ANATOMY OF THE BASAL ORNITHUROMORPH BIRD ARCHAEORHYNCHUS SPATHULA FROM THE EARLY CRETACEOUS OF LIAONING, CHINA

As one of the earliest-known beaked ornithuromorph birds, Archaeorhynchus spathula is important for understanding the early evolution of this derived avian clade. Recently, two new complete and articulated specimens of Archaeohyrnchus spathula were collected from Lower Cretaceous deposits of Jianchang, Liaoning, northeastern China. These specimens preserve new anatomical information, particularly in regards to the morphology of the skull (including the inner structure of the occiput), forelimbs, hind limbs, and sternum. The sternum is similar to that of an unnamed ornithuromorph from the Xiagou Formation, Gansu Province. These new specimens are subadult: together with the holotype of Archaeorhynchus, they constitute the entire Early Cretaceous record of subadult ornithuromorphs and reveal important information regarding the ontogeny of this group. The caudal vertebrae are complete in the two referred specimens and provide information about the development of the pygostyle in basal ornithuromorphs. At least four vertebrae form the pygostyle of Archaeorhynchus and fusion progresses distoproximally. The preservation of gastroliths in all known specimens of Archaeorhynchus signifies that it was likely herbivorous.