INCERTAE SEDIS

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 : 40-46

IGUANIA INCERTAE SEDIS

Isodontosaurus gracilis Gilmore, 1943 Figures 12 , 13

HOLOTYPE: AMNH 6647, incomplete mandibles with well­preserved teeth.

TYPE LOCALITY AND HORIZON: Bayn Dzak (Shabarakh Usu), Mongolian Gobi Desert; Upper Cretaceous Djadokhta Formation.

KNOWN DISTRIBUTION: Djadokhta Formation—Bayn Dzak, Bayan Mandahu, Ukhaa Tolgod, Zos, and Tugrugeen Shireh localities (Gilmore, 1943; Alifanov, 1993a; Gao and Hou, 1996; this paper).

REVISED DIAGNOSIS: Sharing with other iguanians the following derived character states: frontals fused and constricted between orbits, presence of jugal/squamosal contact, parietal foramen opens at frontoparietal suture, and presence of angular process in low­ er jaw. Differing from all acrodontan iguanians primarily in having highly pleurodont dentition.

Distinguished from all nonacrodontan iguanians by a large number of derived character states: squamosal significantly widened with lateral flange; quadrate process of pterygoid strongly expanded and laterally compressed as winglike; coronoid dorsal process not exposed laterally, but entirely covered by coronoid process of dentary; strongly developed coronoid process of dentary carries prominent lateral crest for attachment of bodenaponeurosis of external mandibular adductor muscles; lateral surface of lower jaw develops distinct fossa below coronoid process for the superficialis of external mandibular adductor muscles; angular extends anteromedially to midlevel of dentary tooth row; ventral border of lower jaw develops distinct trochlear notch for M. pterygomandibularis ; marginal teeth reduced in number, containing no more than 14 maxillary and 16 dentary teeth; tooth crowns increasingly dilated posteriorly and compressed laterally, with pos­ terior border lapping anterolateral border of next posterior crown, last tooth much reduced.

REFERRED SPECIMENS: Ukhaa Tolgod— IGM 3/84 (MAE 96­113), nearly complete skull with mandibles; IGM 3/85–3/89 (MAE 27/93­192, 42/93, 49/93­163, 161/93­40, and 94­16­1), all incomplete skulls with mandibles. Zos—IGM 3/90 (MAE 94­54), incomplete skull with mandibles. Tugrugeen Shireh—IGM 3/91 (MAE 261/92­123), nearly complete skull with partial postcranial skeleton; IGM 3/92 (MAE 23/93­28), partial skull with mandibles; IGM 3/93, 3/94 (MAE 221/93­8, 88/93­19), both partial skull with mandibles.

DESCRIPTION

The new specimens referred to Isodontosaurus gracilis include the best preserved skulls known for the species, which reveal taxonomically important features of this poorly known taxon. The species is previously known mostly from tooth­bearing jaw material. In view of the fact that the skull features of the species are extremely poorly known, a description of the cranial morphology is given below, based on the new material.

SKULL ROOF: The premaxillae are fused, bearing six conical teeth (IGM 3/85, 3/86, 3/ 91). The dorsal spine of the element is slen­ der and elongate and is distally slightly spatulate. The nasals are paired, having their anterior one third intervened by the dorsal spine of the premaxillae along the midline. The lateral border of the nasal contacts both the maxilla and the prefrontal, as an anterolateral process of the frontal is not well developed. Like in other iguanians generally, the frontals are fused and constricted between the orbits. A frontal shelf (best shown on IGM 3/84, 3/92) is well developed anteriorly, and the shelf dorsally has a pair of depressions for the posterior processes of the nasals. The anterior part of each frontal is slightly widened, and laterally has a flange for articulation with the prefrontal (shown on IGM 3/84, 3/92; figs. 12A, 13C). The posterior part of the element is very wide, with a maximum width three times that of the interorbital width. The frontoparietal suture is simply transverse, but the posterior border of the frontal is slightly notched for the parietal foramen.

