Birds are the most species-rich class of tetrapod vertebrates. They originated from a theropod lineage more than 160 million years ago. The evolutionary history of Birds is at the root of the paravian radiation, when dromaeosaurids, troodontids, and avialans were diverging from one another. Within the clade Paraves we found the morphology and soft tissue changes associated with the origin of modern avian flight. One of this key changes was the difference of nearly four orders of magnitude in body size, a pivotal element in the origin of powered avian flight. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago.
The Jehol Biota included two formations: the Yixian Formation, and the Jiufotang Formation, and contain the most diversified avifauna known to date. Among them are the long bony-tailed Jeholornis, only slightly more derived than Archaeopteryx, and many fossils of troodontids like Mei long, Sinovenator changii, Sinusonasus magnodens and Jinfengopteryx elegans. Now, the recently described Wulong bohaiensis, from the Jiufotang Formation, shed new light on the evolution of Birds. This small, feathered dromaeosaurid theropod lived in the Early Cretaceous (Aptian) and was discovered by a farmer more than a decade ago. The holotype (D2933) is a complete articulated skeleton (only some ribs are missing)and exhibits special preservation of keratinous structures.
Wulong (meaning “dancing dragon”) is distinguished by the following autapomorphic features: long jugal process of quadratojugal, cranially inclined pneumatic foramina on the cranial half of dorsal centra, transverse processes of proximal caudals significantly longer than width of centrum, presence of 30 caudal vertebrae producing a proportionally long tail, distally located and large posterior process of the ischium, and large size of supracoracoid fenestra (>15% of total area). The holotype has several gross osteological markers of immaturity like the unfused dorsal and sacral vertebrae, but mature feathers are present across the entire body of Wulong.
The feathered dinosaurs from the Jehol Biota are key to understand the origin of birds and dinosaur behavior. In modern birds development of ornamental feathers is generally timed to co-occur with sexual maturity. The presence of such elaborate feathers in the immature Wulong demonstrates that nonavian dinosaurs had a very different strategy of plumage development then their living relatives.
Poust, AW; Gao, C; Varricchio, DJ; Wu, J; Zhang, F (2020). “A new microraptorine theropod from the Jehol Biota and growth in early dromaeosaurids”. The Anatomical Record. American Association for Anatomy. doi:10.1002/ar.24343