Telmatherium validus Marsh 1872

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 : 93-106

Telmatherium validus Marsh 1872

LECTOTYPE: YPM 11120, a partial skull with right I3, C, P1, P3–M2 and left I1, I2, C, P1–M3.

TYPE LOCALITY: Near Henry’s Fork of the Green River, Wyoming, Twin Buttes

Member of the Bridger Formation (Bridger C or D).

SYNONYMS: Telmatherium cultridens Osborn, Scott, and Speir, 1878 ; Manteoceras manteoceras Hay, 1901 ; Manteoceras washakiensis Osborn, 1908a .

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

REFERRED SPECIMENS: (From the Twin Buttes Member [Bridger C–D] of the Bridger Basin, Wyoming) AMNH 1511 , a skull with right I3–M3, and left P2–M3 ; AMNH 1532 , a palate with right P2–M3, left C (partial), and P2–M3 ; AMNH 1545 (associated with USNM 6700 ), a mandible with right p4–m3, left p4–m1, m2 (partial), and m3 ; AMNH 1560 , a mandible with right i3–c, p3, p4, m2, m3 (partial), left i2, c, and p2–m3 ; AMNH 1563 , a mandible with right p2–m3 and left p3–m3 ; AMNH 1566 , a mandible with right i1–m3 and left i3–m3 ; AMNH 1569 (holotype of Manteoceras manteoceras ), a skull with no complete teeth ; AMNH 1587 , a crushed skull with right M1–M3 and left P1– M3, and a right mandibular ramus with p4 (partial) and m1–m3 ; AMNH 12193 , a mandible with right m2, left c, and p4–m3 ; AMNH 12194 , a left maxilla with P4–M3 ; AMNH 12204 , a crushed skull with right P4– M3, left P3–M3, and a crushed mandible with right c, p3, p4, left c, p4, and m1–m3 ; AMNH 12210 , a mandible with p4–m3 ; AMNH 12214 , a right M3 ; AMNH 12678 , a skull with right I1–P4, M2, M3, left I2–C (partial), P1–M1 (severely damaged), M2, and M3 ; AMNH 12687 , a right mandibular ramus with p2–m3 ; AMNH 12681 , a mandible with left p3 (partial), and p4–m3 ; AMNH 12683 , an anterior portion of a skull (in two pieces) with right C–M1, M2–M3 (all partial), and left I1–M3 ; UCM 69355, a mandible with right and left i1–13, c (erupting), and p1–m2; UCM 73736, a crushed skull fragment with left P1–P3; UCMP 31344, a right mandibular ramus with p3– m3; USNM 6700 (associated with AMNH 1545 ), a crushed skull with right P1–M3, left C, P2–M3 ; USNM 12836, a mandible with right p3–m3, left p4–m3, and an isolated canine; USNM 13455, a partial mandible with right p3–m3, left p2, and p4–m3; USNM 13456, a partial skull with right and left maxillae with C–M3; USNM 26113, a right maxilla with P4–M3; USNM 26114, a left maxilla fragment with M1–M3; USNM 26119, a crushed skull with right P2–M3, left C, and P2–M3; USNM 26121, a right mandibular ramus with i2–m3; USNM 26140, a skull with right P3 (partial), P4– M1, and left P3–M1; USNM 26162, a partial skull with right and left P2–M3, and a mandible with right p2–m3; USNM 26302, a mandible with right p4–m3 and left i2–m3; YPM 16415, a left maxilla fragment with M1, M2, and M3 (partial); YPM 16729, a partial skull (in two pieces) with left C, and P2–M3; YPM PU10027 (holotype of Telmatherium cultridens ), a right maxilla with I1–M3 and a left mandible fragment with p2–m3; YPM PU10361, a mandible with m1–m3; (from the lower Adobe Town Member [Washakie A of Granger, 1909] of the Washakie Formation of Wyoming) AMNH 1570 , an anterior portion of a skull with right and left P1–M3 ; AMNH 2353 , a skull with right P2, M1–M3, and left P1–M3 ; AMNH 2354 , a skull fragment with right P1, M2–M3, and left P2–M3 ; AMNH 2356 , a right mandibular ramus with dp3, dp4, m1, and m2 ; AMNH 13165 (holotype of Manteoceras washakiensis ), a partial skull with right M2, M3, and left C–M3 ; AMNH 13176 , a crushed mandible with left p1–m3 ; FMNH PM2328 , a mandible with right i3– m3 and left p1–m3 ; FMNH PM55576 , a partial mandible with right i1–m2, and left i1–p4 ; YPM PU16103, a mandible with right p1–m3 and left c–m3; (from the Aycross Formation, Absaroka Range , Wyoming) AMNH 105429 , associated upper dentition including right P2–M3 and left P2–M2; (no locality data) YPM 47288, associated dentition with right and left P2–M3; YPM PU25022, a palate with right P2–M3 and left P1, P2, P4–M3.

