Diplacodon elatus Marsh, 1875

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 : 267-280

Diplacodon elatus Marsh, 1875

HOLOTYPE: YPM 11180, a flattened skull with right P2 (partial), P3–M3, left C

(partial), P2 (partial), P3–P4, M1 (partial), and M2–M3.

TYPE LOCALITY: Myton Member (Uinta C) of the Uinta Formation, Uinta Basin, Utah.

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

SYNONYMS: Eotitanotherium osborni ( Peterson, 1914b) ; Diplacodon (Pseudodiplacodon) progressus Peterson, 1934 .

REFERRED SPECIMENS: (From the Myton Member of the Uinta Formation of Utah) AMNH 21887 , a skull with right and left P1– M3 ; CMNH 2858 , an anterior portion of a skull with right P2 (partial), P3, left I1- canine, and P2 ; CMNH 2859 (holotype of Eotitanotherium osborni ), an anterior portion of a skull with right I1–C, left I1–M3, a partial mandible with right i3–m3, and numerous postcranial elements ; CMNH 10200 , a skull with right and left P1–M3 ; CMNH 11819 , a fragment of a palate with right M1–M3 ; CMNH 11876 , a mandible with right c–m3 and left i3–m3 ; CMNH 11877 , a complete mandible with right c, p2– m3, and left p1–m3 ; CMNH 11879 (holotype of Diplacodon progressum ), a skull with right I2–I3, P1, P4, M2–M3, left P2–M3, a partial mandible with left p2 (partial), p3–m3, and numerous postcranial elements ; CMNH 11895 , an anterior portion of a skull with right I3, P2, P4, M1–M3, and left P2–M3 ; CMNH 11828 , an anterior portion of a skull with right P2–M3, and left C–M3 ; CMNH 11881 skull (partially prepared) with right C, P2–M3, and left C–M3 ; CMNH 11967 , a partial mandible with right i2–i3, c, p1–m3, and left i2 ; FMNH P14632 skull with right C–M3 and left I3–M3 ; FMNH P14633 , a skull with right C–M3, left C–M3 and isolated right incisors ; FMNH P14634 , a complete mandible with complete lower dentition ; FMNH P14638 , a left mandibular ramus with p2–m3 and a vertebral atlas ; FMNH P14640 (in part), a partial mandible with right i3, c, p1, m2 (partial), m3, and left p2–m3 ; FMNH P14799 dorsoventrally crushed skull with right P1–M3 and left P4– M3 ; FMNH P15446 , a laterally crushed skull with right I1–I2, C–M3, left I1–C, and P2– M3 .

DIAGNOSIS: Diplacodon elatus is somewhat larger than Protitan minor but smaller than Protitanotherium emarginatum with horns of variable size that are positioned high above the orbits. The nasal incision is dorsoventrally deep and it extends posteriorly to the P4. The nasal process is slightly angled downward; it is broad, with thickened and upturned sides, and with a strongly rounded distal edge with a downturned distal tip. The nasal process is not elevated to the peaks of the horns. The orbits are positioned above the M2 and the posterior part of M1. The premaxillomaxillary rostrum hangs down posteriorly and it is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, separate parasagittal ridges that moderately constrict the dorsal width of the cranium posteriorly, lack of a bony emargination surrounding the posterior nares, large ventral sphenoidal fossae, nearly straight zygomatic arches, and a ventrally constricted and mediolaterally angled external auditory pseudomeatus.

Dentally, Diplacodon elatus has three intermediate-sized upper incisors, globular I1 and I2, a more subcaniniform I3, a complex P1, and a distinct P2 metacone. Premolar hypocones are present although the protocone and hypocone of P2 and P3 sometimes take the form of a single lingual crest. The molars of Diplacodon elatus 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 are present. Anterolingual cingular cusps tend to be weakly developed. Paraconules and metalophs are absent. The lower dentition includes three intermediate-sized incisors. The i1 and i2 are semispatulate while the i3 is more subcaniniform. There is a distinct postcanine diastema. There is a distinct metaconid on p3 and p4 but not on p2. The p2 trigonid and talonid are of similar length. The lower molars have shallow basins and the m3 is slender.

