Saichangurvel davidsoni CONRAD & NORELL 2007, new genus and species
Creators: name: CONRAD, JACK L.
name: NORELL, MARK A.
Publisher: name: Zenodo
Year: 2007
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Squamata
Saichangurvel
Saichangurvel davidsoni

description: Saichangurvel davidsoni, new genus and species figures 1, 3–4, 6–7, 8A, 9–12, 13A, 14–18
ETYMOLOGY: Saichan - (Mongolian: ‘ ‘beautiful’’) + gurvel (Mongolian: ‘ ‘lizard’’) and davidsoni after Amy Davidson, who collected and prepared the specimen. Davidson’s beautiful lizard.
HOLOTYPE: IGM 3/858.
TYPE LOCALITY AND HORIZON: Ukhaa Tolgod, Nemegt Basin, Mongolian Gobi Desert; Upper Cretaceous Djadokhta Formation (Loope et al., 1998) (fig. 2).
KNOWN DISTRIBUTION: Known only from the type locality and horizon.
DIAGNOSIS: IGM 3/858 differs from Ctenomastax parva and Temujinia ellisoni (each monospecific; hereafter, Ctenomastax and Temujinia, respectively) in possessing an apparently uncalcified region of the skull roof around the pineal foramen; a parietal fontanelle (similar to that seen in some extant Crotaphytidae). It differs from Temujinia and Zapsosaurus sceliphros (hereafter, Zapsosaurus) in lacking the enlarged paired fossae for the spinalis capitis. It differs from Ctenomastax in lacking caniniform teeth; from Zapsosaurus by the absence of strongly flared marginal tooth crowns and the more robust shape of the retroarticular tubercle; and from Anchaurosaurus gilmorei, Isodontosaurus gracilis, and Polrussia mongoliensis (each monospecific; hereafter, Anchaurosaurus, Isodontosaurus, and Polrussia, respectively) in possessing light dermal sculpturing on the parietal and frontal and the anterolateral orientation of the ectopterygoid. It differs from Isodontosaurus in lacking a posteriorly spatulate nasal process of the premaxilla, which does not make contact with the frontal on the dorsal skull roof; in possessing a weakly inclined anterior margin of the maxillary nasal process; in possessing a jugal that lies mostly dorsal to the maxilla; in possessing a supratemporal; and in possessing a mediolaterally developed postfrontal. It differs from Polrussia in possessing a midline contact of the maxillae posteriorly to the premaxillary nasal process, an elongate supratemporal, a distinct postfrontal, and an anteriorly oriented ectopterygoid. It differs from both Isodontosaurus and Polrussia in possessing a forked medial margin of the postfrontal and in lacking a dorsal process on the squamosal.
DESCRIPTION
Gao and Norell (2000: 106–107; fig. 37) illustrated and briefly mentioned and IGM 3/ 858, noting its exceptional preservation. The entire skeleton is preserved in articulation and lacks only the right postorbitofrontal, squamosal, and quadrate, the right forelimb and manus, the right femur and both hind-limb zeugopodia, and part of the distal tail (fig. 1). These portions of the skeleton were exposed prior to and during the thunderstorm in which IGM 3/858 was discovered. Because the specimen remains ventrally embedded in sandstone matrix, some ventral and medial elements (such as parts of the palate and pectoral girdle) are not visible. As noted by Gao and Norell (2000), the fantastic preservation of the specimen indicates a rapid burial. Rapid burial is typical of many of the Ukhaa Tolgod and Bayn Dzak fossils (see Loope et al., 2005) and is thought to have occurred when semi-stable sand dunes catastrophically mobilized when they became supersaturated with water.
SKULL FORM
The skull is lightly built, with large orbits and a complete supratemporal arch (figs. 1, 3–4). The antorbital snout makes up roughly one quarter of the length of the skull from the tip of the premaxilla to the anterodorsal margin of the foramen magnum. As preserved, the orbital region is slightly more elongate than the snout, making up approximately three quarters of the skull length. The nares are slightly retracted, a condition exaggerated by the fact that the snout tip has rotated slightly dorsally with respect to the rest of the skull (cf. figs. 3–4). The acuminate suborbital fenestra is visible in dorsal view through the orbit and is somewhat smaller than the supratemporal fenestra and approximately one half the length of the orbit. The supratemporal fenestra is posteriorly bordered by the ectopterygoid and pterygoid and anteriorly bordered by the palatine and jugal. The teardrop-shaped supratemporal fenestra is round anteriorly and tapers posteriorly. It is bounded anteriorly by the postorbital and postfrontal, medially by the parietal, laterally by the postorbital and squamosal, and posteriorly by the squamosal, supratemporal, and parietal.
Two ceratobranchials, identified as left and right ceratobranchials I, are preserved extending posteriorly from beneath the skull (figs. 1, 3A). These are elongate rods extending posteriorly to about the level of the posterior margin of the axis.
SKULL ROOF
PREMAXILLA (figs. 1, 3–4): The premaxilla is fused. It is displaced slightly out of its natural articulation with the maxilla, although parts of the articular surfaces remain in contact, and the premaxillary contact with the nasals remains articulated. The premaxilla retains seven visible teeth and probably contains 10 tooth positions. The premaxillary nasal process is broadest at its base and tapers dorsally. It is only about two tooth positions wide at its widest point but is still wider than deep. The premaxillary nasal process is broken near its base and near its contact with the nasal, but extends to near the posterior margin of the external naris. The margin between the nasal process and the main body of the premaxilla is not strongly angulated; instead, it curves down to the dental margin. A maxillary facet is present on the dorsal margin of this curved surface, but apparently was not exposed on the external skull surface. There are no ethmoidal foramina through the premaxilla.
MAXILLA (figs. 1, 3–4, 9): Both maxillae are well preserved, but the left side lacks the premaxillary process, and there is minor dorsal damage to the nasal process. The maxillae are sub-triangular, with an anteriorly placed nasal process that extends vertically along the side of the snout and medially near the naris. This condition also is present in Temujinia, Ctenomastax, and Crotaphytidae. The maxilla is sutured to the premaxilla without a premaxilla–maxilla aperture (sensu Gao and Norell, 1998; Conrad, 2006b, in press). Although only the right premaxillary process is preserved, it is complete and shows that the anterior terminus was forked into septomaxillary and anterior rami (fig. 3). The septomaxillary ramus of the premaxillary process projects medially. Based on the size of the septomaxillary ramus and the articular surfaces present on the premaxilla, as described earlier, it is clear that the septomaxillary rami would have met at the midline posteroventral to the premaxillary nasal process in the complete articulated skull. The shorter, anterior ramus approaches the base of the premaxillary nasal process without actually overlapping it. Posterodorsal to the premaxillary process, the maxilla extends in a gentle slope posteriorly such that the anterior margin of the nasal process is not strongly offset from the dorsal surface of the premaxillary process (fig. 4). The apex of the nasal process overlaps the anterodorsal margin of the prefrontal and extends medially onto the skull roof, where it contacts the nasal. The posterior margin of the nasal process is slightly emarginated, accommodating the anterior projection of the lacrimal. Its margin is posteroventrally oriented toward the dental margin and the posterior terminus of the maxilla. The dental margin extends to about the level of the midpoint of the orbit. Slight damage to the posteriormost tip of the alveolar margin prevents identifying whether the ectopterygoid was exposed on the external surface of the skull behind the maxilla, as it is in Leiocephalus, Oplurus, some iguanids (including Dipsosaurus dorsalis), and various scleroglossans. Five maxillary labial foramina are preserved, all situated anterior to the orbit and well separated from the dental margin.
