Philydrosaurus proseilus, Gao & Fox, 2005

Gao, Ke-Qin & Fox, Richard C., 2005, A new choristodere (Reptilia: Diapsida) from the Lower Cretaceous of western Liaoning Province, China, and phylogenetic relationships of Monjurosuchidae, Zoological Journal of the Linnean Society 145 (3), pp. 427-444 : 428-437

PHILYDOSAURUS PROSEILUS SP. NOV.

( FIGS 2–5 )

Etymology: Philydros + sauros (Gr.), water-loving lizard, referring to the presumed aquatic life style of the animal; proseilos (Gr.), meaning toward the sun, similar in meaning to the place name Chaoyang.

Holotype: PKUP V2001, nearly complete skull and lower jaws in association with partial postcranial skeleton.

Type locality and horizon: China, western Liaoning Province, Shangheshou near the city of Chaoyang; Early Cretaceous Chiufotang Formation.

Known distribution: China, western Liaoning Province, Early Cretaceous Chiufotang Formation.

Diagnosis: A medium-sized monjurosuchid choristodere sharing with Monjurosuchus the following derived character states: frontal markedly narrow and hourglass-shaped; anterior extension of lacrimal close to nasal/prefrontal suture; supratemporal fenestra small; infratemporal fenestra closed by expansion of surrounding elements, including postorbital, quadratojugal and squamosal.

Differing from Monjurosuchus in having a more elongate skull; elongate narial openings and orbits; presence of a ‘supratemporal trough’ formed by the postorbital and postfrontal and containing the supratemporal fenestra; a distinct antorbital ridge on dorsal surface of prefrontal; postfrontal ridge forming medial border of ‘supratemporal trough’; hypertrophied postorbital and squamosal carrying a dorsal ridge to form lateral border of the ‘supratemporal trough’; temporal process of parietal short, roughly 1/ 3 of parietal length; a deep U-shaped post-temporal incision at the midline of the occiput; low-crowned dentition; strongly elongated iliac blade with parallel dorsal and ventral borders; presence of a distinct spike-like posterior process of ischium.

Anatomical description

The holotype ( PKUP V2001) includes a nearly complete skull, mandibles and postcranial skeleton missing minor parts of the limbs ( Fig. 2 ). The entire skeleton is preserved in a volcanic shale slab, and was exposed in dorsal view after preparation of the specimen .

Skull roof. The cranial skeleton is nearly completely preserved, and is dorsoventrally compressed as in other choristoderes ( Fig. 3 ). The general configuration of the skull in dorsal view, however, is sharply different from the type genus Monjurosuchus , as the skull of the new form is substantially more elongated, with the antorbital portion slightly longer than the postorbital portion. The total length of the skull is approximately 103 mm, with the antorbital portion 40 mm, the orbit 25 mm, and the postorbital portion 38 mm long. The external narial openings are paired, elongated and are nearly terminal in position. With a strong interorbital constriction, the orbit is large but more dorsally than laterally orientated. The orbit is longer than wide, differing from that in Monjurosuchus ( Gao et al., 2000) . The supratemporal fenestra of Philydrosaurus proseilus is unique: it is small and connects with an anterior trough formed by the postorbital and postfrontal ( Fig. 3 ). The infratemporal fenestra is completely closed by expansion of the surrounding bones, including the postorbital, quadratojugal and squamosal. The closing of the infratemporal fenestra is apparently a synapomorphic feature of the Monjurosuchidae , as it also occurs in the type genus Monjurosuchus ( Gao et al., 2000) . No post-temporal fenestra is developed: the extremely short temporal process of the parietal is in full contact with the squamosal and the quadrate (see below).

The premaxillae are small, paired elements that meet along a midline suture. The body of the premaxilla is penetrated by several foramina for nerves and vessels, and these are associated with small bumps developed as surface ornamentation. Each premaxilla has a slender but well-defined dorsal process that forms 1/3–1/2 of the medial border of the external nares (the dorsal process of the left premaxilla extends to the level of the posterior border of the narial openings). The posterior end of the dorsal process narrows between the slender anterior processes of the nasals that form the posterior part of the medial border of the external nares. The posterior margin of the premaxillary body is notched for the anterior border of the narial opening, and lateral to the opening is the irregular suture that marks the articulation between the premaxilla and maxilla. Posteroventrally, the premaxilla has a well-developed tooth-bearing process extending to the midlevel of the narial opening.

The nasals are paired, very narrow, elongated (20 mm long) and extend posteriorly to contact the prefrontal at the midlevel of the snout. Anteriorly, the nasal forms the posterior, but not part of the lateral, border of the narial opening. A slender anterior process of the bone contacts the dorsal process of the premaxilla to form a large part of the internarial septum. Laterally, the nasal has a long, straight suture with the maxilla, and posteriorly it is developed as a slender process that pinches between the anterior process of the prefrontal and the dorsal rim of the maxilla.

The prefrontals are paired and elongated, being the same length (20 mm) as the nasals. A midline sutural contact between the prefrontals is clearly visible as in other choristoderes generally; however, a narrow opening is evident within the midline suture; such a prefrontal fontanel has never previously been recorded in other choristoderes, including a small subadult skull of Champsosaurus (RTMP 94.163.1, see Gao & Fox, 1998) from the Upper Cretaceous Dinosaur Park Formation of western Canada. The dorsal surface of the prefrontal of this new animal is unique in having a sharp elongate ridge running anteriorly from the orbital rim to the level of nasal/prefrontal articulation. The prefrontal contacts the nasal anteriorly, the frontal posteriorly, and the lacrimal and maxilla laterally.

The frontals are paired, elongated and hourglassshaped, with a strong interorbital constriction. Having a greatest length of 26 mm, the frontal is longer than the prefrontal, and equal in length to the parietal (26 mm). The anterior part of the frontal is broken bilaterally, but it seems to lack an anteromedial process. The anterolateral processes are in articulation with the prefrontal but barely contact the lacrimal. The anterior and posterior width of the paired frontals is 11 mm, and the interorbital width, 5 mm. The frontal contacts the postfrontal posterolaterally along a short straight suture, and meets the parietal posteriorly at a contact that is roughly transverse to the long axes of these bones. The latter suture is at about the same level as the anterior border of the ‘supratemporal trough’ or the posterior border of the orbits.

The parietals are paired, having a total length of 26 mm, including the short temporal process. The dorsal surface of the parietal table is smooth, but a prominent longitudinal ridge is developed that flanks the lateral border of the table. This lateral ridge also forms the medial border of the small supratemporal fenestra, and is continuous anteriorly with the ridge of the postfrontal that extends to the posteromedial border of the orbit. There is no parietal foramen. The temporal process of the parietal is extremely short (9 mm long), roughly 1/3 of the total length of the bone (26 mm). The two temporal processes form a deep Ushaped post-temporal incision medially on the occiput ( Fig. 3 ).

The maxillae are well preserved on both sides. As in other choristoderes, the maxilla lacks a well-defined dorsal process, but inrolls medially to contact the elongated nasal bone. The maxilla has a short anterior process that is in articulation with the premaxilla and forms the lateral border of the narial opening. Medially, the maxilla has a long suture with the nasal and posteriorly contacts the lacrimal: the lateral wall of the maxilla sharply descends to meet the lacrimal and jugal. The slender posterior process of the maxilla is largely obscured on the right side, but the betterexposed left side seems to show it terminates at a level close to the posterior border of the orbit. This interpretation is supported by the fact that the lower dentition also terminates at this level, as shown by the disarticulated left mandible ( Fig. 3 ).

Anterior to the orbit, the lacrimal is located in a longitudinal depression that is deep and wide in its posterior part but gradually becomes narrow and shallow anteriorly, terminating not far behind the narial opening. The lacrimal is quite narrow and elongated (25 mm long), with its anterior extremity reaching beyond the level of the nasal/prefrontal suture. Medially, it meets the prefrontal, and laterally the maxilla and anterior process of the jugal. The medial edge of the lacrimal contacts the prefrontal in the longitudinal depression developed dorsally on the snout ( Fig. 3 ). The posterior border of the lacrimal is deeply notched for the anterior border of the orbit, and the dorsal rim of the orbit is ornamented with prominent tuberosities. The lacrimal foramen is identifiable on both sides, despite the dorsoventral compression of the skull. This foramen penetrates the lacrimal, the primitive condition differing from the derived pattern in which the foramen is bordered by the lacrimal, prefrontal and palatine as in neochoristoderes (see Gao & Fox, 1998).

