Protitanops curryi Stock, 1936

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 : 298-302

Protitanops curryi Stock, 1936

HOLOTYPE: LACM / CIT 1854 , a skull with complete dentition and a partial mandible with right m3, left m1 (partial), and m2–m3.

TYPE LOCALITY: Lower red beds of the Titus Canyon Formation in Canyon East of Thimble Peak, Grapevine Mountains, California.

AGE: Late Eocene (early Chadronian land mammal ‘‘age’’).

DIAGNOSIS: Protitanops curryi is a large brontothere with large bulbous frontonasal horns. The horns are positioned slightly in front of the orbits and are not elevated high above the orbits. The nasal incision is shallow and extends to the anterior margin of the P3. The anterolateral root of M1 is situated directly below the anterior rim of the orbit. The nasal process is broad, unelevated, slightly angled downward, of nearly constant width throughout its length, strongly round- ed 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 deeply saddle-shaped cranium, a dorsal cranial surface that is moderately constricted posteriorly by parasagittal ridges, a narrow emargination surrounding the posterior nares, nearly straight zygomatic arches with large swellings, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae and postzygomatic processes are absent.

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

Protitanops curryi shares with Eubrontotherium clarnoensis and Notiotitanops mississippiensis the combination of a long upper postcanine diastema and a reduced number of upper incisors. Protitanops curryi differs from E. clarnoensis primarily in the large zygomatic swellings. Protitanops curryi differs from N. mississippiensis in having more posteriorly positioned orbits and a more posteriorly extended nasal incision.

DESCRIPTION

SKULL: The holotype of Protitanops curryi is a complete skull whose shape, from a lateral view, is reasonably intact, although the anterior and posterior views reveal that it has been subjected to lateral shearing distortion (fig. 147). The right horn is incomplete and has been reconstructed with plaster, while the left horn is complete and relatively undamaged. The following description of the skull of P. curryi is based mainly on the holotype. (Note that the holotype skull and jaw in fig. 147a is depicted in an approximately lifelike orientation, with the anterior end tilted down so that the occiput is almost vertical rather than tilted backward. Howev- er, other skulls in this paper are pictured in a more arbitrary horizontal position. The reader should keep this in mind when reading the following description.)

Protitanops curryi is a large horned brontothere similar in size to Eubrontotherium , Notiotitanops , and Duchesneodus . The left horn is large, bulbous, and somewhat ovoid in cross section; however, it is possible that the horn was originally rounder. The entire surface of the horn is roughened. The horn is positioned slightly in front of the orbit and is not highly elevated above the orbit. It is angled in an anterodorsolateral direction. The bones of the facial area (frontal, nasal, maxilla, premaxilla) are fully fused together in this individual and no sutures can be discerned.

The right half of the nasal process is reconstructed with plaster, but the left side is more or less complete. Judging by the left half, the nasal process was about as wide as the premaxillomaxillary rostrum. From a dorsal view the nasal process is nearly of constant width throughout its length and the anterior margin is rounded. From the lateral view the nasal process is about the same length as the rostrum and it is angled slightly downward. The dorsal surface of the nasal process is flat except at the very distal end where it curves downward. The sides of the nasal process are somewhat thickened and curved downward to form shallow lateral walls.

The nasal incision is shallow and extends posteriorly to the anterior margin of the P4. The orbit is positioned directly above the posterolateral root of M1 and the anterolateral root of M2. The anterolateral root of M1 is situated directly below the anterior rim of the orbit. The roots of P4 are entirely anterior to the orbital rim. With respect to the dentition, the orbits of Protitanops curryi are slightly more anterior than those of Eubrontotherium clarnoensis , more posteriorly positioned than those of Notiotitanops mississippiensis , but similar to some specimens of Duchesneodus uintensis .

The premaxillomaxillary rostrum of LACM/CIT 1854 curves upward distally. The rostrum deepens proximally and the dorsal margin slopes steeply posterodorsally, but it does not rise higher than the midpoint of the orbit. The premaxilla is short and does not extend anterior to the canines. The rostral cavity is not covered dorsally by bone behind the symphysis.

The cranium of Protitanops curryi is strongly saddle-shaped. There is no domelike convexity on the dorsal surface of the skull as in Duchesneodus uintensis . The parasagittal ridges are prominent and overhang the sides of the skull. However, the parasagittal ridges converge posteromedially and moderately constrict the posterodorsal surface. The zygomatic arches are thick with massive swellings at the junction of the jugal and squamosal, giving the zygomatic arches a strongly bowed shape. From a lateral view the zygomatic arch of the left side is essentially straight, although its midsection is reconstructed with plaster, rendering its shape dubious. The right zygomatic arch (not shown) is more intact and it is mildly curved from a lateral view.

The nuchal crest is thin and sharply notched medially. From a lateral view the occiput is strongly tilted backward. From the posterior view the dorsal portion of the occiput is similar in width to the ventral portion, and it is slightly constricted in the middle. The dorsal rim of the occiput is nearly flat, though it is notched medially. The occipital pillars are large and the central depression is rather deep, although the whole occiput is not nearly as massive as those seen in some specimens of Megacerops (sensu Mihlbachler et al., 2004b) .