NASAL (figs. 3–4): The paired nasals make contact for approximately one half their length, but are damaged near their middle (fig. 3). Each nasal, as exposed, is subrhomboid, tapering anteriorly and posteriorly. The anterolateral surface is gently curved laterally, forming part of the dorsal margin of the external naris. The premaxilla overlaps the nasals anteriorly, and the frontals invade the internasal suture for more than one third of the length of the nasals. The nasal tapers posteriorly where it dorsally overlies the frontal in a manner suggestive of a nasal lamina of the frontal.
PREFRONTAL (figs. 3–4): Each robust prefrontal is teardrop-shaped in dorsal view, with a broad anterior process and an elongate frontal process. No dermal sculpturing is present on the prefrontal. The prefrontal boss is expressed as a laterally projecting tubercle on the anterolateral corner of the orbit, near the anterior limit of the lacrimal (most visible in fig. 4B). This configuration is more similar to the condition seen in Ctenomastax (fig. 5D), Temujinia (Gao and Norell, 2000), and crotaphytids (Maisano, 2003a) than modern Iguana or Zapsosaurus sceliphros, in which the prefrontal boss is less robust and more contiguous with the rest of the skull surface. The frontal process extends posteriorly to near the midpoint of the orbit, but does not approach the postorbitofrontal. Most of the prefrontal medial border is bounded by the anterior part of the frontal. The prefrontal is blocked from contacting the jugal inside the orbital rim by a lacrimal–palatine contact (fig. 6). A single, small lacrimal foramen is present about midway up the orbital lamina of the prefrontal. It is enclosed dorsally, medially, and ventrally by the prefrontal and laterally by the lacrimal. The prefrontal and palatine are in broad contact in a straight, slightly ventromedially oriented suture.
LACRIMAL (figs. 3–4, 6): Both lacrimals are preserved in articulation with the maxillae and jugals. The lacrimals remain in articulation with the prefrontals, but dorsoventral compression of the skull has shifted the prefrontals slightly ventrally with respect to the lacrimals, jugals, and maxillae.
The lacrimal lies at the anterior end of the jugal. The outline of the lacrimal (its dorsal and ventral margins) is continuous with the anterior part of the jugal with only a small point of maxillary invasion between the two (fig. 4B). The lateral part of the jugal–lacrimal suture is nearly horizontal, and its orbital part is dorsomedially oriented. Dorsally, the lacrimal abuts the ventral surface of the prefrontal boss. The suborbital ridge arises from the point where the prefrontal makes contact with the lacrimal and extends posteriorly along the suborbital process and up the postorbital process of the jugal.
JUGAL (figs. 1, 3–4, 6): Although both jugals are well preserved and in articulation with the maxillae and lacrimals, they have been pushed somewhat out of position from their contacts with the supratemporal arch (figs. 1, 3). The left side is closer to its original position, but still has slid over the anterodorsal surface of the postorbital. The right jugal has turned somewhat dorsolaterally. The jugal is broadly exposed dorsal to the maxilla in lateral view and would have overlapped the ventrolateral surface of the postorbital and the squamosal.
The jugal is composed of a suborbital process and a postorbital process forming an obtuse angle to each other. No posteroventral process is present, but there is a moderately sharp point where the posteroventral process occurs in many lizards (including Anchaurosaurus, Ctenomastax, Temujinia, Zapsosaurus, and many other iguanians). A squamosal ramus of the postorbital process is present. The squamosal ramus is flattened mediolaterally and overlies the lateral surface of the postorbital when in natural articulation. The suborbital ridge extends from its origin on the lacrimal and curves around the ventrolateral and posterior portions of the orbit. The postorbital process of the jugal lacks the anterior and posterior flanges seen in many acrodonts, corytophanids, and some polychrotids; thus, it is not dilated. There is no dermal sculpturing on the jugal.
POSTFRONTAL (figs. 3–4): Saichangurvel davidsoni retains a separate postfrontal and postorbital. The right postorbital has been lost or is incomplete and hidden by the jugal and matrix (fig. 3). Regardless, it is out of articulation with the postfrontal, allowing a clear view of the postorbital facet on the postfrontal.
The postfrontal is a mediolateral bar joining the frontal and parietal with the supratemporal arch. As in Temujinia, the postfrontal is medially forked, with a long frontal process and a shorter parietal process, the latter fitting into a notch on the dorsolateral surface of the parietal. The frontal process is approximately twice as long as the parietal process, but does not approach the prefrontal. As in Temujinia and Ctenomastax, the postfrontal is a mediolaterally oriented bar. It borders the orbit and the supratemporal fenestra. Gao and Norell (2000) noted that this postfrontal morphology is unusual for iguanians and is more like that in scleroglossans. However, even among scleroglossans, the mediolateral-bar morphology of the postfrontal is relatively rare (Conrad, in press).
POSTORBITAL (figs. 3–4): Only the left postorbital is preserved, and it has moved somewhat out of natural articulation. The jugal hides the lateral surface of the postorbital, so the latter’s contribution to the posterior margin of the orbit cannot be determined. The postorbital is triangular in dorsal view. It has a short orbital surface, a long lateral surface, and a slightly curved contribution to the supratemporal fenestra. A forked facet on the postfrontal clasps the medial angle of the postorbital; the postorbital was laterally covered by the jugal in a short overlapping joint. The supratemporal process of the postorbital extends posteriorly in a tapering process that does not reach the midpoint of the supratemporal fenestra. A groove on the medial surface of the squamosal suggests that it accepted the lateral surface of the postorbital in a shallow tongue-and-groove contact.
SQUAMOSAL (figs. 1, 3–4): Only the left squamosal is preserved. It has been displaced somewhat from natural articulation with the supratemporal and postorbital by dorsal movement of the quadrate and by dorsoventral compression. Even so, it is well preserved, without fractures or apparent deformation. It is a hockey-stick–shaped bone with a tapering anterior postorbitofrontal process and a downturned posterior suspensorial process. The dorsal process is absent, an unusual condition for iguanians, but one shared with Temujinia. The squamosal, paroccipital process, and supratemporal articulate with the quadrate in the streptostylic suspension of the quadrate.
