Metatitan khaitshinus ( Yanovskaya, 1954 )

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 : 220-243

Metatitan khaitshinus ( Yanovskaya, 1954)

HOLOTYPE: PIN 3745-1, a skull with a large block of plaster obscuring its dorsal surface, with right and left P1–M3.

TYPE LOCALITY: Khaichin Formation, Khaichin-Ula V, Southwestern Mongolia.

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

SYNONYMS: Metatitan reshetovi ( Yanovskaya, 1980) .

REFERRED SPECIMENS: (From the Khaichin Formation, Khaichin-Ula V, Southwestern Mongolia) PIN 3745-2, a mandible with right i2 (?), c–m3 and left i2–m3; PIN 3745-3, a skull with right P2–M3 and left C–M3; PIN 3754-4, a skull with right P1–P3, P4 (partial), M2–M3 (partial) and left P1–M2; PIN 3745- 8, a partial edentulous skull; PIN 3745-9, a partial edentulous skull; PIN 3745-11 (holotype of Metatitan reshetovi ), a skull missing the nasal process with right C, P2–M3 and left C, P2–M3; PIN 3745-12, the ventral surface of a skull with right and left P2–M3; PIN 3745-28, a mandible with right i2–m3, left i3–c, and p2–m3; PIN 3745-31, a mandible with right p2–m3, left c, and p2– m3.

The specimens listed above are those that I was able to relocate in the PIN collection. Other specimens including two more partial skulls (PIN 3745-5, 6), and many mandibles (PIN 3745 14–17, 19, 22, 23, 26, 27, 30) were referred to M. khaitshinus by Yanovskaya (1980) but could not be located in the PIN collection.

DIAGNOSIS: Metatitan khaitshinus is a large brontothere with small frontonasal horns that are elevated high above the orbits. The nasal process and horns are elevated to the peak of a tall frontonasal process that rises anterodorsally from above the orbit at an angle greater than 45 °. The nasal incision is dorsoventrally deep and its posterior margin extends posteriorly to the anterior margin of M2. The orbit is positioned over the posterior portion of M2 and the anterior portion of M3. The elevated nasal process is horizontal, relatively broad, not strongly rounded distally, and with lateral walls that are deeper proximally and shallower distally. The premaxillomaxillary rostrum deepens posteriorly and it is not enclosed by bone dorsally. The cranium is incompletely saddle-shaped. The posterior end of the cranium is extremely widened. The parasagittal ridges are prominent, but they do not constrict the dorsal surface posteriorly. The zygomatic blades are nearly straight and they extend nearly to the posteriormost end of the skull where they form a 90 ° angle with the lateral zygomatic wing of the squamosal. The external auditory pseudomeatus enters the skull in a mediolateral direction and it is ventrally constricted. A broad postzygomatic process is present. The emargination of the posterior nares is wide and the anterior margin of the posterior nares is positioned between the M3s. Large ventral sphenoidal fossae are present.

Metatitan khaitshinus has a complex P1 and a distinct P2 metacone. Premolar hypocones range from poorly developed to well developed and completely separated from the protocone. Upper and lower postcanine diastemata are absent. 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 is absent. Distinct central molar fossae and small anterolingual cingular cusps are present. Paraconules and metalophs are absent. The mandible has a robust symphysis that extends to p4. The lower dentition of M. khaitshinus includes three moderately sized incisors that form a slightly arched incisor row. The i1 and i2 are semispatulate, while the i3 is somewhat more conical. Metaconids are present on p3 and p4, but not on p2. The p2 trigonid is not much longer than the talonid. The lower molars have shallow basins and m3 is elongate.

Metatitan khaitshinus shares with M. relictus and M. primus the combination of the following traits: horns and a nasal process elevated to the peak of a superorbital frontonasal process, posteriorly positioned orbits, an extremely widened skull, anteroposteriorly shortened basicranium, and lack of postcanine diastemata. However, M. khaitshinus differs from M. relictus and M. primus in having more anteriorly positioned posterior nares, larger lower incisors forming an arched row, and a broad labial notch on p2. M. khaitshinus has a more vertical occiput than M. relictus . It has a less constricted face than M. primus and its lacks the conspicuously bulbous cranium of that species.

DESCRIPTION

SKULL: The following description of the skull of Metatitan khaitshinus refers primarily to the holotype (PIN 3745-1) but additional observations on other specimens are noted. The holotype of M. khaitshinus (PIN 3745-1) is a nearly complete skull (figs. 106 and 107). A large block of plaster is presently fixed to the dorsal surface of the skull, suggesting that it is incomplete or damaged. However, the specimen is not significantly distorted or damaged in any other way. This species is also known from several other reasonably complete specimens; two of them are figured here. PIN 3745-3 is a complete skull that is slightly crushed dorsoventrally (fig. 108). The anterior rim of the orbit is broken and the dorsal portion is displaced medially and ventrally with respect to the ventral portion of the anterior margin; consequently the orbit has become ovalized in contrast to the very rounded orbit of the holotype. Another skull, PIN 3745-11 (fig. 109), is dentally immature with an unerupted M3 crown. This skull is undistorted, but its nasal process and horns are unpreserved. This skull differs from the others in several ways although the differences are probably related to its subadult age.

The frontonasal region of Metatitan khaitshinus resembles M. primus and M. relictus in having closely positioned horns that are elevated above the orbits along with the nasal process. The horns and nasal process are elevated to a degree similar to that seen in M. primus , but to a degree that is greater than

(C) ventral view.

that of M. relictus . The horns of the holotype just barely protrude out from under the front edge of the plaster covering and are positioned over the nasal incision. In PIN 3745-3 the horns and nasal process are not as highly elevated, but the difference is related to the dorsoventrally crushed condition of that specimen. The horns of that specimen form small, rounded dorsolaterally projecting knobs. The contact of the frontal and nasal bone can be clearly discerned on the left horn of PIN 3745-3. The frontal bone forms the posterodorsal surface of the frontonasal process and extends to the peak of the horn. The nasals bone forms the anteroventral portion of the frontonasal process and makes up the base and anterior portion of the horn.

The nasal cavity of Metatitan khaitshinus is voluminous due to the elevated nasal process. In the uncrushed holotype the posterior margin of the nasal incision rises much higher than the orbits. The posterior margin of the nasal incision extends to a point above the M1. Like other species of Metatitan the orbit is positioned rather posteriorly. The posterior part of M2 and the anterior part of M3 are directly beneath the orbit with the anterolateral root of M2 positioned beneath the anterior rim of the orbit. The nasal process is much shorter than the premaxillomaxillary rostrum and it projects from the skull in a horizontal direction. The lateral walls of the nasal process are unthickened and relatively shallow although they slightly deepen proximally. The width of the nasal process tapers slightly distally. The anterior margin of the nasal process is not strongly rounded because of a deep median notch.

