Dolichorhinus hyognathus ( Osborn, 1889 )

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 : 38-49

Dolichorhinus hyognathus ( Osborn, 1889)

HOLOTYPE: YPM PU10273, a mandible with right i1–c, left i3, c, p2, m2 (partial), and m3.

TYPE LOCALITY: Adobe Town Member (Washakie B of Granger, 1909) of the Washakie Formation, Sweetwater County, Wyoming.

SYNONYMS: Dolichorhinus cornutum (Osborn, 1895) ; D. intermedius Osborn, 1908a ; D. heterodon Douglass, 1909 ; D. longiceps Douglass, 1909 ; D. superior ( Riggs, 1912) ; D. fluminalis Riggs, 1912 .

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

REFERRED SPECIMENS: (From the Wagonhound Member of the Uinta Basin , Utah) AMNH 1832 , skull fragments ; AMNH 1836 , a right mandibular ramus with p2–m3 ; AMNH 1837 (holotype of Dolichorhinus intermedius ), a skull with left C–M3 ; AMNH 1840 , a partial left mandibular ramus with p2–m3 ; AMNH 1843 , an anterior portion of skull with partial cheek teeth ; AMNH 1845 , a partial skull missing the nasal and premaxillae with right P4–M3 and left M2–M3 ; AMNH 1847 , a dorsal surface of a skull ; AMNH 1849 , a skull fragment ; AMNH 1850 , a skull with right and left C–M3 ; AMNH 1851 (holotype of D. cornutum ), a skull with complete right and left dentition ; AMNH 1852 , a skull with right P1–M3 and left P2–M3 ; AMNH 1854 , a partial mandible with right i2–c, p2–m1, left c, and p2–m1 ; AMNH 1856 , a complete mandible with complete dentition ; AMNH 1857 , a mandible with heavily worn teeth including right i1–i2, p2–m3, left i1–i2, c, and p1–p4 ; AMNH 1858 , a juvenile mandible with right c (erupting), p1, p4 (?), left p1, p4 (erupting), m1–m2, and m3 (erupting) ; CMNH 2340 (holotype of D. heterodon ), a skull with right P3–M3 and left partial cheek teeth ; CMNH 2347 , a skull with left and right P2–M3 ; CMNH 2865 , a skull with left P4–M3, partial right ramus with m2–m3 ; CMNH 2964 , a skull fragment ; CMNH 3095 , a partial skull with no dentition ; CMNH 3096 , a skull with right M1–M3 (heavily worn) ; CMNH 3117 , a posterior half of a skull with right and left M2–M3 ; CMNH 3119 , a skull fragment ; CMNH 11071 , a complete skeleton ; CMNH 11080 , partial skull with right C–M3 ; CMNH 11081 (holotype of D. longiceps ), a skull with right C, P2–M3 and left P2–M3 ; CMNH 11083 , a skull (partially prepared) ; CMNH 11091 , a skull, broken into two large fragments with right M1–M3, and a partial right mandibular ramus with m2–m3 ; CMNH 11092 , a mandible with left p1, p2 (partial), and p3–m3 ; FMNH P12167 , a skull with right C–M3, left I1–I2, C, P2– M3, and a mandible with right and left p2– m3 ; FMNH P12175 , a skull with right and left C–M3 ; FMNH P12176 , a skull with a left M3 ; FMNH P12182 , a skull with right P2–P3 (partial), P4–M3, and left P3–M2 ; FMNH P12168 (holotype of D. superior ), a skull with right C–P4, M3, and left C, P2–M3 ; FMNH P12193 , a skull with right and left P1–M3 ; FMNH P12200 , a skull with right and left P3–M3 ; FMNH P12205 (holotype of D. fluminalis ), a skull with right I1–M3 and left I1–P2, M1 (partial), M2–M3 ; FMNH P12215 , a skull with complete dentition ; LACM 128402 , a skull with right P3–M3 (partial) and left P1–M3 ; UCMP 31845, a partial skull with left P1–M3; UCMP 31846, a partial skull with partial dentition; USNM 6702, a partial mandible with right p4–m2 and left c–m2; USNM 6703, a skull with right P2–P4 and left P1–2, P4–M3; USNM (uncatalogued), premaxillomaxillary rostrum; YPM PU11241, a skull with right P1–M3 and left C (partial), P1–M3; (from the Adobe Town Member [Washakie B of Granger, 1909] of the Washakie Basin, Wyoming) AMNH 13164 , a complete skull with complete right dentition, left C–M3, and a mandible with right i1–p4, m2–m3 (partial), and left i1–m2, m3 (partial) ; CMNH 9413 , a skull with right P1–M3 and left I2–M3 ; FMNH PM3870 , a skull with right and left I3–M3 ; FMNH PM3873 , a skull with right and left P2–M3 ; FMNH PM26100 , a mandible with damaged teeth .

