417 Journal of Vertebrate Paleontology 20(3):417–421, September 2000  2000 by the Society of Vertebrate Paleontology RAPID COMMUNICATION EXCEPTIONAL FOSSIL MATERIAL OF A SEMI-AQUATIC REPTILE FROM CHINA: THE RESOLUTION OF AN ENIGMA GAO KEQIN 1 , SUSAN EVANS 2 , JI QIANG 3 , MARK NORELL 1 , and JI SHU’AN 3 1 Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A.; 2 Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, United Kingdom; 3 National Geological Museum of China, Xisi, Beijing 100034, China The lacustrine deposits of the Yixian Formation, western Liaoning, China, are renowned for producing remarkable ver- tebrate fossils, including primitive birds (e.g., Hou et al., 1995) and feathered dinosaurs (e.g., Ji et al., 1998). However, the first tetrapod described from these beds was a small reptile, Mon- jurosuchus Endo, 1940, whose relationships have remained problematic because key features of its morphology have been poorly understood. The discovery of five finely-preserved Mon- jurosuchus specimens from the Lingyuan area has resolved the mystery; it is a primitive member of the Choristodera, a clade of distinctive but poorly known aquatic reptiles. For more than a century, knowledge of this group was limited to two highly specialized gavial-like genera, Champsosaurus and Simoedo- saurus, from the Late Cretaceous/Early Tertiary of North Amer- ica and Europe (Cope, 1877; Gervais, 1877). Over the last two decades, an additional nine genera have been recognized. Chor- istoderes had a temporal range spanning at least 190 million years from the Late Triassic to the late Oligocene; a geograph- ical distribution from western North America, across Europe and Asia, to Japan, and from subtropical latitudes to the Arctic (Tarduno et al., 1998). Choristoderes were the only major group of Mesozoic reptiles that survived the K-T transition event and then became extinct. Choristodera are beginning to emerge from the shadows as a group of unexpected phylogenetic and ecological diversity. The newly recovered Monjurosuchus specimens from west- ern Liaoning show unique integumentary structures of this aquatic reptile for the first time. The skin appears to have been rather soft, with the ventral scales smaller than those of the dorsal surface, but there is also a double row of enlarged ovoid and keeled scutes running along the dorsum of the body. The feet are webbed as an adaptation for swimming. In addition, one specimen has preserved intestinal contents. The new spec- imens thus provide important new information on the soft anat- omy and lifestyle of choristoderes. SYSTEMATIC PALEONTOLOGY REPTILIA Linnaeus, 1758 DIAPSIDA Osborn, 1903 CHORISTODERA Cope, 1876 MONJUROSUCHUS Endo, 1940 MONJUROSUCHUS SPLENDENS Endo, 1940 (Figs. 1–3) Holotype—Registration No. 3671 (former Central National Museum of Manchoukuo) from Tanankou, approximately 10.5 km southwest of Lingyuan, western Liaoning, China. The ho- lotype was reportedly lost during the WWII, and repeated at- tempts to trace the specimen in both China and Japan have failed. Neotype—GMV 2167 (Fig. 1), an adult individual, collec- tions of the National Geological Museum of China, Beijing. Although figures of the original holotype exist (Fig. 2), the resurgence of collecting in the Yixian Formation (see Jin, 1996 for discussion) make it desirable to have type material available to maintain the stability of the taxon. Type Locality and Horizon—Niuyingzi, Lingyuan, western Liaoning, China. Upper Jurassic/Lower Cretaceous Yixian For- mation. Swisher et al. (1999) has given an age of 125 Ma to the fossil beds of the Yixian Formation around Sihetun; how- ever, detailed correlation of these beds with those of the Lin- gyuan region (around 130 km to the west) has yet to be un- dertaken and it is therefore unclear whether they are laterally equivalent. Referred Specimens—GMV 2135 and 2162, both nearly complete immature individuals with skin impressions; GMV 2163, a nearly complete adult individual; GMV 2166, a small juvenile individual. Collections of the National Geological Mu- seum of China. Revised Diagnosis—A medium-sized (snout-vent length up to 300 mm) choristoderan reptile characterized by the posses- sion of a small supratemporal fenestra; infratemporal fenestra secondarily closed by expansion of postorbital, quadratojugal and squamosal; frontal markedly narrow and constricted; gas- tralia extremely slender, three or four rows to each body seg- ment, and five elements to each row; fore- and hind feet webbed, with only the claws projecting. DESCRIPTION The skull of Monjurosuchus is dorsoventrally flattened with relatively large orbits, and has antorbital and postorbital regions of roughly equal length (Figs. 2, 3A). The supratemporal fe- nestrae were bordered by deep parietal flanges, while the infra- temporal fenestrae appear to have been closed by the expansion of neighboring (postorbital, squamosal, quadratojugal) bones. The paired frontals are extremely narrow. The parietals are wid- er, but also paired and lack a parietal foramen. Posterolaterally, each parietal is extended into a deep posterior process that met the expanded squamosal. A wide postorbital bar meets the pa- rietal dorsally and the jugal anteroventrally. In the palate, the expanded pterygoids enclose a small, ovoid interpterygoid va-