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| Jaculinykus yaruui Kubo, Kobayashi, Chinzorig & Tsogtbaatar, 2023 Artwork courtesy of Seiji Yamamoto |
Abstract
Alvarezsauria is a group of early-branching maniraptoran theropods that are distributed globally from the Late Jurassic to the latest Cretaceous. Despite recent increases in the fossil record of this group, the scarcity of complete specimens still restricts interpreting their detailed anatomy, ecology, and evolution. Here, we report a new taxon of derived alvarezsaur, Jaculinykus yaruui gen. et sp. nov., from the Late Cretaceous of Mongolia, which represents a nearly complete and articulated skeleton. Our phylogenetic analysis reveals that Jaculinykus belongs to the sub-clade of Alvarezsauridae, Parvicursorinae, and forms a mononphyletic group with Mononykus and Shuvuuia. Its well-preserved manus has only two fingers, composed of a hypertrophied digit I and greatly reduced digit II, which implies an intermediate condition between the tridactyl manus of Shuvuuia and monodactyl manus of Linhenykus. This highlights a previously unrecognized variation in specialization of alvarezsaurid manus. Notably, the preserved posture of the specimen exhibits a stereotypical avian-like sleeping position seen in the troodontids Mei and Sinornithoides. Evidence of this behavior in the alvarezsaur Jaculinykus suggests that stereotypically avian sleeping postures are a maniraptoran synapomorphy, providing more evidence of bird-like traits being distributed broadly among avian ancestors.
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| Geographic and stratigraphic occurrence of the Mongolian alvarezsaurids. (C) The stratigraphic horizon of Jaculinykus yaruui gen. et sp. nov. and other alvarezsaurids in the Nemegt, Baruungoyot, and Djadokhta formations. The composite stratigraphy was modified from Eberth [2018] and Jerzykiewicz and Russell [1991]. |
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| Geographic and stratigraphic occurrence of the Mongolian alvarezsaurids. (A) Plan view of Mongolia. (B) Location of Jaculinykus yaruui gen. et sp. nov. (MPC-D 100/209) and alvarezsaurid occurrences in the Nemegt Basin. The map was generated using Simplemappr (www.simplemappr.net) before modification and modified from Jerzykiewicz et al. [2021]. |
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| Holotype of Jaculinykus yaruui gen. et sp. nov. (MPC-D 100/209). (A) Photograph of the specimen. (B) Explanatory drawing of (A). Highlighted areas refer to the indication of the skeletal elements; skull in green, tail in grey, pectoral girdle and forelimbs in red, pelvis and hind limbs in purple. (C) Reconstruction of Jaculinykus yaruui gen. et sp. nov. Grey areas are missing parts. |
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| Life restoration of sleeping posture of Jaculinykus yaruui (Artwork courtesy of Seiji Yamamoto) |
Systematic paleontology
Dinosauria Owen, 1842
Theropoda Marsh, 1882
Maniraptora sensu Gauthier, 1986
Alvarezsauria Bonaparte, 1991
Alvarezsauridae Bonaparte, 1991
Parvicursorinae Karhu and Rautian, 1996
Jaculinykus yaruui gen. et sp. nov.
Etymology: Jaculinykus is from “Jaculus,” a tiny dragon from the Greek myth, and “onykus,” claw; yaruui, derived from Mongolian word, yaruu (яаруу): speedy (= hasty).
Holotype: The type specimen (MPC-D 100/209) is a nearly complete skeleton with a skull, missing some cranial elements (vomers, nasals, postorbitals, and supraoccipitals), eighth or ninth cervical vertebra, posterior dorsal vertebrae, seven anterior caudal vertebrae, sternum, furcula, right manual phalanx (II-2), right manual ungual and left fibula (Fig 2). It is housed in the Institute of Paleontology of Mongolian Academy of Sciences (IP-MAS), Ulaanbaatar, Mongolia.
Type locality and horizon: Nemegt locality, Ömnögovi Province, Mongolia (Fig 1). The specimen was collected from the upper section of the Baruungoyot (or Barun Goyot) Formation, suggested as the Campanian in age, (Fig 1). The stratigraphic horizon of Jaculinykus belongs to a part of “Big Red” in the transitional stratigraphic interval (Zone 4).
Diagnosis: Jaculinykus differs from all other alvarezsaurs in having a dorsoventrally high narial opening of the premaxilla, medially curved parasagittal crest on the parietal, slender and nearly straight dentaries, triangular-shaped deltopectoral crest being separated from the humeral head by a notch, strong medial tab of metacarpal I, weakly developed proximodorsal process of phalanx I-1, robust medial condyle of the tibia relative to the fibular condyle, and sharply indented base of the ascending process of astragalus. Jaculinykus is also distinguished from other alvarezsaurs by the unique combination of the following characters: slender ischial shaft relative to the pubic shaft, an open popliteal fossa of the femur, and prominent external projection of the ectocondylar tuber of the femur. It differs from the hypothesized sister taxon Shuvuuia deserti in possessing the following additional features: the deltopectoral crest being separated from the humeral head by a notch; the absence of the third manual digit. It also differs from stratigraphically same or older (the Baruungoyot Formation) alvarezsaurid, Ondogurvel alifanovi, from the same locality (the Nemegt locality) in possessing the following features ....
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| Evolution of avian-like sleeping posture in theropod dinosaurs. Skeletal disposition of Jaculinykus yaruui gen. et sp. nov. (MPC-D 100/209) in dorsal (photograph courtesy of Tomonori Tanaka) (A) and ventral (reversed) (B) views. (C), Interpretive line drawing of skeletal disposition. (D), Life restoration of sleeping posture of Jaculinykus yaruui (Artwork courtesy of Seiji Yamamoto). (E), Simplified coelurosaurian phylogeny represents presence for evidence of avian ‘tuck-in’ posture. |
Kohta Kubo, Yoshitsugu Kobayashi, Tsogtbaatar Chinzorig and Khishigjav Tsogtbaatar. 2023. A New alvarezsaurid Dinosaur (Theropoda, Alvarezsauria) from the Upper Cretaceous Baruungoyot Formation of Mongolia provides insights for Bird-like Sleeping behavior in Non-avian Dinosaurs. PLoS ONE. 18(11): e0293801. DOI: 10.1371/journal.pone.0293801
