INCERTAE SEDIS, 2000

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 : 82-89

SCINCOMORPHA INCERTAE SEDIS FAMILY INCERTAE SEDIS New genus and species (unnamed)

Figure 27

SPECIMEN: IGM 3/54 (MAE 29/93­192), incomplete skull articulated with mandibles. LOCALITY AND HORIZON: Ukhaa Tolgod, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Djadokhta Formation.

DESCRIPTION

The specimen IGM 3/54 is an incomplete skull articulated with mandibles. The speci­

men was collected from the Gilvent Wash sublocality at Ukhaa Tolgod, and is preserved in well­cemented sandstone concretion. This type of preservation is different from most of the other lizard specimens from Ukhaa Tolgod, which are often preserved in poorly cemented sandstones.

SKULL ROOF: The premaxillae are paired with a clear midline suture. The anterior surface of the premaxilla is smooth, and has no anterodorsal premaxillary foramina. The nasals are also paired, and each element laterally contacts the dorsal process of the maxilla, but is separated from the prefrontal by a frontal process (fig. 27B). The frontals are fused and strongly narrowed between the orbits. The subolfactory processes of the frontals are not exposed, but the impressions of the posterior part of the frontals show the processes are well developed and may well have a midline contact below the olfactory tract.

The parietal is incompletely preserved. It appears to have had a short and rectangular table, and proportionally long supratemporal processes. Although the table is incomplete, impressions show a parietal foramen is present at the center of the table. The lateral bor­ der of the parietal table is not flanged, indicating a ventral origin of the temporal muscles.

The prefrontal anteriorly sutures with the posterior border of the dorsal process of the maxilla. Medially, the bone contacts the frontal only and is separated from the nasal by a frontal­maxilla contact. The postfrontal and the postorbital are completely fused as a single element (i.e., postorbitofrontal). Such fusion occurs in six extant groups (see Estes et al., 1988 for discussion), and its phylogenetic significance is equivocal among squamate taxa. On IGM 3/54, the posterior process of the postorbitofrontal is very short, and thus the upper temporal bar may be largely formed by a strongly elongated squamosal (not preserved on both sides).

The maxilla is well preserved on both sides. The nasal process of the maxilla is primitively located above the anterior part of the tooth row, and the lateral surface of the process appears to be ornamented with light dermal rugosities (fig. 27A). The maxilla forms a large part of the ventral border of the orbit, and obscures the greatly reduced lacrimal, which is confined to the inner side of the orbital corner. The lacrimal borders a small lacrimal foramen with the prefrontal, without involvement of the maxilla. Anteriorly, the premaxillary process of the maxilla is short and slightly overlaps the lateral side of the premaxilla. No premaxillary aperture is developed at the notched suture with the premaxilla. Posteriorly, the maxilla has a slender process extending to the midlevel of the large and slightly elongate orbit. The posterior end of this process ends with a small notch, which articulates with the jugal (fig. 27A). Such an articulation occurs in some skinks (e.g., AMNH 140794: Corucia zebrata ; AMNH 27296: Tropidophorus queenslandae ), but is highly variable in other skinks (e.g., AMNH 57864: Eumeces schneideri ; AMNH 99684: Tiliqua nigrolutea ).

The jugal is a lightly built element without a posteroventral process. It has an elongate anteroventral process, which extends along the medial side of the posterior process of the maxilla and hence is not exposed in lateral view. The posterodorsal process is extremely slender but forms a complete postorbital bar. As the squamosal is not preserved on both sides it can not be determined wheth­ er a jugal­squamosal contact was present.

PALATAL ELEMENTS: Some palatal elements can be identified in ventral view. The vomers are poorly preserved, and whether the two sides are fused or separate cannot be determined. The palatines are short and wide, and are toothless. The two sides are very close to the midline, and together with the pterygoids strongly restrain the interpterygoid vacuity as a narrow recess. The pterygoids have an oblique suture articulation with the palatine at the midlevel of the suborbital fenestra, and form part of the posterior border of the fenestra together with the ectopterygoid. Like the palatines, the pterygoids are also toothless (fig. 27C).

MANDIBLE: The mandibles are completely preserved on both sides (fig. 27C). The lower jaw is relatively heavily built in relation to the skull. The lateral surface of the dentary is smooth, without any trace of ornamentation. Sutures between the dentary and coronoid can be identified, but the dentary­surangular suture cannot be delimited because of fusion. The posterodorsal part of the dentary seems to have a small notch for the anterior process of the surangular bone. The coronoid bone is small, and its weakly developed dorsal process is anteriorly overlapped by the coronoid process of the dentary bone. The coronoid is laterally exposed as a small wedge between the dentary and the surangular as in most other scincomorphs (see Estes et al., 1988 for evaluation). The anterior surangular foramen is small and located on the surangular behind the base of the coronoid dorsal process. The posterior surangular foramen is barely identifiable and probably located anteroventral to the craniomandibular joint. The surangular­prearticular suture is recognizable on the posterior part but anteriorly blurred because of fusion. The retroarticular process is slender and straight, nondeflected, and medially carries no angular process or prearticular crest.