Diplacodon elatus can be distinguished from Protitanotherium , Rhinotitan , and more plesiomorphic brontotheres such as Protitan by its elevated horns, deep nasal incision, significantly more molarized premolars, and by its autapomorphic nasal morphology with upturned sides. Other brontotheres with premolars that are undifferentiated from those of Diplacodon elatus have smaller incisors, taller lower premolars, and/or a radically different skull shape.

DESCRIPTION

SKULL: The holotype of Diplacodon elatus (YPM 11180) consists of a skull that is preserved in two pieces, an anterior portion (fig. 130), and a posterior portion (not shown). The specimen is extremely flattened and the only parts of the skull that are readily describable are the cheek teeth. However, D. elatus is also known from many complete and undistorted skulls and jaws. Two of the best skulls are FMNH P14632 and FMNH P14633 (figs. 131 and 133) from which the following description is primarily based. However, other specimens offer numerous important details on variation in this species.

Diplacodon elatus is rather small (table 10) for a horned brontothere. It is larger than Protitan minor but is smaller than Protitanotherium emarginatum or Protitan grangeri . There is no discernable frontonasal suture on any specimen. However, it is probable that like other horned brontotheres, the horns are composed of frontal and nasal elements, but these elements are completely fused together. Juvenile specimens may provide a more definitive answer, but none are currently known.

In cross section the horns vary from being round to somewhat elliptical. The surfaces of the horns are rugose, although some specimens have smooth horn surfaces. The horns range in size from being very small to quite massive, although they are never extremely tall as in some specimens of Megacerops (sensu Mihlbachler et al., 2004b) . Many of the specimens have horns that are about the same size as either those of CMNH 2859 (fig. 132a), a gracile specimen, or CMNH 11879A (fig. 132b), a more robust specimen. However, numerous specimens (e.g., AMNH 21887, CMNH 2858, CMNH 11881, FMNH P14632, FMNH P14632, FMNH P14633) have horns that are intermediate in size. A single specimen, CMNH 11828, has horns that are very small and exist only as small rugose ridges.

Despite conspicuous variation in horn size and horn surface texture, the position and orientation on the horns is much less variable. The horns tend to project anterodorsally and laterally to varying degrees. They are positioned high above the orbits and are elevated upon tall superorbital pillars that project from above the orbits in an anterodorsal direction at about a 45 ° angle. This differs from Protitanotherium and Protitan whose horns are lower on the skull, a placement that more closely resembles more derived taxa such as Aktautitan and Metatitan . However, unlike these taxa, the nasal process of Diplacodon elatus is not elevated to the very peak of the frontonasal protuberances.

The nasal incision extends posteriorly to a point that fluctuates between the anterior and posterior margins of the P4. Although the nasal process is not elevated to the apex of the horns as in Metatitan and Aktautitan , it is nonetheless higher than those of Protitanotherium and Protitan , resulting in a dorsoventrally deep nasal incision where the posterior notch of the nasal incision rises higher than the orbit. The orbits of Diplacodon elatus are shifted anteriorly; all the roots of M2 and the posterolateral root of M1 are positioned directly below the orbit. The anterolateral root of M1 is positioned directly below the anterior orbital rim.

The nasal bones of Diplacodon elatus are completely fused. The length of the nasal process varies; it is typically a little shorter than the premaxillomaxillary rostrum, but occasionally, such as in CMNH 2859 (fig. 132a), it is nearly as long as the rostrum. The nasal process is as broad as the rostrum and it has thickened sides. From a dorsal view the nasal process of CMNH 14632 is distally tapered and the anterior end is strongly rounded, although in some specimens (e.g., AMNH 21887) the sides are more nearly parallel and somewhat constricted proximally. The distal tip of the nasal process is roughened and strongly curved downward. The sides of the nasal process are autapomorphically upturned rather than turned downward. The upturned sides of the nasal process are most easily seen from the anterior view where the dorsal surface of the nasal process is concave in a transverse direction

Fig. 130. The holotype of Diplacodon elatus . (Division of Vertebrate Paleontology, YPM 11180. º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.) (A) Ventral view of palate, (B) left P2–P4, (C) right P2–P4.