DIAGNOSIS: Telmatherium validus is an intermediate-sized hornless brontothere. The frontal bone intrudes into the surface of the nasal bone splitting off a small lateral nasal splint from the main body of the nasal. The nasal splint is larger, more strongly arched, and more distinct than those of Wickia brevirhinus or Metatelmatherium ultimum . The nasal incision extends to the P2. The nasal process is 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 are positioned above the M2 and do not project laterally. The premaxillomaxillary rostral cavity is not enclosed by bone dorsally. Other cranial characteristics include a dorsal cranial surface that is either flat or strongly elevated and convex posteriorly, strongly convergent parasagittal ridges that are separated posteriorly by a narrow pit, strongly curved and moderately bowed zygomatic arches, and a ventrally open and mediolaterally angled external auditory pseudomeatus. Ventral sphenoidal fossae are absent.

Telmatherium validus has large subcaniniform upper incisors, a postcanine diastema, a simple P1, a distinct P2 metacone, weak premolar preprotocristae, and short lingual crests occasionally extending posteriorly from the premolar protocones. Premolar hypocones are absent. The molars of T. validus have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. Central molar fossae, a cingular parastyle shelf, and well-developed anterolingual cingular cusps are absent. Metalophs are absent, but vestigial paraconules are occasionally present. The lower dentition of T. validus includes large subcaniniform incisors that are all of similar size, a p1–p2 diastema (variably present), a postcanine diastema, an elongate p2 trigonid, a metaconid on p4 but not on p2 or p3, shallow molar basins, and a slender m3.

Telmatherium validus is most similar to Metatelmatherium ultimum and Wickia brevirhinus . It differs from Metatelmatherium ultimum in the more posteriorly extended nasal incision, thicker parasagittal ridges, broader forehead, and by lacking a ventral zygomatic flange. Telmatherium validus is distinct from Wickia brevirhinus in the more posteriorly extended nasal incision and the less robust premaxillae.

DESCRIPTION

SKULL: Although numerous skulls of Telmatherium validus are known, no single specimen allows for a complete description of the entire skull. The following description of the skull of the T. validus is based primarily on the lectotype YPM 11120 (fig. 38), AMNH 12678 (fig. 39), AMNH 1511 (fig. 40), USNM 26140 (fig. 41), AMNH 13165 (fig. 42), and AMNH 1570 (fig. 43), but additional information from other specimens is provided. T. validus is an intermediate-sized (table 7) hornless brontothere whose skull is most similar to Wickia brevirhinu s and, to a lesser extent, Metatelmatherium ultimum . The frontonasal suture is plainly visible on AMNH 12678, USNM 26140, and most other skulls. The frontal bone forms a pair of short triangular processes just above and anterior to the orbits. On the surface this frontal process appears to protrude into the posterolateral portion of the nasal bone. However, in specimens where the frontal and nasal bones have become detached (e.g., YPM PU10027) it can be seen that the frontal process actually onlaps the nasal bone and sits within a grooved triangular depression in the nasal bone. On the surface, the frontal splits the posterolateral corner of the nasal bone, forming a lateral nasal splint that separates off from the main body of the nasal bone. The nasal splint is strongly arched and broadly contacts the maxillary. The nasomaxillary suture forms a tall arch. The overlapping triangular frontal process and lateral nasal splint of T. validus is shared with many other brontotheres including Metarhinus , Sthenodectes , Qufutitan , Wickia , Metatelmatherium , as well as those brontotheres with frontonasal protuberances (horns). Among hornless brontotheres that are characterized by this particular frontonasal configuration, with the possible exception of Qufutitan , the lateral nasal splint appears to be longer, wider, and more distinctly curved in T. validus , and its contact with the frontal and maxilla is more distinct on the surface of the skull.