The postorbital and postfrontal are separate, and both enter the posterior border of the orbit ( Fig. 3 ). This morphology is primitive in diapsids and differs from the fused condition in some but not all derived choristoderes ( Gao & Fox, 1998; Evans & Manabe, 1999; Ksepka, Gao & Norell, 2005). In dorsal view, the postfrontal is substantially smaller than the postorbital, but overlaps the latter element in the ‘supratemporal trough.’ Dorsally, the postfrontal displays a short but prominent ridge that runs from the posterior border of the orbit to meet the lateral ridge of the parietal. Medial to this ridge, the postfrontal contacts both the frontal and the parietal along a short, straight suture; the posterior extension of this suture enters obliquely into the ‘supratemporal trough’ that separates the postfrontal from a small lateral projection of the parietal. Also within the trough, the postfrontal overlaps the postorbital to form much of the anterior border of the supratemporal fenestra. Lateral to the postfrontal, the postorbital is greatly hypertrophied as the main element that closes the infratemporal fenestra. Dorsally, the postfrontal displays a strongly developed ridge that joins with the ridge from the squamosal to form the lateral border of the ‘supratemporal trough’. Lateral to the dorsal ridge, the postorbital has an extensive sutural contact posteriorly with the squamosal and ventrally with the quadratojugal. As a diagnostic feature of the family Monjurosuchidae , these three elements together have entirely closed the infratemporal fenestra, leaving no trace of the opening.

The jugal and the quadratojugal are preserved on both sides in the holotype. The anterior process of the jugal forms the lower border of the orbit and extends anteriorly to the lateral surface of the posterior process of the maxilla. The anterior tip of the process is damaged on both sides, but it probably terminated at a level slightly anterior to the orbit, as interpreted from the articular surface of the jugal on the maxilla. The postorbital, or dorsal, process of the jugal is low but well defined, forming a small part of the posteroventral border of the orbit. From the postorbital process, the dorsal margin of the jugal descends posteriorly to meet the quadratojugal. The short posterior process of the jugal fits into a small notch on the quadratojugal slightly anterior to the craniomandibular joint. The quadratojugal is a much smaller bone in comparison to the jugal and squamosal. It is located between the jugal and the squamosal, and dorsally contacts the enlarged postorbital.

The squamosal is greatly expanded as a major element that closes the infratemporal fenestra. The dorsolateral exposure of the squamosal is roughly a triangular plate, with a flattened surface rugosely ornamented. In dorsal view, the medial extension of the squamosal forms part of the floor of the ‘supratemporal trough’, and meets the temporal process of the parietal. The anterior border of this medial extension is thin, slightly concave and overlaps the quadrate. Posteriorly, this extension has two prominent processes separated from one another by a small notch ( Fig. 3 ). The medial process forms a small part of the lateral border of the narrow U-shaped post-temporal incision, and the lateral process forms the posterior extremity of the temporal region.

Braincase and palate: Only a small part of the lateral wall of the braincase is exposed in the supratemporal fenestra within the ‘supratemporal trough’, and the lateral wall is badly crushed on both sides. However, it is clear that a downgrowth of the parietal forms a small part of the braincase wall, and it has a dorsally concave border for articulation with a small bone, presumably the neomorph as in other known choristoderes ( Fox, 1968). A neomorph occurs in neochoristoderes and possibly in Cteniogenys ( Evans, 1990) , but this element cannot be identified positively for this new form because of breakage and distortion of the braincase. By the same token, a pterygoquadrate foramen is not identifiable on the new specimen, and the ventral parts of the braincase cannot be exposed without damaging the specimen; therefore, knowledge of the morphology of this part of the skull must await discovery of better preserved material.

