New Data Of Early Cretaceous Dromaeosaurs From Western Liaoning, With Comments On The Origin Of Feathers

Recently, a series of the feathered dinosaurs have been found from the Jehol Biota of western Liaoning, Northeast China, which give extremely strong evidences of the theropod hypothesis of bird origin and of the cursorial hypothesis of the avian flight origin. But the origin and early evolution of feathers have still been unanswered. In late 2000, a well-preserved specimen of dromaeosaurids was excavated from the Early Cretaceous Jiufotang Formation of Shangheshou, suburb of Chaoyang City, western Liaoning. This new specimen (NGMC 00-12-A) is preserved not only by almost complete skeleton but also by many feathers. It is a quite small dromaeosaurid with the total length nearly 1 meter from the snout to the tip of the tail. It shows many typical characters seen in the type species of the genus of Sinornithosaurus, which discovered also from western Liaoning. NGMC 00-12-A can further be included in Sinornithosaurus by the dentary bifurcated posteriorly, unserrated premaxillary teeth, U-shaped furcula, extremely short manual phalanx III-2 in comparison with the length of phalanx III-l, very large and developed obturator process of the ischium, partially arctometatarsalian metatarsal III and so on. This specimen slightly differs from the type species of the same genus Sinornithosaurus milknii in the length proportions of some manual elements. Because its cranial posterior portion is badly preserved and difficult to be compared with the type species, it is temporarily identified as Sinornithosaurus sp. here. The feathers attached to the arms and the tail evidently possess the structures of "modern feathers", which consisting of the central shafts (rachis) and parallel barbs.Dromaeosaurids are unusual group of maniraptoran theropods. They share many similar or the same features with Archaeopteryx or other primitive birds, such as: U-shaped furcula, uncinate process, laterally facing glenoid of shoulder girdle, comparatively long forelimb, large semilunate carple, retroverted pubis, ascending process of astragalus, and "modern feathers" with rachis and barbs etc. These facts indicate not only that dromaeosaurids had developed many bird-like characters in course of the advanced maniraptorans toward birds, but also that there seemed to be the co-evolutionary trend between the feathers and the skeletal bones.Dromaeosaruid theropods can be restricted in the unique family Dromaeosauridae; furthermore, this family may also be subdivided into two subfamilies Dromaeosauridae and Velociraptorinae. About a dozen genera are referred to this group, most of which were collected from the Cretaceous beds including the North American Dromaeosaurus, Deinonychus, Saurornitholestes, Bambiraptor, Utahraptor, and the East Asian Velociraptor, Adasaurus, AchiUobator, Sinornithosaurus, Microraptor as well. But the interrelationships among these genera remain uncertain until now. A great quantity of features recognized in dromaeosaurids supports the view that Dromaeosauridae is the sister group of Avialae. The cladogram proposed in the present paper also suggests that the maniraptoran Troodontidae and Ovkaptoridae should have the branched feathers, and that feathersevolve and initially diversify in terrestrial theropod dinosaurs before the origin of birds and their flight.Up to now, several feathered theropods from western Liaoning have been uncovered, and one kind of another Mongolian theropod has also been proved to possess protofeathers. Sinosauropteryx (compsognathids), Beipzaosaums (therizinosaurids) and Shuvuuia (alvarezsaurids) have the filament-like protofeathers that lack the rachis and barbules. But in Protarchaeopteryx (protarchaeopterygians), Caudipteryx (caudipterygians or oviraptorosaurs) and Sinornithosaurus (dromaeosaurids), not only protofeathers but also the "modern feathers" that have evident rachis and barbules can be observed. These fossils are very important in the study of the definition of protofeathers and in the study of the origin and early evolution of feathers. In the present paper, protofeathers is...