The morphology and function of the manual digits of a troodontid from the Yixian Formation of western Liaoning, China

A well-preserved paravian manus was described from the Jehol Biota of western Liaoning, China. This specimen was inferred as an early-diverging troodontid and reveals new morphological details of the troodontid clade, such as both ends of phalanx II-1 and phalanx II-2 asymmetrical, phalanx II-1 and phalanx III-3 twisted medially, the ventral side of the distal end of phalanx II-2 flat, and the proximal articular surface of phalanx III-1 non-trochoid. The range of motion of this manus was examined based on the CT data of the bones, showing that the range of flexion of this troodontid manus is relatively large while the range of extension is relatively limited, which is consistent with the tendency of the decrease in the manual extension capabilities from early-diverging theropods to maniraptoriforms.