Within the orbit, the pterygoid and ectopterygoid are partly exposed in dorsal view. The short medial process of the ectopterygoid forms the posterior border of the suborbital fenestra and overlaps the short lateral process of the pterygoid. A slender anterior process of the ectopterygoid extends along the medial side of the anterior process of the jugal, terminating slightly anterior to the midlevel of the suborbital fenestra; therefore, the short anterior extension of the process allows a small part of the jugal to enter the lateral border of the suborbital fenestra ( Fig. 3 ). The anterior extension of the pterygoid is greatly widened as a plate that forms the entire medial border of the suborbital fenestra. The midline suture of the pterygoids is obscured by the frontal, and thus, the taxonomically significant feature of the interpterygoid foramen cannot be observed in this specimen. The anterior end of the pterygoid is badly damaged on both sides, and the pattern of the pterygoid/palatine articulation cannot be determined. Posterior to the suborbital fenestra, only a small part of the subtemporal fenestra is exposed behind the medial process of the ectopterygoid; therefore, the actual size of the fenestra is unknown in the holotype of the new taxon.

Disarticulated from the palate, the left palatine is exposed in close association with the skull (partly obscured by the left maxilla). Most of the palatine is a thin plate, but it is slightly thickened posteriorly at the notch for the suborbital fenestra. Along the dorsomedial border of the palatine, the articular surface for the greatly elongated vomer can be seen. This surface is about half the width of the palatine, indicating an extensive articulation between the two bones. Although exposed in dorsal view, the exposed lateral edge seems to show that the palatine has a toothed ventral surface, but the distribution pattern of the palatal teeth cannot be ascertained in the absence of a ventral view.

Also disarticulated from the skull, the right vomer is nearly completely preserved in close association with a hyoid bone. Exposed in dorsal view, the vomer is greatly elongated, with a total length of 50 mm. The anterior extremity of the vomer is narrowly pointed but deeply bifurcated by a fissure 10 mm in length. About 19 mm from the anterior tip, a small vomerine foramen opens close to the medial border of the bone. Along this border, a well-defined ridge runs from the vomerine foramen to the posterior end of the anterior fissure. The vomer posteriorly has a notch for articulation with the pterygoid and posterolaterally an articular surface for the palatine. Lying to the side of the vomer is a slender bone 32 mm in length. This is probably one of the hyoid elements that are also known for some champsosaurs ( Gao & Fox, 1998: fig. 5).

Mandibles: Before burial, both mandibles of the holotype (PKUP V2001) were disarticulated from the skull ( Fig. 3 ). The left mandible is nearly completely preserved and is exposed in medial view; the dentary of the right mandible is exposed in lateral view but the postdentary parts are missing. The total length of the mandible is 83 mm as measured from the nearly complete left element. With a total length of 63 mm, the dentary is the longest mandibular bone and covers a large part of the lateral side of the lower jaw. As shown in the right mandible, the strongly convex lateral surface of the dentary is penetrated by rows of small foramina. The foramina on the anterior part of the jaw are closely spaced, while those on the posterior part are increasingly widely spaced; they are accompanied by narrow and posteriorly extending grooves that housed lateral cutaneous branches of the mandibular ramus of the trigeminal nerve and accompanying blood vessels. The posterior end of the dentary is bifurcated for articulation with the surangular bone (missing on the right mandible along with the other postdentary elements). Medially, the dentary carries a well-developed subdental shelf, the depth of which is roughly equal to the tooth height. Below the shelf is the Meckelian canal that opens medially as a narrow groove for the anterior one-third of the dentary length, but is closed by the splenial for the posterior twothirds. Anteriorly, the mandibular symphysis is extremely short and lacks a posterior extension as seen in advanced neochoristoderes. The splenial has been crushed into the Meckelian canal, but it shows that its posterior extremity terminates as a short tongue slightly beyond the posterior end of the tooth row. This tongue is wedged between the coronoid and the anterior extension of the prearticular, but may have a short ventral sutural articulation with the angular bone. The coronoid is a small triangular bone without a prominent dorsal process. It sends a short posteroventral process to articulate with the splenial and the prearticular, thereby contributing to a small part of the lower border of the mandibular fossa. Behind the fossa is the articular surface for the craniomandibular joint. From the length of the mandible, this joint is interpreted as being located roughly at the same level as the occipital condyle. The articular surface is at the posterior extremity of the jaw, and thus a retroarticular process is undeveloped in this animal.