DIAGNOSIS: Dolichorhinus hyognathus is an intermediate-sized brontothere. The frontal bone does not overlap or protrude into the nasal bone. A small superorbital protuberance, primarily on the nasal bone, is seen in some specimens. The cranium of Dolichorhinus hyognathus is highly dolichocephalic. The nasal incision extends to the posterior margin of the P4. The nasal process is horizontal, unelevated, of relatively constant transverse width, narrow, with thin and relatively deep lateral walls, and without a strongly rounded distal margin. The orbits do not project laterally and are positioned directly over the posterior half of M2 and the anterior half of M3, with the anterolateral root of M2 and the posterolateral root of M1 directly below the anterior orbital rim. The prominent infraorbital process of the jugal also extends onto the maxillary, often to form a double flange. The premaxilla is robust and does not contact the nasal bone. The premaxillomaxillary rostrum does not deepen proximally and it is enclosed dorsally by bone. Dolichorhinus hyognathus lacks a sagittal crest, but the parasagittal ridges strongly constrict the dorsal surface of the skull posteriorly. Other cranial characteristics include a strongly dorsally arched cranium, thin and weakly curved zygomatic arches, a ventrally open and posteromedially angled external auditory pseudomeatus, and disproportionately wide occipital condyles. The functional posterior nares are shifted posteriorly by a bony palatal extension and posteriorly extended maxilloturbinates.

Dentally, Dolichorhinus hyognathus has large subcaniniform upper incisors, a postcanine diastema, a simple P1, a distinct P2 metacone, weak premolar preprotocristae, and short crests extending posteriorly from the premolar protocones. Premolar hypocones are exceedingly rare and inconspicuous when present. 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 and an anterolingual cingular cusp are absent. Central molar fossae are present. D. hyognathus molars occasionally retain vestigial paraconules. All traces of a metaloph are lost. The lower dentition of D. hyognathus includes large subcaniniform incisors all of similar size, a postcanine diastema, a p1–p2 diastema of variable length, an elongate p2 trigonid, a metaconid on p4 but not on p2 and p3, shallow molar basins, and a slender m3.

Dolichorhinus hyognathus is most similar to Sphenocoelus uintensis in its hyperdolichocephalic skull, but it can be differentiated by the absence of both large fossae in the sphenoid and a sagittal crest, and by the presence of a highly specialized premaxillomaxillary rostrum and a more dorsally arched cranium.

DESCRIPTION

SKULL: The following description of the skull of Dolichorhinus hyognathus is primarily based on AMNH 1845, AMNH 1851, AMNH 13164, and FMNH P12175, although other specimens provide additional information on variation. FMNH P12175 (fig. 13a, c,) and AMNH 1851 (fig. 13b, d) are complete and undistorted skulls. AMNH 1845 is another skull that has an intact occiput (fig. 13e). Finally, the skull of AMNH 13164 (figs. 13f, 14a) is slightly crushed dorsoventrally, obscuring some aspects of the shape of the cranium, but otherwise the preservation of this specimen is exquisite and reveals details of the rostrum and ventral surface that are not well-preserved in other skulls.

