New choristoderes (Reptilia : Diapsida) from the upper Cretaceous and Palaeocene, Alberta and Saskatchewan, Canada, and phylogenetic relationships ofChoristodera

New choristoderan fossils from the Late Cretaceous and Palaeocene of Albert a and Saskatchewan, Canada, are described: incomplete maxillae and dentarie s from the Upper Cretaceous Oldman Formation, Alberta, extend the range of the primitive Cteniogenys from the Jurassic in the North American Western I nterior; an incomplete dentary from the Palaeocene Ravenscrag Formation, Sa skatchewan, comprises the first occurrence of the crocodile-like Simoedosau rus in Canada and the earliest record of the genus; well-preserved skulls a nd mandibles from the Oldman and Horseshoe Canyon formations, Alberta, docu ment a new species of Champsosaurus and clarify the status of previously kn own species of the genus. New information about Asian choristoderes support s a Tchoiria-(Ikechosaurus + Simodeosaurus) relationship, contrary to previ ous work. Choristoderes share no convincing synapomorphies with either Lepi dosauromorpha or Archosauromorpha, but occupy a more basal position within Diapsida, possibly as a sister-taxon with Neodiapsida (Younginiformes + Sau ria). (C) 1998 The Linnean Society of London.