Dentition: On the holotype specimen, marginal teeth are observed on both upper and lower jaws. Anteriorly, the upper dentition is exposed on both sides, but its more posterior parts are imbedded in matrix making the total number of the maxillary teeth indeterminable.

As a result of the disarticulation of the lower jaws from the skull, the lower dentition on the left is exposed in medial view and that on the right in lateral view ( Fig. 3 ). The left dentary has as many as 41 teeth preserved, with tooth spaces for 18–19 others. The right dentary carries 46 teeth and tooth spaces for 11 others. The teeth are generally low crowned but sharply pointed, although the anterior teeth are more slender and slightly taller than the posterior ones. The tooth crowns are weakly striated on their medial sides, but are smooth, without striations, laterally.

As best shown on the left mandible, the mode of tooth attachment is subthecodont (referred to as protothecodont by some authors): the teeth are set in a shallow dental groove containing shallow sockets for the tooth bases as in other choristoderes, including Cteniogenys ( Evans, 1990; Gao & Fox, 1998). No enlarged caniniform teeth are developed anteriorly on the marginal tooth row, unlike some other primitive diapsids (see Evans, 1988; de Braga & Rieppel, 1997). None of the upper or lower dentition shows any indication of basal infolding of the tooth enamel. Such basal infolding occurs in Champsosauridae and Simoedosauridae , but is absent in Cteniogenys ( Evans, 1990; Gao & Fox, 1998). Therefore, this character alone places the new choristodere and the Monjurosuchidae outside the Neochoristodera .

Vertebral column and ribs: There are 38 vertebrae preserved in the holotype, including 24 presacrals, three sacrals and 13 anterior caudals ( Fig. 2 ). Most of the presacrals are disarticulated, with ribs, the pectoral girdle and limb elements scattered nearby, while the sacrals and caudals are basically in articulation. All of the vertebrae have a centrum of amphiplatyan type, with essentially flat articular surfaces both anteriorly and posteriorly. The centra of the cervical vertebrae are slightly shorter than those of the dorsal vertebrae, and the caudal centra are generally more elongated than the dorsal centra (see below). In contrast to the condition in neochoristoderes, the neurocentral sutures of this new form are fully closed, and none of the vertebrae show disarticulation of the neural arch from the centrum.

Among the cervical series, the atlas is incompletely preserved, with a disarticulated centrum and pleurocentrum located posterolateral to the skull. The centrum is short with a slightly convex ventral surface, and the pleurocentrum in lateral view is an inverted L-shaped structure as in other choristoderes ( Evans, 1990). The axis is nearly complete with a low crownshaped neural spine extending posteriorly far beyond the centrum. The articular surface of the anterior zygapophysis is oval and faces dorsolaterally at roughly a 45° angle in relation to the sagittal plane. The articular surface of the posterior zygapophysis, also oval in shape, faces ventrolaterally with a similar angle as the anterior zygapophysis. The short centrum (6.5 mm long) of the axis is about half the length of the crownshaped neural spine, with an articular surface anteriorly indicating that the unpreserved odontoid process was free from the centrum as in other choristoderes, but the process itself is not preserved in PKUP V2001. Six rib-bearing vertebrae are also identified as cervicals, as these show a short centrum (6.5–7 mm long) with a well-developed ventral crest, and separated capitulum and tuberculum for articulation with double-headed ribs. Several cervical ribs are scattered nearby the cervicals. These ribs are short and straight, having a spiky distal end but a bifurcated proximal end (capitulum and tuberculum) for articulation with the diapophysis and the parapophysis of the vertebrae.

The remaining 16 presacrals are identified as dorsal vertebrae, because they show the unification of the diapophysis with the parapophysis and thus articulated with single-headed (holocephalous) ribs. In addition, the centrum of all of these is more elongated (8– 9.5 mm long) than that of the cervicals, and has a flat ventral surface lacking a ventral crest. The small spinous process below the postzygapophyses that is present in simoedosaurids ( Sigogneau-Russell, 1981) is absent in the presacrals of the new taxon. More than 20 ribs are preserved in association with the dorsal vertebrae. All of these are unicapitate and are pachyostotic, with a thickened distal end. The anterior surface of the dorsal ribs is convex in cross section, but the posterior surface has a well-developed groove running longitudinally along the ribs.

