Macrocephalosaurus Gilmore, 1943

KEQIN, GAO & NORELL, MARK A., 2000, Taxonomic Composition And Systematics Of Late Cretaceous Lizard Assemblages From Ukhaa Tolgod And Adjacent Localities, Mongolian Gobi Desert, Bulletin of the American Museum of Natural History 2000 (249), pp. 1-118 : 70-74

Macrocephalosaurus Gilmore, 1943

TYPE SPECIES: Macrocephalosaurus ferrugenous Gilmore, 1943 .

KNOWN DISTRIBUTION: Djadokhta and Barun Goyot formations; Mongolian Gobi Desert.

REVISED DIAGNOSIS: Differing from other closely related scincomorphs in having the following derived character states: postorbital elongate, reaching posterior border of supratemporal region; supratemporal lost by fusion with squamosal; parietal short, with parietal foramen near or on frontoparietal suture; suborbital fenestra small or closed; angular process of lower jaw reduced to obtuse angle; teeth high­crowned and slender; crowns flared, laterally compressed, and multicuspate.

Macrocephalosaurus sp.

Figure 21

SPECIMENS: IGM 3/132, partial skull and postcranial skeleton; IGM 3/133 (MAE 51/ 93­25), disarticulated vertebrae.

LOCALITY AND HORIZON: Ukhaa Tolgod, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Djadokhta Formation (Loope et al., 1998).

REMARKS: IGM 3/132 consists of well­preserved premaxillae, maxillae, and partial postcranial skeleton. The premaxillae are fused, and carry five pleurodont and unicuspid teeth and vacant spaces for four others. The maxillary teeth are typical of Macrocephalosaurus , being high­crowned, and having a flared and multicuspid crown pattern. Although the teeth are similar to those of Macrocephalosaurus gilmorei , the specimen cannot be confidently identified to species without reference to other skull morphology. IGM 3/133 consists of several associated dorsal vertebrae. These are generally macrocephalosaur­type, as the vertebral centrum is strongly elongate and posteriorly narrowed. However, lack of cranial material makes it impossible to refer this material to any known species.

It is worth noting that IGM 3/132 and 3/ 133 are the only Macrocephalosaurus material out of over 1000 specimens that were collected from the Ukhaa Tolgod locality. As this remarkably productive locality has been extensively sampled for several years, this ratio likely reflects the rarity of this particular herbivorous lizard in the Cretaceous vertebrate community at Ukhaa Tolgod. The paucity of this and other herbivorous lizards at this locality may or may not reflect stratigraphic differences from the Khermeen Tsav and Khulsan localities, as large herbivorous species are also known from the classic Bayn Dzak locality (Djadokhta Formation). It is likely that the rarity of large herbivorous lizards at some localities indicates environmental differences between these sites.

Macrocephalosaurus chulsanensis

Sulimski, 1975

Figure 22 HOLOTYPE : ZPAL MgR­I/14, nearly complete skull with mandibles and postcranial skeleton.

TYPE LOCALITY AND HORIZON: Khulsan, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Barun Goyot Formation.

KNOWN DISTRIBUTION: Khulsan and Monadnocks, Barun Goyot Formation.

DIAGNOSIS: ‘‘Skull smaller (40­80 mm), more delicate and slender, canine teeth less well developed, tooth shafts projecting farther over parapet of jaw, skull sculpture more symmetrical (forming rosettes) and pectoral girdle more massive than in other species of Macrocephalosaurus ; differs from M. gilmorei in having a more slender lower jaw; differs from M. ferrugenous in having lanceolate, polycuspate teeth’’ (Estes, 1983: 83).

REFERRED SPECIMEN: IGM 3/134 (MAE 215/92­47), incomplete skull with mandibles from Khulsan.

REMARKS: The specimen (IGM 3/134) shows the same skull configuration and tooth

morphology as the holotype of Macrocephalosaurus chulsanensis (fig. 22); also, it has the same pattern of osteodermal ornamentation. The parietal foramen is nearly closed and is located at the frontoparietal suture (fig. 22A). On the basis of these similarities, the specimen is referred to Macrocephalosaurus chulsanensis .

Among the three recognized Macrocephalosaurus species , Macrocephalosaurus chulsanensis is distinguished from Macrocephalosaurus gilmorei in having the parietal foramen opening at the frontoparietal suture and having the suborbital fenestra entirely closed. Other aspects of cranial and tooth morphologies of the two species are astonishingly similar, and these two species are not easily distinguished from the type species Macrocephalosaurus ferrugenous , except for the significant size differences (see Sulimski

lower dentition on the holotype specimen. (2) Anterior maxillary teeth are enlarged and caninelike, as clearly indicated by the significantly thickened tooth bases. (3) Posterior teeth have widened and laterally compressed crowns, indicating multicuspid rather than bluntly chisellike crown pattern (see comments by Estes, 1983).

1975; Estes, 1983). The type species is based on the holotype and only known specimen (AMNH 6520) from Bayn Dzak (Gilmore, 1943). Examination of the type specimen (K. Gao, personal obs., 1998) permits clarification of the following points: (1) The marginal teeth are pleurodont (contra Gilmore, 1943: acrodont; Estes, 1983: subpleurodont), as they have at least half of the tooth height attached to the lateral parapet of the tooth row. The pleurodonty is best shown on the

Erdenetesaurus robinsonae Sulimski, 1975 Figure 23A, B

HOLOTYPE: ZPAL MgR­III/19, incomplete skull with mandibles.

