Philydrosaurus Gao & Fox 2005, GEN. NOV.

GENUS PHILYDROSAURUS GEN. NOV. Type species: Philydrosaurus proseilus sp. nov. Diagnosis: As for the type and only known species. Range: Early Cretaceous, western Liaoning, China. 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 slen- der 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 bor- der 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

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