Bagaceratops sp.

Czepiński, Łukasz, 2020, New protoceratopsid specimens improve the age correlation of the Upper Cretaceous Gobi Desert strata, Acta Palaeontologica Polonica 65 (3), pp. 481-497 : 485-488

cf. Bagaceratops sp.

Figs. 4A, 5 .

Material.—MPC-D 100/551B (field number 00019US FJM Baga), a skull and mandible with a partial articulated skeleton, containing both scapulae, humeri, radii, ulnae, right manus, and ribs ( Fig. 5 ), from the Cretaceous (?middle– late Campanian), Djadokhta or Baruungoyot Formation ( Jerzykiewicz and Russell 1991; Watabe et al. 2010; Kielan-Jaworowska et al. 2003), eolian sandstone beds of the Üüden Sair locality, Ömnögovi Province, Gobi Desert, Mongolia (Fig. 1).

Description.—Sagittal skull length is 230 mm. Skull width between the jugals is 146 mm. Basal skull length is 127 mm, and partial basal skull length is 97 mm.

The rostral was disarticulated from the specimen, revealing the morphology of the anteriormost portion of premaxilla. The rostral bone is rather complete, with a straight ventral margin. The ratio between height and length equals 1.55. In lateral view, the rostral originally reached nearly the level of the dorsal margin of the external nares.

The premaxilla is quite elongated, with the rostral covering only one third of the bone. The ventral margin of both premaxillae is weathered; however, the snout is distinctly narrower than the predentary width. Together with flat lateral sides of the bone this condition suggests a lack of premaxillary teeth. Indeed, in specimens with a premaxillary dentition the premaxilla is usually thicker and bulbous in this region. An edentulous premaxilla is a diagnostic feature of Bagaceratops rozhdestvenskyi and Protoceratops hellenikorhinus ( Maryańska and Osmólska 1975; Lambert et al. 2001). However, a few specimens of Protoceratops andrewsi from Üüden Sair (MPC-D 100/539) and Bayan Zag (AMNH 6431) also lack teeth on the premaxilla ( Handa et al. 2012; Czepiński 2019). The premaxilla contributes to the ventral and, together with the ventral projection of nasal, to the anterior and posterior margins of the elongated external nares.

The maxilla is poorly preserved on both sides of the specimen. The premaxillo-maxillary suture is weathered, and the bone is extremely thin in this area. In the anterior portion of the left maxilla, a part of the possible natural bor- der of the bone is preserved. Due to the poor preservation of the premaxilla’s posterior portion, it cannot be concluded if the “loosening” of the premaxillo-maxillary suture was present, or if the premaxilla simply overlapped the maxilla. However, presence of the “true” accessory antorbital fenestration, with distinct margins and a size similar to that of the external nares (as in B. rozhdestvenskyi ), is not likely, because the eroded area of the bone is limited, being much smaller than the nares. A few maxillary teeth are poorly preserved, and the length of the tooth row and of the maxillary diastema cannot be measured.

The lacrimal contributes to most of the posterodorsal margin of the antorbital fossa. Its contribution to the anterior margin of the orbit is restricted to one third of its height, due to the development of the ventral process of the prefrontal and of the anterodorsal process of the jugal.

The prefrontal is deep, and its descending process reaches the level below the mid-height of the orbit. In dorsal view, the prefrontal is wider rostrally. In anterior view, the prefrontals lean toward the longitudinal axis of the skull.

The “horn-like projection” of the nasals was described in the field report on the specimen ( Watabe and Tsogtbaatar 2004). That part of the bone was no longer attached to the specimen during my visit in MPC, Ulaanbaatar (September 2016). However, its morphology can be reconstructed thanks to the close examination of the preserved part of the bones, and the study of the archival pictures from the field report ( Watabe et al. 2004: pl. 3: 5) and those taken in MPC in the past. Only the midline and right portion of the fused nasals were preserved, forming a low, narrow, elongated horncore. Its dorsal tip nearly reached the level of the dorsal margin orbit. The fused nasals are an autapomorphy of Bagaceratops rozhdestvenskyi ( Maryańska and Osmólska 1975; Makovicky predentary nasal

A 1

external naris premaxilla A 3 A 2 nasal external naris B premaxilla maxilla predentary buccal crest 10 mm dentary dentary

2002; Morschhauser 2012; Czepiński 2019), and are seen also in Ceratopsidae ; however, in the latter the nasal fusion occurs during the ontogeny ( McDonald 2011).

