Protoceratops andrewsi Granger and Gregory, 1923

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 : 482-485

Protoceratops andrewsi Granger and Gregory, 1923

Figs. 2 , 3 , 4B.

Material.—MPC-D 100/505, a weathered skull of a large individual ( Fig. 2 ) from the Zamyn Khond locality, Ömnögovi Province, Gobi Desert, Mongolia, Djadokhta Formation (?middle Campanian; Jerzykiewicz and Russell 1991), and MPC-D 100/551, a nearly complete articulated skeleton with skull and mandible of a medium-sized individual ( Fig. 3 ) from the eolian sandstone beds of the Zhinst Tolgoi site, Üüden Sair area, Ömnögovi Province, Gobi Desert, Mongolia, Djadokhta or Baruungoyot Formation (?middle–late Campanian; Jerzykiewicz and Russell 1991; Watabe et al. 2010; Kielan-Jaworowska et al. 2003).

Description.—MPC-D 100/505 ( Fig. 2 ) has a short and high rostral bone. At least one premaxillary tooth is preserved in the left premaxilla. Maxillary diastema seems to be relatively long (plausibly longer than 30% of the tooth row). However, its length cannot be established with certainty because not all teeth are preserved. As seen on the right side, premaxillo-maxillary suture is not pierced by any fenestra. The nasals are paired and form low, bowed horns. The frontonasal suture is U-shaped posteriorly. The frontals are relatively narrow and short. The frontoparietal depression is restricted to the small area of the posterior portion of frontals and is separated medially by the sagittal crest. The frontoparietal suture is W-shaped. The sagittal crest is slightly bifurcated at the contact with frontals. The skull in anterior view is folded in the postorbital region, with the postorbitals distinctly facing dorsally. The lateral buccal crest of the dentary is U-shaped in cross section. The frill expands posteriorly for nearly a half of the basal skull length. The symmetrical parietal fenestrae of the frill project laterally, nearly reaching the squamosals. The posterior margin of the frill is marked with a low, delicate ridge. Basal skull length (sensu Dodson 1976; measured from the anterior margin of the rostral to the posterior margin of the quadrate) of the specimen equals 223 mm.

Presence of paired nasals, the premaxillary dentition, the absence of the accessory antorbital fenestration and the U-shaped buccal crest of the dentary are diagnostic features of Protoceratops andrewsi ( Makovicky 2002; Morschhauser 2012; Czepiński 2019). MPC-D 100/505 has a folded dorsal surface of the postorbitals, similar to large specimens of P. hellenikorhinus from Bayan Mandahu (e.g., IMM 95BM1/1). The development of the ridge on the posterior margin of the frill varies among P. andrewsi ( Handa et al. 2012) . The lateral expansion of the parietosquamosal frill is typical for individuals of advanced ontogenetic stages ( Hone et al. 2016).

MPC-D 100/551 ( Fig. 3 ) is a nearly complete skeleton measuring ~ 570 mm in total preserved length (measured from the tip of the rostral to the posterior extremity of the ischium). The sagittal skull length equals 216 mm, the height equals 85 mm. Basal skull length equals 120 mm, and the partial basal skull length (sensu Czepiński 2019; measured from the anterior margin of the maxilla to the posterior margin of the quadrate) is 91 mm.

The rostral is nearly complete; however, it is rather small and restricted to the anteroventral portion of the snout. Dorsally it reaches only slightly beyond the level of the ventral margin of external nares. Although the anterior keel is preserved, the lateral portion of the rostral barely shows any rugosities.

The premaxilla is rather short. Two premaxillary teeth are preserved on the left side. They are directed slightly laterally, protruding closely outside the predentary bone, when the jaws are closed, as observed by Varriale (2011). With this condition, it is similar to some other specimens of Protoceratops andrewsi (e.g., AMNH 6430 from Bayan Zag, Djadokhta Formation).

