparietal (fig. 70). The nasal process is mostly missing (although it is has been reconstructed as if very short), and the skull has suffered a minor amount of lateral sheering distortion. Otherwise the proportions of the skull appear to be relatively undistorted. A second specimen, AMNH 26417, a left anterior quarter of a cranium, has a more complete nasal process (fig. 71).

Protitan minor is most similar to that of Protitan grangeri and Protitanotherium emarginatum although it is smaller than these species. The frontonasal protuberances are similar to those of Protitan grangeri and Protitanotherium emarginatum ; they are short and elliptical with a longer anteroposterior axis. The frontonasal protuberances are positioned low on the skull and slightly in front of the orbits. This differs from Protitan grangeri whose horns are positioned far anterior to the orbits. The frontal bone can be seen overlapping the nasal bone and extending to the peak of the frontonasal protuberance.

The nasal incision is dorsoventrally shallow, but it is longer than that of Protitan grangeri and extends to a point above the M1 mesostyle. The anterior rim of the orbit rests over the posterior lateral root of the M1 and the anterolateral root of the M2. The orbit is positioned directly over the posterior half of the M2 and the anterior portion of the M3. In AMNH 26417 the nasal process is nearly horizontal, but it is angled downward slightly and is about as long at the premaxillomaxillary rostrum. The nasal process is much thinner than that of Protitan grangeri . In this respect it is similar to the nasal bones of Telmatherium validus and Epimanteoceras formosus . However, the lateral wall of the nasal process is much shallower than Telmatherium and most other hornless brontotheres.

The premaxillomaxillary rostrum of Protitan minor is typical in most respects. From the lateral view it deepens posteriorly and the dorsolateral margin rises posterodorsally so that the posterior notch of the nasal incision is at the level of the upper rim of the orbit. From the dorsal view of the skull the dorsolateral margins of the rostrum diverge posterolaterally and the rostral cavity (which is presently filled with plaster) is not dorsally sealed by bone. The premaxillae fully contact each other at the symphysis. The premaxillomaxillary suture is visible from the left side of the skull; the premaxilla truncates anterior to the posterior notch of the nasal incision and, therefore, does not contact the nasals.

The postorbital cranium is much longer than the facial region of the skull. The dorsal surface of the cranium of Protitan minor is essentially saddle-shaped. The posterior half of the frontoparietal is clearly concave. The anterior half is more flat, although this portion of the dorsal surface is somewhat warped, and from the dorsal view it can be seen that large portions of skull between the orbits are missing. Otherwise the general shape of the postorbital cranium of AMNH 26416 is similar to Protitan grangeri . The parasagittal ridges converge medially and constrict the dorsal surface of the skull posteriorly, but they do not form a sagittal crest. The zygomatic arches are relatively thin, shallow, and laterally unbowed. From a lateral view, the jugal portion of the zygomatic is horizontal. The squamosal portion of the zygomatic is angled upward posteriorly but at a very shallow angle, and there is essentially no curvature of the zygomatic arch. The posterodorsal end of each of the zygomatic arches exhibits a dorsally projecting postzygomatic process that is similar to that of Protitan grangeri .

Although poorly preserved, the overall shape of the occiput seems to resemble that of Protitan grangeri . The occiput appears to be tilted backward. From a dorsal view of the skull, the nuchal crest is concave. From the posterior view the dorsal half of the occiput is heavily reconstructed, but it appears to have been about as wide as the posterior portion. The occiput appears to have been constricted in the middle.

The posterior nares of AMNH 26416 are slightly more anteriorly positioned in comparison to Protitan grangeri (fig. 72a). The anterior rim of the posterior nares seems to be positioned slightly anterior to the M3.

(AMNH 26417) of ‘‘Protitan’ ’ minor .

However, the elongate posterior narial canal is completely filled with plaster, thus obscuring other details of the posterior nares. Two large ventral sphenoidal fossae are present and are distinctly partitioned by a thin bony septum formed by the basisphenoid. Like Protitan grangeri , the external auditory pseudomeatus forms a broad ventrally unconstricted opening; however, it enters the skull in a posteromedial direction.

