Notiotitanops mississippiensis Gazin and Sullivan, 1942

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 : 303-307

Notiotitanops mississippiensis Gazin and Sullivan, 1942

HOLOTYPE: USNM 16646, the ventral half of a skull with right C–M3, left I1, C–M3, and a partial mandible with right p3–m2 and left p3–m3.

TYPE LOCALITY: Archusa Marl Member of the Cook Mountain Formation (previously considered the Lisbon Formation), about 2.5 miles south Quitman, Clarke County, Mississippi ( Robinson et al., 2004).

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

DIAGNOSIS: Notiotitanops mississippiensis is a large brontothere with a nasal incision that extends to the anterior margin of the P2. The P4 is situated directly below the anterior rim of the orbit. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a narrow emargination surrounding the posterior nares, slightly curved zygomatic arches, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae and postzygomatic processes are absent.

Dentally, Notiotitanops mississippiensis is characterized by two small upper incisors that form a straight row, a complex P1, a long postcanine diastema, a distinct P2 metacone, and distinct premolar hypocones on P3–P4. The upper molars of Notiotitanops mississippiensis 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 anterolingual cingular cusps are present. Paraconules and metalophs are absent. The lower molars have shallow basins and the m3 is slender.

Notiotitanops mississippiensis shares with Eubrontotherium clarnoensis and Protitanops curryi the combination of a long upper postcanine diastema and a reduced number of upper incisors. Notiotitanops mississippiensis differs from P. curryi and E. clarnoensis in having more posteriorly positioned orbits and a nasal incision that does not extend as far posteriorly.

DESCRIPTION

SKULL: Notiotitanops mississippiensis is known from only a single specimen from Mississippi (USNM 16646). This specimen consists of the ventral portion of an undistort- ed skull (figs. 149, 150) and an associated partial mandible (fig. 151). The dorsal portion of the skull is completely weathered away, revealing a transverse cross-sectional view.

The nasal incision extended no farther posteriorly than the anterior margin of the P2. Only the ventral portion of the orbit is preserved. It is positioned more or less directly over M1 and the anterolateral root of M2. The orbit is positioned so far anteriorly, in fact, that the anterior margin of the orbital rim is positioned directly above the anterolateral root of the p4. The orbit is more anteriorly positioned than either that of Protitanops or Eubrontotherium .

The premaxillomaxillary rostrum is short. The rostrum curves upward distally and it deepens posteriorly. The dorsal margin of the rostrum slopes posterodorsally but does not rise higher than the ventral border of the orbit. The short premaxilla does not extend anterior to the canines. The premaxillary symphysis is completely fused. A premaxillomaxillary suture is not discernable. The dorsolateral margins of the rostrum diverge posterolaterally and the rostral cavity is open dorsally.

The left zygomatic arch is incomplete, but aspects of its shape can be interpreted from the remaining part. The zygomatic process of the jugal is straight, angled posterolaterally, and is roughly rectangular in cross section. The zygomatic process of the squamosal originates from the cranium at a point that is higher than the distal end of the zygomatic process of the jugal. Therefore, the zygomatic arch would have had a moderate curvature from a lateral view. It is not clear whether there was a large lateral zygomatic swelling as seen in Protitanops , Dianotitan , Duchesneodus , and Megacerops (sensu Mihlbachler et al., 2004b) .

The weathered state of the skull provides a remarkable view of the internal structure of the skull. The various cranial sinuses and cavities are filled with lightly colored matrix and are clearly outlined by darker bone. Most notable is the teardrop-shaped, voluminous left maxillary sinus. This perspective also reveals the large maxillary tuberosity that underlies the floor of the orbits. The turbinates were apparently destroyed before burial or have been weathered away because they cannot be seen in the matrix filling the nasal chamber. However, the thin walls of the vomer can be seen from the dorsal surface directly above where the posterior nares are located.

From the ventral view of the skull (fig. 150) the anterior margin of the posterior nares is positioned slightly behind the M3 protocones. The posterior nares are rimmed by a narrow horseshoe-shaped emargination. From the ventral view the vomer is totally immersed in the remaining sediment filling the posterior narial canal, except at the posterior end where it emerges to join with the body of the sphenoid. The posterior narial canal shallows posteriorly and extends onto the sphenoid where it is bisected by the sphenoid body. However, by this point the canal is very shallow and large ventral sphenoidal fossae are not present. The foramen ovale and foramen lacerum are well separated. The mastoid process contacts the postglenoid process and forms a small tube-shaped external auditory pseudomeatus.

The preservation of the left side of the basicranium is exquisite and includes an isolated petrosal. Further description of the basicranium of Notiotitanops mississippiensis was provided by Gazin and Sullivan (1942). These details are of potential interest, but they are not repeated here largely because similar details are not yet available for other species.

UPPER DENTITION: The incisors of USNM 16646 are missing except for the left lateral incisor. This tooth is now detached from the skull but can be seen in position in Gazin and Sullivan (1942). The crown of this incisor is small and globular. The incisor alveoli are intact and indicate only two pairs of incisors. The incisor alveoli form a straight row that is positioned between the anterior margins of the canines. The canines are of moderate size. The lateral incisor and canine are separated by a very brief diastema. Notiotitanops mississippiensis retains a postcanine diastema that is somewhat longer than P2.