Telmatherium validus has been described as having a rudimentary horn or frontonasal swelling (Osborn 1895, 1929a; Mader, 1989, 1998). Figures of T. validus , particularly those published by Osborn (e.g., fig. 7 in Osborn, 1895) greatly exaggerate the relief of the overlapping portions of the frontal and nasal bone. I can find no distinct frontonasal protuberances on any of the specimens. The overlapping triangular frontal process is flush with the surface of the nasal bone. However, some specimens present an illusion of small frontonasal protuberances from a lateral viewpoint. For instance, in AMNH 12678 the region above the preorbital concavity appears to be raised. This effect in AMNH 12678 is partly an illusion related to a large crack running transversely across the dorsal surface of the skull. The skull roof above the orbits has been forced downward. Additionally, the shallow preorbital fossa in the maxilla gives the region above it a prominent appearance. However, the triangular frontal process is flush with the dorsal surface of the skull. An actual frontonasal protuberance is not present in USNM 26140, where the facial area is undistorted.

The overlapping frontal and nasal bones of Telmatherium validus have long been thought of as the morphological precursor of the frontonasal horns seen in other brontotheres, thus suggesting that Telmatherium is the ancestor or sister taxon of horned brontotheres ( Osborn, 1929a; Mader, 1989, 1998). It is, no doubt, very likely that the configuration of facial bones of T. validus does represent the ancestral state of brontothere horns, but that does not suggest any particularly close relationship of T. validus with horned brontotheres because the same facial configuration exists among numerous hornless brontotheres.

The nasal incision of Telmatherium validus is longer than those of Wickia brevirhinus and Metatelmatherium ultimum . In AMNH 12678, the nasal incision extends approximately to the protocone of the P2. In all specimens of T. validus the nasal incision extends posteriorly past the anterior margin of the P2. The longest nasal incision seen in a skull of T. validus is found in AMNH 1511, where the nasal incision extends to the posterior margin of P2. The orbits do not protrude laterally as in Metarhinus . The orbit of AMNH 12678 is positioned directly over the M2 and over the alveolus of the M1. The anterior rim of the orbit is positioned directly above the alveolus of the anterior lateral root of the M1. However, the position of the orbit varies slightly from individual to individual, and in specimens with less extremely worn dentition (e.g., AMNH 1511), the posterior lateral root of the M1 is positioned below the orbit.

Fig. 38. Lectotype of Telmatherium validus ( YPM 11120 ). (A) Ventral view, (B) right view, (C) anterior view. (º 2005 Peabody Museum of Natural History , Yale University , New Haven, Connecticut, USA. All rights reserved.)

The nasal bones tend to be poorly fused together. The nasal process is shorter than the premaxillomaxillary rostrum. In AMNH 12678, the nasal process is angled slightly downward, although the downward angle is somewhat exaggerated. In specimens with less dorsoventral crushing, such as USNM 26140, the nasal process is more horizontal, but its dorsal surface is slightly convex from its lateral profile. The lateral walls of the nasal process are vertical and dorsoventrally deep. The sides of the nasal process form deep and essentially vertical lateral walls that are of relatively constant depth throughout the length of the nasal process. The distal edge of the nasal process is thin, rough, and deflected downward (fig. 39c). From the dorsal view, the nasal process is narrower than the premaxillomaxillary rostrum, and is more or less constant in width throughout its length, although it is sometimes slightly constricted proximally (e.g., USNM 26140).