(fig. 131c). In more gracile specimens, such as CMNH 2859 (fig. 132a), the nasal process is thinner and more nearly flat in cross section, although the dorsal surface is still convex in a mediolateral direction. (Note that previously published figures of another specimen, CMNH 2856 [ Peterson, 1914b: pl. 8, and reprinted in Osborn, 1929a: fig. 365] give the

TABLE 10 Summary statistics for selected morphometric variables of Diplacodon elatus See Methods for measurement definitions misleading impression that the lateral margins are not upturned.) In the most robust specimens, such as CMNH 11879A (fig. 132b), a distinct rugosity traces around the edges of the nasal process, the dorsal surface is more strongly concave in cross section, and the distal end is more strongly curved downward. The variation in shape and thickness of the nasal process is generally related to the size and robusticity of the horns although the correlation is not perfect. For instance, CMNH 11828 (not shown) has a relatively thick nasal bone, though the horns are small.

From a lateral view (fig. 131a), the dorsal surface of the premaxilla is flat and steeply angled posterodorsally. Behind the symphysis the dorsal surface of the premaxilla is nearly horizontal. From the anterior view it can be seen that the premaxillary symphysis is rather long. Behind the symphysis the dorsolateral margins of the rostrum diverge laterally and the rostral cavity is not enclosed by bone dorsally. A distinct premaxillomaxillary suture has not been observed in any available specimen of Diplacodon elatus .

The dorsal surfaces of the skulls of Diplacodon elatus are strongly convex or saddle-shaped. The parasagittal ridges remain separate throughout their length and moderately constrict the dorsal surface of the cranium posteriorly but to a lesser degree than seen in Protitanotherium emarginatum . From the lateral view of the skull the zygomatic arch is not very deep. The jugal portion of the zygomatic arch is horizontal, while the squamosal portion is slightly sloped upward posteriorly, but as a whole, the zygomatic arch is nearly straight. From a dorsal view the zygomatic arch is relatively thin, straight, and strongly angled posterolaterally.

The occiput is moderately tilted backward. From a dorsal view the nuchal crest is slightly convex. From the posterior view the dorsal margin of the occiput is barely arched dorsally. The widths of the dorsal and ventral portions of the occiput are nearly the same and the occiput is very slightly constricted in the middle. There are strong occipital pillars of the surface of the occiput and there is a shallow central depression.

The position of the anterior margin of the posterior nares is somewhat variable. It is most often positioned slightly anterior to the M3 protocones as in CMNH 14633 (fig. 133). In a few specimens, such as FMNH P14632, it is slightly anterior to the M3. The bony horseshoe-shaped rim that typically emarginates the posterior nares of many brontotheres is not seen in Diplacodon elatus .

The posterior narial canal is elongate and continues far into the sphenoid via a pair of ventral sphenoidal fossae. In CMNH 14633 the elongate posterior narial canal has been fully cleared of sediment on the left side, thus exposing the deep ventral sphenoidal fossa and the elongate vomer that joins the septumlike basisphenoid to form a continuous partitioning septum. Every specimen of Diplacodon elatus in which the basicranium is preserved has distinct ventral sphenoidal fossae.

The short mastoid process arches anteroventrally and contacts the longer postglenoid process, thus creating a tubelike external auditory pseudomeatus that enters the skull in a mediolateral direction. The configuration of the basicranial foramina is consistent with those of other brontotheres with widely separate foramina of the alar canal, foramen ovale, and foramen lacerum.

UPPER DENTITION: In all of the known skulls the upper incisors are either not preserved or they are heavily worn and/or damaged. One specimen (CMNH 2859) has a set of worn but nearly complete incisors (fig. 134a). Additionally, isolated incisors are associated with FMNH P14633 (fig. 134b, c). Two nearly intact sets of cheek teeth are present in FMNH P14632 and FMNH P14633 (fig. 135). These specimens, along with others including the holotype (YPM 11180, fig. 130), indicate conspicuous intraspecific variation in cheek-tooth morphology that is further described below.