The parietal table is short and roughly trapezoidal. A large part of the parietal table is penetrated by a well­developed fontanelle, which is confluent with the parietal foramen (a feature also seen in several but not all oth­ er nonacrodontan iguanians from the Gobi). The lateral border of the parietal table is flanged for origins of the temporal muscles. The posterior border of the table is also weakly flanged for attachment of the axial muscles. The supratemporal process of the parietal, best preserved on two specimens from Tugrugeen Shireh (IGM 3/91, 3/92), is slightly longer than the parietal table. For most of its length, the process carries a sharp dorsal crest and is sloped both medially and laterally making the process roughly triangular in cross­section. The process distally does not contact the quadrate, but is weakly articulated with the squamosal and the supratemporal bone (see below).

The maxilla is short, and is firmly articulated to the lacrimal and jugal; these together form a roughly rectangular lateral wall of the skull below the orbit. The maxilla has a triangular posterior process bulging laterally. This configuration of the element is similar to that in Mimeosaurus crassus , but it is not fused with the jugal and has no bony ornamentation along its dorsal border. Anteriorly the maxilla bears a well­developed anteromedial process (IGM 3/84, 3/85, 3/89, 3/91), which extends behind the premaxillary spine approaching or contacting the opposite structure. The nasal process of the maxilla is located dorsally above the anterior part of the tooth row. The process curves medially to articulate with the nasal and prefrontal, but seems to have no contact with the frontal as the anterolateral process of the latter is not well developed. Posterior to the nasal process, the maxilla is dorsally articulated with a small lacrimal and the entire length of the anterior process of the jugal; thus, the maxilla does not participate in the formation of the ventral border of the orbit. The maxilla of different individuals variably carries 10– 14 teeth (see below).

The prefrontal is well preserved on two specimens (IGM 3/84, 3/91). The frontal pro­ cess of the element is proportionally long, extending to the midlevel of the orbit along the lateral border of the frontal. The ventral process of the prefrontal (IGM 3/92) curves downward to form the anterior wall of the orbit, and has a small notch on its lateral bor­ der for the lacrimal foramen; therefore, the foramen must open at its suture with the lacrimal bone. The postfrontal is fused with the postorbital to form a postorbitofrontal (IGM 3/84, 3/91), but one specimen (IGM 3/92) shows a remnant ‘‘suture’’ on the left element (no suture at all on the right side).

As mentioned above, the anteroventral process of the jugal forms a large part of the ventral border of the orbit, and it has a triangular base that slightly bulges laterally The element, however, lacks a posteroventral process. The posterodorsal process of the jugal strongly slants posteriorly, and the distal half of the process is spatulate contacting both the postorbital and the squamosal (figs 12A, 13B).

The squamosal (best preserved on IGM 3/ 91) is widened, with a lateral extension for the attachment of the temporal muscles. The lateral border of the wing is curved medially and is posteriorly linked with a hooklike lateral process. A large part of the squamosal covers the cephalic head of the quadrate, but its anterior extension curves laterally and articulates with the jugal bone. The posterior end of the squamosal carries a short but welldefined dorsal (medial) process, forming the posterior rim of the supratemporal fenestra and contacting the supratemporal process of the parietal. The posterior border of the squamosal is slightly notched for articulation with the supratemporal bone (see below).

No supratemporal bone is identified on any of the known specimens of Isodontosaurus gracilis . Such an element is normally developed as a splint wedging between the supratemporal process of the parietal and the squamosal (but see Frost and Etheridge 1989). In this species, however, it appears to be absent as in some agamids and some other lizards (see Estes et al., 1988 for citations).