SUPRATEMPORAL (fig. 3): Only the left supratemporal is preserved. It is robust compared with the ‘ ‘small splint’’ present in Temujinia (Gao and Norell, 2000: 21). As exposed, the deepest part of the supratemporal is nearly as broad as the squamosal. The squamosal is displaced slightly posteriorly as a result of the dorsoventral compression of the skull. It remains in contact with the parietal, but the anterior part of the articular surface for it is visible on the ventrolateral face of the supratemporal process of the parietal. Based on the placement of this articular surface, the supratemporal would have extended for approximately one half the length of the supratemporal process when in life position.
FRONTAL (figs. 1, 3–4, 7): The azygous frontal is completely preserved, with only one major fracture near its anterior end. The ventral surface of the frontal is not visible as preserved, but it is clear that the frontals did not make contact with the prootics or basisphenoid. As is typical for pleurodontan iguanians, the frontal is hourglass-shaped in dorsal view, with the narrowest point occurring near the midpoint of the medial border of the orbit, posterior to the frontal process of the prefrontal. The anterior margin of the frontal has been damaged, but the frontonasal suture is W-shaped, with the anterolateral processes of the frontal approaching the nasal process of the maxilla. Presumably, the anterolateral processes were joined with the median process beneath the nasals, thus forming a nasal shelf. The frontoparietal suture is approximately as broad as the distance between the prefrontal bosses. The suture is transverse, though it is invaded medially by the frontoparietal fontanelle, similar to the condition seen in Zapsosaurus (fig. 5A), Anchaurosaurus, Chalarodon, some crotaphytids, Igua, and Polrussia. Unlike in Igua, Polrussia, and Zapsosaurus, the margin of this fontanelle is not smooth, but, instead, is more like that seen in Anchaurosaurus, Chalarodon, and some Gambelia, wherein the fontanelle is heart-shaped and has a somewhat ragged border. Some specimens of Temujinia show abrasion to this part of the skull, broadly resembling a frontoparietal fontanelle (see Gao and Norell, 2000: fig. 6B). This skullroof damage is not asymmetric and can easily be distinguished from the morphology present in taxa such as Chalarodon madagsacariensis, Gambelia (e.g., G. copei; fig. 5F), and Saichangurvel davidsoni.
PARIETAL (figs. 1, 3–4, 7): The parietal is complete except for the distal tip of the right supratemporal process. A large crack is present along the long axis of the bone, extending anteriorly from the base of the right supratemporal process to the right side of the frontoparietal fontanelle.
The parietal table is rectangular, its mediolateral breadth being greater than its anteroposterior length (figs. 3, 4A). It is invaded anteriorly by the frontoparietal fontanelle, and its lateral margins are medially concave. The jaw adductors originated from the dorsolateral surfaces of the parietal along the parietal table and the supratemporal processes. Unlike Temujinia and Zapsosaurus, which possess two pronounced parasagittal nuchal fossae (figs. 5A, 5B), Saichangurvel davidsoni possesses weak nuchal fossae (figs. 3, 4A). A similar condition is seen in most pleurodontans. By contrast, the nuchal fossae do not extend onto the dorsal surface of the parietal in many iguanians (e.g., Priscagama; fig. 5C). Decensus parietalis are absent. The elongate, narrow, supratemporal processes extend posterolaterally from the main body of the parietal. They are subequal in length to the parietal table, and their short axis is oriented posterodorsally. The long axis shows a gentle posteroventral curve.
PALATE AND QUADRATE
Although skull-roofing bones and matrix obscure most of the palate, some informative parts of the palatine, pterygoid, and ectopterygoid are visible (figs. 3, 6). Although the right vomer is visible through the right naris, it cannot be meaningfully characterized as preserved. Despite some dorsoventral compression of the skull, the palatal bones are mostly in natural articulation. The interpterygoid vacuity is broad posteriorly and tapering anteriorly, especially between the palatines. The right ectopterygoid has been pushed dorsally and moved very slightly out of natural articulation with the exterior skull roofing bones and the pterygoid, but the left side remains undisturbed. The left palatine is missing or hidden by matrix, and the quadrate process on the left pterygoid has been broken and moved dorsally. However, the original articulations remain for the other visible bones of the palate.
PALATINE (figs. 3–4, 6): The visible portion of the palatine shows that it shared broad contacts with both the prefrontal and the pterygoid. The palatine–pterygoid contact is a scarf joint, with the palatine dorsally overlying the pterygoid. In dorsal view, the palatine–pterygoid suture is generally posterolaterally oriented, but with large interdigitations. The infraorbital canal is enclosed dorsally, medially, and ventrally by the palatine, but has a large lateral contribution from the maxilla.
PTERYGOID (figs. 3–4, 6): Most of the dorsal surfaces of the pterygoids are visible, showing that the pterygoids do not make contact with each other or with the vomers. The anterior end of the left pterygoid is visible because the palatine is not preserved. Both transverse processes are anterodorsally covered by the ectopterygoids. The posterior part of the quadrate process is missing on the right side and obscured by matrix on the left side. Presence or absence of palatal teeth cannot be determined.
The palatine process is broad near its base, but tapers anteriorly to form an acute angle at its tip. The palatine facet is very weakly developed—so much so that that it is difficult to make out in dorsal view. The short transverse process is anterolaterally directed and tapers distally. The palatine process of the pterygoid is much broader than the main body of the pterygoid posterior to the transverse process, so the anterior margin of the transverse process is less strongly offset than the posterior margin. A columellar fossa and epipterygoid are preserved in articulation on the dorsal surface of the right pterygoid just posterior to the transverse process. Although a ventral view is not possible, the dorsomedial view through the orbit and the interpterygoid vacuity reveals that a basicranial buttress is lacking from the ventromedial surface of the pterygoid. The quadrate processes of both pterygoids are broken, but the posterior part of the left one is present near the postorbital– squamosal contact. The dorsoventral depth of the quadrate process is slightly less than the maximum breadth of the squamosal.
ECTOPTERYGOID (figs. 3–4, 6): Both ectopterygoids are well preserved; the left is in natural articulation, and the right one is only slightly displaced (fig. 3). Only the dorsal surface of each is visible. The ectopterygoid broadly overlaps the pterygoid in a scarf joint (i.e., it tapers in its narrowest plane distally). Both the ectopterygoid and the transverse process of the pterygoid are anterolaterally directed. By contrast, the transverse process of the pterygoid and the ectopterygoid are somewhat more laterally oriented in many acrodontans and in the pleurodontans Anchaurosaurus, Crotaphytidae, Polrussia, many polychrotids, and Temujinia.
The medial part of the ectopterygoid is bifurcated into dorsal and ventral pterygoid processes. The dorsal pterygoid process tapers medially to a point terminating near the main body of the pterygoid. The anterolateral portion curves gently and forms the posterolateral margin of the suborbital fenestra at the maxillary process. The posterior margin of the ectopterygoid expands at an acute angle from the anterior margin, but is notched to accommodate the anterior portion of the coronoid when the mouth is closed. The breadth of the maxillary contact is equal to about one half the length of the anterior margin of the ectopterygoid.