From a lateral view the posterodorsal slope of the dorsal margin of the premaxillomaxillary rostrum is relatively shallow. The premaxillomaxillary sutures of the lectotype YPM 11120 and many other specimens (e.g., AMNH 12678, AMNH 1569, AMNH 1570, USNM 26140) are plainly visible. The premaxilla truncates before reaching the posterior notch of the nasal incision. Thus, the premaxilla does not contact the nasal bone. The premaxillae are generally thick and well developed although they tend not to be as robust as those of Wickia brevirhinus . In AMNH 12678 the premaxillae are strongly fused at the symphysis. However, in other specimens, including the lectotype (YPM 11120), the premaxillae are not strongly fused. The degree of ossification of the symphysis seems to covary inversely with the size of the canines and the corresponding robustness of the maxillae. In AMNH 12678, the canines are relatively small, the maxillaries are comparatively slender and narrow, and the premaxillae are fully coossified. In contrast, the canines of YPM 11120 and AMNH 1570 are much larger in diameter, the maxillaries are more robust, and the premaxillary symphysis is not strongly coossified. Posterior to the premaxillary symphysis, the nasal processes of the premaxillae diverge laterally. The premaxillomaxillary rostral cavity is not dorsally enclosed by bone.

In comparison to Wickia brevirhinus and Metatelmatherium ultimum , the dorsal roof of the skull of Telmatherium validus is wider and less transversely arched. From a lateral profile, the dorsal surface of the skull above the orbits is slightly concave or it can be flat. Behind the orbits the dorsal surface is flat in AMNH 12678. In some specimens (e.g., AMNH 1511), the postorbital portion of the dorsal cranial surface is strongly convex with an elevated posterior end. The dorsal cranial profile of AMNH 1511 bears a strong resemblance to Palaeosyops , with a steep slope between the elevated posterior portion and the lowered midcranial frontal area.

The parasagittal ridges of Telmatherium validus tend to be prominent, although there is variation in the thickness of the parasagittal ridges. For instance, AMNH 12678 and AMNH 13165 are among the more gracile specimens. One of the most robust specimens is AMNH 1569 (not shown) with extremely thickened parasagittal ridges. The parasagittal ridges of all T. validus specimens strongly constrict the dorsal surface posteriorly, but they never completely join to form a sagittal crest. Instead, they remain separated by several millimeters and a deep narrow pit is formed in the midline of the dorsal surface between the medially constricted parasagittal ridges.

The depth and thickness of the zygomatic arches of Telmatherium validus are variable. Those of AMNH 12678 are among the more gracile. From a lateral view, the jugal portion of the zygomatic arch is essentially horizontal, while the squamosal portion rises posteriorly, giving the zygomatic arch a distinct curvature. The zygomatic arches are moderately bowed laterally. Telmatherium validus lacks a prominent ventral flange on the jugal below the contact with the squamosal as seen in Metatelmatherium . Likewise, there is no infraorbital jugal process as seen in Sphenocoelus .

The nuchal crest is of moderate thickness, and from the dorsal view of the skull of AMNH 12678 it is concave. From a lateral view the occiput of T. validus is moderately tilted backward. From the posterior view, the nuchal crest is dorsally arched; the upper portion of the occiput is narrower than the posterior portion and the occiput is constrict- ed in the middle. The occipital pillars are distinct but not massive and the central depression of the occiput is shallow.

TABLE 7 Summary statistics for selected morphometric variables of Telmatherium validus See Methods for measurement definitions

The palate and posterior nares of Telmatherium validus are best preserved in AMNH 1570. The posterior nares are rimmed anteriorly and laterally by a horseshoe-shaped emargination. The anterior edge of the posterior nares is positioned about at the posterior margin of the M2, although in other specimens the position of the posterior nares fluctuates from between the M2 hypocones to slightly behind the anterior margin of M3. In AMNH 1570 two thin unbroken choanal pouches (maxilloturbinates) protrude beyond the anterior rim of the posterior nares. These choanal pouches are similar in position to those of Metarhinus abbotti , and they are not as posteriorly shifted as those of Dolichorhinus . There are clear remnants of bony choanal pouches in others specimens of T. validus as well (e.g., AMNH, 12678, AMNH 1569; UCMP 6700).

The posterior narial canal is elongate, but it does not extend significantly into the basisphenoid. As in other brontotheres, a thin vomerine septum bisects the posterior narial canal, although it is usually obscured by sediment filling the posterior narial canal, or it was destroyed when the sediment filling the posterior narial canal was removed. Although there are no large ventral sphenoidal fossae in Telmatherium validus , in at least one specimen (USNM 6700), the posterior narial canal extends slightly into the sphenoid, and the anterior end of the sphenoid body narrows anteriorly and forms part of the part of the partitioning septum, along with the vomer. The basicranial foramina of T. validus are typical of brontotheres, with a widely separated foramen ovale and foramen lacerum. The mastoid process does not contact the postglenoid process and the external auditory pseudomeatus is, therefore, unconstricted ventrally.

