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| Shishugounykus inexpectus
Qin, Clark, Choiniere & Xu, 2019
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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.
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| Figure 1: Skeletal anatomy of Shishugounykus inexpectus (IVPP V23567). Skeletal reconstruction showing preserved elements. (A), Skeletal silhouette showing preserved bones (missing portions shown in gray; Scale bar, 200 mm); (B), Partial left frontal in dorsal and ventral view; (C), Partial right frontal and parietal in dorsal and ventral view; (D), Partial right angular in lateral view; (E), Right articular in dorsal view; (F), An anterior dorsal in lateral view; (G), A posterior dorsal in lateral view; (H), Two most anterior sacrals in lateral view; (I), An anterior caudal in lateral view; (J), A posterior caudal in lateral view; (K), Right scapula in lateral view; (L), Partial left humerus in anterior, posterior, lateral and medial view; (M), Proximal end of right ulna; (N), Proximal end of right radius; (O), Right manus in lateral, dorsal and ventral view; (P), Partial left ilium in lateral and medial view; pubis (Q) and ischium (R) in lateral view; (S), Right femur in posterior, lateral, anterior and medial view; (T), Left tibia in anterior, posterior, lateral and medial view; (U), Left and right fibulae in lateral view; (V), Partial left metatarsals II and III, left pedal phalanges III-1 and 2, IV-1, 2, and 4 in dorsal view.
(Figure abbreviations see supplementary materials; Scale bar, 20 mm; The skeletal silhouettes are created by Aijuan Shi using Adobe Illustrator, www.adobe.com/products/illustrator.html).
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Systematic palaeontology
Theropoda Marsh, 1881
Maniraptora Gauthier, 1986
Alvarezsauria Bonaparte, 1991
Shishugounykus inexpectus gen. et sp. nov
Etymology: 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.
Holotype: IVPP V23567, a partial skeleton (Fig. 1) including several cranial elements (possible partial right frontal and partial right parietal, partial left frontal, partial right angular, and right articular), three dorsal vertebrae, four sacral vertebrae, three caudal vertebrae, partial right scapula, partial left humerus, partial right ulna and radius, nearly complete right manus, partial left ilium, ischium, and pubis, complete right femur, partial left femur, nearly complete left and right tibiae, partial left and right fibula, a distal tarsal, partial left metatarsals II and III, left pedal phalanges III-1 and 2, IV-1, 2, and 4, and a few rib fragments and unidentifiable pieces. All recovered bones are clearly from one individual given that they are preserved in a small area (about 0.2 square meters), without any other bone nearby.
Diagnosis: Shishugounykus inexpectus differs from all other alvarezsaurians in having the following unique combination of features (* marks the autapomorphies; we use the II-III-IV identity of manual digits in tetanurans): supratemporal fossa occupying large portion of frontal and with indistinct anterior border (sharp anterior border in early-branching alvarezsaurians such as Haplocheirus sollers and supratemporal fossa occupying a small portion of frontal in late-branching alvarezsaurians); scapula with hollow acromial process but without lateral concavities*; humeral internal tuberosity mediolaterally constricted distally*, giving it a “pinched” appearance; metacarpal III straight in dorsal view (laterally bowed in most other alvarezsaurians including Haplocheirus sollers); ungual III-3 subequal in size to ungual II-2 (considerably smaller in most other alvarezsaurians including Haplocheirus sollers); iliac medial surface with step-wise transition from ischial peduncle to pubic peduncle*; distal end of metatarsal II asymmetrically ginglymoid*.
Locality and Horizon: Wucaiwan, Junggar Basin, Xinjiang, People’s Republic of China, Middle-Upper Jurassic Shishugou Formation34,35,36,37. The holotype-fossil-bearing bed is located between two volcanic tuff layers with radiometric (40Ar/39Ar) ages of 161.2 ± 0.2 and 158.7 ± 0.3 Ma, respectively35,36,37,38. The two tuff layers are separated by a section of fluvial sediments that is 90 meters thick, and assuming constant sedimentation rates this means that each meter of sediment is around 0.0278 million years36, if the sedimentation rate was relatively constant. Based on a section thickness of 36 meters between the holotype-fossil-bearing bed and the lower tuff layer, we infer that the holotype-bearing bed is ∼160.2 Ma. Using a similar method, previous studies estimate the geological ages of the fossil-bearing beds for Aorun zhaoi (about 13 m below the lower tuff) and Haplocheirus sollers (about 40 m above the lower tuff) fossils are ∼161.6 Ma and ~160.1 Ma37, respectively.
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| Figure 4: Time-calibrated alvarezsaurian phylogeny showing alvarezsaurian hand evolution (Scale bar, 10 mm); silhouettes show the size variation both in early-branching and late-branching alvarezsaurians. |
Zichuan Qin, James Clark, Jonah Choiniere and Xing Xu. 2019. A New Alvarezsaurian Theropod from the Upper Jurassic Shishugou Formation of western China. Scientific Reports. volume 9, Article number: 11727. nature.com/articles/s41598-019-48148-7
