Rare fossil reveals behavior dinosaurs may have had in common with birds

Seiji Yamamoto

An illustration of the newly discovered dinosaur species named Jaculinykus yaruui shows it sleeping in a pose similar to that of a modern bird.

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Death in the wild is often brutal and violent. But for one small, birdlike dinosaur in the late Cretaceous Period, the end of its life was relatively peaceful — it curled up to take a nap and never woke up.

That’s what scientists have interpreted from the pose of the dinosaur’s fossil skeleton. With the creature’s head tucked over its limbs and tail snugly wrapped around its body, its cozy posture resembled those of modern birds at rest, hinting that these dinosaurs didn’t just look like birds — they may have behaved like them, too.

Paleontologists excavated the dinosaur’s skull and nearly complete skeleton in the Gobi Desert at the Barun Goyot Formation in Mongolia, and most of the bones were still arranged in the animal’s original death pose, researchers reported Wednesday in the journal PLOS One.

The animal’s long neck wrapped around the right side of its trunk, and its head was tucked close to its side, resting atop the right knee. The hind limbs were folded underneath it, and most of the tail curved around the body’s left side.

The study authors identified it as an alvarezsaurid, a type of small theropod (bipedal meat-eating dinosaur) with a long tail and legs and short front limbs. Alvarezsaurids are part of a larger dinosaur group called maniraptorans, which includes birds and birdlike dinosaurs that were their closest relatives.

The small alvarezsaur’s posture emulated that of two other dinosaur fossils found in Mongolia that were also curled up in birdlike sleeping poses: Sinornithoides youngi and Mei long. Those two are troodontids, another type of dinosaur in the maniraptoran group, and one that’s more closely related to birds than alvarezsaurs were.

Kohta Kubo

Jaculinykus yaruui lived around 70 million years ago. (A) A photo of the fossil skeleton found in the Gobi Desert in Mongolia. (B) An explanatory drawing shows skeletal elements, including the skull in green and pectoral girdle and forelimbs in red. (C) A reconstruction of the dinosaur, with gray areas indicating missing parts.

The new fossil suggests that this sleeping behavior may have been…

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