Wickia brevirhinus, Mihlbachler, 2008

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 : 114-122

Wickia brevirhinus , new genus and species

HOLOTYPE: CMNH 11380 , a skull with right I3, C, P2–M3, left C, P2 (partial), and P3–M3.

TYPE LOCALITY: Sand Wash Basin, Moffat County Colorado.

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

ETYMOLOGY: ‘‘Wick’’, nickname of paleontologist William Berryman Scott ( Scott, 1939). The trivial name, brevirhinus , refers to the short nasal incision and short nasal process of this species.

REFERRED SPECIMEN: (From the Sand Wash Basin, Moffat County Colorado) CMNH 11382 , an anterior portion of a cranium with right I3–M3, left I1–M3, and a mandible with right p1–m3, left i2, and p1– m3; (from the Adobe Town Member of the Washakie Formation , Sweetwater County , Wyoming) UCMP 81300, a skull missing the right zygomatic arch, with right I3, C (partial), P1–M3, and left I3–M3.

DIAGNOSIS: Wickia brevirhinus is a relatively large hornless brontothere, one of several in which the frontal bone intrudes into the nasal bone, thus splitting off a small lateral splint of nasal bone from the main body of the nasal. The size and shape of the nasal splint is similar to that of Metatelmatherium ultimum and less pronounced than that of Telmatherium validus . The nasal incision is short and extends posteriorly to the P1. The nasal process is very short, horizontal, unelevated, of relatively constant transverse width, narrow, with thin and relatively deep lateral walls, and without a well-defined or strongly rounded distal margin. The orbits do not project laterally and they are positioned above M2 with the roots of M1 below the anterior orbital rim. Neither an infraorbital process nor a ventral zygomatic flange is present on the jugal. The premaxilla is extremely robust and does not contact the nasal bone. The premaxillomaxillary rostral cavity is not enclosed by bone dorsally. Other cranial characteristics include a relatively flat dorsal cranial surface, an elevated occiput, deep, strongly curved, and laterally bowed zygomatic arches, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Large ventral sphenoidal fossae are absent. A sagittal crest is absent, but the dorsal surface of the cranium is very narrow and the parasagittal ridges strongly constrict the dorsal surface of the cranium.

Dentally, Wickia brevirhinus has large subcaniniform upper incisors, a simple P1, a distinct P2 metacone, weak premolar preprotocristae, and short lingual crests extending posteriorly from the premolar protocones. Premolar hypocones are absent. The upper molars of W. brevirhinus have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. Small anterolingual cingular cusps are present, but central molar fossae and cingular parastyle shelves are absent in the molars. W. brevirhinus molars appear to retain vestigial paraconules. Anterolingual cingular cusps are present. The upper molars lack paraconules or metalophs. The lower dentition of W. brevirhinus includes a postcanine diastema, a p1–p2 diastema (variably present), an elongate p2 trigonid, a poorly developed p2 talonid, a metaconid on p4 but not on p2 or p3, shallow molar basins, and a slender m3.

Wickia brevirhinus is most similar to Telmatherium validus and Metatelmatherium ultimum , but it is distinct from these species primarily by the combination of a very short nasal incision, a short nasal process, lack of a sagittal crest, lack of a ventral zygomatic flange, and possibly a poorly developed p2 talonid.

DESCRIPTION

SKULL: The following description of Wickia brevirhinus is based upon the holotype skull (CMNH 11380) (figs. 51, 54a) as well as the referred specimens (UCMP 81300) (figs. 52 and 54c) and CMNH 11382 (figs. 53, 54b). W. brevirhinus is a large hornless brontothere that is most similar in size to Metatelmatherium ultimum . All skulls reveal a frontonasal configuration that resembles those of Metatelmatherium and Telmatherium , but the sutures can most clearly be seen on CMNH 11382. A thin, arched splint of the nasal bone can be seen branching in a posteroventral direction from the main body of the nasal. A triangular process of the frontal bone enters the notch between the main body of the nasal bone and the posteroventrally directed nasal splint. From the dorsal views of all skulls, parts of the frontonasal suture can be seen receding in a posteromedial direction. The shape of the nasal splint and the intruding frontal bone is more like that of Metatelmatherium where the nasal splint is relatively short, narrow, and only slightly arched. Likewise, the triangular process that intrudes into the nasal bone is relatively short and inconspicuous. In Telmatherium validus the nasal splint and frontal process are larger and more pronounced.