Three sacrals are identified clearly in the holotype. The second and third are preserved in articulation, but the first has broken away and is preserved in close association with the left pubis. Each sacral has robust transverse processes, and each carries sacral ribs that are not fused to the vertebra. The distal end of these ribs is strongly widened and is in contact with its neighbouring sacral rib to enclose a fenestra. The centrum of the sacrals is 8 mm long, similar to that of the posterior presacrals; however, the ventral surface is narrowed with strong lateral concavities. This morphology of the sacral centra shows the transition from the dorsal to the caudal series.

Of the caudal series, the first six vertebrae are preserved in ventral view in full articulation, while the remaining seven are preserved in lateral view and are disarticulated. The caudal centra vary in length from 8 to 9.5 mm as in the dorsal series, but are more slenderly built and are strongly compressed bilaterally. The ventral surface of the centrum has a groove that is flanked by well-developed ridges. None of the preserved caudals show any sign of neurocentral sutures, indicating the closure of the suture between the neural arch and centrum in the caudal series. The zygapophyses are essentially vertical as in other choristoderes. Each of the six anterior caudals bears ribs. The ribs are expanded in a horizontal plane, and are tightly articulated with the vertebrae, although a suture is visible. The first pair of ribs is the most robustly built in the caudal series, but the remainder progressively diminish posteriorly in length and thickness.

Gastralia: As in Monjurosuchus , a mass of gastralia is developed on the ventral aspect of the trunk region ( Fig. 2 ). The gastralia (or the so-called ventral ribs) are slender and rod-like structures scattered in the abdominal region. The individual elements are cylindrical and more or less spindle-like with pointed ends, but their original pattern cannot be determined because of their postmortem disarticulation.

Pectoral girdle and forelimb: The interclavicle is a Tshaped structure, with the crossbar slightly shorter than the stem (23 mm vs.? 25 mm). Anteriorly, the crossbar has a well-developed shelf that reinforces the articulation with the clavicles. Beneath the shelf is a groove for the interlocking articulation of the interclavicle with the lower half of the clavicle. Because the interclavicle is exposed in ventral view, the entire groove can be observed in this specimen. The stem is a narrow, lanceolate structure. The posterior end of the stem is concealed by the left coracoid plate. Judging from the exposed part, the stem is at least the same length (23 mm) as the crossbar, and may well be slightly longer than the latter. Both clavicles are well preserved, and slightly disarticulated from the other girdle elements ( Fig. 4 ). The clavicle is a boomerangshaped structure, with a thick, curved middle part and thinner and straight lateral ends. Both clavicles are exposed in anterior view, and thus the posterior edges that interlock in the groove of the interclavicle cannot be observed.

The scapula and the coracoid plate, although disarticulated, are well preserved on both sides of this specimen. The scapula is shorter than the humerus (27 mm vs. 34 mm), as measured from the left elements. The scapular blade is narrow and tall, with the dorsal end only 5.5 mm wide; the ventral end of the scapula is much broader, about 12 mm wide at the level of the glenoid. Posteriorly at the same level, a small triangular supraglenoid buttress borders the dorsal rim of the glenoid cavity. The buttress displays a small depression, but a supraglenoid foramen is not evident in the depression. Slightly above the buttress a small but prominent tubercle is developed for attachment of pectoral muscles. To our knowledge, such a structure is not previously known in choristoderes, and its function and distribution among choristoderes requires further investigation. The anterior border of the scapular blade is straight dorsally but curves more ventrally, where it matches the curvature of the clavicle. At the base of the scapular blade is a smooth bulge that may represent the remnant acromion process (see de Braga & Rieppel, 1997 for evaluation of this structure).

The coracoid is a single element, taking the form of a suboval plate with a rounded lower border. A second coracoid ossification is present in primitive reptiles ( Captorhinidae and Palaeothyris ) including early diapsids (Araeoscelidia), and the loss of this ossification has been interpreted as a derived condition for neodiapsids ( Gauthier, Kluge & Rowe, 1988: Sauria; Evans, 1988; but see also Rieppel, 1993; de Braga & Rieppel, 1997). The middle part of the dorsal border is significantly thickened for the glenoid cavity, which is short and less screw-shaped than in other choristoderes. A small coracoid (supracoracoid) foramen opens anteroventral to the glenoid cavity; this penetrates the coracoid plate and serves for the passage of the supracoracoid nerve and its associated vessels ( Romer, 1956).