The premaxillomaxillary rostrum is wide; it deepens posteriorly and its dorsal margin is sloped posterodorsally. A premaxillomaxillary suture is not visible on any of the specimens. The dorsolateral margins of the rostrum diverge laterally behind the premaxillary symphysis. The rostral cavity is open to the nasal cavity and not dorsally enclosed by bone.

The skull of Metatitan khaitshinus is similar to other species of Metatitan in its exceptional width. The postorbital portion of the cranium most closely resembles M. relictus and does not take on the extremely swollen appearance seen in the cranium of M. primus . While most horned brontotheres have saddle-shaped skulls, the crania of M. relictus and M. primus are incompletely saddle-shaped. However, this character is difficult to determine for M. khaitshinus . The dorsal surface of the holotype is obscured. The dorsal surface of PIN 3745-3 is concave from the horns to the occiput although this may be an artifact of crushing. However, several other specimens with less distorted crania, such as PIN 3745-4, PIN 3745-8, PIN 3745-9, and PIN 3745-11, indicate an incompletely saddle-shaped skull with a deep convexity in the center of the skull, but with a flat or more concave surface

(PIN 3745-11).

over the posterior portion of the cranium. The central parietal dome seen in M. primus is distinctly absent in skulls of M. khaitshinus .

The parasagittal ridges of M. khaitshinus are prominent. In the holotype (PIN 3745-1) they can be seen originating from the postorbital process of the frontal and sloping in a posterodorsal direction toward the occiput. From the dorsal view of PIN 3745- 3 the parasagittal ridges appear to strongly overhang the sides of the skull. The parasagittal ridges are widely separated and they do not constrict the posterodorsal surface of the skull.

From lateral views of the skulls of Metatitan khaitshinus the zygomatic blade is straight, although it is deflected strongly upward at the posterior end where there is a tall posterior zygomatic process similar to that of M. relictus . From dorsal views of the skulls the zygomatic blades are thin, straight, and they diverge strongly posterolaterally, thus creating a strongly wedge-shaped skull. Like other species of Metatitan , the zygomatics extend nearly to the posterior end of the cranium, and the lateral wings of the squamosals are positioned at the posteriormost end of the skull and form an abrupt, nearly 90 ° angle with the zygomatic blades.

The nuchal crest of the holotype is obscured by plaster. From a dorsal view of PIN 3745-3 the nuchal crest is strongly concave. From a posterior view the dorsal margin of the occiput is arched dorsally. The occiput itself is extremely broad and like M. primus it is vertical. The dorsal half of the occiput is similar in width to the ventral half and the occiput is not strongly constricted in the middle. The surface of the occiput has narrow occipital pillars with a shallow triangular depression between them.

The posterior nares of the holotype are partially damaged while those of PIN 3745-3 are intact. However, in both specimens the posterior nares are constricted by a wide horseshoe-shaped emargination similar to that of Metatitan relictus . However, the posterior nares are more anteriorly situated than those of M. relictus ; the anterior margin of the posterior nares of M. khaitshinus is consistently positioned roughly between the protocones of M3 while the posterior nares of M. relictus are entirely behind M3. This is one of the major features distinguishing these two species. Other aspects of the ventral surface of M. khaitshinus closely resemble M. relictus . The posterior narial canal is elongate and extends well into the sphenoid bone where it broadens notably, forming a round- ed depression. The elongate vomer is not preserved in the holotype, but remnants of it are seen in PIN 3745-3 bisecting the elongate posterior narial canal and ventral sphenoidal fossae. In PIN 3745-3 the left sphenoidal fossa is clear of sediment while the right sphenoidal fossa is still filled with matrix. The basicranium is very wide and anteroposteriorly compressed. The width of the skull, measured from the left and right mastoid processes, is much greater than the width across the M3s. Ratios calculated from the width of the basicrania of PIN 3745-1 and PIN 3745-3 divided by the width across the M3s yield values (1.73 and 1.47, respectively) that are similar to those calculated for M. relictus . The external auditory pseudomeatus is tube-shaped, enters the skull in a mediolateral direction, and it is positioned at the posteriormost end of the cranium.

UPPER DENTITION: The upper incisors of Metatitan khaitshinus are unknown. The incisor alveolar border of PIN 3745-1 suggests a broad and somewhat arched incisor row (fig. 106) This interpretation agrees with the lower incisors (see below). The juvenile skull, PIN 3745-11, has a pair of small canines (fig. 109). Another specimen, PIN 3745-12 (not shown), has a pair of similarly sized canines that are very heavily worn. A postcanine diastema is not present in any specimen. The short postcanine gap in PIN 3745-11 is the result of P1 having fallen out.

The premolars of Protitan khaitshinus show a remarkable amount of intraspecific variability that is, nonetheless, a pattern typical of brontothere species. The labial aspects of the premolars are morphologically fairly stable, while the lingual sides are morphologically variable. To demonstrate this pattern of intraspecific variability close-ups of the premolars of the holotype (PIN 3745-1) (fig. 110a) and three other specimens (PIN 3745-3, PIN 3745-11, PIN 3745-12) (fig. 110b–d) are shown.

The P1 of the holotype is heavily worn. PIN 3745-3 has a less worn P1 and has a morphology that is typical of many advanced brontotheres. The crown is rounded in outline with a distinct paracone, a metacone of roughly similar size, and a small lingual heel with a small lingual crest. P2–P4 have parallel anterior and posterior sides. P2 is slightly oblique in outline, while P3–P4 are more nearly rectangular. The parastyle of P2 is straight, but those of P3 and P4 are deflected anterolabially. The metastyles of P2–P4 are nearly straight. A broad labial paracone rib can be seen on the P2. The labial paracone ribs of P3 and P4 are much narrower and progressively smaller. Mesostyles are absent on all premolars.

As noted above, the lingual heels of the P2–P4 are variable. The holotype specimen is the least molariform, while the premolars of the remaining specimens are more molariform. The P2 of PIN 3745-1 has only one distinct lingual cusp, consisting of a large central ovoid cusp with a very short lingual crest at its peak. A small preprotocrista arches anterolabially from the peak of the lingual cusp, but it does not reach the paracone. The P2 of PIN 3745-3 is similar, but it appears to have two lingual cusps that are strongly fused together. PIN 3745-11 has two well-separated lingual cusps that are connected by a small lingual crest. The lingual crest continues posterior to the apex of the hypocone and ends at the posterior cingulum. Finally, the P2 of PIN 3745-12 has two equally sized and fully separated lingual cusps.