The quadrate is vertically positioned, without distortion (IGM 3/91). It has a thin and straight tympanic crest, lacking a strongly concave conch on its posterior margin. The cephalic condyle is partially covered by the widened squamosal, but is exposed posteriorly as a strong tubercle above the tympanic crest. As a matter of preservation, the anterior surface of the quadrate cannot be observed without further preparation; but the posterior aspect of the bone is observable on two specimens (IGM 3/91, 3/92). Like in many other lizards generally, the quadrate is posteriorly divided into two parts by a prominent and curved posterior crest that runs dorsoventrally from the cephalic condyle to the ventral condyle. The medial part of the posterior aspect is about half the width of the lateral part, and the medial part has a posterior opening of the quadrate foramen located about one third the height of the quadrate bone above the ventral condyle. The ventral condyle does not show significant expansion, but remains small and slightly saddle­shaped fitting in the articular fossa of the lower jaw.

PALATAL ELEMENTS: Slightly displaced palatal elements can be observed on IGM 3/ 85 and 3/92 (fig. 13D). Both the palatine and pterygoid are toothless elements. The palatine is narrow and elongate, forming the medial border of the suborbital fenestra. Along the medial border of the palatine is a narrow groove (on IGM 3/92), in which fits the slen­ der posterior process of the vomer which approaches or may even contact the pterygoid. The pterygoid has a short anterior process, which is articulated to the medial edge of the palatine. An even shorter lateral process articulates with the ectopterygoid. The posterior (or quadrate) process has a medial trough as in other lizards generally, but the process carries a strongly expanded dorsal wing. A similar condition is seen in extant Uromastyx (Saksena, 1942: text­fig. 7).

BRAINCASE: The supraoccipital is well exposed posterior to the parietal table (IGM 3/ 91, 3/92). It has a strongly convex dorsal surface, but lacks a clearly defined sagittal crest, nor does it develop a processus ascendens at its anterior border. The element laterally contacts the prootic with a longitudinal suture, and posteromedially has a notch at the dorsal rim of the foramen magnum. The posterolateral border of the bone contacts the exoccipital with a slanted suture. The paroccipital process is short, and has the prootic extended onto its anterolateral surface (figs. 12A, 13C). Also in dorsal view, the anterior and posterior semicircular canals are clearly recognizable in several specimens.

The braincase floor is slightly elongate and rectangular in shape (exposed on IGM 3/84, 3/92). Anteriorly, the cultriform process of the parasphenoid is poorly ossified, as only the basal part is preserved. The basipterygoid process is short, having a slender shaft but strongly expanded end for articulation with the pterygoid. The basisphenoid/basioccipital suture is medially horizontal but laterally diagonal, a primitive condition (Gao and Norell, 1998). The basioccipital composes a smaller part of the braincase floor than the basisphenoid. The spheno­occipital tubercles are short but proportionally quite robust. The tubercles are more ventrally than laterally directed. Other features of the braincase cannot be observed without further preparation of the specimens.

The epipterygoid is exposed on several specimens (IGM 3/84, 3/85, 3/94). This is a slender pillar as seen in other lizards generally, but its actual position and its contact with the lateral wall and roof of the braincase cannot be observed.

MANDIBLE: The mandible of the species shows a set of peculiar features, which are apparently related to its specialized feeding mechanism evidenced by its highly durophagous dentition. The jaw is robustly built. The dentary portion is slightly longer than the postdentary portion. Posterodorsally the dentary bears an extremely well­developed coronoid process, which entirely covers the lateral surface of the dorsal process of the coronoid bone (leaving no lateral exposure of the latter element). This process carries a prominent lateral crest for insertion of the bodenaponeurosis of the external mandibular adductors (see Oelrich, 1956; Rieppel, 1980c). The lateral surface of the mandible below this crest forms a distinct fossa (or depression), providing extra surfaces for insertion of the superficialis portion of the external jaw adductor muscles (see Oelrich, 1956). The fossa is ventrally bordered by a prominent adductor crest formed by both the surangular and the dentary bone. Within the fossa is the meandering dentary/surangular suture, and the anterior surangular foramen opens on the suture ventral to a short posterior extension of the dentary (fig. 12C).

