Abstract: A specimen collected from the Early Cretaceous lowest part of Yixian Formation of Liaoning, northeastern China, represents a new genus and species of dromaeosaurid theropod. It comprises a fragmentary maxilla with some teeth, a few caudals, almost complete fore limbs, and partial hind limbs and is here named Graciliraptor lujiatunensis gen. et sp. nov. Distinctive characteristics of the new species include a laminal structure connecting the postzygapophyses of middle caudals, middle caudals extremely long and slender, small manual ungual I, proximal end of metacarpal III strongly expanded, proximal tibiotarsus shaft rectangular in cross section, astragalar medial condyle significantly expanded posteriorly, metatarsal II distally much wider than the other metatarsals and long and slender pedal phalanx III-1. Being the earliest definitive dromaeosaurid species known to date, G. lujiatunensis provides new information important for understanding the early evolution of the group. On one hand, G. lujiatunensis displays a few features similar to those of basal birds, such as caudals significantly elongated, semilunate carpal small and primarily contacting metacarpal II, and manual digit I short, providing further evidence for a close relationship between the Dromaeosauridae and the Aves; on the other hand, it is similar to troodontids in some features on the caudals. The discovery of G. lujiatunensis also indicates a high diversity of the Dromaeosauridae in the Early Cretaceous Jehol Biota. Combined with other lines of evidence, it is inferred that the Dromaeosauridae rapidly diversified taxonomically but remained relatively stable morphologically in the early evolution of the group. Key words W

摘要： 我国辽西早白垩世义县组底部陆家屯层近年来产出大量恐龙和其他脊椎动物化石,其中包括兽脚类恐龙当中的伤齿龙类和窃蛋龙类(Xu , 2002) 。本文报道该层位产出的一件驰龙类化石标本。通过对比研究,我们确认这一标本不同于热河群已知的其他3 种驰龙类,并建立了驰龙类的一个新属种:陆家屯纤细盗龙(Graciliraptor lujiatunensis gen. et sp. nov. ) 。 驰龙类最初发现于北美晚白垩世地层中(Matthew and Brown , 1922) ,随后在蒙古晚白垩世地层中