UPPER DENTITION: The description of the upper dentition of Telmatherium validus is primarily based on the lectotype (YPM 11120) (figs. 38, 44), but other specimens are noted to document variation. The lectotype includes a complete set of lightly worn upper teeth indicating an unreduced dental formula (3-1-4-3). The incisors are large and increase in size laterally. The crowns of I1 and I2 are short, round in outline, pointed at the apex, and slightly lingually curved. Each incisor is bordered lingually by a distinct cingulum. The I3 crown is taller but otherwise similar to I1 and I2. The incisor row strongly arches anterior to the canines and it is separated from the canines by a short precanine diastema.

The lectotype canine is large and tall with two distinct ridges that extend down the sides of the crown. The canines of Telmatherium validus are variable in size, but they could never be characterized as being very small.

Fig. 44. The lectotype of Telmatherium validus (YPM 11120) (A) Occlusal view of left premolars, (B) lateral view of left premolars, (C) occlusal view of left molars, (D) occlusal view of incisors and canine, (E), labial view of left I2, I3, C. (º 2005 Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. All rights reserved.)

The length of the postcanine diastema is very short in YMP 11120, but in other specimens it can be slightly longer than the P2.

The P1 is small and simple with a single cusp, a posterior heel, and a thin but distinct lingual cingulum. A very short P1–P2 diastema is not seen in the lectotype. In the remaining skulls a P1–P2 diastema is most often absent, but it is occasionally present (e.g., AMNH 1569). The anterior margin of the P2 of YPM 11120 is steeply angled posterolingually. There is a marked discontinuity in shape from the rather oblique P2 crown to the more rectangular crowns of P3 and P4.

The parastyle of P2 arches slightly lingually, while the parastyle of P3 is nearly straight, and the P4 parastyle is angled labially. The P2 metastyle is slightly angled lingually, while those of P3 and P4 are more or less straight. The labial ribs of the paracones are prominent, although they follow the typical pattern of becoming narrower in progressively posterior premolars.

The lingual heel of the P2 is small and posteriorly shifted. A small protocone is seen on the lingual shelf. In the lectotype the small preprotocrista is angled anteriorly and forms part of the anterior cingulum. The lingual heels of P3 and P4 are much wider so that the anterior and posterior sides of the crowns are nearly parallel and the protocones are more centrally positioned between the paracone and metacone. Tiny paraconules can be seen on the P3 and P4 of the lectotype, although they are most often absent in other specimens. More typically, a very low preprotocrista connects the protocone to the lingual side of the ectoloph of P3 and P4 (e.g., YPM PU10027, AMNH 13165).

The premolars (P2–P4) of the lectotype lack distinct lingual crests ; however, on other specimens, such as YPM PU10027, there is a minor lingual crest on P3 and a barely discernable lingual crest on P4. Occasionally, P2 lacks a distinct protocone; instead, a tall crest of enamel arches around the lingual side of the crown (e.g., YPM 47288 ). In one specimen ( YPM PU10027 ) P2 has a small hypocone ; however, a hypocone does not occur on the premolars of any other specimen of Telmatherium validus .

The labial cingula of the P2–P4 are thin and incomplete. In the lectotype the lingual cingulum of P2 is continuous around the base of the protocone. The anterior and posterior cingula of the P3 and P4 arch around the lingual side of the crown, but they are not connected lingually. However, in other specimens continuous cingula wrap around the anterior, lingual, and posterior sides of the crowns of P3 and P4.