Dolichorhinus hyognathus is an intermediate-sized (table 2) brontothere that is most notable for its extremely dolichocephalic skull and dorsally arching cranium. Only Sphenocoelus uintensis has a similarly elongate cranium. D. hyognathus lacks conspicuous horns, although many specimens, such as AMNH 1851 possess small but distinct nasal protuberances. Others, such as FMNH P12175, have much weaker nasal protuberances. The nasal protuberance of AMNH 1851 is small, elliptical, projects laterally, and is positioned directly above the orbit. The frontonasal suture runs behind the nasal protuberance and recedes posteromedially, but it is acutely redirected anteriorly near the midline. D. hyognathus does not possess a forward projection of the frontal bone like that seen in Telmatherium validus . The hornlike protuberance rests entirely on the nasal bone. Thus, D. hyognathus is the only brontothere known to have a hornlike protuberance that is predominantly on the nasal bone rather than on both the frontal and nasal bone.

The nasal incision of Dolichorhinus hyognathus is shallow and long. It extends as far back as the posterior margin of the P4. From a lateral view the orbits are elliptical. The bottom of the orbit is positioned directly over the posterior half of M2 and anterior half of M3. The anterolateral root of M2 and the posterolateral root of M1 are positioned directly below the anterior orbital rim. The orbits do not protrude laterally as in Metarhinus .

The nasal bones tend to be poorly fused together. In FMNH P12175, the nasal process is slightly longer than the premaxillomaxillary rostrum, although in other specimens (e.g., AMNH 1851) the nasal process and the rostrum are about the same length. The nasal process is horizontal. The lateral margins of the nasal process form dorsoventrally deep and thin vertical walls. Typically, the depth of the lateral walls is nearly constant, but they become shallower near the distal end. The anterior margin of the nasal process is thin, roughened, and strongly deflected downward. From the dorsal view the nasal process is narrow and of nearly constant width throughout its length, although it is sometimes slightly constricted at its proximal end.

The premaxillomaxillary rostrum of Dolichorhinus hyognathus is long and slightly upturned. From a lateral view, the dorsal margin of the rostrum is horizontal. Normally, the rostral cavity, which houses the vomeronasal organ, is bordered laterally and ventrally by the maxillaries and is open dorsally, thus forming a continuous osteological space with the nasal cavity. However, in D. hyognathus , the rostral cavity is completely covered by bone dorsally and is separated from the nasal chamber. This condition is shared with Metarhinus . In most specimens, the elements of the rostrum are completely fused into a solid dorsal cover and a distinct dorsal ridge of bone (the apparent osteological marker for the cartilaginous nasal septum) runs the length of the rostrum and extends into the nasal cavity of the skull.

TABLE 2 Summary statistics for selected morphometric variables of Dolichorhinus hyognathus See Methods for measurement definitions

The skull of AMNH 13164 allows for a more precise description of the specialized rostrum. The premaxillae are joined at the midline for almost their entire length and they form a domelike roof above the incisors. The posterior tips of the premaxillae diverge posterolaterally forming a posterior notch between the premaxillae. A thin pair of bones emerges horizontally from the nasal cavity and covers the dorsal surface of the rostrum, inserting into the notch formed by the premaxillae. Laterally, these small bones are sutured to the maxillaries. The origin of this unusual bony cover is uncertain. It continues posteriorly into the skull, but most specimens are filled with sediment or they are too heavily damaged to trace this structure internally. It seems most likely that these bones represent extensions of the maxilloturbinates, which, in the cross-sectional view of AMNH 1851 appear to extend into the rostral cavity below the surface of the bony covering (see Osborn, 1929a: fig 254c).

The dorsal surface of the skull above the orbits is slightly concave. Behind the orbits, the dorsal surface is strongly convex because the posterior half of the skull is strongly arched dorsally. The parasagittal ridges do not join to form a sagittal crest; instead, the parasagittal ridges remain separate throughout their length, although they strongly constrict the dorsal surface of the skull posteriorly.

The zygomatic arches are thin, shallow, and slightly bowed laterally. The jugal portion of the zygomatic arch is horizontal, while the squamosal portion is angled posterodorsally, thus giving the zygomatic arch a weak curvature. A conspicuous rounded infraorbital process extends ventrolaterally from the jugal. This process is similar to those of Mesatirhinus and Sphenocoelus , though it is generally larger and more laterally projected. In most specimens of D. hyognathus the flange created by the infraorbital process extends onto the maxillary forming a distinct secondary process. The degree to which the jugal and maxillary infraorbital processes are separated varies; sometimes it appears as a single flange (e.g., CMNH 3117).

From a dorsal view of the skull the nuchal crest has a shallow but wide median notch. From the posterior view, the nuchal crest is strongly arched dorsally. The width of the upper portion of the occiput is similar to the width of the ventral portion in AMNH 1845, although in other specimens the dorsal portion of the occiput is narrower. The width of the occiput is not strongly constricted in the middle as in Mesatirhinus . Small occipital pillars are visible on the posterior surface of the occiput, although the middle of the occiput is not deeply recessed between these structures. The occipital condyles of Dolichorhinus hyognathus seem disproportionately large and are almost as wide as the entire occiput.

Peterson (1924) first noted many of the peculiar aspects of the posterior nares of Dolichorhinus hyognathus . A distinct rim of bone, seemingly the original position of the posterior nares, is positioned between the anterior margins of the M3s. The position of this rim fluctuates between the hypocones of the M2 (e.g., CMNH 3117) to between the M3 protocones (e.g., FMNH P12167). A raised palatal extension is seen behind the original margin of the posterior nares. In AMNH 13164, this palatal extension is about three cm long and shifts the position of the posterior nares to a point between the posterior margins of the M3s. However, this palatal extension is commonly longer and shifts the posterior nares to well behind the M3s (e.g., AMNH 1845). A short median process extends posteriorly from both the original border of the posterior nares and from the palatal extension. Behind the palatal extension, an elongate posterior narial canal is bisected lengthwise by a thin vomerine septum. Two elongate pouches of bone (choanal pouches) project beyond the palatal extension and fill roughly the anterior two thirds of the posterior narial canal. The bony choanal pouches that extend beyond the palatal extension are similar to those seen in Telmatherium validus and Metarhinus abbotti , although they are more elongate and positioned much farther posteriorly in D. hyognathus . The thin pouches appear to be extensions of the maxilloturbinates, as seen in a cross section of AMNH 1851 (see Osborn 1929a: fig 254c). Finally, the functional posterior nares are situated near the end of the posterior narial canal. In AMNH 13164, the posteriorly shifted right functional posterior naris can be clearly seen behind the bony choanal pouch. The left posterior naris of this specimen is still filled with sediment.

Dolichorhinus hyognathus lacks large paired ventral sphenoidal fossae as seen in Sphenocoelus uintensis , however in AMNH 13164 a pair of very small ovoid pits can be seen on the ventral surface of the basisphenoid. The pits are separated by a rod-shaped structure that joins the thin vomerine septum. However, most other specimens of D. hyognathus lack these small pits entirely and have a relatively normal basisphenoid.

The external auditory pseudomeatus enters the skull at a posteromedial angle, a condition shared with Sphenocoelus . Other aspects of the basicranium of Dolichorhinus hyognathus are more typical. For instance, the external auditory pseudomeatus is wide and unconstricted ventrally. The main basicranial foramina such as the foramen lacerum and the foramen ovale are widely spaced.

UPPER DENTITION: The following description of the upper dentition of Dolichorhinus hyognathus is based primarily on AMNH 13164 (fig. 14), although additional information from other specimens is provid- ed. The three upper incisors are large and form an arched row that extends anterior to the canines. The incisors increase in size laterally. The crowns are subcaniniform: they are short, pointed, and lingually curved. Each crown is only slightly bilaterally asymmetrical from the lingual view. Each incisor has a thick lingual cingulum. No specimen of D. hyognathus exhibits labial incisor cingula. The precanine and postcanine diastemata are shorter than the P2. The canine of AMNH 13164 is small, as is typical of many specimens of this species.

The P1 crown is relatively simple with a single cusp and a narrow posterior heel. Other specimens (e.g., AMNH 1852) have slightly less elongate P1s. There is no P1–P2 diastema, although a p1–p2 diastema is found in the mandible (see below). The remaining premolars (P2–P4) become progressively larger and less oblique posteriorly. The anterior margin of the P2 crown is strongly posterolingually angled, although in other specimens the P2 is more nearly square (e.g., AMNH 1851). The parastyle and metastyle of P2 arch slightly lingually. The parastyle and metastyle of P3 are nearly straight, while those of P4 are angled labially. The labial paracone ribs of the P2–P4 become shorter and narrower in more posterior premolars. The metacone of P2 is shifted lingually in comparison to those of P3 and P4. Because of these differences, the labial wall of the P2 is rounder than those of P3 and P4. A small bulge of enamel can be found at the labial base of the metacone of P4. This small bulge is not present in every specimen, and in some others it is enlarged and forms a short mesostyle (CMNH 11081, FMNH P12182).

The lingual side of the P2 crown exhibits a large protocone, a distinct lingual crest extending posteriorly from the protocone, and a small preprotocrista. In P3, these crests are faint. P4 lacks these crests altogether, but there is a tiny beadlike paraconule. The lingual sides of the premolars of Dolichorhinus hyognathus are morphologically unstable. For instance, in FMNH PM3870 there are no lingual crests extending posteriorly from the protocones and each premolar (P2–P4) has a small but distinct preprotocrista. Other specimens have large lingual crests extending posteriorly from the protocones, or even distinct hypocones, but this last condition is very rare. For instance, CMNH 11081, a specimen with heavily worn dentition, has a small hypocone on P2. The shape of the wear facets on P3 and P4 of that specimen suggests that hypocones were present on those premolars as well, but were worn off.

The labial cingula of the premolars are thin but distinct. They tend to be discontinuous around the proximal base of the paracone in P2 and P3, but continuous around the base of the paracone of P4. The anterior and posterior premolar cingula are thick and stretch around the lingual sides of the crowns. They are not connected lingually in AMNH 13164, but in other specimens (e.g., AMNH 1850) they form continuous lingual cingula.

The molars of Dolichorhinus hyognathus exhibit numerous brontotheriine apomorphies such as tall, lingually angled ectolophs, weak labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual borders of the paracone and metacone visible in molars that are not heavily worn. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. D. hyognathus molars lack anterolingual cingular cusps. Shallow central molar fossae are invariably present. Each of the molars of AMNH 13164 retains a vestigial paraconule, however the size and presence of paraconules in the molars of D. hyognathus is variable (e.g., smaller in FMNH P12182, absent in FMNH P12175). All traces of a metaloph on M1 and M2 are lost. The M3 of AMNH 13164 has a small hypocone and small metalophlike ridge. However, the presence and size of these structures varies. For instance, in FMNH P12175 and FMNH P12188 the M3 hypocone is as large as that of the M2. On the other hand, FMNH PM3873 lacks a M3 hypocone altogether.

MANDIBLE AND LOWER DENTITION: The description of the mandible and lower dentition of Dolichorhinus hyognathus is based on two nearly perfectly preserved and essentially identical mandibles (AMNH 13164 and AMNH 1856) with complete and lightly worn teeth (fig. 15). The holotype mandible (YPM PU10273) is also pictured (fig. 16). The mandible of D. hyognathus is distinctive in the slenderness of the horizontal ramus. The ascending ramus is short. The coronoid process is long, curves posteriorly, and is taller than the mandibular condyle. The symphysis is long and relatively slender in comparison to most other brontotheriids. The inferior margin of the symphysis has a very shallow angle (much less than 45 °), although the symphyses of AMNH 13164 and AMNH 1856 are somewhat steeper than that of the holotype. The symphysis extends posteriorly to the trigonid of the p3, although in some specimens it extends only to the p2 talonid (e.g., AMNH 1587).