The dental formula of Diplacodon elatus is unreduced (3-1-4-3). The incisors are small in comparison to Protitan and other more plesiomorphic brontotheres, but they have not reached the vestigial state as seen, for example, in Duchesneodus . The incisors are not clearly differentiated from those of Protitanotherium emarginatum although they arch slightly more anterior to the canines. The I1 and I2 appear to have been subglobular, particularly those of FMNH P14633. The I1 and I2 of CMNH 2859 retain small lingual cingula. The substantial amount of wear on this specimen obscures other details. The I3 is larger than I1 or I2 and though worn in both specimens, it is clearly taller and more caniniform. The left incisors of CMNH 2859 are separated by very short diastemata. There is always a distinct diastema between the I3 and canine. The canines tend to be small with a weak distal cingulum. The postcanine diastema varies in length from slightly longer than the P2 to slightly shorter. The canines tend to be smaller than those of Protitanotherium , the postcanine diastema tends to be longer, and the distal end of the rostrum is somewhat narrower.

The P1 of FMNH P14632 is well worn, but two labial bulges in the crown indicate that both a paracone and metacone were present. The P1 crown is rounded. A lingual heel was clearly present, but further details are obliterated by wear. The P1 of FMNH P16633 is less worn. In that specimen there is single large protocone on the lingual heel. This protocone is connected to the paracone by short preprotocrista. Additionally, there is a short lingual crest that arches around the posterolingual border of the crown.

The P2–P4 of FMNH P14632 are subrectangular with nearly flat lingual margins. The anterior and posterior sides are nearly rectangular, but the lingual sides of P2 and P3 are slightly narrower than the labial sides. In other specimens, such as FMNH P14633, the P2 can be slightly more oblique in shape due to a more strongly distolingually angled anterior margin. The parastyle and metastyle of the P2 are straight. The parastyle of P3 is slightly angled labially while the metaconid is straight. The parastyle and metastyle of P4 are strongly labially directed. The labial sides of the P2 paracone and metacone form small swellings. In P3 and P4 there are more distinct labial paracone ribs. Although a P4 mesostyle is occasionally seen among specimens of brontotheres such as in Pachytitan or Protitanops , none of the P4s of Diplacodon elatus has a mesostyle.

The lingual features of the P2–P4 tend to be higher in relief in comparison to Protitan , Protitanotherium , or Rhinotitan . A distinct preprotocrista can be seen on P2–P4, although this structure tends to be larger on more anterior premolars. The P2–P4 of Diplacodon elatus tend to have two lingual cusps, a protocone and a hypocone. The exact morphology of the lingual side of the crown is variable. In AMNH 14632 the P2– P4 have distinct protocones and smaller hypocones. The P2 hypocone is not connect- ed to the protocone by a lingual crest. However, the P3 and P4 hypocones are connected to the protocones by a thin lingual crest. In each of these premolars the hypocone is positioned distantly from the protocone. FMNH P14633 exhibits a different morphology. In that specimen, there are no distinct lingual cusps on the P2, although a rounded crest arches around the lingual margin of the crown. In P3, the lingual crest is much thicker than that of P2 and it completely absorbs the protocone and hypocone. There is a distinct protocone and hypocone on P4, although, in this instance, the cusps are positioned very closely together and are of approximately the same size.

The upper premolars of the holotype specimen (fig. 130) demonstrate bilateral asymmetry. In that specimen there is a lingual crest on the right P2, but there are no distinct lingual cusps. On the left P2 there are two widely spaced cusps connected by a lingual crest. On the right P4 the small hypocone is connected to the protocone by a crest, while on the left P4 there is no connecting crest.

Variation in the size and distinctness of the lingual cusps, and their degree of separation is seemingly random in each individual tooth. However, when one examines all the specimens the lingual side of P2 ranges from having a single tall lingual crest with no distinct cusps to having two distinct lingual cusps that are weakly connected by a thin crest. P3 most often has a distinct protocone and a smaller hypocone. The connecting crest itself is sometimes nearly as tall as the lingual cusps, but it can also be much shorter. P4 is more variable in this respect. There are always two distinct cusps that are usually connected to varying degrees by a connecting crest of variable height, but occasionally, as in FMNH P14632, the hypocone is much smaller and it is positioned on the base of the posterior slope of the protocone. Despite the overall variability, a general pattern does emerge; P2 shows a tendency to have large lingual crests that often absorb the lingual cusps, while P4 tends to have more distinct labial cusps with the connecting crest reduced or absent. P3 is usually intermediate.

