Fossil range: Early Cretaceous, 129.7–122.1 Ma
Illustration of Tianyuraptor.
| Scientific classification
Tianyuraptor is a newly described genus of short-armed dromaeosaurid dinosaur that lived during the Early Cretaceous and whose remains have been found in the Jehol Group Yixian Formation of western Liaoning, China. It is based on a nearly complete skeleton and is similar to other Liaoning dromaeosaurids, with the exception of being somewhat less developed. The type specimen, formally described in 2009, also exhibits features unknown in previously described Laurasian dromaeosaurids but present in the Gondwanan dromaeosaurids and basal avialans, and is described as a transition that reduces the morphological gap between these groups of animals. The newly described animal also differs from previously described dromaeosaurids in that it possesses a comparatively small furcula and proportionally short forelimbs compared to body length.
The generic name of Tianyuraptor has a split meaning, with the first part, 'Tianyu', referring to the Shandong Tianyu Museum of Nature where the holotype specimen is stored. The second part, 'raptor' is in reference to the Latin word for 'robber', referring to the action of grabbing one's prey, often used in naming dromaeosaurids. The specific epithet, 'ostromi', is in honor of John Ostrom, who contributed greatly to the study of dromaeosaurid fossils, including Deinonychus and feathered dinosaurs.
The type specimen is based on 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. 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. The Dawangzhangzi Bed specifically has been dated to about 122 million years ago. 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.
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, the tail is relatively long at 960 millimeters (38 in), nearly 4.8 times as long as the femur.
The forelimbs are also comparatively short in comparison to the hindlimbs, being only 53% of the hindlimbs' length. This differs greatly from the known skeletal elements of other dromaeosaurids, most of which have relatively long forelimbs that are more than 70% of the hindlimbs' length.
While Tianyuraptor is larger in size than all other known microraptorines, it also has comparably elongated lower hindlimbs compared to other microraptorines. In this regard, it is different from most other dromaeosaurids, which have relatively short lower legs.[Note 1] Aside from elongate hindlimbs, Tianyuraptor is different from other members of Microraptorinae in regards to the relative lengths of the forelimb elements. The forelimbs of Tianyuraptor are proportionally much shorter than those of larger dromaeosaurids. For example, a similarly sized Velociraptor specimen shows an arm/leg length ratio of approximately 0.75., while Tianyuraptor has an arm/leg ratio of 0.53.
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 and Rahonavis have been suggested to have been aerodynamic and may have glided. 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.
Phylogenetic analysis and classificationEdit
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 et al. also noted that while Tianyuraptor shares some features with the monophyletic subfamily Microraptorinae,[Note 2] it also lacks many other microraptorine features.[Note 3] Zheng et al. goes 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. Furthermore, in the cladistic analysis performed, it was found that Tianyuraptor was closer to the Eudromaeosauria than the Unenlagiinae.
- ^ For example, Tianyuraptor has a tibiotarsus/femur length ratio of greater than 1.30, while Velociraptor mongoliensis, a creature of similar size, exhibits a ratio of less than 1.10.
- Source: Norell, M. A. & Makovicky, P. J. (1999) Important features of the dromaeosaurid skeleton. II. Information from newly collected specimens of Velociraptor mongoliensis. Am. Mus. Novit. 3282, 1–45.
- ^ Microraptorine features include:
- A laterally sculpted maxilla.
- A significantly shortened manual phalanx III-2.
- A spatulate pubic symphysis.
- ^ These include:
- ^ a b c d e f g h Zheng, X., Xu, X., You, H., Zhao, Q. and Dong, Z. (2009). "A short-armed dromaeosaurid from the Jehol Group of China with implications for early dromaeosaurid evolution." Proceedings of the Royal Society B, published online before print August 19, 2009. doi:10.1098/rspb.2009.1178
- ^ Ostrom, John H. (1969). "Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana". Bulletin of the Peabody Museum of Natural History 30: 1–165.
- ^ "At Last, His Theory Flies". May 5, 2000. Olivia F. Gentile. Hartford Courant.
- ^ Chang, S.–C.; Zhang, H.; Renne, P.R.; and Fang, Y. (2009). "High–precision 40Ar/39Ar age for the Jehol Biota". Palaeogeography, Palaeoclimatology, Palaeoecology 280 (1–2): 94–104. doi:10.1016/j.palaeo.2009.06.021.
- ^ Zhou, Z. (2006). "Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives." Geological Journal, 41: 377-393.
- ^ Senter, P., Barsbold, R., Britt, B. B. & Burnham, D. A. (2004) Systematics and evolution of Dromaeosauridae (Dinosauria, Theropoda). Bull. Gunma Mus. Nat. Hist. 8, 1–20.
- ^ Currie, P. J. (1997) Dromaeosauridae. In Encyclopedia of dinosaurs (eds P. J. Currie & K. Padian), pp. 194–195. San Diego, CA: Academic Press.
- ^ Xu, X., Zhou, Z.-H., Wang, X.-L., Kuang, X.-W., Zhang, F.-C. & Du, X.-K. (2003) Four-winged dinosaurs from China. Nature 421, 335–340. doi:10.1038/nature01342
- ^ Chatterjee, S. & Templin, R. J. (2007) Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceedings of the National Academy of Science USA 104, 1576–1580. doi:10.1073/pnas.0609975104