The molars of Telmatherium validus exhibit the derived characteristics typical of brontotheriines including tall, lingually angled ectolophs, thin labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone. The anterior cingulum is thin and passes proximally to the distal peak of the parastyle. T. validus molars lack an anterolingual cingula cusp or central molar fossae. All traces of a metaloph are lost, although each of the molars of the lectotype retains a tiny paraconule. In other specimens tiny paraconules are variably present. In YPM PU10027, for instance, a paraconule is present on the M1, but this feature is not detectible in the M2. Other specimens lack distinct paraconules, but they have a very low preprotocrista-like crest at the lingual base of the paracone (e.g., AMNH 2353). Yet others show no evidence of paraconules or preprotocristae (e.g., AMNH 12194), but this is the least frequent condition. None of the specimens of T. validus has an M3 hypocone, although the posterolingual cingulum of the M3 is rather thick and raised in the lectotype. The labial cingula of the molars tend to be thin and discontinuous around the mesostyles. The anterior cingulum of each molar terminates at the lingual base of the protocone and does not continue around the lingual side of the crown.

MANDIBLE AND LOWER DENTITION: A number of Telmatherium validus skulls are associated with mandibles: AMNH 1587, AMNH 2353, AMNH 12204, and YPM PU10027. There are additional mandibles without associated skulls from the upper Bridger C–D and the lower Washakie formations whose molars are far too slender for Palaeosyops , fall well above the upper size range of Mesatirhinus junius , and are within the size range of T. validus . The most complete mandibles that are referred to T. validus are AMNH 1560, AMNH 1566 (fig. 45b, d), FMNH PM2328 (fig. 45a), FMNH PM55576 (fig. 45c), and USNM 26121. The general shape of these mandibles does not differ notably from the contemporaneous taxon, M. junius . The inferior margin of the symphysis is typically steep ($ 45 °). The symphysis extends to a point between the talonid of the p2 (e.g., AMNH 1560) and the trigonid of the p3 (e.g., AMNH 1566).

The lower dentition of T. validus is not differentiated from the contemporaneous species, M. junius in any way other than size. FMNH PM55576 includes a complete set of lower incisors. The incisors are large, positioned closely together, and form a row that arches anterior to the canines. The apex of i1 is worn off. The apex of i2 is lightly worn but the crown is still rather pointed. Finally, the i3 is essentially unworn and has a pointed apex. All of the incisors are roughly of the same size; however the i3 is more mesiodistally elongate than the other incisors. Each incisor has a thin but distinct lingual cingulid. A precanine diastema is generally absent. The postcanine diastema is generally about the same length as p2 or slightly shorter. The p1– p2 diastema is typically very short (e.g., AMNH 1566) or absent (e.g., FMNH PM2328); however one mandible (AMNH 1563) has a p1–p2 diastema that exceeds the anteroposterior length of the p2.

The p1 is a small and simple tooth with a single cusp and a short talonid heel. (Note that the posterior end of the p1 is broken in AMNH 1566 [fig. 45d]). The p2 trigonid is nearly twice the length of the talonid. The p3 trigonid is also longer than the talonid, while the p4 trigonid is slightly shorter than the talonid. The p2 trigonid and talonid are of similar width. The p3 trigonid is about the same width as the talonid or slightly narrow- er. The p4 talonid is slightly wider than the trigonid. The paralophids of p2 and p3 are slightly angled lingually, creating a very small lingual notch in the trigonids. The protolophids of the p2 and p3 are essentially straight. However, in comparison to that of p2, the p3 protolophid is positioned more lingually with respect to the protoconid. Both p2 and p3 lack metaconids. The trigonid of p4 is significantly more molariform. The paralophid arches fully lingually. The p4 protolophid arches 90 ° lingually and is connected to a large, lingually positioned metaconid. The lingual orientation of the p4 paralophid and protolophid results in a very broad lingual trigonid notch. The talonid of the p2 is narrow with a weakly developed cristid obliqua and a very short hypolophid. The lingual side of the p2 trigonid forms a slightly concave sloped surface. The p3 and p4 have more developed cristids obliqua and longer hypolophids. The lingual-talonid notches of p3 and p4 are broader than that of p2, although they do not quite form molariform basins. Lingual premolar cingulids are absent, while the labial premolar cingulids tend to be either weak or absent.

The lower molars of Telmatherium validus are typical with relatively thin enamel, shallow talonid and trigonid basins, and an elongate m3. Labial molar cingulids are generally weak while lingual molar cingulids are absent.