Labial premolar cingula are always weak or absent in Diplacodon elatus . The lingual cingula of the P2–P4 are always thicker. It is consistently thickest on P2 and it is never discontinuous. Occasionally the lingual cingulum of P3 and P4 is slightly discontinuous around the lingual margin of the crown.

The upper molars of Diplacodon elatus show numerous advanced features, 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. Distinct central molar fossae are present. In the holotype specimen (fig. 130) the anterolingual cingulum of the molars thickens and rises slightly but does not form a strong anterolingual cingular cusp. Other specimens, such as FMNH P14632 (fig. 135c), reflect the condition seen in the holotype. However, other specimens, such as FMNH P14633 (fig. 135d) show strong anterolingual cingular peaks on the molars.

Mader (1989, 1998) noted that paraconules or metaconules are occasionally present on the molars of Diplacodon (5 Pseudodiplacodon ). However, I found no evidence of paraconules or metaconules in any of the specimens. Nor is there any evidence of a vestigial metaloph. Mader (1989, 1998) also noted the occasional presence of a hypocone (or ‘‘pseudhypocone’’) on M3, but none of the specimens has a distinct hypocone. Although no hypocone is present, the lingual side of the M3 is broad and squarish with a large space posterior to the protocone and a thick cingulum that wraps around the distolingual corner of the crown. Labial molar cingula are generally weak and lingual molar cingula are very weak between the protocone and hypocone, or they are absent altogether.

MANDIBLE AND LOWER DENTITION: The only skulls of Diplacodon elatus that are associated with mandibles are CMNH 2859 and CMNH 11879. However, each of these mandibles is incomplete. The mandible of CMNH 2859 is missing its incisors, but that specimen has the most unworn and complete set of lower premolars available for Diplacodon elatus , and is therefore figured in close up (fig. 136c). There are also several mandibles and partial mandibles from the Myton Member of the Uinta Formation that are not associated with skulls but that are consistent with CMNH 2859 and CMNH 11879 and are referable to D. elatus . Among these, FMNH P14634 is the most complete D. elatus mandible (fig. 136a, b, d, e).

The mandible of FMNH P14634 has a thin coronoid process that rises above the mandibular condyle. The inferior margin of the symphysis is angled slightly less than 45 °. From the dorsal view of the mandible the mandibular symphysis is longer and more slender than that of Protitanotherium emarginatum and with a longer postcanine diastema. The posterior portion of the symphysis extends to the talonid of p 4 in FMNH P 14634, but the symphysis more typically ends at the posterior edge of the p3 (CMNH 11876, 11877, 11967).

Like the upper incisors, the lowers are smaller than those of Protitan grangeri , but they have not reached a vestigial stage. The lower incisors of Diplacodon elatus are not clearly differentiated from those of Protitanotherium , although the incisor row is more arched. The i2 is the largest incisor; this is most obvious from the labial view (fig 136d). The crowns of the i1 and i2 are semispatulate with rounded apices and narrow lingual cingulids. The i3 is shorter and more mesiodistally elongate in comparison to i1 or i2. A single left i3 is also preserved with CMNH 2859 (not shown). The i3 of that specimen seems taller and somewhat more subcaniniform than the i3 of FMNH P14634. There are no diastemata between any of the lower incisors or canines. The lower canines are of small to moderate size and the postcanine diastemata of FMNH P14634 and other specimens of D. elatus are consistently longer than those of Protitanotherium emarginatum .

The lower premolars of Diplacodon elatus are relatively slender and low-crowned. In general, their proportions do not differ notably from Protitanotherium emarginatum . The p1 is a simple, single-cusped tooth with a talonid heel and a weak lingual cingulid. The trigonid of p2 is slightly longer than the talonid. This differs from Protitanotherium , whose p2 trigonid is nearly twice the length of the talonid. The p3 trigonid of D. elatus is slightly longer than the talonid, while the trigonid and talonid of p4 are of similar length. The trigonids of p2–p4 are all barely narrower than their talonids. The paralophid of p2 arches lingually but at an angle less than 45 °, thus creating a small lingual trigonid notch. The protolophid of p2 is nearly straight, but it is lingually positioned. The paralophid of p3 is relatively long and arches lingually about 45 °, while the p4 paralophid is molariform and arches 90 ° lingually. The protolophids of p3 and p4 are molariform and arch almost completely lingually. The trigonid of p3 has a very broad lingual notch, while the trigonid of p4 has an essentially molariform basin. The p2 lacks a metaconid, but p3 and p4 possess a large lingually positioned metaconid. The talonids of p2–p4 have well developed cristids obliqua and hypolophids. These elements are more elongate in more posterior premolars, creating increasingly molariform talonid basins. The labial premolar cingulids are weak to absent and lingual premolar cingulids are absent.

The lower molars of Diplacodon elatus are typical. They have relatively thin lingual enamel, shallow trigonid and talonid basins, and the m3 is elongate. There are no lingual cingulids, but labial molar cingulids are distinct although they tend to be discontinuous around the paraconids and metaconids.

REMARKS

Marsh (1875) based Diplacodon elatus on a dorsoventrally crushed skull (YPM 11180) with comparatively undistorted cheek teeth (fig. 130). Marsh (1875) asserted that D. elatus was hornless. The dorsal surface of the holotype does appear to lack horns as Marsh (1875) had observed, but the specimen is severely flattened and it was apparent to others that the specimen was far too damaged to realistically determine the presence or absence of horns in D. elatus from its holotype ( Hatcher, 1895; Osborn, 1929a). The only readily describable parts of the holotype are the cheek teeth, which indicate a brontothere that is somewhat smaller than Protitanotherium emarginatum , also from the Myton Member of the Uinta Formation, and with significantly more molarized premolars.

Several complete and less distorted specimens of Diplacodon elatus from the Myton member of the Uinta Formation have since been discovered, although many of them have been assigned to a variety of other names. Peterson (1914b) named a new species, Diploceras osborni , from the anterior portion of a skull and mandible (CMNH 2859) (figs. 132a, 134a, 136b). Peterson (1914b) relegated another partial skull (CMNH 2858) as the cotype. The later specimen differs distinctly from the former with its larger and more rugose horns, which Peterson (1914b) attributed to sexual dimorphism. Peterson (1914c) later renamed the genus to Eotitanotherium after discovering that Diploceras was preoccupied by a mollusk. Peterson (1914b) did not clearly differentiate Eotitanotherium osborni from Diplacodon elatus , although he mentioned that Eotitanotherium osborni had shorter, blunter canines, a less ridgelike deuterocone (protocone) on the P2, and more distinct lingual cusps on the P3. None of these observations warrant a clear taxonomic distinction considering (1) the poor state of preservation of the canine in the holotype of D. elatus , (2) and the large degree of essentially continuous morphological variation seen in the upper premolars of brontotheres.

Peterson (1934) named yet another species, Diplacodon progressum , based on a skull (CMNH 11879A) (fig. 132b) and nearly complete skeleton (CMNH 11879). The horns of that specimen are robust and similar to those of Peterson’s paratype of Eotitanotherium osborni . Peterson (1934) differentiated D. progressum from D. elatus with the following observations, (1) ‘‘Facial region is shorter in D. progressum ’’, (2) ‘‘the premolar series in the new species is farther advanced in molarization than in D. elatus , the individual teeth being more perfectly quadrate in D. progressum , P2 being especially advanced’’ ( Peterson, 1934: 353). The first observation was based only on the slightly shorter premolar length and might simply relate to the fact that the specimens are of slightly different sizes. Moreover, the extremely crushed and incomplete condition of the holotype of D. elatus (YPM 11180) complicates any comparison. The second observation is questionable as well; the slight difference in the squareness of the premolars is consistent with a pattern of intraspecific premolar variation generally found in brontotheres and does not represent a taxonomically significant difference.