UPPER DENTITION: The teeth of the holotype skull (AMNH 26416) are complete (except for the lost canines), undamaged, and for the most part, they are only lightly worn (fig. 72). The incisors of Protitan minor are relatively large but with small diastemata between them. They form a distinct arch anterior to the canines. The tips of the incisors are worn, but each incisor appears to have been subcaniniform with a short, lingually curved crown and a distinct lingual cingulum. The I3 is distinctly larger and taller than the I1 or I2 and it is more caniniform. Relatively large canines were once present and can be seen in older photographs of AMNH 26416 (fig. 70a). However, the canines are now missing (fig. 72a). There are both short precanine and postcanine diastemata.

The cheek teeth are not differentiated from those of Protitan grangeri . The P1 is a single-cusped tooth with an elongate posterior heel. A small lingual cingulum is also visible on the P1. The anterior margin of P2 is angled distolingually, giving the tooth a strongly oblique shape. The anterior and posterior margins of P3 and P4 are closer to parallel. The parastyle of P2 is straight, although the ectoloph and the metastyle are directed in a posterolingual direction. The parastyle and metastyle of P3 are nearly straight, while the parastyle and metastyle of P4 are distinctly angled labially. The labial margin of the P2 paracone is very convex, while the paracones of P3 and P4 have distinct labial ribs. The metacone of P2 is lingually shifted, while those of P3 and P4 are more labially positioned. Because of these differences the outer surface of P2 is rounder than those of P3 and P4.

Like Protitan grangeri the lingual features of the premolars have low topographic relief. Only a single lingual cusp (protocone) is present on P2–P4. A small preprotocrista is seen on the P2. A faint preprotocrista can be discerned on the P3. Additionally, the protocones of P2 and P3 are each followed by a short lingual crest. The lingual heel of P4 is devoid of any crests. The anterior and posterior premolar cingula wrap around the lingual side of the crowns and nearly join together, but they do not form completely continuous lingual cingula.

The molars show 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. Central molar fossae are present, but they are very shallow and almost indistinct. Anterolingual cingular cusps are absent. All traces of paraconules and metalophs are lost. M3 lacks a hypocone. Labial molar cingula are weak, and lingual molar cingula are absent.

REMARKS

Protitan minor Granger and Gregory (1943) is based on a nearly complete skull (AMNH 26416) from the Camp Margetts area of Inner Mongolia. Along with P. grangeri , P. minor is one of only two brontotheres with conspicuous frontonasal horns that retains a ventrally unconstricted external auditory pseudomeatus. Granger and Gregory (1943) did not explicitly state how P. minor differed from P. grangeri although there are several differences. In addition to being distinctly smaller, P. minor can clearly be differentiated by its shorter more constricted face, more posteriorly positioned horns, and thinner nasal bone.

No mandibles or lower teeth can presently be referred to Protitan minor . However, considering that the upper dentitions of P. grangeri and P. minor are undifferentiated, it seems likely the lower dental morphologies of these species were similar. Granger and Gregory (1943) referred several mandibles from the Camp Margetts area of Inner Mongolia to P. minor ; however, the large metaconid seen on the p3 of these specimens is inconsistent with what one would expect of the lower dentition of P. minor . Therefore, I have removed these specimens from P. minor and reassigned them to an unnamed taxon, informally referred to as Camp Margetts ‘‘taxon A’’ (see section on dubious and problematic taxa).

Yet another mandible (AMNH 26419) from the ‘‘Houldjin’’ beds of Camp Margetts, with extremely worn incisors, a complete and moderate-sized left canine, and right and left premolars, closely resembles what one would predict for Protitan minor in terms of size and overall morphology. Unlike the mandibles assigned to Camp Margetts ‘‘taxon A’’, the p3 of this specimen lacks a metaconid. This specimen could represent P. minor . Additionally, Dong and Ai (2001) referred a left p1–p4 and right m1 from the lower part of the Tunggur Formation of Inner Mongolia to P. minor . These referrals were based primarily on size and there is no conclusive evidence that they actually belong to P. minor .