Medially, the splenial is reduced to cover the posterior two thirds of the Meckelian canal. The anterior one third of the canal tends to be closed, but remains open as a narrow and medially faced fissure. The posterior extension of the splenial reaches at least the level of the posteroventral process of the coronoid bone, but its actual extent cannot be identified because of fusion with the prearticular. The anterior inferior alveolar foramen and the anterior mylohyoid foramen are close to one another, and both lie at a level about two thirds of the way back along the tooth row. The anterior part of the angular can be delimited as a very slender splint, but it is posteriorly fused with the prearticular and the surangular.

DENTITION: The paired premaxillae carry a total of nine fine teeth (the right side has five and the left side four). The left maxilla carries about 22 teeth, and the tooth row posteriorly extends to the midlevel of the orbit. The teeth are extremely tiny, simple and peglike, and closely spaced from one another. This type of dentition, in keeping with the relative small size of the skull, indicates the lizard may have fed largely on ants. The low­ er dentition cannot be exposed for observation; however, the number of dentary teeth can be estimated at 22­24, as the dentary normally carries a few more teeth than the maxilla. All the marginal teeth are presumably pleurodont, although they are not exposed in medial view.

COMPARISON AND DISCUSSION

Morphologically, the specimen (IGM 3/ 54) shows a unique mosaic of characters Presence of frontal fusion and interorbital constriction seems to indicate an iguanian affinity, but many other character states such as presence of the parietal foramen near the center of the parietal table and a ventral origin of the temporal musculature indicate that the above­mentioned character states are homoplastic relative to similar conditions in iguanians. On the other hand, fusion of the frontals with descending processes in contact below the olfactory tract suggests a gekkotan affinity of this lizard; however, this possible affinity is disproved by other character states (see below).

Despite the above­mentioned uncertainties, the unnamed new taxon is referred to the Scincomorpha based on a combination of the following features: the nasal and prefrontal are separated by an anterolateral process of the frontal; origin of the adductor muscle is on the ventral aspect of the parietal; and the coronoid is laterally exposed as a small wedge between the dentary and the surangular. Within the Scincomorpha, two character states (paired premaxillae, and nearly closed Meckelian canal by dentary with strong reduction of the splenial) seem to indicate a possible affinity of the new taxon to the Scincoidea; however, a slender and straight retroarticular process disproves this affinity.

The species is known from a single and relatively poorly preserved specimen, and there is still much to learn about this lizard before a confident taxonomic assignment can be made. For this reason, we leave the genus and species unnamed, and hence, no type designation and diagnosis are provided However, the following character states are potential autapomorphies of this species fused frontals are extremely narrow; marginal teeth are extremely fine and closely arranged along the tooth row; postdentary bones are partially fused in the lower jaw.

Slavoia darevskii Sulimski, 1984 Figure 28

HOLOTYPE: ZPAL MgR­I/8, skull with mandibles and postcranial skeleton.

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

KNOWN DISTRIBUTION: Djadokhta Formation—Ukhaa Tolgod locality; Barun Goyot Formation—Khulsan and Khermeen Tsav localities (Sulimski, 1984; this paper).

REVISED DIAGNOSIS: Scincomorph lizard differing from other members of the group in having the following derived character states: skull very short and wide; nasal­prefrontal contact lost owing to widening of frontals; significantly wide frontals; great reduction of orbit into narrow teardrop­shaped opening; parietal foramen rudimentary or entirely closed; prefrontal­postfrontal contact forming medial rim of orbit; anteroventral process of jugal nearly lost; strong reduction in number of marginal teeth, with short entirely antorbital maxillary tooth row; suborbital fenestra strongly reduced; ectopterygoid contacts palatine anteriorly and posteriorly, excluding both maxilla and pterygoid from entering suborbital fenestra.

REFERRED SPECIMENS: Ukhaa Tolgod— IGM 3/145 (MAE 96­1), nearly complete skull with mandibles and partial postcranial skeleton (fig. 28C); IGM 3/146, incomplete skull with mandibles and articulated postcranial skeleton (fig. 28D); 3/147–3/153 (MAE 178/92­21, 160/93­40, 95­7, 94­66­1, 94­66­ 2, 94­66­3, 94­66­4), all incomplete skull with mandibles (total: 9 specimens from Ukhaa Tolgod). Khulsan—IGM 3/154 (MAE 63), nearly complete skull with mandibles and partial postcranial skeleton (fig. 28A, B); Khermeen Tsav—IGM 3/155–3/159 (MAE 175/92­21, 176/92­21, 181/92­21, 185/92­21, 198/92­25), all incomplete skulls with mandibles (total: 5).

