Acta Palaeontologica Polonica

The choristoderan reptile Monjurosuchus from the Early Cretaceous of Japan

Ryoko Matsumoto, Susan E. Evans, and Makoto Manabe

Acta Palaeontologica Polonica 52 (2), 2007: 329-350

The choristoderan reptile Monjurosuchus is described from the Lower Cretaceous Tetori Group of Japan on the basis of an associated specimen from the Kuwajima Formation, Ishikawa Prefecture, and more fragmentary remains from the contemporaneous Okurodani Formation, Gifu Prefecture. This is the first report of Monjurosuchus from Japan, but a long-necked choristodere, Shokawa, has already been recorded from these deposits. Monjurosuchus was first described from the Lower Cretaceous Jehol Biota of China, although it has only recently been recognised as a choristodere. As reconstructed, the Japanese Monjurosuchus differs from the type species, Monjurosuchus splendens, in the structure of the postorbital region, reduction of the quadratojugal, a slender parietal with a deep groove along the interparietal suture, and elongation of the jugal. As in M. splendens, the lower temporal fenestrae are closed. A cladistic analysis was performed in order to place Japanese and Chinese taxa, including the incompletely described Chinese long-necked Hyphalosaurus lingyanensis, into choristoderan phylogeny. The results support the monophyly of Neochoristodera and of a Sino-Japanese clade of long necked choristoderes. The placement of the European Tertiary Lazarussuchus remains problematic, but the analysis supports its placement within Choristodera rather than on the stem. The identification of Monjurosuchus from Japan provides an additional link between the fossil assemblages of the Tetori Group and those of the slightly younger Jehol Biota of China.

Key words: Reptilia, Choristodera, Monjurosuchus, Cretaceous, Tetori Group, Kuwajima Formation, Japan.

Ryoko Matsumoto ucgarym@ucl.ac.uk and Susan E. Evans ucgasue@ucl.ac.uk, Department of Anatomy & Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK; Makoto Manabe manabe@kahaku.go.jp, Department of Geology, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan.