A Small Derived Theropod from Öösh, Early Cretaceous, Baykhangor Mongolia

Abstract

A new theropod dinosaur, Shanag ashile, from the Early Cretaceous Öösh deposits of Mongolia is described here. The new specimen (IGM 100/1119) comprises a well-preserved right maxilla, dentary, and partial splenial. This specimen exhibits a number of derived theropod features, including a triangular anteriorly tapering maxilla, a large antorbital fossa, and maxillary participation in the caudally elongate external nares. These features resemble the Early Cretaceous dromaeosaurids Sinornithosaurus millenii and Microraptor zhaoianus, as well as the basal avialan Archaeopteryx lithographica. A comprehensive phylogenetic analysis including 58 theropod taxa unambiguously depicts the new Öösh theropod as a member of Dromaeosauridae. Relative to other dromaeosaurids, Shanag ashile is autapomorphic in its lack of a promaxillary fenestra and in the presence of interalveolar pneumatic cavities. The discovery of IGM 100/1119 expands our knowledge of Early Cretaceous dromaeosaurids and the faunal similarity between the Öösh and the Jehol biotas.