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| Zhongjianosaurus yangi
Xu & Qin, 2017
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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 yangi gen. 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.
Key words: Lower Cretaceous, Jehol Group, Theropoda, Dromaeosauridae, small size, morphological variation, niche differentiation
Systematic paleontology
Theropoda Marsh, 1881
Tetanurae Gauthier, 1986
Dromaeosauridae Matthew et Brown, 1922
Zhongjianosaurus yangi gen. et sp. nov.
Etymology: The genus name and specific epithet are in honor of Yang Zhongjian (C.C. Young), who is the founder of vertebrate paleontology in China.
Locality and Horizon: Sihedang, Lingyuan County, Liaoning Province, China. Possibly Yixian Formation, Aptian, Cretaceous (Swisher et al., 2001).
Diagnosis: A tiny microraptorine theropod distinguishable from other microraptorines in the following autapomorphies: proportionally long ossified uncinate processes fused to dorsal ribs, widely arched furcula with slender and posteriorly curved clavicular rami, humeral proximal end strongly offset medially from humeral shaft, humeral internal tuberosity short, presence of large fenestra within humeral deltopectoral crest, humeral ulnar condyle hypertrophied, ulna slightly longer than humerus, ulnar olecranon process with posterior margin mediolaterally pinched, ulnar distal end bending anteriorly and strongly expanded laterally, proximal end of metacarpal II with strong ventrolateral extension, metacarpal III laterally bowed with longitudinal ventral groove, phalanx II-2 without proximodorsal lip and lacking strong dorsal arching, femoral head stout and lower than trochanteric crest, medial condyle of tibiotarsus distal end with a prominent distal extension, arctometatarsalian pes, and metatarsal II without ginglymus on distal end.
Xing Xu and Zi-Chuan Qin. 2017. A New Tiny Dromaeosaurid Dinosaur from the Lower Cretaceous Jehol Group of western Liaoning and Niche Differentiation Among the Jehol Dromaeosaurids. Vertebrata PalAsiatica. In press.
