Abstract: The Early Cretaceous Jehol dromaeosaurids are taxonomically and morphologically diverse, and one of them, Microraptor zhaoianus, has been suggested to be among the smallest known non-avialan theropods. However, this idea is based on specimens of relatively early ontogenetic stages, and the lower limit of the mature body mass of Jehol dromaeosaurids thus remains unknown. Here we describe a new dromaeosaurid, Zhongjianosaurus yangigen. et sp. nov., based on a specimen from the Lower Cretaceous Yixian Formation (the middle section of the Jehol Group) from Sihedang, Lingyuan County, Liaoning in Northeast China. While this new taxon is referable to the Microraptorinae, it differs from other microraptorine dromaeosaurids in numerous features, most notably the fusion of proportionally long uncinate processes to dorsal ribs, a humerus with a strongly medially offset proximal end and a large fenestra within the deltopectoral crest, an ulna slightly longer than the humerus, and an arctometatarsalian pes. Most significantly, the estimated 0.31 kg mass of the Z. yangi holotype of an adult individual confirms that some Jehol dromaeosaurids are among the smallest known non-avialan theropods. Our preliminary analysis demonstrates niche differentiation among the Jehol dromaeosaurids, a phenomenon rarely reported among Mesozoic dinosaurian faunas.  

摘要： 早白垩世热河生物群的驰龙类恐龙在分类和形态上多样化程度很高，其中的赵氏小盗龙曾被认为是已知体型最小的非鸟兽脚类恐龙之一。然而这个观点依据的标本都处于相对早期的生长发育阶段，因此热河生物群驰龙类恐龙的体型下限仍不明确。本文依据一件产自辽宁省凌源县四合当下白垩统义县组(热河群中部)的标本，描述驰龙类一新种，杨氏钟健龙。这一新种可归入小盗龙类，但它在很多特征上不同于其他小盗龙类，其中最明显的区别是其相对长的钩状突愈合于背肋上，肱骨具有强烈内倾的近端，肱骨三角嵴上有一窗孔，尺骨略长于肱骨，并具有窄跖型的足。最重要的是，已为成年个体的杨氏钟健龙正模估计体重约为0.31 kg, 这证实了热河生物群的一些驰龙类恐龙属于已知体型最小的非鸟恐龙。初步分析显示热河生物群驰龙类恐龙有生态位分化的情况，这一现象在中生代恐龙动物群中报道很少。