Myrmecodaptria microphagosa, KEQIN & NORELL, 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 : 47-52

FAMILY INCERTAE SEDIS Myrmecodaptria microphagosa , new genus and species

Figure 14

ETYMOLOGY: myrmex + daptria (Gr., f.), meaning anteater; mikrophagos (Gr.), meaning eating little.

HOLOTYPE: IGM 3 /95 (MAE 271/93­65), incomplete skull with mandibles.

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

KNOWN DISTRIBUTION: Known only from the type locality and horizon.

DIAGNOSIS: Sharing with other gekkotans derived character states such as: frontals

fused and having descending processes in contact below olfactory tract; postorbital absent; lacrimal lost; pterygoid teeth lost; crista prootica extends onto basipterygoid process quadrate suspension by paroccipital abutting and retroarticular process deflected medially

Differing from all other members of the group in having the following autapomorphies: Skull strongly elongate, having parietal table roughly equal length with frontals presence of well­defined ventral process anteriorly on ventrolateral flange of parietal supratemporal arch formed by slender squamosal only; squamosal extends anteriorly to level of frontoparietal suture, and abutting jugal; reduced number of small marginal teeth that are widely spaced and peglike.

DESCRIPTION

The holotype (IGM 3/95) is the only known specimen of this taxon. Although having a rounded snout, the skull is strongly elongate, and roughly rectangular in dorsal view. The snout is heavily covered with thick osteoderms that have pitted surfaces, and the external narial opening is not retracted.

SKULL ROOF: The premaxillae are fused and have a relatively wide dorsal spine that forms the medial border of the narial opening. The spine is covered by a thick osteoderm, and thus the pattern of the premaxillanasal articulation cannot be observed. The nasal is partly preserved on the left side, and is also covered with an osteoderm, like the premaxillae. The frontals are fused (fig 14B), and the olfactory flanges are well developed and in contact ventral to the olfactory tract (gekkotan synapomorphy, see Estes et al., 1988). The frontal articulates anteriorly with the nasal; other articulations are not preserved. Posteriorly the frontal is expanded and has a straight transverse suture with the parietal. The dorsal surface of the frontal is scarred, suggesting a covering of osteoderms

The parietals are fused without a trace of a midline suture (fig. 14B). The parietal table is narrow and very elongate. In length, it is roughly equivalent to the frontal bone. The dorsal surface of the parietal table is ornamented with osteoderms. The parietal foramen is a minute opening, which is located on the anterior one third of the parietal table

Fig. 14 . Continued. The ventrolateral border of the parietal table has a vertical flange, indicating a ventral origin of the temporal muscles. The anterior part of the flange bears a well­developed ventral process (downgrowth of Estes et al., 1988), which is laterally compressed and anteroposteriorly broadened. Such a process is absent in extant gekkonids and pygopodids (Estes et al., 1988). The process ventrally contacts the prootic (poorly preserved), but it cannot be determined whether it contacts the epipterygoid, as the latter element is not preserved on either side of the skull. The supratemporal process of the parietal is very short, having a length equal to about one third that of the parietal table. The supratemporal process has a roughly triangular base and a slender extension that contacts the paroccipital process (shown on the right side of the specimen).

The maxilla has a very short anterior process articulating with the premaxillae (fig. 14A). A slightly longer anteromedial process extends posterior to the tooth­bearing part of the premaxillae, but does not meet its equivalent counter part. The nasal (or dorsal) process of the maxilla is high, and is located dorsal to the anterior half of the maxillary tooth row (a primitive condition in squamates). The process dorsally articulates to the nasal and the prefrontal, and may contact the anterolateral process of the frontal (indicated by a gap between the nasal and the prefrontal). The lateral surface of the maxilla is covered with osteoderms, and is penetrated by a horizontal row of very small foramina (lateral superior alveolar foramina) parallel to its ventral border. The posterior process of the maxilla forms the ventral border of the orbit, and extends to the midlevel of the orbit where it articulates with the jugal.

The dorsal surface of the prefrontal is small and triangular, and is ornamented with osteoderms. The prefrontal forms the entire anterior wall of the orbit, and ventrally contacts the palatine within the orbit. Like in other gekkotans, the lacrimal is absent and a small lacrimal foramen opens at the prefrontal­maxilla suture (but see Rieppel, 1984). Below the lacrimal foramen is another small opening, representing the inferior orbital foramen. The postorbital is completely lost, leaving the elongate squamosal to form the entire bar of the supratemporal fenestra. The postfrontal (preserved on both sides) is forked medially to clasp the frontoparietal suture (see discussion above), and is laterally articulated with both the jugal and the squamosal (fig. 14B).

