This dinosaur slept like a bird, and it did so 70 million years before birds existed.
Seventy million years before the first bird took flight, a small dinosaur curled into the same sleeping posture a duck assumes today — and was preserved that way in the Gobi Desert long enough for us to find it. The newly named Jaculinykus yaruui, an alvarezsaurid no larger than a house cat, offers the first definitive fossil evidence that birdlike thermoregulatory sleep behavior evolved in non-avian dinosaurs well before powered flight emerged. Its discovery gently erases another boundary we had drawn between dinosaurs and birds, reminding us that evolution does not honor the categories we impose upon it.
- A nearly complete, articulated skeleton found in Mongolia's Barun Goyot Formation was still locked in its original sleeping pose — head tucked, neck curved, tail wrapped around the body — exactly as the animal died 70 million years ago.
- The posture was immediately recognizable to ornithologists and paleontologists alike: it is the same heat-conserving curl seen in resting ducks and pigeons today, raising urgent questions about how far back bird behavior truly reaches.
- Two earlier Mongolian fossils hinted at this behavior in troodontids more closely related to birds, but Jaculinykus belongs to a more distant branch, suggesting the sleeping posture was widespread across non-avian dinosaur relatives — not an isolated quirk.
- Researchers now argue that thermoregulatory sleep behavior evolved as a survival strategy driven by miniaturization and cooling climates, long before the ancestors of birds ever left the ground.
- The discovery is landing as a meaningful reframing of the dinosaur-to-bird transition — not a single dramatic leap, but a gradual accumulation of shared behaviors embedded in creatures that looked nothing like the birds they prefigured.
Seventy million years ago, a small dinosaur lay down to sleep in what is now Mongolia's Gobi Desert and never woke up. When paleontologists excavated the fossil from the Barun Goyot Formation, they found its skeleton still arranged exactly as it had fallen — head tucked against its side, neck curved around its trunk, tail wrapped snugly around itself. The posture was immediately familiar: this dinosaur slept like a bird.
The creature has been named Jaculinykus yaruui, a newly identified alvarezsaurid — a group of small theropods with long legs but oddly shortened forelimbs. Just over three feet long, it was a modest animal, yet its nearly complete and articulated skeleton tells a story that reaches deeper into evolutionary history than scientists had previously documented. Lead researcher Kohta Kubo of Hokkaido University identified it as the first definitive record of this sleeping posture in alvarezsaurs and in the broader early maniraptoran lineage.
Two earlier Mongolian fossils — the troodontids Sinornithoides youngi and Mei long — had suggested similar behavior, but those finds stood somewhat alone. Jaculinykus, belonging to a more distantly related branch, implies the posture was far more common among non-avian dinosaur relatives than anyone had realized. Jingmai O'Connor of the Field Museum put it plainly: the sleeping position is identical to what we see in ducks today, and now it has deep roots in dinosaur ancestry.
The significance extends beyond posture. In modern birds, curling tightly conserves body heat — a critical strategy for small, warm-blooded animals. Jaculinykus, tiny and living during a period of cooling climate, likely slept this way for the same reason. Crucially, this behavior evolved before powered flight ever appeared, suggesting that the physiological and behavioral traits we associate with birds were already being assembled in creatures that looked nothing like them.
Fossils that capture behavior rather than mere anatomy are extraordinarily rare. This one offers a window not into violence or struggle, but into the quiet, ordinary end of a life — and in that quietness lies evidence that the boundary between dinosaurs and birds was always far blurrier than we imagined.
Seventy million years ago, in what is now the Gobi Desert of Mongolia, a small dinosaur lay down to sleep and never woke up. Its body remained exactly as it fell—head tucked against its side, long neck curved around its trunk, tail wrapped snugly around itself like a blanket. When paleontologists excavated the fossil from the Barun Goyot Formation, they found the skeleton still arranged in that original pose, preserved in stone. The posture was unmistakable to anyone who has watched a duck or pigeon settle in for rest: this dinosaur slept like a bird.
The creature has been named Jaculinykus yaruui, a newly identified species belonging to a group called alvarezsaurids—small theropods with long tails and legs but oddly shortened front limbs. At just over three feet long from nose to tail, it was a modest animal, yet its fossil tells a story that reaches far deeper into the evolutionary history of birds than scientists had previously documented. The skeleton, described in a study published Wednesday in PLOS One, is nearly complete and articulated, meaning the bones remained in their original arrangement rather than being scattered and jumbled by scavengers or currents. Lead researcher Kohta Kubo, a doctoral candidate at Hokkaido University in Japan, noted that this represents the first definitive record of such a sleeping posture in alvarezsaurs and in the broader group of early maniraptoran dinosaurs.