The paired frontals are roughly triangular in dorsal view. Anteriorly, proximal to the nasal horncore, the frontals are elevated medially, and the nasofrontal suture is V-shaped. The frontals meet with the postorbitals at a nearly straight suture. The interfrontal suture is slightly convex along most of its longitudinal length, with the posterior portion of the suture being concave. A distinct frontoparietal depression occurs on both sides of the posterior portion of the interfrontal suture. The frontals form a small contribution on the anterior margin of the supratemporal fenestra.

The left jugal is better preserved than the right one. However, the bone is deformed in the region of the contact with the maxilla. The bone contributes to the whole ventral margin of the orbit, and the short ascending anterior process forms the anteroventral corner of the margin. Its quadratojugal portion is deformed. Posteriorly, the jugal contributes to the formation of the anterior margin of the infratemporal fenestra, excluding the postorbital. The epijugal is not preserved and no trace of its attachment is preserved on the jugal.

The postorbitals are elongated, reaching half of the parietosquamosal frill length. Small rugosities occur on the bone at the posterodorsal margin of the orbit. A small tubercle occurs on the suture between the postorbital and frontal. Such a tubercle was also observed in some specimens of B. rozhdestvenskyi (e.g., ZPAL MgD-I/125, 129, IVPP V12513; Czepiński 2019).

The squamosals are widely separated from the margin of the parietal fenestra. In lateral view, the squamosals lean posteroventrally, with a high posteriormost margin. The posterior tip of the squamosals is covered with longitudinal rugosities. The parietal fenestrae are small and asymmetrical. The left fenestra is nearly circular, whereas the right is elongated mediolaterally. The parietal fenestration is small, even when compared with smaller specimens of P. andrewsi (i.e., the parietal fenestrae area of MPC-D 100/551 is more than twice as large as in the cf. Bagaceratops sp. specimen, although the basal skull length of the latter is 7% greater). It is similar to the known material of B. rozhdestvenskyi , where the fenestration is relatively smaller than in Protoceratops spp. ( Czepiński 2019). The ridge at the posterior margin of the parietal frill is subtle, and the sagittal crest is somewhat bifurcated at its contact with the frontal, resembling the holotype specimen of B. rozhdestvenskyi (ZPAL MgD-I/126).

The predentary is elongated, but its lateral surface is poorly preserved. It is longer than in P. andrewsi (MPC-D 100/551), extending anterior to the premaxilla. The dentary bears a buccal crest, nearly V-shaped in cross-section. The crest is distinct; however, it does not expand laterally to the same degree as in some specimens of B. rozhdestvenskyi e.g., ZPAL MgD-I/144). There is a posteroventral (angular) projection of the dentary, similar to the condition seen in Protoceratops spp. ( Czepiński 2019).

Given the presence of the fused nasals and V-shaped buccal crest of dentary, the specimen is similar to Bagaceratops . However, it lacks the well-developed premaxillo-maxillary fenestration (accessory antorbital fenestra) and the angular is not projecting anteroventrally. The mosaic of plesiomorphic and apomorphic features makes the morphology of the specimen intermediate between P. andrewsi and B. rozhdestvenskyi ( Fig. 4).

Remarks.—The specimen was collected on July 19–20, 2000 at Üüden Sair by the HMNS-MPC expedition to the Gobi Desert. It was photographed in the report of the HMNS expedition with the field number 000719 US FJMBaga (Watabe andTsogtbaatar2004).Itwas,probablyerroneously,described with the field number 00720 US by Tsogtbaatar (2004).

Figures

Fig. 5. Ceratopsian dinosaur cf. Bagaceratops sp. (MPC-D 100/551B, field number 000719 US FJMBaga), from the Late Cretaceous, Üüden Sair, Ömnögovi, Mongolia. Skull in left lateral (A1, A2), dorsal (A3, A4), and right lateral (A5, A6) views. Slab with the skeleton in left laterodorsal view (A7). Abbreviations: l, left; r, right.