The squamosal is a slender bar extending anteriorly to the level of the frontoparietal suture, where it abuts the jugal and medially contacts the postorbital. The bone is hockeystick shaped without a trace of a dorsal process. The squamosal has a short contact with the supratemporal process of the parietal posterior to the supratemporal fenestra, but posteriorly this contact is separated by the splintlike supratemporal bone.

The quadrate, preserved on both sides, is in an oblique position slanting dorsoposteriorly. As in extant gekkonids (Rieppel, 1984), the quadrate has a convex anterior slope that forms part of the origin for the external adductor muscles. The cephalic condyle dorsally articulates with the squamosal and the supratemporal, and medially bears a well­defined process contacting the anteroventral part of the paroccipital process (a gekkotan synapomorphy, see Rieppel, 1984; Estes et al., 1988). The medial border of the quadrate is vertically straight, lacking any trace of a pterygoid lappet. The ventral condyle is expanded transversely, and a small quadrate foramen is recognizable anteriorly above the condyle.

PALATAL ELEMENTS: In palatal view, the vomers and the palatines are disarticulated and slightly displaced, indicating a reduced contact between these elements (Rieppel, 1984). The palatine is short, wide, and toothless. The anterior half of the pterygoid (toothless) is a broad elongate plate, making the interpterygoid vacuity (pyriform recess) narrow throughout most of its length. Although incompletely preserved on both sides, the quadrate process of the pterygoid is proportionally thick (fig. 14C). It is dorsally convex, but ventrally flattened. The ectopterygoid has a slender anterior extension, which meets the palatine excluding the jugal and the maxilla from the suppressed suborbital fenestra.

BRAINCASE: The braincase on IGM 3/95 is poorly preserved. In ventral view, the preserved braincase floor includes part of the basisphenoid and part of the basioccipital with the occipital condyle. The braincase floor is narrow and elongate, in keeping with the elongation of the skull. In articulation with several fragmentary bones including part of the atlas vertebra, the occipital condyle, formed by the basioccipital and the exoccipital, is partially exposed in ventral view, and seems to be rounded differing from the bipartite condition in extant gekkotans (Rieppel, 1984; Estes et al., 1988).

The lateral wall of the braincase (prootic) is poorly preserved, but it shows an alar process contacting the ventral process of the parietal. Although incompletely preserved, part of the medial wall of the recessus vena jugularis extends onto the basipterygoid process, suggesting that the crista prootica may have extended forward to this point. In dorsal view, the supraoccipital is better preserved and anterodorsally bears a prominent process (processus ascendens; processus anterior tecti of Jollie, 1960). Such a process is absent in extant gekkotans (Bellairs and Kamal, 1981; Rieppel, 1984; Estes et al., 1988). Posterolaterally, the supraoccipital has a sutural articulation with the exoccipital, which forms most of the lateral border of the foramen magnum, and is fused to the opisthotic forming the short paroccipital process. The exoccipital contributes to about one third of the occipital condyle on each side.

MANDIBLE: The mandibles are preserved on both sides of the specimen. The articular fossa of the lower jaw accepts the ventral condyle of the quadrate, and has a prominent buttress at its anterior border (fig. 14A). The dentary has a smooth lateral surface with no osteodermal ornamentation but a horizontal row of small mental foramina. Posterodorsally, the dentary sutures with the coronoid and the surangular, and at the junction of the three elements is a small opening representing the surangular foramen. The dentary has a long posteroventral process extending far beyond the apex of the coronoid to the midlevel of the surangular bone. Because such a long process occurs in some other gekkotans and xantusiids (see Rieppel, 1984), in some iguanians (Pregill, 1984; Etheridge and de Queiroz, 1988; Frost and Etheridge, 1989), and in more basal rhynchocephalians, it may well represent a primitive condition for squamates.

In medial view, the splenial covers about three fourths of the Meckelian canal below the tooth row. It has a slender tongue extending anteriorly, but posteriorly terminating in front of the posteromedial process of the coronoid bone. The anterior inferior alveolar foramen opens at the splenial­dentary suture at the level of the anterior one third of the tooth row, and the anterior mylohyoid foramen is close but slightly posteroventral to the former foramen.

