Tuesday 21 November 2023

Jaculinykus yaruui: A new species of Alvarezsaurid Dinosaur from the Late Cretaceous of Mongolia.

The Alvarezsaurids are a poorly understood group of Maniraptoran Theropod Dinosaurs, in which later members became miniaturized and independently obtained a number of features associated with Birds, including s a lightly built, kinetic skull, a keeled sternum, a fused carpometacarpus, and a retroverted pubis and ischium. This was combined with several features unique to the group which have been hard to interpret functionally, including a short robust forelimb with a greatly elongated first finger, which has been suggested as a possible adaptation to digging and hunting burrowing Insects, combined with elongate hind limbs, apparently adapted to fast running. It has also been suggested that the group show adaptations towards vision in low-light environments and highly acute hearing, possibly comparable to that of modern Barn Owls, which is likely an indication of nocturnal behaviour.  

The Alvarezsaurids had a global distribution, but the majority of fossils are known from South America or Asia. The most derived Alvarezsaurids are known from the Late Cretaceous Nemegt Basin of Mongolia, from where eight species have been described to date, although most of these are known from fragmentary remains, of little help in reconstructing the ecology of the living Dinosaurs, or the way in which they were related to one-another.

In a paper published in the journal PLoS One on 15 November 2023, Kohta Kubo of the Department of Natural History and Planetary Sciences at Hokkaido University, Yoshitsugu Kobayashi of the Hokkaido University Museum, Tsogtbaatar Chinzorig of the Department of Biological Sciences at North Carolina State University and the Institute of Paleontology of the Mongolian Academy of Sciences, and Khishigjav Tsogtbaatar, also of the Institute of Paleontology of the Mongolian Academy of Sciences, describe a new species of Alvarezsaurid Dinosaur from the Nemegt Basin.

The new species is described from a single nearly complete and articulated skeleton, from the Baruungoyot-Nemegt interfingering interval at the Nemegt locality of the Nemegt Basin in the Gobi Desert of Mongolia. It is named Jaculinykus yaruui, where 'Jaculinykus' is a combination of 'Jaculus' a small Dragon from Greek mythology, and 'onykus', meaning 'claw', while 'yaruui'- derives from the Mongolian word 'yaruu', meaning 'speedy'.

Holotype of Jaculinykus yaruui (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. Grey areas are missing parts. Kubo et al. (2023).

The specimen comprises is a nearly complete skeleton with a skull, missing the vomers, nasals, postorbitals, and supraoccipitals, and a post-cranial skeleton missing the 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. 

Surprisingly, it was discovered with the neck curved posteriorly on the right side of the body; the tail positioned on the left side and curled around the flexed hind limbs to the right. This is very different from the typical 'flexed' position seen in small Theropod Dinosaur remains, and closely resembles the sleeping posture in Birds, where the head is tucked under the wing. Such a posture has previously been recorded in numerous Troodontids, Dinosaurs closely related to Birds. However, Alvarezsaurids are much less closely related within the Maniraptora. This raises two possibilities; either this sleeping posture arose before or with the emergence of the first Maniraptorans, and can be expected in all members of the group, or is a convergent adaptation to heat conservation in sleeping small feathered Dinosaurs.

Evolution of Avian-like sleeping posture in Theropod Dinosaurs. Skeletal disposition of Jaculinykus yaruui  (MPC-D 100/209) in dorsal (A) and ventral (reversed) (B) views. (C), Interpretive line drawing of skeletal disposition. (D), Life restoration of sleeping posture of Jaculinykus yaruui . (E), Simplified coelurosaurian phylogeny represents presence for evidence of avian ‘tuck-in’ posture. Kubo et al. (2023).

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