The P3 shows a similar degree of variability in relative molarization. The least molarized P3s have partially separated protocones and hypocones (PIN 3745-1, PIN 3745-3). Other specimens have two fully separated lingual cusps of similar size. One of these, PIN 3745-11 has a small lingual crest, while the other, PIN 3745-12, does not. The lingual heel of the P4 shows a lesser amount of variation and a lesser degree of overall molarization. Three of the figured upper premolar rows have a single lingual cusp without a preprotocrista or lingual crest on the P4. Only one of these specimens (PIN 3745-11) has a small lingual crest. The posterior portion of this crest thickens into an elongate hypoconelike structure. Labial premolar cingula are distinct but thin. The lingual premolar cingula are thick and continuous around the lingual sides of the crowns.

PIN 3745-11 (fig. 110e) has the least worn and best-preserved molars available for Metatitan khaitshinus . Other than the fact that they are less worn, they do not differ in any notable way from those of the holotype or other specimens belonging to this species. The upper molars are elongate. The molars of other specimens tend to be shorter, but this is primarily related to wear. Typical brontotheriine apomorphies are present, 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, shallow central molar fossae and small anterolingual cingular cusps are present. There is no evidence of paraconules or metalophs. There is no hypocone on the M3 although the distolingual cingulum of the M3 is thickened and raised. Labial molar cingula are weak and lingual molar cingula are absent or extremely weak.

MANDIBLE AND LOWER DENTITION: Yanovskaya (1980) referred numerous mandibles to Metatitan khaitshinus although I was able to locate only one of these, PIN 3745-31 (fig. 111). This specimen is nearly complete but it lacks the condyles. The angle of the ventral margin of the symphysis is near 45 °. The symphysis is broad and extends to the P4. The dental formula is unreduced (3-1-4-3).

The incisors are of moderate size and they form a slight arch anterior to the canines. This differs from other species of Metatitan in which the incisors form a straighter row and are reduced to a very small and essentially vestigial condition. The lower incisors of M. khaitshinus more closely resemble those of Brachydiastematherium transylvanicum . In general the lower incisors have a semispatulate morphology. The apex of the i1 is worn flat. The i2 and i3 have arched apices. The i2 is the largest incisor. Each lower incisor has a distinct lingual cingulid. Labial cingulids are absent. There are no diastemata between the incisors or canines. The postcanine gap seen in PIN 3745-31 is the space for the missing p1, for which signs of the alveolus are still present. A genuine postcanine diastema (gap between canine and p1) is not present.

The remaining premolars (p2–p4) show a degree of molarization similar to other species of Metatitan and Brachydiastematherium transylvanicum , although some distinctions can be found and are noted below. The trigonids of p2–p4 are similar in length and width to their talonids. The paralophid of p2 arches strongly lingually generating a small lingual trigonid notch. The protolophid is straight, but it is angled in a posterolingual direction. There is no p2 metaconid although it might have been variably present as in M. relictus . The labial notch formed by the p2 protolophid and cristid obliqua is broad, shallow, and points lingually. This trait resembles B. transylvanicum but differs from both M. relictus and M. primus in which the labial p2 notch forms a deep narrow posterolingually angled groove. The right p3 that is shown in close up (fig. 111c) is damaged. However, the left p3 (fig. 111b) has a fully lingually arched paralophid and protolophid, creating a nearly molariform trigonid basin. The trigonid of the p4 is equally molariform. Both p3 and p4 have large lingually positioned metaconids. The talonids of p2–p4 are well developed with long hypolophids, long cristids obliqua, and broad talonid basins. The p4 of M. khaitshinus lacks the unusual crest of enamel extending posteriorly from the middle of the cristid oblique that is seen in B. transylvanicum . Labial premolar cingulids are weak and lingual premolar cingulids are absent.

The molars of Metatitan khaitshinus are typical with shallow trigonid and talonid basins and thin lingual enamel. The m3 is very elongate. Labial molar cingulids are distinct. Lingual molar cingulids are absent. The m3 cingulid does not wrap completely around the hypoconulid.

REMARKS

Three diagnosable brontothere species can be assigned to Metatitan : M. primus Granger and Gregory (1943) (the type species), M. relictus Granger and Gregory (1943) , and M. khaitshinus ( Yanovskaya, 1980) . Granger and Gregory (1943) erected Metatitan for a group of brontotheres characterized by extremely widened skulls, a short basioccipital, small frontonasal horns, small incisors, no postcanine diastema, and relatively advanced premolars. Granger and Gregory (1943) considered these species to be derived from Rhinotitan and to have bridged the morphological gap between the bizarre battering ram of Embolotherium and the more typical paired frontonasal horns of most other horned brontotheres. The type species, Metatitan primus , ‘‘points the way to Embolotherium , since its horns are displaced forward in front of the orbit and joined by a transverse connecting crest’’ ( Granger and Gregory, 1943: 367). Granger and Gregory (1943) seem to have been referring to the fact that the frontal and nasal bone forms a single large process that rises above the orbits at a steep angle. This was thought to be the precursor of the ram of Embolotherium , a hypothesis that is further supported by the phylogenetic analysis of Asian horned brontotheres in Mihlbachler et al. (2004a). However, unlike Embolotherium , the nasal processes are retained in Metatitan and they are elevated to the peak of the frontonasal pillar, whereas in Embolotherium they are lost. Aktautitan hippopotamopus , a taxon not known to Granger and Gregory (1943), shares the Metatitan -like frontonasal morphology, but differs from Metatitan primarily in having large incisors, a postcanine diastema, less molarized premolars, and a more saddle-shaped cranium.

In addition to the type species, Metatitan primus, Granger and Gregory (1943) assigned two other species to Metatitan , M. relictus and M. progressus . Both of these species are valid, although the latter species is now placed in a new genus, Nasamplus . Most of the brontothere material collected in the Camp Margetts area during the 1930 Central Asiatic Expedition of the American Museum of Natural History represents M. relictus . M. relictus can be distinguished from M. primus by its less swollen cranium, more prominent parasagittal ridges, and deeper nasal incision. It is possible that that premolar hypocones occur less frequently in M. relictus , but this cannot be confirmed without additional specimens of M. primus .

Additional Metatitan fossils from Mongolia were described by Yanovskaya (1980) although she misidentified much of the material. Yanovskaya (1980) erroneously referred a skull, mandible, and numerous postcranial elements from the Ergilin Dzo of Inner Mongolia to M. relictus . Mihlbachler et al. (2004a) recognized that this material was not Metatitan and found it to be more similar to Parabrontops gobiensis . In this revision, that material is assigned to Eubrontotherium clarnoensis .