The bones of the forelimbs are disarticulated but are in close association with the remainder of the specimen ( Fig. 2 ). The left humerus is exposed in lateral view, while the right is exposed ventrally. The humerus is shorter but more massively built than the femur, and is longer than the scapula (34 vs. 27 mm). The shaft of the humerus is short but is strongly constricted (4 mm at its narrowest). The proximal end of the bone is expanded (11 mm), while the distal end is even wider (14 mm). The two ends are twisted relative to each other at a small angle of less than 45°, as observed on both sides of the specimen.

In dorsal view, the proximal articular surface of the humerus is well ossified, indicating that this individual was mature at death. The anterior border of the bone is straighter than the posterior border, with the greatest curvature at the anterior border of the shaft. The proximal dorsal surface of the bone is generally convex, but is anterodorsally concave at the deltoid crest. The delto-pectoral crest is well developed, curving from the anterior border of the articular surface and then extending towards the shaft. Distally, the dorsal surface is slightly concave, in contrast to the proximal dorsal surface. Anterodistally, the ectepicondyle is a poorly defined structure with a low, smooth ridge running towards the shaft. Anterior to the low ridge is a deep and narrow fissure representing the ectepicondylar foramen ( Fig. 4 ). The supinator process (supracondylar process) is also a weakly developed structure that is set off from the ectepicondyle by the ectepicondylar groove. The posterodistal region of the humerus is expanded to a much greater degree than the anterodistal region, although the entepicondyle itself is weakly developed. Proximal to the condyle and distal to the shaft, another narrow fissure is present on the posterior border of the humerus. This fissure is the posterodorsal opening of the entepicondylar foramen (see below), representing the primitive morphology within choristoderes.

In ventral view, the proximal ventral surface is deeply concave with a triangular depression tapering towards the shaft. Anterior to the depression is a welldefined crest that supports a tubercle (slightly damaged on the right humerus of the holotype) for the attachment of the supracoracoideus muscle ( Romer, 1956). A large part of the distal ventral surface is damaged on the right humerus; consequently, the structure of epicondyles, trochlea (ulnar condyle) and the capitellum (radial condyle) cannot be observed. However, a narrow fissure-like structure is clearly identifiable as the entepicondylar foramen. This foramen occurs in primitive diapsids ( Reisz, 1981), but is lost in many advanced diapsid groups including most neochoristoderes. In this new choristodere, the fissure opens on the anterodorsal border of the humerus at the base of the shaft and at a level slightly higher than the ectepicondylar foramen ( Figs 3 , 4 ).

The disarticulated epipodial elements of the forelimb are completely preserved on the left side, but partly missing on the right. The epipodial is roughly two-thirds the length of the humerus; the ulna (24 mm) is slightly shorter and thicker than the radius (25 mm). Evans (1988) regarded a radius longer than the ulna as diagnostic of the Younginiformes , while Rieppel (1994) and de Braga & Rieppel (1997) later showed that this feature had evolved independently within Eosauropterygia, Prolacertiformes and Younginiformes . As in other choristoderes, there is no development of either an olecranon process or a sigmoid notch on the proximal end of the ulna in this new form. Loss of the process and notch has been recognized as a neodiapsid condition (see Evans, 1988; see also de Braga & Rieppel, 1997).

All of the elements of the manus, including carpals, are scattered on the shale slab, and thus the relative length of the digits and the phalangeal formula of the manus are indeterminable on the holotype. Six carpal elements can be identified on the associated part of the left manus.

