Tianyuraptor

The generic name of Tianyuraptor combines Tianyu, referring to the Shandong Tianyu Museum of Nature where the holotype specimen is stored, with raptor, the Latin word for 'robber', referring to the action of grabbing prey, often used in naming dromaeosaurids. The specific epithet, ostromi, is in honor of John Ostrom,[1] who contributed greatly to the study of dromaeosaurid fossils, including Deinonychus and feathered dinosaurs.[2][3]

Artist's life restoration

Tianyuraptor is a medium-sized dromaeosaurid that has several derived features that separate it from other dromaeosaurids. These include the length of the middle caudal (tail) vertebrae being more than twice that of the dorsal (back) vertebrae, a small and extremely slender furcula, and an unusually long hindlimb that is roughly three times as long as the entire series of dorsal vertebrae. As in other dromaeosaurid fossils discovered in Liaoning,[4] the tail is relatively long at 960 millimetres (38 in), nearly 4.8 times as long as the femur.[1]

The type specimen is STM1–3, a nearly complete and fully articulated skeleton that is only missing the extreme distal end of the tail. A total of 25 fully articulated caudal vertebrae are preserved and at the end three at most are estimated to be missing. The fossil was discovered in the Dawangzhangzi Bed of the Yixian Formation (Jehol Group), located in Lingyuan in western Liaoning, China.[1] The Yixian Formation is an Early Cretaceous rock unit, dated to between approximately 129.7 and 122.1 million years old, in the Barremian and Aptian faunal stages.[5] The Dawangzhangzi Bed specifically has been dated to about 122 million years ago.[6] STM1–3 is believed to be a sub-adult, with features including the incomplete fusion of skeletal parts during ontogeny. The holotype of Tianyuraptor preserves no soft tissues, unlike many other theropod specimens from the Jehol Group.[1]

The stark difference in arm length compared to other dromaeosaurids implies that the function of the arms in Tianyuraptor was different from that of other dromaeosaurs. Members of the subfamily Microraptorinae, such as Microraptor, have been suggested to have been aerodynamic and may have glided.[9][10] Microraptorines are usually noted for their long and robust forelimbs and large, asymmetrical flight feathers. However, the shortened forelimbs, small furcula, and the transversely wide coracoid in Tianyuraptor suggest that it was not suited for aerodynamic gliding or flight.

A phylogenetic analysis performed by Zheng et al. showed Tianyuraptor to be a basal member of a group containing Laurasian dromaeosaurids. Tianyuraptor seems to possess several features that are unknown in other Laurasian dromaeosaurids, but which are seen in basal avialans and Gondwanan dromaeosaurids, including Austroraptor, Buitreraptor, Neuquenraptor, Rahonavis, and Unenlagia. Zheng and colleagues also noted that Tianyuraptor shares some features with the monophyletic subfamily Microraptorinae, though they went on to say that this mixture of features suggests a basal placement for Tianyuraptor within Microraptorinae, as evidenced by their phylogenetic analysis which indicated maximum parsimony in six of the 30 results recovered by the analysis. The authors then go on to suggest that since Tianyuraptor is considered a short-armed microraptorine, more derived long-armed microraptorines might have independently evolved flight capability. However, it is also equally possible, as argued by Zheng et al., that Tianyuraptor may in fact be a basal member of a clade containing all other Laurasian dromaeosaurids with the exception of Microraptorinae. This is indicated by the other 24 out of 30 most parsimonious trees recovered from the analysis. The discovery of Tianyuraptor sheds new light on the early evolution of dromaeosaurs and further exemplifies the great diversity this group enjoyed at an early stage.[1]

• Timeline of dromaeosaurid research
• Paraves