In the same monograph, Yanovskaya (1980) described numerous Metatitan fossils but misidentified them as Protitan . Yanovskaya (1980) erected two species based on a large collection of brontothere material from the Khaichin Formation of southwestern Mongolia; P. khaitshinus ( Yanovskaya, 1980) was based on a complete skull (PIN 3745-1) (fig. 106) while P. reshetovi was based on a subadult skull (PIN 3745-11) (fig. 109). Mihlbachler et al. (2004a) first demonstrated the close similarity of these species with Metatitan , and they considered Yanovskya’s Protitan synonymous with Metatitan . Working from descriptions and figures published by Yanovskaya (1980), Mihlbachler et al. (2004a) preliminarily suggested that (1) P. khaitshinus is a junior synonym of M. relictus , (2) that P. reshetovi is possibly a valid species, and that (3) Brachydiastematherium transylvanicum (the only brontothere species named from Europe) might actually be synonymous with a species of Metatitan .

After directly examining the material in the PIN collection these conclusions must be modified. Based on more substantial observations taken directly from the specimens, M. khaitshinus can be differentiated from both M. relictus and M. primus and thus constitutes a valid species. Although similar to M. relictus and M. primus , M. khaitshinus differs in substantial ways. M. khaitshinus has more anteriorly positioned posterior nares, a more arched lower incisor row, larger lower incisors, and it lacks the long narrow groove-like labial notch in the p2 that characterizes M. primus and M. relictus . M. khaitshinus appears to have a more vertical occiput than M. relictus . It has a less constricted face than M. primus and its lacks the autapomorphic dorsal dome of that species.

Secondly, Metatitan reshetovi cannot be differentiated from M. khaitshinus and is therefore a junior synonym of the later. Yanovskaya (1980) diagnosed M. reshetovi as having a narrower skull, narrower nasal, and more molarized premolars. However, the differences in relative skull width and nasal width between M. khaitshinus and the type of M. reshetovi (PIN 3745-11) are barely noticeable. For instance, Yanovskaya (1980) found only a 5 % difference in skull width (calculated as a ratio with skull length). Considering the possible effects of subtle distortion and the subadult age of PIN 3745- 11, these differences are of dubious taxonomic significance. Among the specimens attributed to M. khaitshinus and M. reshetovi there is conspicuous variation in the relative degree of premolar molarization. Specimens without premolar hypocones or poorly developed premolar hypocones were assigned to M. khaitshinus by Yanovskaya (1980), while others with more developed premolar hypocones were assigned to M. reshetovi . However, the variation in premolar molarization in these specimens is arguably continuous. Moreover, conspicuous intraspecific variation in lingual premolar morphology is a common pattern of intraspecific variation that is seen within many other brontothere species.

Thirdly, to the extent that they can be compared, M. khaitshinus is very similar to Brachydiastematherium transylvanicum from Europe. Currently B. transylvanicum is known only from its type specimen, a partial mandible that is almost identical to the mandible of M. khaitshinus . They are of similar size, both lack postcanine diastemata, and both differ from M. relictus and M. primus in the same ways: namely, each has larger, less globular incisors with lingual cingulids and each has a broader and more lingually pointed labial notch on the p2. B. transylvanicum and M. khaitshinus differ in only one detectable way: B. transylvanicum has an unusual crest extending posteriorly from the cristid oblique that is absent in M. khaitshinus . For this reason I consider these species to be distinct, but further discoveries may indicate that these species are synonymous.

Nasamplus progressus ( Granger and Gregory 1943) , new genus

HOLOTYPE: AMNH 26014 , a skull fragment including portions of the right orbit, frontal, nasal, maxilla, and P4–M1.

TYPE LOCALITY: Ulan Gochu , Jhama Obo , East Mesa, Shara Murun Region, Inner Mongolia, China. (Ulan Gochu [used in quotes] denotes uncertain correlation with the type Ulan Gochu beds at Baron Sog and refers to a faunal zone rather than a formation; see Radinsky, 1964.)

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

ETYMOLOGY: The genus name, Nasamplus , is a combination of the Latin terms nasus (‘‘nose’’) and amplus (‘‘large’’). This name refers to the elevated nasal process of this species, which results in a large nasal cavity.

DIAGNOSIS: Nasamplus progressus is a large brontothere in which the frontonasal horns are fused into a single transverse crest. The frontonasal crest and nasal process are elevated high above the orbits on a frontonasal process (ram). The nasal incision is very deep and extends posteriorly about to the anterior margin of M1. The orbit is positioned above the posterior portion of M1. The elevated nasal process is horizontal and with lateral walls that are deep proximally. The P4 of Metatitan primus has a distinct hypocone that is well separated from the paracone. A central fossa is present on M1.

The skull fragment representing Nasamplus progressus is morphologically intermediate between Aktautitan and Metatitan on the one hand and Protembolotherium and Embolotherium on the other. Nasamplus progresses resembles Aktautitan and Metatitan in having an elevated horizontal nasal process, but like Protembolotherium and Embolotherium the horns are fused into a transverse frontonasal crest at the peak of the frontonasal ram.

DESCRIPTION

SKULL AND UPPER DENTITION: The single fragmentary specimen of Nasamplus progressus (AMNH 26014) (fig. 112) indicates a large brontothere with a frontonasal morphology that is intermediate between the condition seen in Aktautitan and Metatitan , and the more derived condition of Protembolotherium and Embolotherium . As in Metatitan and Aktautitan the frontonasal protuberance is elevated high above the orbit, although it is more highly elevated in N. progressus . The angle of the superorbital frontonasal process upon which the frontonasal protuberance rests is steeper than 45 °. The frontonasal contact is visible from the lateral view on the upper half of the frontonasal process. The frontal bone forms the posterodorsal surface of the frontonasal process while the nasal bone forms the anteroventral surface. The frontonasal contact is even more distinct from the dorsal view of the cranial fragment. At the peak of the frontonasal process the posterolateral border of the nasal bone is bordered by a thick section of frontal bone. The line of frontonasal contact recedes posteromedially and at the midline the frontal bone is thinnest. The frontal bone does not form a pair of distinct protuberances. Instead, there is a single transverse crest formed at the peak of the frontonasal process that is formed by both the frontal and nasal bones. The surface of this crest is roughened and from the anterior view it is dorsally arched. The configuration of the frontal and nasal bones and the transverse frontonasal crest at the peak of the frontonasal process of Nasamplus progressus closely resembles the rams of Protembolotherium and Embolotherium .

However, unlike Protembolotherium and Embolotherium , Nasamplus progressus retains a large horizontal nasal process that, like Metatitan and Aktautitan , is elevated to the peak of the frontonasal process. Only the proximal end of the nasal process is preserved. The lateral walls of the proximal end of the nasal process are very deep.