REMARKS

The taxonomic history of Telmatherium validus and its various synonyms is remarkably convoluted. Marsh (1872) named T. validus from ‘‘the greater portion of a skull with teeth, and portions of several other skeletons’’ that were collected by the Yale expedition of 1871 to the Green River Basin. Later, Marsh (1880) amended the genus name to Telmatotherium and this spelling was adopted by numerous authors (e.g., Earle, 1892; Hatcher, 1895; Osborn, 1895), although Osborn (1929a) eventually reverted to the original spelling, Telmatherium .

Marsh (1872) did not specify which of the specimens was to be the holotype of Telmatherium validus , although his description depended heavily on a partial skull. Osborn (1929a) considered the holotype to be YPM 11120. No other specimen in the Yale Peabody Museum matches the description and measurements provided by Marsh (1872). Therefore, Osborn’s assignment of YPM 11120 as the holotype specimen is undoubtedly accurate. However, the specimen is a lectotype because ‘‘portions of several other skeletons’’ were initially reported along with YPM 11120.

Osborn et al. (1878) named Leurocephalus cultridens from a partial skull and mandible from the Bridger Basin. No number was given to the specimen, but their figure of L. cultridens clearly matches YPM PU10027. Earle (1891, 1892) subsequently reassigned this species to the genus Telmatherium , but he continued to recognize it is as distinct from the type species, T. validus .

Eventually, Earle (1892) and Osborn (1895) began to use the genus Telmatherium as a dumping ground for middle Eocene brontotheres. Several species that eventually were assigned to other genera (e.g., Metatelmatherium , Dolichorhinus , Mesatirhinus ) were grouped into Telmatherium at one time or another. Among these was Palaeosyops vallidens Cope (1872) , a taxon that was reassigned to Telmatherium by Osborn (1895). Telmatherium vallidens (Cope) (not to be confused with Telmatherium validus Marsh ) is represented by a partial mandible (AMNH 5098, a lectotype assigned by Osborn [1929a]). This species is presently considered a nomen dubium. However, in 1895, Osborn considered Telmatherium vallidens (Cope) to be valid and went so far as to conjecturally refer two additional skulls to this species (AMNH 1569 and AMNH 1570).

Osborn’s (1895) referral of these two skulls to Telmatherium vallidens (Cope) was a grave mistake because it initiated a string of taxonomic blunders that was continued by Hatcher, Osborn, Matthew, and Hay, thus generating a confusing series of invalid taxonomic revisions related to these two skulls. Going on a recommendation made by J. L. Wortman, Hatcher (1895) reassigned T. vallidens (Cope) to a new genus, Manteoceras ; however, Hatcher’s diagnosis of Manteoceras vallidens (Cope) does not actually refer to the lectotype jaw (AMNH 5098). Instead, Hatcher’s (1895) diagnosis is exclusively made up of cranial characters, and his description and figure ( Hatcher, 1895: pl. 39: fig. 2) of Manteoceras vallidens were clearly based on the skulls (AMNH 1569 and AMNH 1570) that Osborn had conjecturally referred to that species.

Another species was reported by Matthew (1899), Palaeosyops manteoceras Osborn ex ms., although no holotype was reported. Hay (1901) designated a holotype for this species, AMNH 1569, one of the skulls that had been conjecturally referred to Manteoceras vallidens by Osborn (1895). Hay (1901) also reassigned Palaeosyops manteoceras to Hatcher’s genus Manteoceras . A third species, Manteoceras washakiensis , was eventually named by Osborn ( Osborn, 1908a) with yet another skull (AMNH 13165).

Ultimately, in contrast to his opinion of 1895, Osborn (1929a) concluded that the skulls he had referred to Manteoceras vallidens (Cope) (AMNH 1569 and AMNH 1570) did not really belong to that species. He removed those specimens from M. vallidens (Cope) , reassigned it to the genus Dolichorhinus , and redefined Dolichorhinus vallidens (Cope) based only on the lectotype jaw (AMNH 5098).

