A new compsognathid theropod dinosaur from the oldest assemblage of the Jehol Biota in the Lower Cretaceous Huajiying Formation, northeastern China.

Compsognathids — long regarded as an assemblage of 'prototypical' small theropods including the historically significant Compsognathus and Sinosauropteryx — have patchy distributions. The majority of definitive members of the clade come from the Upper Jurassic Solnhofen Archipelago and the Lower Cretaceous Yixian Formation. Chronologically falling between these two major compsognathid localities, a new compsognathid described in this paper, Xunmenglong yingliangis gen. et sp. nov, occurs in the Huajiying Formation of northeastern China. Xunmenglong is distinguished from other compsognathids by one autapomorphy (pedal phalanx IV-4 longer than IV-3) and a unique combination of postcranial traits, including the disproportionately long lower legs. Nested among the Jehol and Solnhofen compsognathids in a phylogenetic analysis, Xunmenglong is an intriguing addition to the known range and diversity of the clade. The taxon is stratigraphically the lowest among Asian compsognathids, and its holotype specimen is the smallest among compsognathids (unless Scipionyx is treated as a compsognathid). Questions remain about habitat preferences and diversification patterns of compsognathids, but Xunmenglong offers two broader implications. (1) As previously proposed on the basis of the diverse enanthiornithine assemblage, the Huajiying Formation may preserve the oldest fauna of the Early Cretaceous Jehol Biota, which is distinct from the Late Jurassic Daohugou Biota and includes the Yixian Formation from which other Asian compsognathids were discovered. (2) Xunmenglong provides a useful calibration point with which to reconstruct biogeographic/stratigraphic patterns and to estimate allometric trends in the clade. [ABSTRACT FROM AUTHOR]