The orbit is positioned above the posterior portion of M1 with the anterolateral root of M1 below the anterior orbital rim. Due to the elevated nasal process the nasal incision is very deep and it extends posteriorly about to the anterior margin of M1.

Only two teeth, P4 and M1, are preserved in the fragmentary holotype and these are heavily worn. P4 is essentially rectangular with a labially oriented parastyle and a straight metastyle. A labial paracone rib is not discernable. On the lingual side of the tooth, there is both a protocone and a smaller but well-developed hypocone. The hypocone is positioned on the very distal side of the crown and connected to the protocone by a minor lingual crest. The labial cingulum of P4 is very weak, and the lingual P4 cingulum is thick but slightly discontinuous. M1 is extremely worn and the ectoloph is not preserved. A small remnant of the central fossa is visible in the center of M1.

REMARKS

Granger and Gregory (1943) originally assigned Nasamplus progressus to Metatitan . Mihlbachler et al. (2004a) found that Metatitan progressus did not group into a monophyletic clade with other species of Metatitan , but that it grouped closer to Embolotherium . Unfortunately, no specimen other than the fragmentary holotype has ever been reported and although the holotype is only a very small cranial fragment, the right combination of features are preserved to determine that this specimen represents a distinct taxon that is essentially intermediate between Metatitan and Embolotherium . Therefore, this species is assigned a new genus name, Nasamplus .

This specimen led Granger and Gregory (1943) to conclude that the distinctive ‘‘battering ram’’ of Embolotherium was derived from a Metatitan -like morphology, involving the increasing height of the frontonasal process and the eventual loss of the nasal process. Nasamplus progressus seems to represent an intermediate stage in which the frontonasal process is increased in height and forms a battering ram–like crest at the peak but the horizontal nasal process has not yet been reduced or lost.

Protembolotherium efremovi Yanovskaya, 1954

HOLOTYPE: PIN 473-311, a skull lacking the nasal elements with right P2–M3 and left P3–M3.

TYPE LOCALITY: Ergilin Dzo (lower part), Dornogobi Province, Outer Mongolia

AGE: Late Eocene (Ulangochuian [Ergilian] land mammal ‘‘age’’).

REFERRED SPECIMENS: (From Ergilin Dzo [lower part, Outer Mongolia) PIN 473-310, a partial skull missing its left and ventral surfaces; PIN 3109-40, a partial skull missing its left and ventral surfaces.

DIAGNOSIS: Protembolotherium efremovi is a large brontothere that is very similar to Embolotherium . Its most distinctive features are a pair of small secondary horns positioned above the orbits, and a tall, nearly vertical process of bone (a ‘‘ram’’), probably composed of the frontal and nasal bones. The ram rises from above the orbits and arches backward. The ram tends to be shorter than those of Embolotherium andrewsi and E. grangeri . The peak of the ram is a transverse crest that forms the distal margin of the anteroventral nasal channel, sometimes with rugose swellings at the distolateral corners. The nasal incision extends posteriorly to the anterior margin of P4 and the orbits are positioned above the M2 and the posterior portion of M1. A horizontal nasal process can be absent or it persists in a diminished form as a flat triangular process that is elevated to the peak of the ram. The dorsal margin of the rostrum is strongly sloped so that the premaxillomaxillary rostrum deepens posteriorly. The rostrum is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, a robust occiput, and weakly curved zygomatic arches with prominent lateral swellings. The external auditory pseudomeatus enters the skull in a posteromedial direction and is ventrally constricted.

Dentally, Protembolotherium efremovi is characterized by a distinct P2 metacone, and hypocones on P2–P4 that are connected to the protocone (to varying degrees) by lingual crests. The molars of P. efremovi 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. Deep central molar fossae and large anterolingual cingular cusps are present. Paraconules and metalophs are absent.

Protembolotherium efremovi is one of three species with an enlarged frontonasal battering ram. It can be differentiated from Embolotherium in the following ways: there is a pair of small secondary horns positioned above the orbits, the dorsal margin of the rostrum is steeply sloped upward, the skull is less deeply saddle-shaped, the occiput is shorter and more erect, the ram is somewhat shorter, and occasionally a reduced remnant of the nasal process is seen in an elevated position at the peak of the ram. It can be further differentiated from E. andrewsi by its less robust occiput, less prominent parasagittal ridge, and more laterally bowed zygomatic arches. It can be further differentiated from E. grangeri by its shorter nasal incision, more anterior position of the ram, and the backward curvature of the ram.

DESCRIPTION

SKULL: The holotype skull of Protembolotherium efremovi (PIN 473-311) is missing the frontonasal region and a small portion of the right basicranium and occiput (fig. 113). In other respects the skull is essentially complete and not greatly distorted. Additional skulls referred to P. efremovi are PIN 473-310 (fig. 114) and PIN 3109-40 (fig. 115). These three skulls indicate a species that is large, but perhaps smaller on average than Embolotherium andrewsi and Embolotherium grangeri . The two referred skulls (PIN 473- 310 and PIN 3109-40) have relatively complete and undistorted right and dorsal sides but they are lacking complete left and ventral surfaces. Both of these specimens exhibit two conspicuous features in the frontonasal region that are not preserved on the holotype specimen but that are, nonetheless, critical to the identification and characterization of this species. These features are (1) a large battering ram–like structure similar to that of Embolotherium and (2) an autapomorphic pair of short and widely spaced secondary hornlike bony protuberances positioned directly above the orbits. The hornlike protuberances of both of these specimens are rather narrow and they project dorsally, posteriorly, and laterally. In Protembolotherium efremovi the ram is massive and well developed although it is shorter than those of Embolotherium andrewsi and Embolotherium grangeri . Several aspects of the ram specifically resemble Embolotherium andrewsi . For instance, the ram originates from a position above and slightly anterior to the orbits, it protrudes from the skull in an anterodorsal direction at an angle greater than 45 °, and curves slightly backward, thus resulting in a concave posterodorsal surface and a convex anteroventral surface. However, because of its shorter length the backward curvature of the ram is somewhat less pronounced and the distal end does not achieve the vertical angle that is seen in some specimens of Embolotherium andrewsi (such as AMNH 26001, see fig. 117). The ram of Protembolotherium efremovi broadens distally and a deep nasal channel runs along the anteroventral surface and continues into the nasal cavity of the skull. Like E. andrewsi , the nasal cavity extended to the distal end of the ram and was extraordinarily tall, although nostril position (elevated to the peak of the ram, or positioned more normally in a low position) remains unknown ( Mihlbachler and Solounias, 2004). This nasal channel is bordered laterally by thick but rather shallow walls. The lateral walls moderately constrict the nasal channel at the proximal end of the ram, but not to the degree seen in Embolotherium andrewsi , nor is the nasal channel extremely deepened at the proximal end of the ram.

