A new postcranial skeleton of Sinovenator changii (Theropoda, Troodontidae) from the Jehol Biota of the Ningcheng Basin, Inner Mongolia, China

A new, well-preserved troodontid postcranial skeleton from the Jehol Biota of the Ningcheng Basin, Inner Mongolia, China, is described and assigned to Sinovenator changii based on an autapomorphy: the presence of two pneumatic foramina on the lateral surface of the anterior cervical vertebrae. This specimen is inferred to be an adult based on the presence of an external fundamental system (EFS) in the ulna, as well as the complete fusion of the cervical neural arches and centra. This represents the first record of a troodontid dinosaur from the Jehol Biota in Inner Mongolia. The discovery of this new specimen reveals additional morphological details of Sinovenator changii and expands the paleogeographic range of this taxon. Notably, this new specimen bears a moderately developed pubic ambiens process, in contrast to the anteroposteriorly elongated condition in Late Cretaceous troodontids. This morphology may reflect an early evolutionary phase in the elongation of the ambiens process among troodontids and might suggest a comparatively constrained hindlimb locomotor capacity relative to that of their Late Cretaceous relatives. In addition, body size comparisons among deinonychosaurians reveal that dromaeosaurids exhibit much greater size disparity than troodontids, which may reflect greater diversity in dromaeosaurid locomotion, ecological niches, or growth strategies.