The posterior surangular foramen is small and opens anterolateral to the craniomandibular joint. Below the anterior surangular foramen, the ventral border of the mandible develops a distinct trochlear notch probably for a tendinous bundle of the pterygomandibularis muscle (of Oelrich, 1956; M. pterygoideus of Lakjer, 1926; Haas, 1973). This notch is formed largely by the angular, which has a small lateral exposure and twists medially at the trochlear notch. The retroarticular process is slender, posteriorly directed, and terminates with a tubercle for the insertion of the M. depressor mandibularis (Oelrich, 1956) . Ventromedially at the base of the retroarticular process, an angular process is well developed and slightly hooked anteriorly (fig. 12B).

The medial aspect of the mandible can be observed on two specimens (IGM 3/84, 3/ 92). A noticeable feature on this side of the jaw is the long anterior extension of the angular bone. The anterior extent is twice the length of the posterior extension exposed on the lateral surface of the jaw. Anteriorly the angular terminates at the midpoint of the tooth row, anterior to the anterior inferior alveolar foramen. This anterior extension has a long sutural contact with the splenial as in some extant acrodontans (Jollie, 1960), but such a condition cannot be polarized satisfactorily in comparison with rhynchocephalians (Etheridge and de Queiroz, 1988; Frost and Etheridge, 1989). The posterior mylohyoid foramen (or angular foramen of other authors) penetrates the angular at the level slightly anterior to the coronoid summit of the jaw (fig. 13D).

The subdental shelf is present and curves along the basal line of the tooth row; a sulcus dentalis (dental gutter), however, is completely lost. The Meckelian canal anteriorly turns ventrally and opens as a groove, but posteromedially is covered by the splenial and the angular bone. The splenial is just slightly deeper than the angular and has a posterior extension terminating at the level of the distinct trochlear notch of the jaw where it contacts the anteroventral process of the coronoid. The anterior inferior alveolar fo­ ramen and the anterior mylohyoid foramen lie closely together (IGM 3/84).

DENTITION: As recognized by Gilmore (1943), Isodontosaurus gracilis is peculiar in having a highly durophagous dentition. All the marginal teeth are highly pleurodont with approximately two thirds of the tooth shaft attached to a relatively deep lateral parapet of the jaw (fig. 13D). Tooth replacement is of the typical iguanid type, where new teeth erupt within resorption pits. The tooth shafts are slender and slightly anteroposteriorly compressed; the crowns, however, are strongly dilated anteroposteriorly and compressed laterally, with posterior borders lapping the outside of the next posterior crown Generally, anterior teeth are small, increasingly becoming enlarged posteriorly; but the last maxillary tooth is often significantly smaller than the adjacent anterior tooth. The ultimate maxillary tooth is more medially located than others and is therefore slightly set off from the main tooth line.

Both small and larger specimens show essentially the same typical dentition; however the number of teeth in a complete tooth row varies considerably among specimens. For the maxillary tooth row, a complete tooth count ranges from as few as ten teeth (IGM 3/89, 3/90) to as many as 14 (IGM 3/84, IGM 3/91, 3/94); two other specimens (IGM 3/85, 3/86) have 12 maxillary teeth on each side. Dentary teeth can be observed on five specimens (IGM 3/84, 3/85, 3/90 –3/92), while other specimens have them concealed by the upper dentition. A complete dentary tooth row contains as few as 12 positions (IGM 3/90) or as many as 16 (IGM 3/84, 3/ 91, 3/92). One specimen (IGM 3/85) has 14 positions. Therefore, the dentary tooth row normally contains two more positions than the maxillary tooth row in the same specimen.

COMPARISON AND DISCUSSION

Gilmore (1943) named Isodontosaurus gracilis , and provisionally referred it to the Anguidae on the basis of its resemblance to Peltosaurus in tooth morphology. Estes (1983) removed Isodontosaurus from the Anguidae and hinted at a scincomorphan relationship. Ignoring the fact that the species has a highly pleurodont dentition, Borsuk­Bialynicka (1991a) regarded Isodontosaurus gracilis as a ‘‘true agamid,’’ and Alifanov (1993a) formally classified it in the Agamidae . The monotypic taxon has been given subfamilial and familial ranks (Alifanov, 1993a), which provide no help in elucidating its relationships.