Peterson (1934) distinguished Diplacodon progressum from Eotitanotherium osborni on the observations that the (1) nasals ‘‘are shorter and heavier’’, and (2) ‘‘the alveolar border of the premaxillary border is noticeably shorter’’ ( Peterson, 1934: 353). Upon reexamining the specimens, the second observation cannot be confirmed. The first observation relates to the fact that the holotype of D. progressum is more robust in comparison to the holotype of E. osborni . This distinction contradicts Peterson’s (1914a) earlier concept of E. osborni whereby differing degrees of robustness were attribut- ed to sexual dimorphism. Specimens that have since been referred to either E. osborni or D. progressum vary only in the general robustness of the skull (including horn size and nasal thickness) and in the degree to which the lingual cusps of the premolars are separated or connected by a lingual crest.

Based on these aspects of variation Mader (1989, 1998) continued to accept Eotitanotherium osborni and Diplacodon progressum . Mader (1989) suggested that E. osborni was probably a junior synonym of D. elatus but he preferred to view the later as a nomen dubium because of the crushed nature of the holotype of that species. Because Diplacodon was not considered valid, Mader (2000) erected a new genus, Pseudodiplacodon , for the progressum species. Mader (2000) continued to distinguish E. osborni and P. progressum based on horn size, nasal thickness, and the degree of separation of the lingual cusps on the P4.

In considering all of the specimens, Mader’s (2000) distinctions of Eotitanotherium and Pseudodiplacodon seem to represent continuous intraspecific variation. For instance, many of the specimens assigned to Pseudodiplacodon progressum by Mader (2000) have horns that are intermediate in size between the holotypes of E. osborni and P. progressum (e.g., AMNH 21887, CMNH 2858, CMNH 11881, CMNH 14632, FMNH P14632, and FMNH P14633) (for example, compare specimens in fig. 131 to fig. 132). In addition, specimens assigned to P. progressum by Mader (2000) distinctly vary in the degree to which the lingual premolar cusps are separated. For example, in FMNH P14632 the hypocones and premolars are well separated, although in FMNH P14633 the lingual cusps are very poorly separated (fig. 135). Finally, the nasal bones of one of the three specimens that was earlier referred to E. osborni by Mader (1989) has a nasal bone that is as thick as those specimens referred to P. progressum by Mader (2000).

The variation in horn size, nasal thickness, and lingual premolar morphology among these specimens is seemingly continuous and is generally consistent with a pattern of intraspecific cranial variation found in other brontotheres. Additionally, the lingual premolar morphology is found to be variable in most brontothere species. Given the continuous nature of this variation, the specimens in question cannot be easily partitioned into discretely diagnosable units. Therefore, if specimens previously referred to Eotitanotherium osborni or Pseudodiplacodon progressum represent a single taxon, as is concluded here, Diplacodon elatus Marsh is a valid species. E. osborni (Peterson) and P. progressum (Peterson) are junior synonyms of D. elatus .

Specimens referred to Diplacodon elatus are confined to the Myton Member of the Uinta Formation. However, a large undescribed Diplacodon -like skull (AMNH 117163) with very heavily worn dentition was collected by Malcolm McKenna from Mudstone Peak in the Wiggins Formation of Wyoming. Previously, Mader (1998) suggest- ed this skull belongs to Protitanotherium , but this is almost certainly not correct. The skull very closely resembles Diplacodon , particularly in its dual lingual premolar cusps, deep nasal incision, and distinctively upturned lateral margins of the nasal process. The skull, AMNH 117163, differs from typical Diplacodon elatus from the Uinta Formation in only a few ways. It is considerably larger, has more elliptical horns (with the longest axis anteroposterior), and the parasagittal ridges constrict the dorsal surface of the skull more severely than typical D. elatus skulls. This specimen could represent a new species of Diplacodon , but it has not yet been thoroughly studied. Some or all of these differences may relate to sexual dimorphism or intraspecific variation. For the time being its taxonomic identity is uncertain.