QUADRATE (figs. 3–4): Only the left quadrate is preserved, and it has been moved slightly out of its natural articulation; it has rotated clockwise around its long axis so that its anterior face is directed primarily medially. Despite this movement, the quadrate remains in contact with the cranium and the mandibular glenoid. Apparently concomitant with the quadrate movement, a portion of the pterygoid has broken off and is wedged between the supratemporal arch and the braincase so that it partly obscures the quadrate–articular contact in dorsal view. The medial face of the quadrate is still mostly visible so that it can be determined that a strong pterygoid flange of the quadrate is absent.
BRAINCASE
Although hidden anterodorsally by the parietal, much of the braincase is visible. Absence of the right supratemporal arch offers a clear view of the right prootic and the anterior surface of the paroccipital process of the otooccipital (fig. 7). Most of the supraoccipital and the posterior part of the paroccipital processes are visible in dorsal view behind the parietal (figs. 3–4). Matrix and the surrounding bones hide the basisphenoid, basioccipital, and occipital condyle.
PROOTIC (figs. 4B, 7): The right prootic is visible in right lateral view. Its relationships with the major foramina of the braincase (e.g., the facial and vagus foramina) are not exposed, but it is largely complete at the level of the prootic crest and above. A short alar crest (crista alaris prootica of some authors) is present and extends dorsally toward the
+
preserved (not restored). B–C, left scapulocoracoids of the extant taxa: B, Hoplocercus spinosus (AMNH 93807); C, Crotaphytus sp.
parietal 94(1), as in many iguanians (see Etheridge and de Queiroz, 1988; Frost and Etheridge, 1989; Maisano, 2003a, b; TorresCarvajal, 2003), but this is different from the anteriorly oriented plate seen in scleroglossans. A swelling of the anterior semicircular canal is visible at the base of the alar process. The well-preserved prootic crest (crista prootica of some authors) extends laterally and ventrally. It is slightly damaged posteriorly, but the anterior part is well preserved and shows that it was extensive. The otooccipital process extends posteriorly onto the paroccipital process.
SUPRAOCCIPITAL AND OTOOCCIPITAL (fig. 3– 4, 7): The supraoccipital and otooccipital are fused, although a faint indication of a suture remains on the left side. However, this may also be a crack created during diagenesis. The right paroccipital process is broken distally, and vertebrae and matrix obscure the exits of the vagus and spinal accessory nerves.
The supraoccipital has been somewhat compressed, and its anterior margin is obscured by the overlying parietal, yet it is well preserved generally. The bony base from which the processus ascendens originates is visible just posterior to the parietal at midline. The dorsal margin of the foramen magnum is slightly damaged. The supratemporal and opisthotic areas around the foramen magnum are contiguous and set off from the paroccipital processes. A swelling for the posterior dorsal semicircular canal is present posterolaterally near the base of the paroccipital process (fig. 7), similar to the condition in Gambelia copei (see fig. 5F).
MANDIBLE
Both mandibles are preserved in position but are mostly hidden anteriorly by matrix and the overlying skull bones. The posteromedial parts of both dentaries are narrowly visible through the orbits and suborbital fenestrae. Both coronoids are preserved in their natural articulation (or very close to it), as well as both surangulars and articulars/ prearticulars. The presence of an angular cannot be confirmed. Loss of the supratemporal arch has left the right posterior mandible exposed and only slightly damaged. The left mandibular fossa is well preserved and visible through the orbit posterior to the ectopterygoid. The right adductor fossa is preserved. It is small and dorsoventrally narrow, but only slightly smaller than that of similarly sized extant crotaphytids (fig. 8A, 8B). The right quadrate remains in contact with the right mandibular glenoid, but has rotated clockwise along its long axis in dorsal view as described above.
CORONOID (fig. 3, 4B, 8A): Both coronoids are well preserved and visible through the orbits, but with some parts obscured by the ectopterygoid, pterygoid, and jugal. The coronoid process is tall, narrow and bladelike, and projects well above the main body of the mandible. Distally, it is slightly twisted posteromedially. Its dorsal margin is rounded, rather than square, in lateral view. The ventromedial margin of the bone is arched, with anterior and posterior medial flanges. The anteromedial flange extends anteriorly to underlie the posterior part of the dentary. Anteriorly, the medially exposed margin of the flange tapers to a point between the splenial and the subdental shelf. However, disarticulated coronoids in some extant forms demonstrate that the coronoid may expand deep to these contacts, thus adding support to the dentary–splenial contact posteriorly. The posteromedial flange projects posteriorly almost to the level of the adductor fossa but does not contribute to the margin of adductor fossa as preserved. The posterior margin of the coronoid posteromedial process is vertically oriented. The posteromedial process possesses small interdigitations with the surangular, where the two contact dorsal to the adductor fossa and with the prearticular ventrally.
SURANGULAR (fig. 8A): Both surangulars are preserved. The left surangular is visible primarily in medial view, and the right one is visible in medial and lateral view. However, the right surangular has slight damage near the mandibular glenoid and has pulled away from the prearticular/articular element where the two meet at the anterior rim of the mandibular glenoid. This separation demonstrates absence of fusion between the surangular and prearticular/articular. The suture at the contact between the surangular and the prearticular/articular is faint, but remains undisturbed, just anteroventral to the adductor fossa. The lateral contact between these elements is hidden by matrix.
The lateral surface of the surangular is laterally convex along most of its length, just as in crotaphytids, Temujinia, and Ctenomastax. This condition is variable among iguanian groups. The iguanid Ctenosaura possesses a similar condition, with a longer dorsolateral slope, but Iguana possesses no lateral convexity at all. The surangular forms all the margin of the adductor fossa except for the ventral portion, which is formed by the prearticular/articular.
PREARTICULAR/ARTICULAR (figs. 3, 8A): The articular and prearticular commonly fuse in squamates and other reptiles (de Beer, 1937; Romer, 1949, 1956; Rieppel, 1993) into a single structure hereafter referred to as the prearticular. The prearticular is elongate in Saichangurvel davidsoni, forming the ventral and posteroventral margins of the adductor fossa, the mandibular glenoid, and the retroarticular process.
The articular glenoid is relatively small compared with the overall width of the mandible, more similar to the condition seen in polychrotids than crotaphytids (e.g., fig. 8B) or iguanids. The glenoid is weakly divided between medial and lateral portions and anteromedially oriented. It is bounded anteriorly and laterally by the surangular. The retroarticular process is elongate and possesses a dorsal pit or fossa. This fossa is laterally bounded by a weak tympanic crest and medially by a mandibular depressor crest. The robust articular tubercle lies along the medial margin of the retroarticular process, extending from the level of the articular glenoid posteriorly for about two thirds the length of the retroarticular process. The preserved part of the articular tubercle is expressed as a broad crest (visible in figs. 3, 8A). Damage along both the medial and lateral surfaces indicates that the preserved portion does not represent the entire tubercle,
(IGM 3/858) in posterolateral view.
and it is likely that a finger-like process was originally present along the anteromedial margin of the tubercle, perhaps similar to the condition in Dipsosaurus dorsalis (see Maisano, 2003b).