REMARKS: Although Sulimski ( 1984) suggested that a relationship of Slavoia darevskii with the gymnophthalmine teiids ‘‘seems to be more probable,’’ he made no formal familial assignment of this taxon. Borsuk­Bialynicka (1991a: 10) ambiguously stated that ‘‘ Eoxanta , and possibly Slavoia darevskii , become sister groups of both Xantusiidae and Scincidae .’’ We interpret that this statement implies a possible Slavoia darevskii ­ Scincidae sister group relationship. More recently, Alifanov (1993a) without explanation listed Slavoia darevskii under the Acontiidae , which is commonly treated as a subfamily in the Scincidae (Greer, 1970; Estes, 1983; Estes et al., 1988).

In terms of skull morphology, Slavoia darevskii shares a single palatal character state with the acontine skinks (ectopterygoid contacts palatine anteriorly and posteriorly, excluding maxilla and pterygoid from entering the suborbital fenestra), but lacks many derived character states of the latter group (e.g., loss of limbs, loss of supratemporal arch, prefrontal and squamosal reduced, and Meckelian canal closed with fusion). Presence of palatine scrolling indicates that the taxon may be scincid related, but lack of oth­ er features (such as the jugal­squamosal contact) may prevent classifying Slavoia darevskii in the Scincidae . The relationships and classification of Slavoia darevskii within the Scincomorpha remain ambiguous, pending thorough phylogenetic study of this highly specialized form and its likely relatives.

Globaura venusta Borsuk­Bialynicka, 1988 Figure 29

HOLOTYPE: ZPAL MgR­III/40, incomplete skull with mandibles.

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

KNOWN DISTRIBUTION: Barun Goyot Formation—Khermeen Tsav and Khulsan localities (Borsuk­Bialynicka, 1988; this paper); Djadokhta Formation—Bayn Dzak and Ukhaa Tolgod (this paper).

DIAGNOSIS: ‘‘Small non­teiioid lacertoid with paired premaxillae, and frontals fused and constricted. Ratio of minimum frontal width/sagittal length 0.16­0.19. Postfrontal posterior extension subject to variability. No osteoderms. Snout/skull length ratio 0.3­ 0.33. Ratio of length of tooth row underlying orbit/orbit length 0.18­0.27’’ (generic diagnosis of Borsuk­Bialynicka, 1988: 214).

‘‘Skull length 14­25 mm. Modal skull length of 21.5 mm, as in holotype. Postfrontal rarely extends posteriorly more than half the length of the parietal table. Otic regions of the brain case globose’’ (specific diagnosis of Borsuk­Bialynicka, 1988: 215).

REFERRED SPECIMENS: Ukhaa Tolgod— IGM 3/160 (MAE 95­89), nearly complete skull with mandibles; IGM 3/161–3/163 (MAE 21/93­176, 95­24, 95­40), all incomplete skulls with mandibles. Khulsan—IGM 3/164 (MAE 217/92­61), nearly complete skull with mandibles. Khermeen Tsav—IGM 3/165–3/167 (MAE 173/92­21, 172/92­21, 183/92­21), all incomplete skulls with mandibles.

REMARKS: Borsuk­Bialynicka (1988) described Globaura venusta and classified it in the Lacertoidea. Later, Alifanov (1993a) list­ ed this species under the Xenosauridae without explanation. There is little doubt about its affiliation with the Scincomorpha, but the relationships of Globaura venusta to other scincomorphs are far from clear. Presence of interdigitation of the frontoparietal suture (fig. 29A, D) is the single character state supporting the referral of Globaura venusta to the Lacertiformes, based on the phylogenetic framework provided by Estes et al. (1988). Two other character states (pyriform recess narrow throughout most of its length, and facial region elongated and snout region laterally compressed) are incorrectly described for this species (fig. 29). Lack of a pterygoid lappet of the quadrate and a noninflated mandibular fossa are two other features that disprove the close relationship of Globaura venusta with the Lacertiformes. The enlarged postfrontal can be interpreted as either a lacertid or scincid feature, while fused and constricted frontals, and the presence of a jugalsquamosal contact ambiguously suggest possible relationships with either the Teiidae or the Scincidae . Because of conflicting evidence, we tentatively place Globaura venusta in the Scincomorpha without further referral to any subgroup.

Figures

Fig. 27. Scincomorpha, family incertae sedis, new genus and species (unnamed): A–C, IGM 3/54 incomplete skull with mandibles, lateral, dorsal and ventral views.

Fig. 28. Scincomorpha, family incertae sedis, Slavoia darevskii: A, B, IGM 3/154, incomplete skull with mandible and partial postcranial skeleton from Khulsan, dorsal and ventral views; C, IGM 3/145, nearly complete skull with mandibles and partial postcranial skeleton from Ukhaa Tolgod, dorsal view D, IGM 3/146, incomplete skull with mandibles and articulated postcranial skeleton from Ukhaa Tolgod dorsal view.

Fig. 29. Globaura venusta: A–C, IGM 3/164, nearly complete skull with mandibles from Khulsan, dorsal, ventral, and right lateral views; D–F, IGM 3/160, nearly complete skull with mandibles from Ukhaa Tolgod, dorsal, ventral, and left lateral views.