The left and right cheek-tooth rows are complete, but curiously the right side is more worn (fig. 150). The P1 of Notiotitanops mississippiensis is small, but with a relatively complex morphology. It is nearly rounded in outline. There are two bulges on the labial wall of the ectoloph indicating two labial cusps, a paracone, and a smaller metacone. There is a small lingual shelf with a small protocone and a low preprotocrista. The anterior and posterior margins of the P2–P4 are nearly parallel. The parastyle of P2 is directed anteriorly, the parastyle of P3 is angled slightly labially, and the P4 parastyle is strongly angled labially. The metastyles of P2 and P3 are essentially straight, while the P4 metastyle is strongly angled labially. The labial side of the P2 paracone is convex, while P3 and P4 have very weak labial paracone ribs. The P4 exhibits a mesostyle that is prominent but not as well developed as those of the molars. There is also a slight bulge on the P3 ectoloph between the paracone and metacone that could be interpreted as a rudimentary mesostyle. The protocone and hypocone of P2 are positioned very closely together and are almost fully absorbed by the lingual crest. However, two distinct lingual wear facets with exposed dentin can be seen on each P2, a small. rounded protocone facet, and a small, elongate hypocone facet. The protocone and hypocone of P3 and P4 are more distinct and more distantly separated, although they are strongly connected by a lingual crest. Small but distinct preprotocristae are seen on P2–P4. The preprotocrista of P2 is the longest and tallest, while that of P4 is the lowest and shortest. The labial cingula of P2–P4 are faint. Anterolingual cingular cusps are not seen on the premolars. Lingual premolar cingula have been smoothed by wear but they are continuous around the protocones of P3 and P4 and slightly discontinuous on the P2.

The upper molars of Notiotitanops mississippiensis show typical brontotheriine traits, including tall, lingually angled ectolophs, very weak labial ribs, thin lingual ectoloph enamel (though most of it is worn away), and wedge-shaped lingual margins of the paracone and metacone. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. Distinct central molar fossae are seen on M2 and M3. Anterolingual cingular cusps can be seen on M2 and M3, and they have well-developed wear facets. M1 is too worn to reveal these last two traits. The M3 exhibits a prominent hypocone that is almost as large as those of M1 and M2. Labial molar cingula are very weak and lingual molar cingula are absent.

MANDIBLE AND LOWER DENTITION: The holotype mandible is missing the anteri- or part of the symphysis and the ascending rami (fig. 151). The symphysis extends to the talonid of p4. The lower incisors, canines, p1, and p2 are not preserved. The most complete cheek-tooth row is that of the left side with p3–m3. The p3 trigonid is about as long as the trigonid, while the p4 trigonid is slightly shorter than the p4 talonid. The trigonids of p3 and p4 are narrower than the talonids. The paralophids and protolophids of p3 and p4 are strongly angled lingually. The trigonid of p3 has a comparatively narrow lingual notch, while that of p4 is broad and molariform. Each of these premolars exhibits a large lingually positioned metaconid. The talonids of p3 and p4 have well-developed cristids obliqua and hypolophids with shallow and broad basins. The molars are typical with thin lingual enamel, shallow basins, and an elongate m3. A thin, beaded cingulid can be seen tracing around the distal margin of the m3.

REMARKS

Notiotitanops mississippiensis Gazin and Sullivan (1942) is known only from its holotype, which represents a rare occurrence of a brontothere fossil east of the Mississippi River. Further details of the discovery and provenience of this interesting specimen are provided by Gazin and Sullivan (1942) and are not recounted here. Gazin and Sullivan (1942) initially compared Notiotitanops mississippiensis to several North American horned brontotheres such as Protitanotherium emarginatum , Diplacodon progressum (5 D. elatus ), Duchesneodus uintensis , and others. However, they were apparently unaware that Stock (1936) had named a very similar species, Protitanops curryi , six years before. Gazin’s and Sullivan’s (1942) diagnosis of N. mississippiensis also fits P. curryi . Nonetheless, Notiotitanops mississippiensis has continued to be accepted as a distinct species ( Mader 1989, 1998), although Krishtalka et al. (1987) suggested that it might belong to the genus Protitanops .

Mader’s (1989, 1998) diagnoses of Notiotitanops and Protitanops differ in only two respects. Notiotitanops was described as having poorly separated lingual cusps on P2–P4 and a rudimentary hypocone on M3. Protitanops was described as having two lingual cusps and a hypocone on M3. The characterization of Notiotitanops as having a rudimentary hypocone is incorrect. The holotype of N. mississippiensis has a well-developed M3 hypocone. The point seems moot, at any rate, since the presence and size of the M3 hypocone is variable within many species. Secondly, given the intraspecific variability seen in the morphology of the lingual premolars of brontotheres, the difference between having poorly separated cusps in one specimen and two lingual cusps in another does not justify two species.

Other than minor premolar variation, the only clear way in which Notiotitanops mississippiensis differs from Protitanops curryi is in the proportions of its face. The nasal incision of N. mississippiensis does not extend as far posteriorly as that of P. curryi . Additionally, the anterior orbital rim of the holotype of N. mississippiensis is positioned above P4, but in the holotype of P. curryi , P4 is completely anterior to the orbital rim. These differences are subtle to say the least, and there tends to be some fluctuation in the exact position of the orbits and nasal incision in other brontothere species, but the differences between the holotypes of Notiotitanops mississippiensis and Protitanops curryi seems to exceed the typical degree of intraspecific variation found in these characters. For this reason, N. mississippiensis is considered valid.