From the ventral view of the holotype skull (fig. 148a), the anterior rim of the posterior nares is situated slightly posterior to the M3 protocones. However, the exact edge of the posterior nares is covered by sediment. The posterior narial canal is elongate. The thin vomer forms a septum that divides the canal but it has not been exposed from the sediment filling the posterior narial canal. However, the vomerine septum is seen emerging posteriorly where the posterior narial canal begins to become shallow. Sediment has been left on the ventral surface of the basisphenoid where the posterior narial canal shallows, although there do not appear to be large ventral sphenoid fossae in this specimen. As with other brontotheres, the foramen ovale and foramen lacerum are widely separated. The mastoid process, best preserved on the right side (fig. 147b), curves anteriorly, contacts the postglenoid process, and forms a tube-shaped external auditory pseudomeatus.

UPPER DENTITION: The right incisors are complete. There are only two. The incisor crowns are very small, globular, and essentially featureless. They are positioned along the anterior edge of the canines in a straight row. There is no precanine diastema. The canines are relatively small. The postcanine diastema is similar in length to the P2.

The left premolars are shown in close-up (fig. 148b). The P1 is much smaller than the other premolars. Both right and left P1s are heavily worn, but the left side has two labial cusps (paracone and metacone) and a small lingual heel whose features are worn off. The remaining premolars (P2–P4) are heavily worn. Neither the right nor the left P2 is in good condition. The P3 and P4 are nearly rectangular in outline with parallel anterior and posterior sides. Although damaged, the P2 parastyle appears to have extended in an anterior direction. The parastyles of P3 and P4 are deflected labially. None of the premolar metastyles are angled labially. The lingual sides of the right and left P2s exhibit a tall anteroposteriorly oriented lingual crest that almost completely absorbs the protocone and hypocone. However, both a distinct protocone and hypocone are seen on the P3 and P4. The hypocones are slightly smaller than the protocones and connected to them by a lingual crest. Small preprotocristae are discernable on P2–P4. On P3 and P4, the preprotocristae are visible as small beads of enamel that connect the protocone to the lingual base of the metacone. No anterolingual cingular cusps are seen on the premolars, although they are occasionally seen in Duchesneodus uintensis and Megacerops . The lingual cingula of the premolars are thick and continuous around the protocone.

The upper molars of Protitanops curryi show typical brontotheriine characteristics, including tall, lingually angled ectolophs, very weak labial ribs and thin lingual ectoloph enamel. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. All of the labial molar cusps are worn beyond their wedge-shaped lingual margins except the metacone of the right M3. Distinct central molar fossae and anterolingual cingular cusps are seen on M2–M3. The M1 is too worn to preserve these characters. There is no distinct hypocone on M3, although a crest extends from the distolingual corner of the M3 to the lingual base of the metacone. Labial and lingual molar cingula are essentially absent.

MANDIBLE AND LOWER DENTITION:

The holotype specimen includes a partial mandible that is missing most of the symphysis, although it was reconstructed with plaster (fig. 147a). No incisors, canines, or premolars are preserved, although the roots of the premolars allow for measurement of tooth-row length. The remainder of the mandible is of little interest, other than that it indicates that the molars are typical, with an elongate M3, thin lingual enamel, and shallow talonid and trigonid basins.

REMARKS

Stock (1936) named Protitanops curryi from a skull and partial mandible from the early Chadronian Titus Canyon Formation of California ( Emry et al., 1987). Stock (1936) recognized Protitanops curryi as more advanced than middle Eocene horned brontotheres such as Diplacodon elatus and Protitanotherium emarginatum due to its more greatly developed horns and smaller incisors. Protitanops curryi shares with Eubrontotherium , Dianotitan , Notiotitanops , Duchesneodus , and Megacerops (sensu Mihlbachler et al., 2004b) a reduced number of upper incisors. Only three of these taxa, Protitanops , Notiotitanops , and Eubrontotherium , retain postcanine diastemata. The conspicuous zygomatic swellings of Protitanops curryi most clearly differentiate this species from Eubrontotherium clarnoensis . It is more difficult to differentiate Protitanops curry i from Notiotitanops mississippiensis . Protitanops curryi has a longer nasal incision and a more posteriorly positioned orbit; it is possible that with more specimens these two species will be found to overlap (see remarks for N. mississippiensis ).

Mader (1989, 1998) continued to accept Protitanops curryi as a valid species but only referred the holotype to that species. Other specimens could represent Protitanops curryi . Stock (1936) reported a partial skull, LACM/ CIT 2007, from a similar stratigraphic position as the type of P. curryi , but he referred it to it as ‘‘Menodontine (?)’’. Realistically, the specimen is too incomplete to identify to a species level. A palate, LACM-CIT 2143, from the Sespe Formation of California with two small incisors, originally referred to Duchesneodus californicus by Stock (1938), was later referred to Duchesneodus uintensis by Lucas and Schoch (1989b) and Kelly (1990). This specimen retains a postcanine diastema and should be excluded from D. uintensis for that reason. The nasal incision of that specimen extends farther posteriorly than that of Notiotitanops mississippiensis , but it is consistent with Protitanops curryi ; however, CIT 2143 cannot clearly be differentiated from Eubrontotherium clarnoensis . Finally, Hanson (1996) suggested that brontothere material from the Hancock Quarry Local fauna of the Clarno Formation, Wheeler Oregon belonged to Protitanops . Lucas et al. (2004) also suggested that the Clarno brontothere could be Protitanops curryi although newer observations indicate that it is a new species, Eubrontotherium clarnoensis ( Mihlbachler, 2007) . Therefore, the holotype, LACM/CIT 1854, is the only specimen referable to Protitanops curryi .