Protembolotherium efremovi has a pair of small secondary horns positioned above the orbits that are completely separated from the ram. On first impression the horns of P. efremovi appear to be homologous to the paired frontonasal horns typical of many brontotheres while the ram has the superficial appearance of an enlarged and upturned nasal process, the portion of nasal bone that in less derived brontotheres typically forms the dorsal margin of the nasal incision. Evidence of the sutural configuration of the facial bones would be valuable in determining the best hypothesis of homology of these structures, but unfortunately none of the relevant sutures are discernable on any of these specimens. However, there seems little doubt that the ram itself is homologous to that of Embolotherium andrewsi , in which there is clear evidence that the distal surface of the ram is homologous to the tips of the frontonasal horns of other horned brontotheres. Therefore, the small secondary horns of P. efremovi do not appear to be homologous with the paired frontonasal horns of other horned brontotheres.

The distal end of the ram of PIN 3109-40, in particular, is unique and takes on an intermediate condition between Embolotherium andrewsi and less derived species, such as Nasamplus , Metatitan , and Aktautitan . From the anterior view, the distal surface of the ram is concave. At each distal corner is a large pachyostotic swelling. From the lateral view the pachyostotic swelling is continuous with the lateral emargination of the ram. A more peculiar feature present in PIN 3109-40 but not seen in PIN 473-310 is the relatively flat process that projects anteriorly and slightly ventrally from between the two distolateral swellings. This process appears to be the free end of a reduced nasal process that in less derived species (e.g., Protitan , Metatitan ) extends anterior to the frontonasal horn and forms the dorsal margin of the nasal incision. In Embolotherium andrewsi this structure is entirely lost. In the other specimen, PIN 473-310, the lateral swellings at the distal end of the ram are less pronounced; however, the free end of the nasal bone is essentially absent (or absorbed by the ram) as in E. andrewsi . If Yanovskaya (1954, 1980) and I are correct in assigning these skulls to a single species, the reduced nasal process was variable within P. efremovi , sometimes appearing in diminished state as in PIN 3109-40, and sometimes completely diminished as in PIN 473-310. Unfortunately both of these specimens are edentulous, so it is difficult to gauge their ontogenetic ages, although variation at the distal end of the ram might have been related to the ontogenetic process.

In the holotype ( PIN 473-311 ) the posterior border of the nasal incision extends as far back as the posterior margin of P3. This is similar to Embolotherium andrewsi . Likewise, the position of the orbit is similar ; it is positioned directly above M2 with the anterior rim of the orbit positioned approximately over the anterolateral root of M1. The nasal incisions of PIN 3109-40 and PIN 473-310 are complete and their posterior margins are entirely anterior to the orbits; however, the position of the nasal incision and the orbits cannot be determined with respect to the dentition due to the edentulous condition of these specimens.

None of the three skulls of Protembolotherium efremovi has a complete premaxillomaxillary rostrum although most of it is preserved with the type skull (PIN 473-311). Unfortunately premaxillomaxillary sutures are not clearly discernable. From a lateral view the rostrum is rather short. The ventral surface of the rostrum is curved slightly upward. The dorsal margin of the rostrum is flat and steeply angled posterodorsally so that it rises to a level just slightly higher than the orbits. From anterior and dorsal views the rostrum is broad and the lateral margins diverge posterolaterally to create a broad cavity on the dorsal side of the rostrum that is continuous with the nasal cavity. In these respects the rostrum is unspecialized and different from the rather shallow and abbreviated rostrum of Embolotherium andrewsi or the elongate Tapirus -like rostrum of Embolotherium grangeri . PIN 3109-40 has a less completely preserved rostrum; its dorsal surface appears to have been flatter than that of PIN 473-311, although it is so incomplete that I am not confident that the shape of this specimen can be accurately interpreted.

The entire dorsal surface of the skull of Protembolotherium efremovi is concave (saddle-shaped) though not as deeply as that of Embolotherium andrewsi . The parasagittal ridges extend along the sides of the skull from the postorbital process of the frontal bone to the upper corner of the occiput. These ridges are not as prominent as those of E. andrewsi . From a dorsal view the parasagittal ridges are widely separated and they barely constrict the dorsal surface posteriorly.

In the holotype (PIN 473-311), the zygomatic arches are massive and laterally expanded, with a large swelling at the junction of the jugal and squamosal bones. From the dorsal view the zygomatic process of the jugal is relatively straight anterior to the swelling and it projects posterolaterally. The degree to which the zygomatic arches bow out laterally is intermediate between Embolotherium andrewsi on the one hand, in which the jugal processes project in a predominantly posterior direction, and Embolotherium grangeri on the other, whose zygomatic arches are more strongly bowed. The remaining skulls of Protembolotherium efremovi demonstrate conspicuous intraspecific variation in the size of the zygomatic swellings. In both PIN 473-310 and PIN 3109-40 the zygomatic arches are thinner with lesser amounts of swelling in the center of the arch. Similar intraspecific variation in the thickness of the zygomatic swellings is documented in other species that possess such swellings and probably represents sexual dimorphism ( Mihlbachler et al., 2004b). From lateral views of the skulls the jugal portion of the zygomatic arch anterior to the swelling is shallower than the squamosal portion of the zygomatic arch. There is a small ventral flange on the jugal below the massive swelling. The jugal portion of the zygomatic is essentially horizontal while the squamosal portion is angled slightly upward, thus giving the zygomatic arch a weak curvature.

From a dorsal view the nuchal crest is thickened with posterolaterally angled winglike expansions. The posterior margin of the occiput is concave, although it lacks the deep median notch seen in Embolotherium andrewsi . The overall degree of robustness of the occiput is more or less typical of large horned brontotheres, although it is not nearly as massive as the occiput of E. andrewsi . From a lateral view the occiput is rather short and it is not as strongly tilted backward as the occiputs of E. andrewsi and E. grangeri . The best-preserved occiput is that of PIN 3109-40. The dorsal margin of the occiput is slightly concave, the width of the occiput is slightly constricted in the middle, and the dorsal and ventral portions of the occiput are of similar width. The posterior surface of the occiput has two distinct but relatively weak occipital pillars with a shallow central depression between them. It these respects the occiput most closely resembles E. grangeri .