Several specimens (IGM 3/84, 3/91, 3/92), collected from Ukhaa Tolgod and Tugrugeen Shireh localities, represent the best preserved skulls known for Isodontosaurus gracilis . These specimens, together with several others, show character states that support the referral of I. gracilis to the Iguania: frontals are fused and constricted between orbits; presence of a jugal/squamosal contact; parietal foramen opens at the frontoparietal suture (confluent with a fontanelle); and presence of an angular process on the mandible. More importantly, the new specimens reveal several characters that were previously unknown for this lizard (see discussion below).

The peculiar dentition of this bizarre iguanian was noted by Gilmore (1943), but a comparison of the new specimens reveals some previously unknown information. As described above, the number of maxillary teeth ranges from as few as ten to as many as 14, and the number of dentary teeth ranges from 12 to 16. No clear pattern can be identified for correlation of size with number of teeth, as a higher number of teeth occurs in both smaller and larger specimens. However, one recognizable pattern is that the dentary tooth row has two more positions than the maxillary tooth row in the same individual.

Besides the dentition, several other peculiar features of Isodontosaurus gracilis should be noted. One of these is the presence of a large parietal fontanelle confluent with the parietal foramen. A similar condition of the foramen is known not only in some nonacrodontan iguanians from the Gobi (e.g., Anchaurosaurus , Zapsosaurus , Igua , and Polrussia ), but also to a much smaller extent in some extant iguanians: e.g., Oplurus (Blanc, 1977) , some Sceloporus (Wiens and Reeder, 1997) , and acrodontan Uromastyx (Beddard, 1905; El­Toubi, 1945; Jollie, 1960). Presence of such a fontanelle is a noticeable feature for these Cretaceous iguani­ ans, although the phylogenetic significance of this character requires careful evaluation.

Several other peculiar features of Isodontosaurus gracilis are detectable in its lower jaw, and may have developed in association with its specialized feeding mechanism. These include: an unusually well­developed coronoid process of the dentary that covers the entire lateral side of the coronoid bone with a prominent lateral crest for attachment of the bodenaponeurosis of external mandibular adductor muscles; the lateral surface of the mandible has a well­developed fossa for the superficialis muscle; and the ventral bor­ der of the mandible develops a distinct trochlear notch that serves as a possible passageway for a tendinous bundle of the pterygomandibularis.

Except for its pleurodont dentition, Isodontosaurus gracilis shows striking similarities in skull configuration and palatal features to extant Uromastyx , which is commonly placed in the Agamidae (or in Leiolepidinae of the Chamaeleonidae, Frost and Etheridge, 1989 ). They share similarities such as: parietal foramen is confluent with the parietal fontanelle; jugal is posterodorsally spatulate; posterolateral process of basisphenoid extends onto spheno­occipital tubercle; dentary covers lateral part of the coronoid process; angular strongly extends anteriorly to, or surpassing, midlevel of tooth row. However, these features must be carefully evaluated to determine whether they are phylogenetically significant.

Based on the available evidence, Isodontosaurus gracilis can be broadly referred to the Iguania. The relationships of this bizarre lizard within the Iguania, however, are far from clearly understood; and for this reason, we classify it as ‘‘Iguania Incertae sedis .’’

Figures

Fig. 12. Iguania incertae sedis, Isodontosaurus gracilis: A–C, IGM 3/84, incomplete skull with mandibles from Ukhaa Tolgod, dorsal, ventral, and lateral views.

Fig. 13. Iguania incertae sedis, Isodontosaurus gracilis: A, B, IGM 3/91, cranial part of a skeleton preserved in concretion from Tugrugeen Shireh, dorsal and left lateral views; C, D, IGM 3/92, incomplete skull with mandibles from Tugrugeen Shireh, dorsal and ventral views.