The three lower incisors are rather large and form a broad semicircular arch. The incisors are mesiodistally elongate and roughly of the same size. The crowns are short, lingually curved, slightly bilaterally asymmetrical, and with blunt distal points. From the lingual view, the incisors each have a prominent lingual rib and distinct lingual cingulid. It is apparent from specimens with more heavily worn incisors that these characters tend to fade with wear. No labial cingulids are seen on the lower incisors. There are no gaps between any of the incisors, although there is occasionally a very short precanine diastema. The lower postcanine diastema is typically longer than the upper postcanine diastema. In AMNH 1856 and AMNH 13164, the postcanine diastema is about the length of the p2. The lower canine is typically small.

Fig. 16. Holotype of Dolichorhinus hyognathus (courtesy of Division of Vertebrate Paleontology , YPM PU10273. º 2005 Peabody Museum of Natural History , Yale University , New Haven , Connecticut, USA. All rights reserved.). (A) Left view, (B) dorsal view.

The p1 is a small elongate tooth with a single cusp and a narrow bladelike talonid heel. A p1–p2 diastema is always present, although its length is variable. In AMNH 13164, for instance, it is visibly shorter than that of the holotype jaw, YPM PU10273. The trigonid of the p2 is much longer than the talonid, although the trigonid and talonid are of similar width. The paralophid of p2 is essentially straight, although in some specimens it can curve slightly lingually, creating a small lingual notch in the trigonid. The p2 protolophid is short, straight, and extends in a posterior direction from the protoconid. The p3 trigonid is slightly longer and narrower than the talonid. The paralophid of the p3 curves in a slightly lingual direction, creating a distinct lingual notch in the trigonid. The p3 protolophid extends about equally lingually and posteriorly from the protoconid. The p4 trigonid is clearly shorter and narrower than the talonid. The paralophid of p4 strongly arches lingually creating a very broad lingual notch in the trigonid. The p4 is the only premolar with a large lingually positioned metaconid. The p2 talonid has a short cristid obliqua and hypolophid with a shallow sloping notch on the lingual side of the crown. The talonids of p3 and p4 have longer and more well-developed cristids obliqua and hypolophids with broad- er and more nearly molariform talonid basins.

The lower molars of Dolichorhinus hyognathus have relatively thin lingual enamel, shallow talonid and trigonid basins, and an elongate m3. The cheek teeth of AMNH 13164 lack lingual cingulids and the labial cingulids are thin and discontinuous around the bases of the cusps. The distinctness of the labial cingulids is variable; this is at least partly related to the degree of dental wear.

REMARKS

Dolichorhinus hyognathus ( Osborn, 1889) was based on a mandible, YPM PU10273 (fig. 18). The holotype can be distinguished from Sphenocoelus , Mesatirhinus , Telmatherium , Sthenodectes , and Metarhinus by the combination of its more slender proportions, shallow angle of the ventral margin of the symphysis, and retention of a p1–p2 diastema. Osborn (1889) originally assigned this species to the genus Palaeosyops . Shortly thereafter, Earle (1892), who described YPM PU 10273 in more detail, reassigned this species to the genus Telmatotherium (a variation on the spelling of Telmatherium ).

In 1895, Osborn described a complete skull (AMNH 1851) that he referred to Telmatotherium cornutum ; he also referred several skulls to the same species, including AMNH 1850, AMNH 1847, AMNH 1848, AMNH 1852, and AMNH 1837. These specimens represent a species that was ‘‘remarkable for its very long flat-topped cranium and its incipient knoblike osseous horns borne chiefly upon the nasals, but partly upon the frontals’’ (Osborn, 1895: 92). None of these skulls was associated with a jaw that would allow direct comparison with the type jaw of T. hyognathus . Nonetheless, Osborn conjecturally referred several mandibles (AMNH 1857, AMNH 1858, AMNH 1854, and AMNH 1855) to T. cornutum . Based on these jaws, he distinguished T. cornutum from T. hyognathus ‘‘by the presence of two incisors’’ (p. 92). However, this was clearly a mistake, because one of these mandibles (AMNH 1854) shows three unmistakable incisor alveoli. The other specimens are too damaged or the teeth are too worn to determine the number of incisors.