DENTITION: Saichangurvel davidsoni is heterodont in that the anterior teeth (those of the premaxilla and maxilla) are conical or peglike, whereas more posterior teeth are distinctly tricuspid (see fig. 4B). The premaxilla apparently contains 10 tooth positions. All of the preserved premaxillary teeth lack cusps. Only the right maxillary tooth row is exposed, and it has 19 tooth positions. The anterior two maxillary teeth are similar in size and form to those of the premaxilla. All of the more posterior teeth are tricuspid with a strong medial cone flanked by two slightly smaller accessory cusps. Only a few damaged dentary teeth are preserved at the posterior end of the tooth row (figs. 4B, 8), so the dentary-tooth count and dentary-tooth morphology are unavailable.
POSTCRANIAL AXIAL SKELETON
Fifty-three vertebrae are preserved, representing all regions of the axial column (figs. 1, 9–12). Representatives from 17 presacral rib pairs and both pairs of sacral ribs are present. The first 23 presacral vertebrae are preserved in articulation. The final dorsal vertebra is the first in an unbroken string also containing both sacral vertebrae and 23 caudals. Following this is a gap, then another two caudals, another gap, and one more caudal. The preserved posterior caudals are not significantly shorter than those at the end of the continuous series of 23, so a significant portion of the distal part of the tail may be missing.
Most of the vertebrae are in articulation, but the few dislocations demonstrate that the vertebrae are procoelous and lack a notochordal canal. None of the articulations between the anterior presacrals are visible, but the contact between the penultimate and the last dorsal shows that there was no obliqueness to the condyle–cotyle contact of the dorsal vertebrae. Zygosphenes and zygantra are absent. Nearly all of the vertebrae are preserved with intact neural spines, demonstrating that they are similar to those seen in many pleurodontans such as crotaphytids, iguanids, polychrotids, and hoplocercids.
PRESACRAL VERTEBRAE (figs. 1, 9, 12): Saichangurvel davidsoni possesses 24 presacral vertebrae. Differentiation of these vertebrae into cervicals and dorsals is problematic, given that the standard landmark for diagnosing these anatomical regions (i.e., first rib with a sternal contact; McDowell and Bogert, 1954; Romer, 1956) is not well preserved. Moreover, the presence of elongate dorsal ribs does not necessarily indicate the presence of a sternal contact. Many squamates possess one or more pairs of elongate posterior cervical ribs, making the identification of the cervical–dorsal vertebral boundary especially difficult in fossil taxa (Conrad, 2006a, b). Neural spines and centrum lengths vary independently within the presacral series and vary without clear correlation to any vertebral regionalization.
The atlas is preserved in articulation with the skull and the axis (figs. 1, 7, 9). Each atlantal arch is oriented vertically, with an anterodorsal process that arches medially (fig. 7). The atlantal arch possesses a posteriorly directed transverse process that is developed well enough to overlap the anterior portion of the axis in lateral view. Dorsal to this, the arch is posteriorly excavated by a round intervertebral fenestra. A short postzygapophysis forms the dorsal margin of the intervertebral fenestra and is posteromedially overlapped by the axis prezygapophysis.
The dorsally exposed portion of the axis is similar in general form to the succeeding vertebrae, except that the neural spine is more elongate (fig. 9). In lateral view, the transverse process is well developed (fig. 7B) and may have originally supported a lateral ossification, although a free, fully formed rib was probably absent; the morphology of the transverse process/synapophysis is similar to that present in the anguimorph Shinisaurus crocodilurus (Conrad, 2006a). The anterior extension of the axis into the ring of the atlas is visible in lateral view, but little else of its morphology is visible.
The axis and the two succeeding vertebrae are noticeably more elongate than the immediately succeeding vertebrae (fig. 9). Presacral vertebra 5 is intermediate in length between presacrals 2–4 and presacrals 6 and 7. Posterior to presacral 7, the vertebrae generally increase in length to about the level of vertebra 18, whereupon they become more or less uniform in length to the penultimate presacral. Presacrals 9–13 are subequal in length to the elongate anterior vertebrae, but those posterior to presacral 13 are slightly longer. Elongate vertebrae 2–4 are certainly cervicals, but so are vertebrae 5 and 6 and probably 7 and 8 under the current definition. Presacral vertebra 6 is the most anterior vertebra preserved with ribs. The ribs remain in articulation and are complete. They are very short—only about 1.5 times the length of the vertebra—and would not have been connected to the sternum. Thus, there is much variation in cervical vertebral length. Crotaphytus and Iguana are much more consistent in the lengths of their cervical vertebrae, although the penultimate cervical (cervical 7) in Iguana is slightly shorter than the adjacent vertebrae.
The neural spines are variable in their anteroposterior length and do not directly correspond with the length of the vertebrae or the presence or absence of elongate ribs (see fig. 9). The axis and cervical vertebrae 3– 5 possess anteroposteriorly broad neural spines. Vertebra 6 has an anteroposteriorly abbreviated neural spine; vertebra 7 possesses a more elongate neural spine; and the neural spines for vertebrae 8–16 are consistently elongate. These elongate neural spines are more robust posteriorly than anteriorly. In lateral view, the anterior margin of each describes a posterodorsally slanting surface that reaches its apex and flattens just posterior to the level of the synapophysis. Posterior to vertebra 16, the thin anterior lamina recedes, and only the robust posterior portion of the neural spine remains, giving the structure a narrower lateral profile with a more sloping anterior border. Although some of the neural spines are damaged, this neural spine pattern seems natural and differs from the more uniform morphology of the neural spines present in other observed pleurodontans.
As is common among squamates, the anterior synapophyses are more robust than the posterior ones. Although no ribs are preserved anterior to vertebra 6, synapophyses complete with fully formed articular surfaces are present, beginning with the third vertebra.
The holocephalus ribs are best preserved on the left side of the specimen. The rib articulating with vertebra 6 is very short, but the ribs increase in size posteriorly to about the level of vertebra 15 (fig. 1). Vertebrae 15–17 appear to have the longest dorsal ribs, posterior to which the ribs become shorter approaching the sacrum. A rib remains associated with presacral vertebra 23, lying in the thyroid fenestra of the pelvis and just lateral to the vertebra. The final presacral vertebra does not have an associated rib preserved, but it retains a damaged remnant of a synapophysis of the left side. This suggests that the final presacral vertebra carried an unfused rib and that there was no lumbar region.