The face of Wickia brevirhinus is tall. The maxilla extends upward to the level of the upper rim of the orbit, where it meets the ventral edge of the nasal splint and forms an arched nasomaxillary suture. The maxilla forms a very shallow preorbital fossa. The nasal incision of W. brevirhinus is shorter than that of Telmatherium but similar to Metatelmatherium ; the posterior notch of the nasal incision is positioned between the canine and the P 2 in CMNH 11380. The nasal incision extends as far back as the P 1 in CMNH 1382 and UCMP 11380. The orbit of W. brevirhinus is positioned directly above M2. The posterolateral root of M1 is rooted below the anteriormost edge of the orbital floor. The anterolateral root of M1 is situated directly below the anterior orbital rim.

The nasal process of the holotype is not preserved, but it is complete in the referred specimens. The nasal bones are poorly fused together. The nasal process is shorter than the premaxillomaxillary rostrum in UCMP 81300, but it is of similar length to the rostrum in CMNH 11382. The nasal process is slightly curved downward, resulting in a convex dorsal surface. The lateral margins of the nasal process form dorsoventrally deep and thin vertical walls that are of relatively constant depth throughout most of their length, but become shallower near the distal end. From a dorsal view the nasal process is narrower than the premaxillomaxillary rostrum and it tapirs slightly distally with a somewhat convex distal edge. The anterior edge of the nasal process is thin, roughened, and deflected downward.

From the lateral view the dorsal margin of the premaxillomaxillary rostrum rises steeply posterodorsally so that the posterior notch of the nasal incision is at the level of the upper half of the orbit. The rostral cavity is open dorsally and continuous with the nasal cavity. The premaxillomaxillary sutures are clearly visible in CMNH 11380. The nasal processes of the premaxillae diverge laterally in a posterior direction and they are truncat- ed before reaching the posterior notch of the nasal incision. The premaxillae of CMNH 11380 are massive and they do not contact each other medially, thus the premaxillary symphysis is not ossified. The premaxillae of UCMP 81300 and CMNH 11382 are somewhat less massive than those of the holotype and they contact each other mesially, although they are not strongly co-ossified. Overall, the premaxillae of Wickia brevirhinus seem more massive than those of Metatelmatherium or Telmatherium . The degree of premaxillary robusticity is somewhat variable in these species and relates to variation in canine size.

The skull of Wickia brevirhinus is rather tall and the posterior portion of the cranium is elevated. From a lateral view the dorsal surface of the skull above the orbits is flat. Behind the orbits the dorsal surface of the skull is slightly convex in lateral profile. From a dorsal view the dorsal surface of the skull is postorbitally very narrow, although

(C) anterior view.

the parasagittal ridges do not meet to form a true sagittal crest. An elongate shallow pit resides in the constriction of the parasagittal ridges at the posterior end of the skull.

The zygomatic arches of Wickia brevirhinus are deep and very strongly bowed laterally. The jugal portion of the zygomatic is essentially horizontal while the zygomatic process of the squamosal is steeply angled posterodorsally, giving the zygomatic arch a strongly curved appearance. The large ventral flange that extends from the inferior margin of the zygomatic process of the jugal in Metatelmatherium is not seen among the skulls of W. brevirhinus . W. brevirhinus also lacks an infraorbital jugal process like that seen in Sphenocoelus uintensis .

The nuchal crest of CMNH 11380 is concave from the dorsal view. From the lateral view the occiput is moderately tilted backward. From the posterior view the nuchal crest is dorsally arched, the upper half of the occiput is narrower than the lower portion and the middle is slightly constricted. The occipital pillars are prominent like in Telmatherium validus and the central occipital pit between them is shallow. The nuchal crest of CMNH 11380 is very rugose on its posterior surface, while the remainder of the occipital surface is much smoother. Wickia brevirhinus lacks a distinctive notch with facetlike margins above the foramen magnum that is seen in Metatelmatherium ultimum .

