Paleontologists in Mongolia have unearthed a nearly complete and articulated skeleton of a previously unknown genus and species of alvarezsaurid dinosaur that lived more than 72 million years ago. Notably, the preserved posture of this specimen exhibited a stereotypical bird-like sleeping position.
Life restoration of sleeping posture of Jaculinykus yaruui. Image credit: Seiji Yamamoto.
The newly-discovered dinosaur roamed our planet during the Campanian age of the Late Cretaceous epoch, between 84 and 72 million years ago.
Dubbed Jaculinykus yaruui, the species was a type of alvarezsaurid, a group of early maniraptoran theropod dinosaurs that were distributed globally from the Late Jurassic to the latest Cretaceous.
“Alvarezsauria is a group of bizarre maniraptoran theropods whose latest-branching members possess remarkably avian homoplasies,” said Hokkaido University paleontologist Kohta Kubo and colleagues.
“The fossil record of alvarezsaurs is globally widespread but known mostly from either Asia or South America.”
“Especially, fossil remains of late-branching alvarezsaurids are abundant in the Nemegt Basin of Mongolia and eight genera have been documented so far.”
“Despite the recent increase in the fossil record of alvarezsaurids, a detailed anatomy of the members is still limited due in parts that most of the fossil remains are fragmentary.”
“This often leads to difficulty in interpreting of their ecology and poorly resolved phylogenetic interrelationships.”
“Jaculinykus yaruui belongs to the sub-clade of Alvarezsauridae, Parvicursorinae, and forms a mononphyletic group with Mononykus and Shuvuuia.”
Holotype of Jaculinykus yaruui: (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 gray, pectoral girdle and forelimbs in red, pelvis and hind limbs in purple; (C) reconstruction of Jaculinykus yaruui; gray areas are missing parts. Image credit: Kubo et al., doi: 10.1371/journal.pone.0293801.
The nearly complete skeleton of Jaculinykus yaruui — with a skull, missing some cranial elements — was unearthed at the Nemegt locality of the Nemegt Basin in the Gobi Desert of Mongolia.
The specimen was collected from the upper section of the Baruungoyot (or Barun Goyot) Formation.
It is housed in the Institute of Paleontology of Mongolian Academy of Sciences in Ulaanbaatar, the capital of Mongolia.
“Jaculinykus yaruui represents one of the most complete alvarezsaur skeletons reported so far,” the paleontologists said.
“It also represents the ninth genus of alvarezsaurids from the Nemegt Basin.”
The specimen displays a stereotypical avian-like sleeping position with the neck and tail arched as well as hind limbs folded under the pelvis.
“The avian-like sleeping posture of Jaculinykus yaruui shows the following features: the hind limbs folded on either side of the body; the left forelimb folded next to the body with the elbow; 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,” the researchers said.
“Despite displacements of both forelimb elements of Jaculinykus yaruui, these bones are tucked underneath the body as in the hind limb.”
“This posture obviously differs from the posture commonly seen in theropod dinosaurs, in which the body lies on one side with both neck and tail arched dorsally.”
“It confirms that this avian-like behavior was already present in maniraptorans prior to paravians.”
The results were published online in the journal PLoS ONE.
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K. Kubo et al. 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
