Abstract
Alvarezsaurian dinosaurs, a group of bizarre theropods with greatly shortened and modified forelimbs, are known mostly from the Cretaceous of Asia and South America. Here we report a new alvarezsaurian, Shishugounykus inexpectus gen. et sp. nov, based on a specimen recovered from the Middle–Upper Jurassic Shishugou Formation of the Junggar Basin, western China. Together with two other alvarezsaurians from this formation, i.e., Haplocheirus sollers and Aorun zhaoi, these Shishugou forms represent the only known Jurassic alvarezsaurians worldwide. Similar to the two other Shishugou alvarezsaurians, this new alvarezsaurian displays early stages in the development of the highly modified alvarezsaurian forelimb, but it possesses a number of manual features closer to the typical coelurosaurian theropod condition. Combining morphological and histological features, our analysis indicates that the earliest known alvarezsaurians are variable in size and other important morphological features, and in particular display a mosaic distribution of forelimb features.
Highlights
Alvarezsauria was first recognized when Bonaparte named the Alvarezsaurus calvoi from the Coniacian-Santonian of Argentina[1]
The generic name is a combination of Shishugou (Chinese Mandarin for the formation which produced the holotype specimen of the new animal; translates as “rock” “tree” “wash” for the abundant petrified wood in the formation) and onyx (Greek, “claw); the specific name refers to the unexpected discovery of a new alvarezsaurian species from the Middle-Late Jurassic Shishugou Formation, which has produced fossils of two other Jurassic alvarezsaurians, i.e., Haplocheirus sollers and Aorun zhaoi
The alvarezsaurian affinities of these three species are supported by the following synapomorphies: internal tuberosity of www.nature.com/scientificreports humerus offset from humeral head by distinct notch; lack of collateral ligament fossae on the distal condyles of metacarpal II; ungual II distinctly larger than other unguals; the proximal end of the lateral grooves of manual ungual II-2 partially enclosed by lateral notches; the distal projection of lateral femoral distal condyle is distinctly further than medial condyle; a low and rounded fibular crest; and the bracing for the ascending process of the astragalus on the tibia is step-like and running proximodistally
Summary
Alvarezsauria was first recognized when Bonaparte named the Alvarezsaurus calvoi from the Coniacian-Santonian of Argentina[1]. Fossil remains of this group have been recovered from the Upper Cretaceous of North America[2,3], South America[1,4,5], Asia[6,7,8,9,10,11,12,13,14,15], and Europe[16]. We describe a new specimen recovered from the Shishugou Formation, which represents the third alvarezsaurian species in this formation, and which demonstrates significant anatomical variability previously unappreciated among the earliest known alvarezsaurians
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