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EXCEPTIONAL FOSSIL MATERIAL OF A SEMI-AQUATIC REPTILE FROM CHINA: THE RESOLUTION OF AN ENIGMA

The lacustrine deposits of the Yixian Formation, western Liaoning, China, are renowned for producing remarkable vertebrate 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, Monjurosuchus Endo, 1940, whose relationships have remained problematic because key features of its morphology have been poorly understood. The discovery of five finely-preserved Monjurosuchus 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 Simoedosaurus, from the Late Cretaceous/Early Tertiary of North America and Europe (Cope, 1877; Gervais, 1877). Over the last two decades, an additional nine genera have been recognized. Choristoderes had a temporal range spanning at least 190 million years from the Late Triassic to the late Oligocene; a geographical 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 western 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 specimens thus provide important new information on the soft anatomy and lifestyle of choristoderes.