SACRAL VERTEBRAE (figs. 1, 10): The sacral vertebrae are well preserved, although they have been shifted somewhat from their natural articulation with each other and from the dorsal vertebrae. As preserved, the last dorsal vertebra now dorsally overlies the first sacral. The anterior end of the first sacral has been pushed ventrally with respect to the rest of the vertebral column, pulling it slightly out of natural articulation with the second sacral and exposing the articular surfaces of the two bones. Despite this movement and associated damage, the left sacral ribs remain in contact with one another; those on the right side are damaged and partly obscured by the remnants of the right ilium.
The neural spines are not markedly different between the posterior dorsal and the sacral vertebrae. Both sacral ribs are preserved, the first showing some damage on the dorsal surface, probably caused when the overlying weight of the sediment forced the separation of the ilium from the sacrum. The sacral ribs are proximally narrow, but expand distally into broad blades making contact with one another at the ilium. The first sacral rib is somewhat posteriorly deflected. The contact between the sacral ribs occurs only very distally, is very narrow, and is unfused.
CAUDAL VERTEBRAE (figs. 1, 10–11): The tail is well preserved and curled around so that the end of the preserved caudal series lies close to the skull (fig. 1). Because the vertebrae are ventrally embedded in matrix, the chevrons are not visible, and the presence or absence of pygal vertebrae is impossible to identify. No fewer than 29 caudal vertebrae made up the tail, and it is likely that there were many more. The sacral vertebrae and first 23 caudals are preserved in articulation, after which four more are preserved separated by two gaps. The first gap is bounded by most of the preceding vertebra and a nearly complete succeeding vertebra. The gap is the appropriate size to have included the missing portions of the surrounding vertebrae and one more complete vertebra. The second gap is bounded by two complete vertebrae and is long enough to accommodate a single vertebra. The final preserved vertebra (reconstructed as caudal vertebra 29 based on the description earlier) is still robust and retains a relatively welldeveloped neural spine compared with those of nearby vertebrae, suggesting that it was far from the tail tip; certainly, it was not the last vertebra in the caudal series.
The caudal transverse processes are single, contrasting the double caudal transverse processes present in species of Anolis and many scleroglossans (Etheridge, 1967; Estes et al., 1988). Autotomy septa are present in the caudal vertebrae, but they are somewhat faint. An apparent autotomy septum is present on caudal vertebra 4 (figs. 10, 11A). That vertebra has a symmetrical transverse crack in the same position as a very faint septum line on caudal 5. Caudal vertebra 8 is also cracked, but not along septum line that occurs in the same position as that on caudal 5. Several vertebrae posterior to caudal 8 show remnants of autotomy septa. Hecht and Costelli (1969) note that autotomy septa sometimes fuse in older individuals of various lizards. The faintness of the septa in this specimen suggests their partial fusion.
The caudal vertebrae vary in their morphology along the length of the tail (fig. 11). The preserved caudals have subequal central lengths, becoming much more slender posteriorly but not shorter. This slimming of the caudals comes from a reduction in size of the neural spines and transverse processes, as well as a reduction in centrum diameter.
Anteriorly, the neural spines are extensive and similar to the general morphology present in the anterior presacrals with a posteriorly robust portion and a thinner anterior lamina (e.g., fig. 11A). However, the anterior caudal neural spines are somewhat taller than those of the anterior presacral vertebrae. Development of the thin anterior lamina is most extensive in the first four caudals and gradually reduces over the next three caudals to be absent or so poorly calcified/ossified as to preserve no remnant in caudal vertebrae posterior to caudal 8 (e.g., fig. 11B). Thus, from caudal vertebra 4–8, the neural spine transitions from a broad, square-shaped profile in lateral view to a posteriorly placed vertical tab with a slight posterior deflection. Caudal vertebra 8 retains a vertical neural spine, but caudal 9 (fig. 11B) and the following vertebrae have much shorter neural spines that are oriented more posteriorly. In addition, these vertebrae possess a sagittal lamina that is more robust than that seen in the more anterior caudal vertebrae, although it is still delicate relative to the main portion of the neural spine. The lamina extends anteriorly to the level of the transverse process, where it joins a pseudospine associated with the autotomy septum (fig. 11). The neural spine is further reduced in more posterior vertebrae. It does not extend posterior to the level of the postzygaphophyses in vertebrae posterior to caudal 15 (e.g., caudal vertebra 20; fig. 11D). At the level of caudal vertebra 21, the neural spine is gone, leaving only the sagittal lamina.
Transverse processes are present at least to caudal vertebra 23 and may have been present in more posterior caudals, as well, but the three vertebrae posterior to caudal 23 are not well enough preserved to confirm their presence or absence (fig. 1). The first two pairs of transverse processes are somewhat posteriorly deflected and placed posteriorly so that their distal tips extend beyond the level of the postzygaphophyses (fig. 10). The third pair of transverse processes distally extends to about the same posterior level as the postzygapophysis. All subsequent transverse processes extend laterally.
The anterior transverse processes are notably longer than those posteriorly. The transverse processes on the second caudal are more elongate than the length of the vertebra itself. The lengths of the caudal transverse processes decrease posteriorly, and the transverse process on caudal 9 is shorter than the vertebral length. Caudal vertebra 12 and all succeeding caudals bear transverse processes that are equal only to about half the length of their centra or less; there are no transverse processes preserved posterior to caudal 18.
PECTORAL GIRDLE AND FORELIMB
Both sides of the pectoral girdle are preserved and much of the pectoral girdle is visible, despite being mostly embedded in matrix ventrally (figs. 1, 12). Parts of the clavicles, scapulae, precoracoids (sensu Barrows and Smith, 1947; Costelli and Hecht, 1971; Rieppel, 1980b) and coracoids are preserved and visible on both sides of the animal. Note that the broader homology of the precoracoid within gnathostomes remains questionable (R.M. Shearman, personal commun.). Also, a thin bar of bone is visible extending from beneath the precoracoid toward the angle of the clavicle. This structure probably represents the left arm of the interclavicle; alternatively, it may represent a calcified portion of the epicoracoid. Surprisingly, remnants of both suprascapulae are preserved. The suprascapulae are preserved only as amorphous masses of calcified tissue that have folded down onto the suprascapulae. Both forelimbs are represented. Whereas the right one is preserved only from the shoulder to the proximal part of the zeugopodium, the left is almost completely preserved.
CLAVICLE (figs. 1, 12): The right clavicle is damaged and partly hidden by matrix, but the left clavicle is nearly complete and remains in contact with the scapula, suprascapulae, and interclavicle. The angled clavicle possesses a broad medial process, but there is no notch or foramen such as that seen in some iguanians (e.g., some polychrotids, corytophanids, and some phrynosomatids, among others). The ascending process is narrower than the medial process.
CORACOID/PRECORACOID AND EPICORACOID (figs. 12–13): Ribs and vertebrae hide parts of the endochondral component of the pectoral girdle, but the absence of visible sutures suggests that the scapula, precoracoid, and coracoid were fused. Most of the morphology of the coracoid/precoracoid unit may be reconstructed when the two sides of the specimen are compared (fig. 13).
