Pachytitan ajax Granger and Gregory, 1943

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 : 265-267

Pachytitan ajax Granger and Gregory, 1943

HOLOTYPE: AMNH 21612 , a poorly preserved anterior fragment of a skull with left I3, C, and P2–M3.

TYPE LOCALITY: Shara Murun Formation, four miles north of Baron Sog Lamasery, Baron Sog Mesa, Shara Murun region, Inner Mongolia, China.

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

DIAGNOSIS: Pachytitan ajax is a very large brontothere with short robust horns. The nasal incision is dorsoventrally deep and extends to the anterior edge of M1. The nasal process is unelevated, relatively broad, strongly rounded, thick, downturned anteriorly, and with very thick lateral walls that deepen proximally and are angled ventromedially. The premaxillomaxillary rostrum deepens posteriorly.

Dentally, Pachytitan ajax is characterized by three large or intermediate-sized upper incisors. There is a distinct P2 metacone. Premolar hypocones are well developed and only weakly connected to the protocone. The molars of Pachytitan ajax 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. Central molar fossae are present, but anterolingual cingular cusps are absent. Paraconules and metalophs are absent.

Pachytitan ajax is most similar to Rhinotitan andrewsi and Diplacodon elatus . It differs from the former primarily in the more massive nasal process with a more strongly rounded and downturned anterior margin, and by its more molariform premolars. Pachytitan ajax differs from Diplacodon elatus most clearly in the deep proximal lateral walls of the nasal process and the flat rather than upturned distal lateral margins of the nasal process.

DESCRIPTION

SKULL: Pachytitan ajax is a large horned brontothere that is known only from the holotype specimen ( AMNH 21612 ), which consists of a poorly preserved anterior portion of a skull (fig. 129). The right view shows a complete horn and nasal process ; the left side includes a partial premaxillomaxillary rostrum, a poorly preserved I3 and canine, and a complete but heavily damaged cheek-tooth row. The two primary pieces, (1) the right horn and nasal and (2) the left maxilla are plastered together, but these two pieces do not actually appear to contact each other. Therefore, some aspects of the shape of the skull, such as the orientation of the nasal process are not necessarily reliable.

No sutures are discernable on this specimen, although it is probable that the horn is composed of both frontal and nasal components. The surface of the horn is rugose. The horn is large, projects in a dorsal direction, and is elliptical in cross section with its longest axis in an anteroposterior direction. However, the shape of the horn and its orientation seem to be affected by lateral crushing. The lateral side of the horn appears to have been smashed and dislocated medially, so that shape and orientation of the horn has been altered. The position of the horn with respect to the orbit is not clear, but it appears to have been located above and behind the posterior margin of the nasal incision. Judging from the left side, the nasal incision extended nearly to the anterior edge of M1. The nasal incision appears to have been rather deep, like that of Rhinotitan . The nasal process is reconstructed as if it had been angled slightly upward; Granger and Gregory (1943) described the nasal process of this specimen as being ‘‘sharply upturned’’ (p. 366). However, considering the copious amount of plaster in the specimen, the original orientation of the nasal process is uncertain. It is clear, however, that the dorsal surface of the nasal process is not turned upward with respect to the dorsal surface of the skull that is posterior to the horn. This suggests, rather, that the nasal process was not sharply upturned (contra Granger and Gregory, 1943).

The external surface of the nasal process is nearly intact on the right side. The nasal process is massive. The dorsal surface of the nasal process is flat, and the distal end is curved downward. The lateral walls are very thick and are very deep at the proximal end of the nasal process. However, the lateral walls shallow distally so that the anterior end of the nasal process lacks lateral walls. From the anterior view (fig. 129d) it can be seen that the thick lateral walls curve ventromedially and nearly partition the nasal chamber of the skull into two openings. In this respect the nasal processes of Pachytitan ajax resemble Rhinotitan andrewsi , although the lateral walls of the nasal process of P. ajax are much deeper and do not curve medially as sharply as in R. andrewsi . The distal end of the nasal process is strongly rounded and the anterior margin is thick and quite rugose.

The premaxillomaxillary rostrum is similar in length to the nasal process. From a lateral view the rostrum deepens posteriorly and its lateral dorsal surface is sloped dorsoventrally. Little else of the cranial morphology of Pachytitan ajax can be described from the fragmentary holotype.

UPPER DENTITION: The crowns of I1 and I2 are not preserved and the crown of I3 is too poorly preserved to precisely judge its morphology. However, the incisors appear to have been either large or intermediate in size, but they do not appear to have reached a small vestigial state. Additionally, the incisors are positioned completely anterior to the canine rather than between the canines. The canine itself is of moderate size. There is a short precanine diastema. The postcanine diastema is longer than the P2.

P1 is not preserved. P2–P4 are complete but badly damaged. In outline, P2–P4 are nearly rectangular although the anterior margin of P2 is slightly angled more posterolingually. The parastyle and metastyle of P2 are straight. The parastyles of P3 and P4 are more strongly angled labially. The P3 metastyle is straight while the P4 metastyle is deflected labially. The labial side of the P2 paracone is rounded while P3 and P4 have slight labial paracone ribs. P4 has a well-developed mesostyle that is situated closer to the metacone than the paracone. The other premolars lack mesostyles. Although it is possible that a P4 mesostyle is a characteristic of this species, this seems unlikely considering that specimens of other brontothere species occasionally show a P4 mesostyle, but it never seems to occur in any brontothere species as a fixed state.

The lingual margins of the P2–P4 are nearly flat. The lingual morphology of P2 is uncertain, but there appears to have been two lingual cusps or a single lingual crest. P3 and P4 have two distinct lingual cusps that are weakly connected. The hypocones of P3 and P4 are positioned well behind the protocones and are similar in size to the protocones. Labial premolar cingula are weak. P4 has a continuous lingual cingulum, but the lingual cingulum of P3 is slightly discontinuous.

The molars are too damaged for precise description and measurement. However, the shape of the wear facet is consistent with those of other brontotheriines. The ectoloph is substantially taller than the lingual cusps, the inner band of enamel is thinner than the outer band, and the lingual sides of the paracone and metacone are wedge-shaped. Central molar fossae are present, but anterolingual cingula cusps appear to have been absent. There is no evidence of paraconules on the molars, nor does the M3 have a hypocone.

REMARKS

Granger and Gregory (1943) based Pachytitan ajax on a single specimen (AMNH 21612) and differentiated it from Rhinotitan by its larger size, more massive upturned nasals, large horns, and more advanced premolars. Due to the poor condition of the specimen, the interpretation of the nasal bone as upturned is conjectural. Likewise, when one considers the apparent variability of horn size in other brontothere species the larger horns of AMNH 21612 are not strong evidence for a distinct species. However, the premolars of Pachytitan ajax are distinctly more molarized than those of Rhinotitan due to the well-developed hypocones that are only weakly connected to the protocones. In this respect the premolars of P. ajax more closely resemble those of Diplacodon elatus and more advanced brontotheres such as Embolotherium or Parabrontops .

Cranially, Pachytitan ajax seems most similar to Rhinotitan andrewsi and Diplacodon elatus . Pachytitan ajax shares with Rhinotitan andrewsi (and differs from Diplacodon elatus ) in its deep lateral nasal walls that are strongly angled ventromedially. However, the nasal bone of P. ajax is more massive than R. andrewsi and it has a thick, strongly rounded, and downwardly curved distal end; in these respects the nasal process of P. ajax more closely resembles Diplacodon elatus .