Most of the postdentary part of the jaw is formed by the surangular, which has a round­ ed dorsal surface and a lateral crest for the adductor muscles attached to the jaw. The posterior surangular foramen opens anterior to the craniomandibular joint and lateral to the buttress of the articular fossa. The suture line between the angular bone and its adjacent elements cannot be delimited, and thus, the nature of the angular is uncertain. Medially, the mandibular fossa is narrow and elongate, extending from the posteroventral process of the coronoid to slightly anterior to the craniomandibular joint. The articular and the prearticular are fused without trace of suture. The retroarticular process of the jaw is broad at the base, but narrow toward the end (as opposed to extant gekkotans). The retroarticular process is slightly deflected medially resembling extant gekkotans, but differing from them in having no lateral notch.

DENTITION: The marginal teeth are strongly reduced both in size and in number. Most of the premaxillary teeth are not preserved, and the number of teeth on this element cannot be determined, although it can be estimated as six or seven. The maxillary and dentary teeth are peglike, widely spaced from one another with no basal expansion at all. There are approximately 13 tooth positions on each maxilla, but the actual number cannot be determined due to inadequate preservation.

The dentary teeth are better preserved than those on the maxillae. The left dentary has six teeth preserved, and the right side has five, but a complete dentary tooth row probably contains about 15 positions on each side of the jaws. The teeth are pleurodont in terms of implantation, having about half of the tooth shaft attached to the lateral parapet of the jaw.

COMPARISON AND DISCUSSION

On the basis of the phylogenetic framework presented by Estes et al. (1988), two subgroups are included in the Gekkota (but see Kluge, 1967 for different classification): the Gekkonidae (geckos) and the Pygopodidae (snake lizards). The latter family includes about 30 species, which are confined to Australia and its neighboring islands (Mattison, 1989). The group has a fossil record from Australia (Hutchinson, 1997), but is unknown from elsewhere. The former group, Gekkonidae , includes more than 800 species, and has a distribution throughout the tropical and subtropical world including hundreds of oceanic islands (Mattison, 1989). The group has Cenozoic fossil records in both western and eastern hemispheres (see Estes, 1983), but the Mesozoic record of the Gekkonidae is extremely poor and is known only from fragmentary specimens from the Gobi Desert (Alifanov, 1989b; Borsuk­Bialynicka, 1990).

The referral of the new genus and species to the Gekkota is supported by several character states as mentioned above. However, the relationship of the new taxon within the Gekkota remains unknown in the absence of a phylogenetic analysis. In recent literature, there are considerable discussions regarding the higher level phylogeny of gekkotan clades (Rieppel, 1984; Kluge, 1967, 1987; Grismer, 1988; Estes et al., 1988), and conflicting evidence from extant taxa has been the source of disagreement among authors on the placement of the Pygopodidae and Eublepharidae (see Estes et al., 1988 for comments). It has been demonstrated that fossil evidence can overturn a phylogenetic hypothesis based only on extant groups (Gauthier et al., 1988; Kemp, 1988); therefore, incorporating a primitive gekkotan from the Gobi (such as Myrmecodaptria microphagosa ) with extant taxa in a phylogenetic analysis may provide important insights into the character evolution of the group as a whole and help resolve the differences among authors.

Although the exact relationships of the new taxon cannot be ascertained at this stage, it is clear that the new lizard retains many primitive character states in comparison to extant gekkotans. These include: retention of a rudimentary parietal foramen; a well­developed jugal that forms a complete postorbital bar; the retention of a supratemporal fenestra with a complete upper temporal arch; presence of a processus ascendens of the supraoccipital; a well­developed supratemporal bone; Meckelian canal closed by the splenial without fusion of the dentary tube; splenial extending beyond the midpoint of the dentary tooth row; retroarticular process that is narrow posteriorly; and retroarticular process lacking a lateral notch, and thus, a narrow waist at the base of the process is not developed. Retention of these primitive character states (see Estes et al., 1988 for character evaluation) suggests that the new species from the Gobi may well represent a phylogenetically important basal taxon in the gekkotan clade.

Figures

Fig. 14. Gekkota incertae sedis, Myrmecodaptria microphagosa, new genus and species: A–C, IGM 3/95 (holotype), incomplete skull with mandibles from Ukhaa Tolgod, left lateral, dorsal, and ventral views.