IGUANIANS FROM THE UPPER CRETACEOUS DJADOCHTA FORMATION, GOBI DESERT,CHINA

Newly discovered lizard fossils from the Upper Cretaceous Djadochta Fo rmation in the Gobi Desert of Inner Mongolia China, include early igua nids and agamids that are taxonomically and biogeographically importan t. Two iguanids reported here, Anchaurosaurus gilmorei, gen. et sp. no v. and Xihaina aquilonia, gen. et sp. nov.. document the geologically earliest record of the family from Asia, and extend the fossil record of the group by 11-13 Myr. The geological age and primitive structure of these Cretaceous iguanids, coupled with other material of the same group from the slightly younger Barun Goyot Formation, suggest that th e group had a long evolutionary history on ?he northern continents, an d hence, challenge the widely accepted South American origin hypothesi s of the Iguanidae (sensu late). Three taxa of agamids are documented in the new collection: Mimeosaurus crassus Gilmore, 1943, Priscagama g obiensis Borsuk-Bialynicka and Moody, 1984, and Pleurodontagama aenigm atodes Borsuk-Bialynicka and Moody, 1984. Several well-preserved skull s of M. crassus show a structure that requires a taxonomic revision of this genus. Priscagama and Pleurodontagama are poorly documented. It has been long held that the iguanids and agamids have different center s of origin, and that the two groups have been separated geographicall y from one another since the beginning of their evolution. With the di scoveries of new iguanids and agamids together from the Gobi, it is no w dear that the two families were sympatric during their early evoluti on on the northern continents.