From the ventral views of CMNH 11380 and UCMP 81300 the posterior nares are rimmed by a distinct horseshoe-shaped emargination. The anterior rim of the posterior nares is positioned between the hypocones of the M3. A distinct anteroposteriorly oriented ridge emerges from the midline of the palate and extends posteriorly beyond the anterior rim of the posterior nares. Neither specimen bears evidence of posteriorly extended maxilloturbinates, although they could have been present but were not preserved.

The vomer forms a thin septum that bisects the elongate posterior narial canal. The posterior narial canal of CMNH 11380 extends into the sphenoid bone, but it does not extend past the foramen ovale. The morphology of the posterior narial canal of the referred specimen, UCMP 81300, is presently obscured by a large mass of wax, although the posterior narial canal does not appear to extend quite as far posteriorly in this specimen. Other aspects of the basicranium of Wickia brevirhinus are typical. For instance, the foramen ovale is widely separate from the foramen lacerum. The external auditory pseudomeatus enters the skull in a strictly mediolateral direction and is not constricted ventrally by the mastoid process.

UPPER DENTITION: All three specimens of Wickia brevirhinus have partial sets of upper incisors (fig. 54). The incisor row forms a short arch anterior to the canines. Though the incisor rows are incompletely preserved, the premaxilla shows room for three large incisors. Little else is noteworthy about the incisors of CMNH 11380. In UCMP 81300, only the left and right I3s are preserved. The nearly intact I3s are large with pointed and lingually curved subcaniniform crowns. A thin cingulum can be seen tracing around the lingual base of the right I3. The I3s are separated from the canines by a distinct diastema on both specimens. The canines themselves are relatively large. The left I1 and I2 of CMNH 11382 are preserved, but they are heavily worn. These incisors are large, yet they are smaller than the large caniniform I3.

The P1s of CMNH 11380 are lost and their alveoli are mostly reabsorbed, nearly erasing all evidence of P1. Due to the absence of P1 the length of the postcanine diastema cannot easily be judged from this specimen, although the gap between the canine and P2 is quite long. The P1s are still present in CMNH 11382 (fig. 54b) and UCMP 81300 (fig. 54c). In these specimens the postcanine diastema is relatively short. P1 is small with a simple crown with a single cusp, a posterior heel, and a thin lingual cingulum. There is no P1– P2 diastema.

The crowns of the P2–P4 of UCMP 11380 are heavily worn and reveal only basic details, while those of UCMP 81300 and CMNH 11382 are substantially less worn. The anterior margin of P2 is angled posterolingually, giving the crown of that tooth a somewhat more oblique outline. P3 is less oblique in outline while the anterior and posterior sides of P4 are essentially parallel. The parastyle and metastyle of the P2 arch slightly lingually, while those of P3 are approximately straight. The parastyle of P4 is strongly angled anterolabially, while the metastyle of P4 is straight. The labial sides of the paracones of P3 and P4 are more strongly convex than those of the metacones.

In CMNH 11380 and CMNH 11382, the lingual side of the P2 is heavily worn. However, on the P2 of UCMP 81300 a short crest extends anterolingually from the protocone and continues along the anterior side of the crown. The lingual side of P2 exhibits a short preprotocrista as well as a short lingual crest extending posteriorly from the protocone. In P3 there is a small but distinct preprotocrista and a short lingual crest. Finally P4 has a very indistinct preprotocrista and there is no lingual crest. However, a lingual crest is present on the P4 of CMNH 11382. None of the premolars has a hypocone.

The molars of CMNH 11380 are extremely worn, and their proportions have been significantly altered by interstitial wear. The molars of UCMP 81300 and CMNH 11832 serve as better sources for the molar morphology of this species. The molars of Wickia brevirhinus exhibit typical brontotheriine traits including tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in molars that are not heavily worn (e.g., M3). The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. W. brevirhinus molars exhibit small anterolingual cingular peaks, but they lack central molar fossae. The molars retain vestigial paraconules. The M3 lacks a hypocone.

MANDIBLE AND LOWER DENTITION: The holotype of Wickia brevirhinus lacks an associated mandible, however, a referred specimen, CMNH 11382, includes a complete mandible with partially preserved incisors and complete cheektooth rows (fig. 55). The ramus of CMNH 11382 is similar in proportion to those of Telmatherium validus and Metatelmatherium ultimum , including a tall coronoid process and relatively steeply angled (,45 °) ventral margin of the symphysis. The symphysis extends posteriorly to the p3 metaconid.

The remnants of the three incisors of CMNH 11382 are relatively large and form a short arch anterior to the canines, although they are too damaged to describe other details of their morphology. There is no precanine diastema, but the postcanine diastema is somewhat longer than the p2.

The p1 of CMNH 11382 is small and simple with a single cusp and a short talonid heel. There is no p1–p2 diastema. The p2 of CMNH 11382 is slender and ovoid in outline. The trigonid of the p2 is nearly twice the length of the talonid, the p3 trigonid is somewhat longer than the talonid, and the p4 trigonid is slightly shorter than the talonid. The p2 trigonid and talonid are of similar width, while the trigonids are narrower than the talonids in p3 and p4. The p2 paralophid projects anteriorly from the protoconid and there is only a slight lingual notch in the p2 trigonid. The p2 protolophid is short and posteriorly directed. The paralophid of p3 is angled about 45 ° lingually, creating a distinct lingual notch in the trigonid. The p3 protolophid is straight but positioned lingually. Finally, the p4 trigonid is essentially molariform with a lingually arching paralophid and protolophid. A distinct metaconid is seen only on the p4. The p2 talonid of CMNH 11382 is very simple; the cristid obliqua is not well developed and there is essentially no talonid basin or hypolophid. The p3 and p4 of CMNH 11382 have more well-developed cristids obliqua, elongate hypolophids, and broader basins.

(C) dorsal view.

The molars of Wickia brevirhinus are typical, with thinner lingual enamel, shallow talonid and trigonid basins, and an elongated m3. The labial cingulid is discontinuous around the bases of the cusps, but it is continuous between them. The m3 cingulum does not wrap around the distal end of the m3.

REMARKS

Wickia brevirhinus is based on a skull (CMNH 11380) from the Sand Wash Basin of Colorado. In addition, a partial skull and mandible (CMNH 11380) from the Sand Wash Basin and a partial skull (UCMP 81300) from the middle Adobe Town Member of the Washakie Formation of Wyoming are referable to W. brevirhinus . West and Dawson (1975) originally assigned two of these specimens, CMNH 11380 and CMNH 11382, to Manteoceras pratensis Cook (1926) . However, M. pratensis is based upon a poorly preserved partial mandible with deciduous teeth and is considered a nomen dubium in this paper.

Nonetheless, West and Dawson (1975) noted that CMNH 11380 and CMNH 11382 share attributes with Manteoceras manteoceras (5 Telmatherium validus ) and Metatelmatherium ultimum . However, Wickia brevirhinus distinctly differs from both of these species due to a unique combination of characters. The skulls of W. brevirhinus differ most conspicuously from Telmatherium validus in the shorter nasal incision although there are other minor differences such as the narrower and shorter lateral nasal splint, less prominent parasagittal ridges, and minor anterolingual cingular cusps on the molars. W. brevirhinus differs from Metatelmatherium ultimum most conspicuously in the lack of a true sagittal crest, the lack of a large ventral flange on the zygomatic arch, and in having more prominent occipital pillars. Moreover, W. brevirhinus lacks the notch on the anterior rim of the foramen magnum that is seen in the type skull of Metatelmatherium ultimum . Finally, the p2 talonid of W. brevirhinus is poorly developed in comparison to Telmatherium validus or Metatelmatherium ultimum .

In many ways Wickia brevirhinus is morphologically intermediate between Telmatherium validus and Metatelmatherium ultimum . Temporally, it also bridges the gap between the late Bridgerian T. validus and the late Uintan M. ultimum . W. brevirhinus specimens from the Sand Wash Basin of Colorado are either latest Bridgerian or Early Uintan in age. The Washakie Basin skull, UCMP 81300, from above level 17 of Granger (1909), which occurs within the middle Adobe Town Member of the Washakie Formation (TWKA2), suggests an earliest Uintan age for W. brevirhinus ( McCarroll et al., 1996b) .