The coracoid/precoracoid contributes the posterior margins to each a primary (anterior) and a secondary (posterior) coracoid fenestra. These coracoid fenestrae are given their shape partly by two precoracoid bars lying anteromedially. The posterior coracoid bar and the very base of the anterior coracoid bar are visible on the left side of the skeleton.
SCAPULA AND SUPRASCAPULA (figs. 12– 13): Both robust scapulae are well preserved, but the left scapula is more visible than the right. Whereas the anterior margin of the scapula is gently concave, the posterior margin is straight. There is no scapular bar, and there is no subdivision of the scapular fenestra, as is seen in some iguanians. Both suprascapulae are preserved, but little of their original morphology remains. These appear to have been composed of calcified cartilage and to have collapsed onto the dorsomedial parts of the scapulae.
HUMERUS (figs. 1, 9, 12, 14): Both robust humeri are preserved in dorsal view, in contact with their glenoids and in articulation with their zeugopodia. The left humerus has been prepared slightly more than the right, but both retain matrix only on their ventral surfaces near the midlines of their shafts. The humeri are uncrushed and undistorted. Only very faint suture lines remain between the epicondyles and the main body of the bone humerus both proximally and distally, again indicating the maturity of this specimen (fig. 14B).
The humerus in Saichangurvel davidsoni is comparable to that of a similarly sized Crotaphytus but somewhat more robust (the minimum diameter of the humerus is approximately 12% of the humerus length in the former and 9% in the latter). Both humeral heads are slightly obscured by their contacts with the glenoids, but their general morphology is clearly preserved and is similar to that of other limbed squamates. The triangular deltopectoral crest is well developed and extends for about 0.4 cm, with its apex situated in its distal one third. Muscle scars are visible on it lateral surface. Based on comparisons of published Iguana dissections (Romer, 1922), the clearest of these was the origin of the M. triceps c. humeralis medialis. The robust medial tubercle is similarly shaped but with a proximal apex directed posterodorsally (fig. 14). A muscle scar near the base of this tubercle is similar to the insertion point of the scapulo-humeralis anterior in Iguana as described by Romer (1922). Distally, the epicondylar region is nearly 2.5 times the width of the humerus at midshaft. The ectepicondyle arcs outward a little more dramatically than the entepicondyle; the former produces a tiny point at its dorsolateral margin, whereas the latter is expressed as a gentle curve that reaches its apex more distally. An ectepicondylar foramen is present near the proximolateral margin of the ectepicondyle 174(0). It is preserved on both right and left humeri. Unfortunately, the radial and ulnar condyles are mostly hidden behind the zeugopodia.
FORELIMB ZEUGOPODIUM (figs. 1, 9, 12, 14): The morphology of the radius and ulna is known mostly from the left side. The distal ends of both the radius and ulna have been cracked and the end of the ulna has tuned dorsolaterally nearly 90 degrees. The exposed portion of the proximal head is rounded, but somewhat oblong, around its short axis. The ulna is longer than the radius and proximally terminates in an olecranon process. The olecranon apex occurs somewhat posterior to the main axis of the bone. Both the radius and the ulna possess narrow shafts that expand distally. As with the humerus, faint suture lines are visible at the epiphyseal–shaft contacts of both the radius and ulna, but the epiphyses were at least partly fused at the time of death. An ulnar patella is preserved on the dorsal surface of the humerus, lying just proximal to the olecranon process (fig. 14). This element is relatively small compared with the patella preserved in the left knee (below) and sits in the very faint olecranon trochlea as preserved.
MANUS (figs. 1, 12, 14): The left manus is preserved with the wrist in a flexed position. Consequently, only the ventral surface of the manus is visible as preserved, and some of that surface is partly obscured by the radius, ulna, and manual sesamoids (fig. 14).
The robust radiale possesses a well-developed medial tubercle. A thin layer of matrix remains mesially between the radius and ulna, but this has been reduced through preparation to the point of being translucent, so the absence of an intermedium can be confirmed. A large pisiform is preserved in contact with the posterodistal margin of the ulna. The pisiform is rounded but more elongate proximodistally than mediolaterally. Distal carpal 5 is very narrowly visible distal to the pisiform. Two palmar sesamoids (heterotopic calcifications of the palmar aponeurosis) are present medial to the pisiform and distal to the radiale (fig. 14). They hide much of the carpus as preserved, and the larger of the two (the medial element) narrowly overlies the proximal margins of metacarpals I and II. Similar palmar sesamoids are present in many extant lizards and were recently described for a fossil shinisaurid (Conrad, 2006b).
All five metacarpals and digits are preserved. The metacarpals are indistinguishable from those of iguanians such as Crotaphytus, Iguana, and Plica. Metacarpals III and IV are of subequal length. Digits II, V, and I are of progressively lesser length. The first phalanx is missing from digit I (although it may be preserved lying distal to digit I; fig. 14A), but the ungual appears to be lying in its natural position, allowing the length of the first phalanx to be reliably estimated and the manual digital formula to be identified as 2–3–4–5–3. The penultimate phalanges are subequal in length with the next-to-penultimate ones (fig. 14). The unguals are not strongly curved and are slightly shorter than the penultimate phalanges. Ungual I is the longest in the series and appears is subequal in length to the space left by the proximal phalanx in digit I.
PELVIC GIRDLE AND HIND LIMB
All of the pelvic bones are preserved except for the right ilium (figs. 10, 15). The bones of the left side remain in natural articulation, and the partial right pubis remains articulated with the left pubis. The right ischium is preserved only ventral to the ilio-pubic peduncle and projects ventromedially beneath the first sacral and last dorsal vertebrae, probably retaining its contact with the left ischium. The pelvic bones were fused together. Very weak sutures remain visible only between the ilium and ischium (figs. 15–16).
The thyroid fenestra has been deformed somewhat by the shifting pelvic bones. However, its original shape may be inferred to have been wider than long. As is common among squamates, it was posteriorly invaded by the ischia and, perhaps, hypoischia.
ILIUM (figs. 10, 15–16): Only the left ilium is preserved. It has come apart from the sacral vertebrae but is undistorted and is exposed in dorsal and medial views. The lateral surface, including the acetabulum, is hidden.
The robust ilium is elongate and dorsoventrally shallow, but robust mediolaterally. A robust anterior process is present and is slightly anterolaterally deflected, just out of the main axis of the iliac blade. In natural articulation, its straight dorsal margin would have been oriented somewhat posterodorsally. The extreme distal tip of the posterior process has been damaged, so the presence or absence of a distinct fossa for the origin of the caudoiliac ligament cannot be confirmed. The ventral margin of the iliac blade is slightly arched ventrally, with a small constriction of the blade just posterior to the descending portion of the ilium. The medial surface of the iliac blade possesses a bulbous medial expansion, tapering anteriorly, that served as a buttress for the sacral contact in natural articulation (fig. 16). This bulbous buttress is developed to varying degrees in limbed lizards; it is prominent in Iguana, but weakly developed in groups such as Crotaphytidae and Polychrotidae.