The maxilla is well preserved. The premaxillo-maxillary suture is seen at the right side of the specimen and it shows no evidence for the “true” accessory antorbital fenestration (sensu Czepiński 2019). However, the anterior margin of the maxilla that forms the suture is slightly concave at its midpoint, resembling the natural border of the bone, suggesting that some “loosening” (where the bones did not overlap along the entire premaxillo-maxillary suture, but had some loose contact at mid-height of the suture, however, without the formation of the “true” accessory antorbital fenestration with well-defined margins and a size similar to that of the external nares; Czepiński 2019: fig. 20) was present. However, due to the poor preservation of the area, it remains ambiguous. Nine maxillary teeth are preserved. The maxillary diastema is rather long, reaching 30.8% of the tooth row length.

The lacrimal forms up to two-thirds of the anterior orbit margin. The prefrontal is shallow, placed at the dorsal tip of the orbit. Preserved on the right prefrontal the descending process projects slightly below the midpoint of the orbit.

The nasals are not fused and form paired nasal horns, as can be noticed from the morphology of the preserved left nasal. It is covered by rugosities and slightly arched along the longitudinal axis of the bone.

The frontals are paired and covered by rugosities. They meet the postorbitals along slightly oblique sutures. A small and shallow frontoparietal depression is present. Frontals contribute extensively in the formation of the anterior margin of the supratemporal fenestra.

The jugals are complete. Although epijugals are not preserved, scars for the epijugal attachment occur on both sides of the specimen. The postorbitals are covered by rugosities. A small tubercle is visible beyond the posterodorsal corner of the orbit, especially on the right postorbital. The posterior process of the postorbital is short.

The squamosals form a wide contact with the parietal bone, as seen in dorsal view. In lateral view, the posteriormost portion of the squamosal is low and elongated. The posteromedial process of the squamosal is expanding anteriorly, and contributes to the lateral margin of the parietal fenestra. A low ridge occurs along the posterior margin of the parietal, as seen in other specimens of P. andrewsi from the same locality ( Handa et al. 2012). The posterior margin of the parietal is slightly concave, resulting in a heart-shaped morphology in dorsal view. The sagittal crest is low and straight in lateral view.

The tip of the predentary does not project anteriorly, being placed behind the anterior tip of the premaxilla. The dentary is low, lacking a distinct buccal crest ( Fig. 4B). A posteroventral process of the bone is present, as in other specimens of P. andrewsi and P. hellenikorhinus .

Presence of the premaxillary dentition, paired nasals, and a short maxillary diastema, and the lack of the distinct dentary buccal crest and accessory antorbital fenestration indicate that MPC-D 100/551 belongs to Protoceratops andrewsi .

Remarks.—MPC-D 100/505 was collected in 1977 by MPC (Ulziitseren Sanjaadash, personal communication 2016); MPC-D 100/551 was collected by the HMNS-MPC Joined Paleontological Expedition on July 20, 2000, with a field number 000720 US-Zhinst ( Watabe and Tsogtbaatar 2004). The latter specimen was listed as Protoceratops in a report of the HMNS-MPC expedition by Watabe and Tsogtbaatar (2004). It is the same specimen described as Bagaceratops GI SPS 100/588 by Varriale (2011) and GI SPS 100/528 by Nabavizadeh and Weishampel (2016). Only recently has it been referred to as Protoceratops andrewsi ( Czepiński 2019) .

Stratigraphic and geographic range.—Upper Cretaceous, Gobi Desert, Mongolia.

Figures

Fig. 2. Ceratopsian dinosaur Protoceratops andrewsi Granger and Gregory, 1923 (MPC-D 100/505), from the Late Cretaceous, Zamyn Khond, Ömnögovi, Mongolia. Skull in right (A1) and left (A2) lateral views.

Fig. 3. Ceratopsian dinosaur Protoceratops andrewsi Granger and Gregory, 1923 (MPC-D 100/551), 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:, left; r, right.