TYPE LOCALITY AND HORIZON: Khermeen Tsav II, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Barun Goyot Formation.

KNOWN DISTRIBUTION: Only known from the type horizon.

DIAGNOSIS: ‘‘Skull high, well arched. Parieto­occipital region elongated. Maxilla moderately high, slightly inclined medially dorsal. Postfrontal with poorly developed distal processes. Supratemporal fossa small, oval, slit shaped. Supratemporal bone squamous, vestigial. Parietal foramen in parietal. Posterior processes of the parietal diverge at an angle of about 80°. Frontals partially fused, posteriorly widened. Descending thickening of the parietal well developed. Ectopterygoid with a long anterior process. Lower jaw slen­ der with slight posterior backward inclination of coronoid process. Retroarticular process wide and long. Dentition pleurodont, almost homodont. Tooth replacement of iguanidtype. Cheek tooth crowns rounded in outline, mediolaterally compressed with six­seven radially arranged denticles’’ (from Sulimski, 1975: 78).

REFERRED SPECIMEN: IGM 3/135 (MAE 129/92­31), incomplete skull with mandibles from Khermeen Tsav.

REMARKS: IGM 3/135 is a small specimen (20 mm long), the size of which is only about half of the holotype of the species (ZPAL MgR­III/19: 55 mm). However, it shows diagnostic features of the species in tooth morphology and jaw structures: the teeth are short crowned, having rounded crowns that are aligned with no more than seven cusps; the mandible is slender, lacking the typical deepening and lateral swelling of the jaws in Macrocephalosaurus species. On the basis of these features, the specimen IGM 3/135 is referred to Erdenetesaurus robinsonae .

Sulimski ( 1975) noted the close similarities of Erdenetesaurus robinsonae with Cherminsaurus kozlowskii , and placed the two species in the Polyglyphanodontidae The two species are similar in general configuration of the skull, position of the parietal foramen, reduction of the suborbital fenestra pattern of the postorbital­squamosal articulation, but are clearly different from one another in proportion of the parietal table and the orientation and cusp pattern of the crowns along the tooth row (see Sulimski 1975). These two species may be closely related to one another, but their relationships to Macrocephalosaurus , Adamisaurus , and Polyglyphanodon are far from clearly understood.

Cherminsaurus kozlowskii Sulimski, 1975 Figure 23C, D

HOLOTYPE: ZPAL MgR­III/24, skull with mandibles.

TYPE LOCALITY AND HORIZON: Khermeen Tsav, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Barun Goyot Formation.

KNOWN DISTRIBUTION: Only known from the type locality and horizon.

DIAGNOSIS: Polyglyphanodontine diagnosed by the following characters: ‘‘Skull length 65 mm; skull contours relatively unmodified; snout region high, arched, but not widened; jaw robust but not deepened; dentition essentially homodont, subpleurodont tooth number low (19 dentary teeth); supratemporal and squamosal separate; parietal foramen in parietal; weak sagittal crest on parietal; ectopterygoid with long anterior process; suborbital fenestra open; teeth large crowns diagonally arranged at about 30° angle to axis of jaw; cutting edges of teeth with 5­6 low, blunt cusps; coronoid and dentary in contact’’ (from Estes, 1983: 79).

REFERRED SPECIMEN: IGM 3/136 (MAE 256/92­23), incomplete skull with mandibles and partial postcranial skeleton; IGM 3/137 (MAE 267/92­35), partial skull; both specimens from Khermeen Tsav locality, Barun Goyot Formation.

REMARKS: As noticed by Sulimski ( 1975), the most distinctive features of Cherminsaurus kozlowskii are the oblique arrangement of the marginal teeth along the tooth row and the chisellike crowns with five to six low cusps. The two specimens from Khermeen Tsav clearly show these diagnostic features and they are accordingly referred to this species.

Sulimski ( 1975) placed Cherminsaurus kozlowskii together with Erdenetesaurus robinsonae in the Polyglyphanodontidae . This placement is accepted by Estes (1983) with change of ranking of the familial group as a subfamily (Polyglyphanodontinae). Howev­ er, the phylogenetic positions of the two taxa in relation to other Gobi taxa (e.g., Macrocephalosaurus , Adamisaurus ) and Polyglyphanodon need to be carefully investigated, before a reliable classification scheme can be provided.

Figures

Fig. 21. Macrocephalosaurus sp.: IGM 3/132, A, fused premaxillae anterior view; B, left maxilla medial view; C, D, right maxilla, lateral and medial views.

Fig. 22. Macrocephalosaurus chulsanensis: IGM 3/134, A, B, incomplete skull articulated with mandibles from Khulsan, dorsal and lateral views; C, close­up shot of right maxillary teeth.

Fig. 23. Erdennetesaurus robinsonae: A, B, IGM 3/135, incomplete skull with mandibles from Khermeen Tsav, dorsal and lateral views; Cherminsaurus kozlowskii: C, D, IGM 3/136, left mandible from Khermeen Tsav, lateral and medial views.