Bird-Like Dinosaurs Slept Like Modern Birds, New Fossil Reveals

Jaculinykus yaruui had a similar sleeping posture to modern birds.

Prehistoric Bird Sleep: A dinosaur species discovered in Mongolia appears to have slept in a manner similar to that of modern birds over 75 million years ago. The fossil of Jaculinykus yaruui reveals that the creature folded its legs under its body while simultaneously bending its tail and neck, adopting a posture reminiscent of sleeping geese or swans. Researchers infer that Jaculinykus exhibited bird-like characteristics, possibly including the presence of a complex feather covering.

Every extant bird today originates from bipedal, walking dinosaurs. Among these early avian ancestors were the Alvarezsaurids, small, nocturnal carnivores with a penchant for consuming meat and insects and sporting a feathered plumage. A distinctive feature was their highly reduced hands; some species possess only a single claw-like finger, likely used for probing insects, such as termites.

A Nimble Dragon From Mongolia

Fossil and skeletal reconstruction of Jaculinykus. Image: PLoS ONE/CC-by 4.0

The skeletons of Alvarezsaurids already exhibited some similarities to those of modern birds. For instance, their sternum was equipped with a sort of ridge that increased the surface area for musculature, and their carpal and metacarpal bones were fused. A recent fossil discovery from the Gobi Desert could further support the notion that not only the anatomy but also the behavior of Alvarezsaurids resembled that of modern birds.

In the Nemegt Basin in southern Mongolia, paleontologists led by Kohta Kubo from Hokkaido University in Japan uncovered the nearly complete skeleton of a previously unknown Alvarezsaurid. They named this creature Jaculinykus yaruui, translating to “nimble little dragon with claws.” Paleontologists claim that Jaculinykus had two fingers on each hand and lived during the Late Cretaceous period, which lasted from 83.6 to 72 million years ago.

Buried in Sleep

The sleeping position of Jaculinykus compared with other fossil finds and the sleeping posture of modern birds (right).
The sleeping position of Jaculinykus compared with other fossil finds and the sleeping posture of modern birds (right).

The uniqueness of the fossil lies in its preservation of the small dinosaur in a deep sleep. Kubo and his colleagues believe that Jaculinykus was once caught by a mudslide or sandstorm and buried while it was asleep. This preserved its sleeping position in detail until today.

This discovery provides valuable insights into the posture in which the dinosaur rested. “The neck curves to the back on the right side, with the skull above the right knee. The majority of the tail lies on the left side of the body, bending around the flexed hind limbs to the right, then curving forward under the skull,” describe Kubo and his team regarding Jaculinykus’s posture.

Sleeping Posture Similar to Today’s Birds

The sleeping posture of the small dinosaur, Jaculinykus, closely resembles that of modern birds, according to paleontologists. Its curved neck and tail, along with the folded hind limbs beneath the pelvis, bear a strong resemblance to the sleeping posture observed in contemporary birds such as geese or swans. These birds typically sleep by bending their necks and tucking their heads into their feathers, aiding in retaining warmth during sleep.

The researchers suggest that the similarity in sleeping posture, coupled with Jaculinykus being relatively small, implies that this posture might have served the purpose of warmth retention for this dinosaur as well. The resemblance to sleeping birds could also suggest that Alvarezsaurids, to which Jaculinykus belongs, shared more similarities with modern birds than previously thought. According to Kubo and his colleagues, it is possible that these dinosaurs had intricate feathers with distinct shafts instead of just a feathered covering.

Jaculinykus is the third bipedal dinosaur whose bird-like sleeping posture has been identified. The other two instances include Mei long, dating back 130 million years, and Sinornthoides youngi, dating back 125 million years.

Source: PLoS ONE