Pelvic girdle and hind limb: All three ossifications of the pelvis (ilium, pubis and ischium) are preserved on both sides in close association, although disarticulated ( Fig. 5 ). Both ilia are exposed in lateral view. The dorsal iliac blade of this new form differs from that of Monjurosuchus in having a greater elongation posteriorly, parallel dorsal and ventral borders, and in lacking a constricted neck between the blade and the acetabulum. However, the blade retains a primitive morphology, including its lack of an anterior extension and the steep angle it makes with the horizontal plane (see character coding below). The lateral surface of the blade is convex and mostly smooth, but the dorsal border and the posterodorsal part are rugous for attachment of axial muscles ( Romer, 1956). The triangular base of the ilium is occupied by the acetabulum, with a well-developed supra-acetabular buttress forming the dorsal border.

The puboischiadic plate consists of a well-ossified pubis and ischium. The pubis is a broad plate with rounded dorsal and ventral rims. The straight posterior border is tightly articulated with the ischium and lacks a thyroid fenestra (see Carroll & Currie, 1991; de Braga & Rieppel, 1997; for evaluation). The lower part of the anterior border is weakly notched, but a pectineal tubercle (term of Romer, 1956) is absent. Posterodorsally, the pubis has a small obturator foramen ( Romer, 1956: for the nerve of the same name) opening anteroventral to the acetabulum. Above the foramen, the dorsal rim of the pubis is slightly thickened for articulation with the ilium and contributes little to the acetabulum.

The ischium of this animal is unique among choristoderes in having a well-developed spike (a horn-like process) projecting posteriorly from the middle part of the posterior border of the ischiadic plate ( Fig. 5 ). Such a strong process may have provided attachment for ligaments or tendons of a powerful tail in this animal. Anterior to the spike, the ischiadic plate is dorsally narrow and ventrally expanded in lateral view. The dorsal rim of the plate is thickened, forming a small part of the acetabulum.

The femur (44 mm long) is more slender than the humerus and is roughly 130% the length of the latter element. The bone is weakly sigmoid (best shown on the right femur) as in adult specimens of Monjurosuchus (see Gao et al., 2000). The proximal end of the femur has a greatest width of 10 mm, and the distal end, 10.5 mm; the diameter of the shaft is 4 mm at its narrowest. The right femur is partly damaged, but shows a shallow proximal depression representing the reduced intertrochanteric fossa. Anterior to the fossa is a longitudinal ridge (damaged), the proximal end of which is the broken base of the internal trochanter. As in Champsosaurus ( Romer, 1956) , this trochanter is set off slightly from the femoral head and is in a ventral position. There is no indication of a fourth trochanter, although the proximal base of the femoral shaft is damaged. The distal end of the femur lacks a clearly defined median notch, and thus the two distal condyles are poorly separated from one another.

The tibia and fibula are only preserved for the right hind limb, whereas those on the left are missing, together with the left pes. As in the forelimb, the epipodial segment of the hind limb is significantly shorter than the propodial (femur). The tibia is slightly longer than the fibula (33 mm vs. 31 mm), and is more robustly built than the latter. The proximal end of the tibia has a greatest width of 11 mm, while the distal end narrows to 6.5 mm. The condition of the cnemial crest cannot be interpreted, as the anterior surface of the tibia is badly damaged. The fibula is straight and is much more slender than the tibia; its widened distal end is about 6 mm, while the narrowed proximal end is only 4 mm wide. The right pes is incompletely preserved, with digital elements scattered but in association with the epipodial. Only two tarsals can be found on the specimen; others are not preserved. Four metatarsals can be identified, among which is metatarsal V. This has an expanded proximal end, but the short shaft is straight with no plantar tubercles. Like the manus, the relative length and phalangeal formula of the pes are indeterminable because of poor preservation.

Figures

Figure 2. Philydrosaurus proseilus gen. et sp. nov., holotype (PKUP V2001) from Shangheshou near Chaoyang, western Liaoning Province; Early Cretaceous Chiufotang Formation.

Figure 3. Philydrosaurus proseilus gen. et sp. nov., photograph and line drawing of the holotype skull and mandibles (PKUP V2001).

Figure 4. Philydrosaurus proseilus gen. et sp. nov., pectoral girdle and fore limb of the holotype (PKUP V2001). Note arrows pointing to ect- and entepicondylar foramina.

Figure 5. Philydrosaurus proseilus gen. et sp. nov., pelvic girdle, hind limbs and anterior caudal vertebrae of the holotype (PKUP V2001). Double arrows pointing to the ischium spike.