The ventral surface of the skull of Protembolotherium efremovi can only be described from the holotype (PIN 473-311) (fig. 116). The anterior margin of the posterior nares is positioned near the posterior margin of M3; this position is slightly more anterior to the posterior nares than those in Embolotherium andrewsi and E. grangeri in which the anterior margin is typically a short distance behind M3. Other characteristics of the ventral surface of PIN 473-311 are essentially indistinguishable from E. andrewsi and E. grangeri . The anterior and lateral sides of the posterior narial opening are rimmed by a wide U-shaped emargination. The ridge demarcating this emargination is faint, but it can be seen upon examination of the specimen. This emargination is widest on the anterior margin and tapers along the lateral margins. The vomer, which would have bisected the posterior narial canal, is not preserved although a remnant in the form of a small ridge of bone can still be seen running along the upper (dorsal) surface of the posterior narial canal. The posterior narial canal seems to intrude slightly into the sphenoid bone (a typical condition of brontotheres), but large sphenoidal pits like those of Metatitan or Protitan , for instance, are not present in P. efremovi . The basicranium seems rather narrow, although this effect is exaggerated because part of the right side is broken away. The total width of the basicranium at the position of the mastoid processes does not exceed the distance across the right and left M3s and the basicranium is not nearly as widened as in Metatitan . The configuration of the basicranial foramina is typical as well with a widely separated foramen ovale and foramen lacerum. The mastoid process is shorter than the postglenoid process and it curves anteroventrally forming a tube-shaped external auditory pseudomeatus. (This character can also be seen in PIN 3109-40.) The external auditory pseudomeatus extends into the basicranium in a posteromedial direction, a condition shared with E. andrewsi and E. grangeri .

UPPER DENTITION: Of the three known skulls of Protembolotherium efremovi only the holotype (PIN 473-311) includes upper dentition, consisting of an incomplete set of moderate to heavily worn and rather poorly preserved upper teeth, which includes the right P2–M3 and the left P3–M3 (fig. 116). Neither the upper incisors nor their alveoli are preserved. The crowns of the canines are not preserved, although bits of the roots of both right and left canines remain inside their alveoli. The canines appear to have been small though well developed. Apparently the canines have not reached the tiny essentially vestigial state seen in Embolotherium andrewsi . The transverse distance across the roots of the canines is about the same as the distance across the P1s, further indicating that the rostrum is not as reduced as that of E. andrewsi . (In E. andrewsi the transverse distance across the canines is notably less than that of the P1s.) Like E. grangeri there is a short postcanine diastema, although unlike E. grangeri the width of the rostrum is not constricted at this point.

The P1s are missing although well-preserved alveoli indicate that single-rooted P1s were present in life. The remaining premolars are nearly rectangular in outline with nearly parallel anterior and posterior margins. The parastyle of P2 is missing. The parastyles of P3 and P4 are directed anterolabially. The metastyles of the P2, P3, and P4 are essentially straight. Minor labial paracone ribs can be found on P3 and P4 (P2 is too damaged to discern this character). The labial paracone rib of P3 is broader than that of P4. None of the premolars has a distinct mesostyle. Labial cingula are discernable on the premolars, but they are very thin and poorly developed.

The lingual features of P2 consist of a protocone and nearly equally sized hypocone. Each of these cusps is embedded in a strongly developed lingual connecting crest. The connecting crest arches around the anterolingual corner of the crown forming a small preprotocrista. The protocone of P3 is a well-developed tall cusp, while the hypocone is significantly smaller. The lingual morphology of P4 is similar to that of P3, although the hypocone is more completely disconnected from the protocone. The protocone and hypocone of P3 are barely connected by a poorly developed lingual crest. Preprotocristae are not discernable on either P3 or P4, indicating that they were either absent or so poorly developed that they have been worn off. The lingual premolar cingula tend to be discontinuous or barely continuous around the lingual margins of the protocones and they tend to be particularly thick next to the hypocones, most notably on P4.

The upper molars are heavily worn and fragmented although many typical brontotheriine traits can be discerned on them, including tall (though now very worn) and lingually angled ectolophs, very weak (and on some teeth absent) labial ribs, thin lingual ectoloph enamel, and wedge-shaped lingual sides of the paracone and metacone in the least worn molars. The anterior cingulum (though impossible to see on the photographs) is thin and passes proximally to the apex of the parastyle. Deep central molar fossae and tall anterolingual cingular cusps are present. Parts of the central molar fossae can still be seen on all three molars. The anterolingual cingular cusp, however, is completely worn away on the M1s, but it is still discernable on M2 and M3. There are no traces of paraconules or metalophs on any of the molars. Both right and left M3 appear to have had a rather small hypocone. Labial molar cingula are very weak and lingual molar cingula are mostly absent or have been worn off. A very thin cingulum appears to wrap around the anterolingual corner of M3.

REMARKS

Yanovskaya (1954) initially described Protembolotherium efremovi from two skulls and a variety of other specimens from the late Eocene Ergilin Dzo Formation of Outer Mongolia. P. efremovi is a large Asian brontothere that is most easily recognized by its Embolotherium -like ‘‘battering ram’’ that protrudes upward from the anterior end of the skull, as well as an autapomorphic pair of short secondary horns that are positioned above the orbits. It is a curious fact that Yanovskaya (1954) assigned holotype status to a skull, PIN 473-311, in which the frontonasal portion, and consequently all evidence for these diagnostically useful characters, is not preserved. Another skull (PIN 473-310) that preserves the battering ram and secondary horn was referred to P. efremovi by Yanovskaya (1954). Decades later, Yanovskaya (1980) referred a third skull (PIN 3109-40) from Ergilin Dzo to P. efremovi . This specimen is unique in retaining an anteriorly projecting nasal process at the peak of the ram.

In addition to these three skulls several additional fragmentary specimens were initially referred to Protembolotherium efremovi (PIN 473-139, 473-141, 473-216, 473-217, 473-397, 473-654, 473-656, PIN 473-708) by Yanovskaya (1954). I was unable to find these additional specimens in the PIN collection, however, I am reasonably certain that it would be difficult or impossible to narrow down the specific identity of most of them due to their fragmentary condition. One of these, PIN 473-139, is a partial mandible with left p2–m2 that Yanovskaya (1954: fig. 7 and pl. 3) figured; however, based on these figures I am unable to differentiate that specimen from mandibles of Embolotherium andrewsi or E. grangeri . Presently, there are no mandibles or lower dentitions associated with diagnostic P. efremovi fossils.

To summarize, the specimens that are relevant to the diagnosis and characterization of Protembolotherium efremovi are three skulls: the holotype ( PIN 473-311 ) is missing the entire frontonasal region ; a second specimen ( PIN 473-310 ) has an Embolotherium -like ram and an additional horn above the orbit. Finally, the third specimen ( PIN 3109-40 ), in addition to having a ram and additional horns, retains a short nasal process projecting from the peak of the ram. It is necessary to assess the cranial morphology of the type ( PIN 473-311 ) in more detail to determine what features (if any) distinguish it from other brontothere species, and if there is truly enough evidence to conclude that the remaining skulls (with the more diagnostic elements that are not preserved in the type) are indeed the same species as the type .