Faint scars marking the attachment points for two of the major pelvic muscles are present anterodorsal and posteroventral to the tapered anterior end of the sacral buttress. Based on comparisons with a published description of Iguana by Romer (1922), the identity of these muscles and others (below) may be determined. The dorsal edge of the blade is narrowed into an ilio-costal crest. Posterodorsal to the sacral attachment points and ventral to the iliocostalis scar is an origin site for the M. longissimus dorsi.
The suture between the ilium and ischium remains visible posteriorly, but becomes faint anteroventrally (figs. 10, 15–16). It is raised posteriorly into a ridge, but this flattens near the presumed point of common contact between the ilium, ischium, and pubis, and the suture is partly obliterated by intergrowth of the bones. The origin site of the cloacal muscle is preserved as a small tubercle at the posterior margin of the ilio–ischial contact. The ilio–pubic contact is mostly obliterated and is visible primarily as a change in contour where the two with intermittent remnants of the suture. It is dorsally arched as it extends anterolaterally. An anteroventral process of the ilium is present anterolateral to the obturator foramen (the latter fully within the pubis; see below) and is broad and rounded ventrally rather than tapered.
PUBIS (figs. 10, 15–16): The left pubis is well preserved and exposed dorsally. Diagenesis has deformed the symphysial process so that it now points slightly more anteriorly than it would have originally.
The pubes meet directly at the midline so that an eipipubis did not extensively invade the contact. An epipubis may have been present originally, but it was likely small as in other iguanians [contrast with the condition in, for example, eublepharid geckos (Grismer, 1988: fig. 17) and anguimorphs (Rieppel, 1980b: fig. 8; Conrad, 2006a: fig. 7)]. The obturator foramen is very large and elongate within the main axis of the pubis. This is apparently a real condition (rather than an artifact of preparation) because the margins of the foramen are smooth and unabraded. The shape of the pectineal tubercle is hidden, but the tubercle itself may be identified lying just medial (distal) to the obturator foramen, at about the midpoint of the pubis. Thus, the tubercular and symphysial portions of the pubis are of subequal length.
ISCHIUM (fig. 16): Both ischia are partly covered by the final dorsal vertebra and the first sacral, and the right ischium is very incomplete. The ischium is narrow proximally where it contacts the ilium and pubis, but expands and is rectangular distally. The anterior margin of the ischium (the posterior margin of the thyroid fenestra) is bowed posteriorly. The posterior margin expands posteriorly to form a short, pointed tubercle near the midpoint of the bone posteriorly, visible only on the right ischium. Little of the posteroventral margin is visible, but it seems to run anteroventrally from the posterior tubercle toward the midline.
FEMUR (figs. 1, 15, 18): Only the left femur is preserved, and it is preserved with its posterodorsal surface facing dorsally. Its head is hidden by the overlying ilium and remains partly embedded in matrix proximally, with some damage to the internal trochanter (sensu Romer, 1956), but is otherwise well preserved and exposed. It remains articulated with the pelvis and the hind-limb zeugopodium.
The femur is more strongly curved than in extant Iguana and Crotaphytus or the fossil taxon Anchaurosaurus (Gao and Hou, 1995, 1996). It curves distoventrally from the femoral head to the level just beyond the internal trochanter and then forms a gentle arc to the distal tip. Besides being robust, little about the internal trochanter is apparent. A patella is preserved on the dorsal surface of the femur, just proximal to the femoral trochanters (figs. 15, 18).
HIND- LIMB ZEUGOPODIUM (figs. 1, 15, 17– 18): Both the tibia and fibula are well preserved without deformation or serious damage, although the most proximal part of the proximal fibular head is missing. The fibula has been completely freed of matrix except for small areas near the tibial contact, but the tibia is still embedded, and its anterior surface is thus obscured. The fibula is slightly longer than the tibia, and the fibula is the only long bone preserved that retains a clear (open?) suture with its distal epiphysis on both sides. Loss of the proximal tip of the left fibula may indicate that the proximal epiphysis also was unfused. Besides a slight deflection of the fibula proximally, the tibia and fibula are both straight. The tibia has a narrow shaft that expands somewhat proximally at the trochanteric surface. The surface for articulation with the femur is triangular in dorsal view, with a weak and slightly rounded cnemial crest forming the anterior angle. A well-developed crest for the origin of pedal/ digital flexors is present along the anterolateral surface of the tibia. Both the tibia and the fibula end in distally convex articular surfaces without notching to receive the tarsus.
PES (figs. 1, 15, 17): Extreme dorsiflexion of both pedes hides much of the tarsal region. Consequently, presence or absence of astragalus-calcaneum fusion cannot be confirmed, and the number and morphological characterization of the distal tarsals remains unknown. Because of uncertainty regarding the fusion of the astragalus and calcaneum, they will be referred to individually. A full complement of phalanges is present in neither pes. However, the preserved bones of the pedes are complete, uncrushed, and unweathered.
Surrounding bones mostly hide the anterior surfaces of both astragali, but part of the lateral articular buttress is visible on the left astragalus, and most of the posterior surface is visible on the right astragalus. The anteromedial surface of the astragalus possesses a robust rim underlying the tibial fossa. A ridge separates the tibial and fibular articular fossae of the astragalus and calcaneum, respectively, and the two articular surfaces are set at nearly right angles to each other.
Both the left (fig. 17) and right distal tarsals 4 are preserved in articulation with the rest of their respective pedes, but the surrounding bones mostly hide each. The metatarsals are preserved on both sides, but only digit I is complete on the right pes and digits I and V on the left.
Metatarsal IV is the longest, being only slightly longer than III. Metatarsals II, I, and V follow in decreasing length. Metatarsal I is notably thin and delicate looking, but the digit is complete and retains a robust ungual. The hooked metatarsal V is extremely short and is only about half the length of metatarsal I. Its lateral process and lateral plantar tubercle are well developed. The lateral plantar tubercle occurs somewhat distally, near the midpoint of the bone. A complete phalangeal formula cannot be reconstructed. Digit I possesses two phalanges and digit V possesses four. Digits II and III are each known only from two phalanges, and digit IV is known only from one. The preserved unguals are similar in form to those of the manus.
description: Published as part of CONRAD, JACK L. & NORELL, MARK A., 2007, A Complete Late Cretaceous Iguanian (Squamata, Reptilia) from the Gobi and Identification of a New Iguanian Clade, pp. 1-48 in American Museum Novitates 3584 (1) on pages 2-27, DOI: 10.1206/0003-0082(2007)3584[1:ACLCIS]2.0.CO;2, http://zenodo.org/record/5388390