Shortly thereafter, Hatcher (1895) reassigned the species Telmatotherium cornutum to a new genus, Dolichorhinus , but made no mention of T. hyognathus . Subsequently, Osborn (1908a) named another species, D. intermedius . This species was based on a skull (AMNH 1837) that was essentially the same as those he had referred to D. cornutum . Douglass (1909) and Riggs (1912) continued the trend of erecting new species based on skulls similar to those that Osborn had referred to D. cornutum . Douglass (1909) erected D. heterodon (CMNH 2340) and D. longiceps (CMNH 2347) , while Riggs (1912) erected D. fluminalis (FMNH P12205) and D. superior (FMNH P12168). (The holotype of D. superior was mistakenly reported by Riggs (1912) to be FMNH P12188. However, records at the FMNH indicate that the holotype specimen has always been catalogued as FMNH P12168 [William Simpson, personal commun., 2004]). When all of these supposed species were distinguished from others, differentiaion was based on (1) minor size differences or (2) taphonomic distortion and/or damage. However, all of these holotype skulls strongly resemble the holotype skull of Dolichorhinus cornutum , and I have not been able to find any compelling morphological differences between them. Peterson (1924) expressed doubt over the validity of the large number of Dolichorhinus species, although he did little to rectify this problem and continued to ‘‘provisionally’’ accept all of the species that had been named by Osborn, Douglass, and Riggs.

Osborn (1908a, 1929a) ultimately conclud- ed that the neglected holotype jaw of Telmatotherium hyognathus (Osborn) (YPM PU10273) represented the same species as the holotype skull of Dolichorhinus cornutum (AMNH 1851) . Osborn (1929a) reassigned T. hyognathus to the genus Dolichorhinus and considered D. cornutum to be a junior synonym of D. hyognathus . At that point, D. hyognathus became the type species of Dolichorhinus . Despite this one revision, Osborn (1929a) continued to recognize all of the other species of Dolichorhinus as valid, despite the similarities of the holotype skulls of each of these supposed species.

Mader (1989) expressed doubt about Osborn’s (1908a) decision to synonomize D. cornutum with D. hyognathus ; he considered D. cornutum to be the type species and considered D. hyognathus to be a nomen dubium. Subsequently, Mader (1998) accept- ed only two species of Dolichorhinus , D. hyognathus and D. intermedius , but he mistakenly reassigned these species to the genus Sphenocoelus . However, it can be shown that Dolichorhinus (sensu Hatcher and sensu Osborn) is clearly distinct from Sphenocoelus (sensu Osborn) . There are at least six conspicuous differences between Sphenocoelus and Dolichorhinus : (1) Sphenocoelus lacks the specialized morphology of the premaxillomaxillary rostrum seen in Dolichorhinus . (2) Sphenocoelus has large ventral sphenoidal fossae, while Dolichorhinus does not. (3) Sphenocoelus has a sagittal crest, while Dolichorhinus does not. (4) The posterior half of the cranium of Dolichorhinus is more strongly arched than that of Sphenocoelus . (5) Dolichorhinus molars have distinct central fossae, whereas central molar fossae are apparently variable in Sphenocoelus . (6) Finally, there is a p1–p2 diastema in the mandibles of Dolichorhinus , whereas Sphenocoelus lacks a p1–p2 diastema.

Fortunately, there are at least two associated skulls and mandibles (e.g., AMNH 13164 and CMNH 11017) that allow one to confirm that the holotype mandible of Dolichorhinus hyognathus does belong to the same species as the highly elongate, distinctive skulls that represent other supposed species of Dolichorhinus ( D. cornutum , D. intermedius , D. heterodon , D. longiceps , D. superior , and D. fluminalis ). The characteristics found to be variable among these specimens is consistent with intraspecific variation found in most species of brontotheres and does not suggest multiple species. These variable characters include the presence or absence of M3 hypocones, the presence or absence of molar paraconules, the presence or absence of a minor P4 mesostyle, the variable lingual morphology of the premolars, and the size and distinctness of hornlike protuberances. Therefore, all other species of Dolichorhinus are considered junior synonyms of D. hyognathus ( Osborn, 1889) .