Despite the lack of preservation of the ram in the type (PIN 473-311), certain characteristics of this specimen show a combination of states very similar to Embolotherium andrewsi and E. grangeri ; this includes the very widened dorsal cranial surface, the fully saddle-shaped skull, prominent swellings in the zygomatic arches, the high relief of the lingual premolar cusps, the relatively deep central molar fossa, and the well-developed anterolingual cingular cusps on the molars and the sharp posteromedial angle of the external auditory pseudomeatus. No other brontotheres share this last condition except two small North American brontotheres ( Dolichorhinus , Sphenocoelus ) that are otherwise clearly very different from PIN 473-311. Therefore, it is evident that the type of P. efremovi belongs to an Embolotherium -like species.

On the other hand, the type of P. efremovi exhibits a number of features that are structurally very different from both Embolotherium andrewsi and E. grangeri . For instance, the skull is not as deeply saddle-shaped. The occiput is shorter and not tilted backward to the degree seen in E. andrewsi or E. grangeri . In other respects the occiput is similar to E. grangeri , but it is not nearly as massive as that of E. andrewsi . The nuchal crest is not as thick and rugose, the occipital pillars are not nearly as prominent, and the dorsal portion of the occiput is not broader than the ventral portion. Further differences with E. andrewsi include the much less prominent parasagittal ridges and more laterally bowed zygomatic arches. The rostrum differs substantially from both species of Embolotherium . The dorsal margin of the rostrum is steeply sloped posterodorsally, so that the rostrum deepens proximally while those of E. andrewsi and E. grangeri do not. Furthermore, the rostrum of P. efremovi is much broader than that of E. andrewsi . At the same time, the rostrum of PIN 473-311 is not nearly as elongate as that of E. grangeri and there is no constriction at the postcanine diastema. Moreover, the nasal cavity of E. grangeri is much longer with the nasal incision extending above the orbits, whereas in PIN 473-311 the nasal incision appears to be entirely anterior to the orbit. Finally, though they are not completely preserved, the canines of PIN 473-311 seem larger and more developed than the globular incisiform or cuplike vestigial canines of E. andrewsi and E. grangeri .

The additional skulls referred to Protembolotherium efremovi (PIN 473-310 and 3109- 40), to the extent that they are preserved, share the same combination of similarities and differences with Embolotherium andrewsi and E. grangeri that are seen in the type specimen of P. efremovi (PIN 473-311). Based on the morphological congruence of these three skulls it is probable that they represent the same species. Unfortunately, it is not feasible to compare the type of P. efremovi with Nasamplus progressus from the ‘‘Ulan Gochu’’ faunal zone (sensu Radinsky, 1964) of Inner Mongolia, a species known only from a single cranial fragment. However, the two skulls referred to P. efremovi, PIN 473- 310 and PIN 3109-40, are quite distinct from Nasamplus , primarily (to the extent that they can be compared) because of their well-developed rams with diminished (or absent) nasal processes.

Protembolotherium efremovi is the third valid species found to possess the ‘‘battering ram’’ that was first discovered in the genus Embolotherium . The extremely derived and bizarre ram structure was initially interpreted by Osborn (1929b) as having a separate origin from the paired frontonasal horns of other brontotheres. Later Granger and Gregory (1943) interpreted the ram as homologous with the frontonasal horns of other brontotheres. More recently reported evidence suggests that Granger’s and Gregory’s (1943) idea is the better hypothesis ( Mihlbachler et al., 2004a; Mihlbachler and Solounias, 2004). The evolution of the ram of Embolotherium and Protembolotherium is best explained by a morphological transformation series starting from a condition similar to that of Metatitan where the paired frontonasal horns and the free end of the nasal process are elevated on a frontonasal pillar. The ram itself is a grossly amplified frontonasal process that initially formed the base of the horns and the frontonasal pillar, the portion of the skull upon which the horns rested. The original free end of the nasal process is lost or, in the words of Granger and Gregory (1943), ‘‘absorbed’’ by the ram. The distal surface of the ram is homologous to the peaks of the frontonasal horns, fused into a transverse crest. Some evidence for this is seen in the apparent remnants of a sutural pattern of facial bones in Embolotherium andrewsi and Nasamplus progressus . Nasamplus represents an intermediate condition one step derived from Metatitan and Aktautitan , in which the frontonasal horns have been fused into a single transverse crest at the top of an enlarged frontonasal process, but the free end of the nasal process is still present and unreduced. Protembolotherium efremovi represents a further derived condition in which the ram is more fully developed, although it is still shorter than that of E. andrewsi and has shallower lateral margins that do not constrict the nasal cavity as strongly. Perhaps more significantly, the process that protrudes from the peak of the ram of PIN 3109-40 seems to be the reduced remnant of the free end of the nasal bone. If this interpretation is correct the small hornlike protuberances above the orbits of PIN 3109-40 and PIN 473-310 are secondary horns and they are not homologous with the paired frontonasal horns of other horned brontotheres.

If Yanovskaya (1954, 1980) and I are correct in referring PIN 3109-40 and PIN 473-310 to the same species, this implies that the nasal process, which is still present in Nasamplus but lost or absorbed in Embolotherium , was occasionally present in a diminished state (PIN 3109-40) but also was occasionally absent (PIN 473-310) in Protembolotherium efremovi . It seems obvious that, as with Embolotherium , the ram of this species would have undergone considerable ontogenetic change throughout life. After all, it seems very unlikely that embolotheres could have given birth to calves with fully developed rams. It is entirely possible that the loss of an already diminished nasal process was part of the ontogenetic process. Unfortunately, both PIN 473-310 and PIN 3109-40 are edentulous, rendering it impossible to determine the relative ontogenetic ages of these skulls even though the lack of unfused sutures in these skulls suggests that all of the skulls were well into their adult years. The North American species Megacerops coloradensis (sensu Mihlbachler et al., 2004b) is known from hundreds of specimens and exhibits considerable variation in the size and shape of the nasal process. In that species the variation is clearly related to the size of the frontonasal horns (Mihlbachler, 2004). In smaller specimens of M. coloradensis with more gracile horns the nasal process is well developed and takes on more typical proportions; however, in many of the larger specimens with huge swollen frontonasal horns the nasal process is reduced to a small triangular remnant and most of it seems to have been absorbed by the bony growth related to the massive horns. In Megacerops , this variation seems related to sexual dimorphism. Given these possible explanations (ontogeny, sexual dimorphism) for variation in the nasal process, it is difficult to justify the differences between PIN 473-310 and PIN 3109-40 as taxonomically significant (i.e., indicating two species). Therefore I presently accept Yanovskaya’s (1954, 1980) conclusions that these specimens are conspecific and that both belong to Protembolotherium efremovi .