At that point, it would have been more reasonable of Osborn to consider Manteoceras the senior synonym of Dolichorhinus . However, because Hatcher’s (1895) earlier diagnosis of Manteoceras was based entirely on skulls (AMNH 1569 and AMNH 1570) rather than the lectotype jaw (AMNH 5098) of Manteoceras vallidens (Cope) , Osborn (1929a) rationalized that Manteoceras should continue to be applied to the species to which these skulls were now thought to have belonged, that is, Manteoceras manteoceras Hay.

Regardless of how Manteoceras was ultimately used by Osborn (1929a), that genus name is invalid because Hatcher’s (1895) original diagnosis of Manteoceras is based on an invalid species concept, Manteoceras vallidens sensu Osborn (1895) (i.e., it was based on two skulls that were incorrectly referred to the type species of Manteoceras ). Despite the confusing series of taxonomic errors associated with these skulls, it was never recognized by any of the involved authors that AMNH 1569 and AMNH 1570 actually belong to Telmatherium validus Marsh (1872) .

Three previously named species can be considered junior synonyms of Telmatherium validus Marsh. These include Telmatherium (Leurocephalus) cultridens ( Osborn et al., 1878) , Manteoceras manteoceras Hay , and M. washakiensis Osborn. Osborn (1929a) believed Manteoceras and Telmatherium to be two distinct ‘‘lineages’’. However, most of the differences noted by Osborn (1929a) between the Telmatherium and Manteoceras ‘‘lineages’’ were are actually based on comparisons of specimens belonging to Telmatherium validus (but assigned by Osborn to M. manteoceras and M. washakiensis ) with another species altogether, T. ultimum Osborn (1908a) , a species that Osborn had believed was the terminal member of the Telmatherium ‘‘lineage’’. Telmatherium ultimum (described below) is clearly distinct from T. validus or any of its synonyms, including T. cultridens , M. manteoceras , and M. washakiensis ; it was finally removed from Telmatherium by Granger and Gregory (1943) and assigned to Metatelmatherium . Excluding observations involving Metatelmatherium ultimum , the actual differences noted by Osborn (1929a) between Manteoceras and Telmatherium are minor and can easily be attributed to dental wear, damage, and/or intraspecific variation. The similarities between Manteoceras and Telmatherium (sans Metatelmatherium ultimum ) were attributed by Osborn (1929a) to parallel evolution, although without justification. Mader (1989) concluded that Manteoceras was synonymous with Telmatherium . In a later revision, Mader (1998) considered T. cultridens , M. manteoceras , and M. washakiensis to be junior synonyms of T. validus Marsh ; these revisions are all upheld here, although justification of these revisions (none were provided by Mader, 1998) are needed.

Telmatherium cultridens was thought by Osborn (1929a) to represent a less ‘‘progressive’’ stage of Telmatherium , although the minor differences in size and in the premolar morphology of its holotype (YPM PU10027) could just as easily represent individual variation. The holotype of Manteoceras manteoceras (AMNH 1569) is a skull that lacks teeth. It is therefore not possible to directly compare it to the lectotype of T. validus (YPM 11120), which mostly consists of upper teeth. However, numerous skulls similar to that of AMNH 1569 have teeth that are consistent in size and morphology with those of the lectotype of T. validus . The holotype of M. washakiensis (AMNH 13165) is one of the more gracile skulls among this group, but nonetheless one that is consistent cranially and dentally with other specimens of T. validus .

Finally, Telmatherium validus Marsh can be considered valid for the following reasons. (1) The dentition of the lectotype of T. validus (YPM 11120) is similar to that of Mesatirhinus junius (a contemporaneous Bridgerian species), but it is larger than the upper size limit of M. junius . (2) Additionally, its P2 is somewhat different from that of Mesatirhinus junius , with a less lingually positioned metacone. (3) The lectotype is morphologically consistent with and falls within the size range of a group of more complete specimens (including the holotype skulls of Manteoceras manteoceras and Manteoceras washakiensis ) from the upper Bridger (C–D) and lower part of the Washakie Basin (Lower Adobe Town Member of McCarroll et al., 1996b) that represents a species of hornless brontothere. (3) Telmatherium validus is most similar to Wickia brevirhinus and Metatelmatherium ultimum , although it can be distinguished from both by a deeper nasal incision; additionally, the splint of nasal bone that separates the triangular frontal process from the maxilla on the surface of the skull is larger and more prominent in T. validus .