Two other dinosaur fossils found in Mongolia had previously suggested this behavior—Sinornithoides youngi and Mei long, both troodontids that were more closely related to modern birds than alvarezsaurs were. But those discoveries stood somewhat alone. The new fossil suggests the sleeping posture was far more common among the non-avian relatives of early birds than anyone had realized. Jingmai O'Connor, an associate curator of fossil reptiles at the Field Museum of Natural History in Chicago, put it plainly: "We've all seen ducks sleeping with their heads tucked under their wings. And then you see this little dinosaur with the exact same sleeping posture." She emphasized that this finding offers tangible evidence of a behavior once thought unique to birds, now proven to have deep roots in dinosaur ancestry.
The significance lies not merely in posture but in what the posture reveals about physiology and evolution. In modern birds, curling up tightly conserves body heat—a thermoregulatory strategy essential for small, warm-blooded animals. Jaculinykus yaruui, being tiny and living in an era when the climate was cooling, likely adopted the same strategy for the same reason. What makes this discovery particularly striking is that it demonstrates this heat-conserving behavior evolved in non-avian dinosaurs before powered flight ever emerged. The alvarezsaurids underwent what researchers call "drastic miniaturization" over their evolutionary history, shrinking steadily in size. That size reduction may have driven them to adopt the same sleeping posture their avian cousins would later perfect.
Jaculinykus yaruui's closest known relative is another alvarezsaurid called Shuvuuia deserti, but the new species distinguishes itself through several anatomical features: the shape of its nostrils, the way jaw muscles attached to its skull, and the specific geometry of its limb bones. These details matter because they establish Jaculinykus as a distinct species, not merely a variant of something already known. O'Connor called it "a nice specimen," noting that new dinosaur diversity, especially among alvarezsaurids, remains genuinely exciting to the field.
Fossils this well-preserved—ones that capture actual behavior rather than merely skeletal anatomy—are extraordinarily rare. Most dinosaur fossils show us what creatures looked like; this one shows us how one small dinosaur spent its final moments, curled in sleep. It is a window not into violence or struggle but into the quiet, ordinary end of a life, and in that quietness lies evidence that the line between dinosaurs and birds was far blurrier than the popular imagination suggests. The behaviors we associate with birds today have roots stretching back millions of years into the Cretaceous, embedded in the bodies of creatures that looked nothing like the birds we know but acted, at least in sleep, exactly like them.
Citas Notables
We've all seen ducks sleeping with their heads tucked under their wings. And then you see this little dinosaur with the exact same sleeping posture.— Jingmai O'Connor, Field Museum of Natural History
This specimen represents a nearly complete and articulated skeleton preserving its life posture. It is the first definitive record of this posture in alvarezsaurs.— Kohta Kubo, lead researcher, Hokkaido University
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that we found a dinosaur in a sleeping position? Isn't that just one fossil?
It matters because sleeping posture tells us about behavior—something we almost never get to see in the fossil record. Most fossils show us anatomy. This one shows us how the animal actually lived, moment to moment.
But there were already two other dinosaurs found in similar poses. What does a third one add?
The first two were troodontids, which were more closely related to birds. This one is an alvarezsaur, a different branch of the family tree. It suggests the behavior wasn't isolated to one lineage—it was widespread. That changes how we understand dinosaur evolution.
You mentioned thermoregulation. Why would a dinosaur need to conserve heat that way?
These were small animals, and they lived during a cooling period in the Cretaceous. Small bodies lose heat faster. Curling up tightly reduces surface area and traps warmth. It's the same reason a shivering person huddles. We know birds do this; now we know their dinosaur ancestors did too.
So this proves dinosaurs were warm-blooded?
It's evidence pointing that way, yes. But more specifically, it shows that certain dinosaurs had behavioral strategies for managing body temperature—strategies we thought were uniquely avian. That's a significant shift in how we understand the dinosaur-bird connection.
The dinosaur was only three feet long. Does size matter here?
Absolutely. Smaller animals need to conserve heat more urgently. The fact that alvarezsaurs underwent "drastic miniaturization" over time—getting smaller and smaller—may have pushed them to adopt this sleeping behavior. It's evolution responding to physical constraint.
What's the next question paleontologists will ask?
Probably whether other small dinosaur groups did the same thing. And whether we can find more fossils in similar poses. Each one is rare, but each one tells us the behavior was more common than we thought. Eventually, we might realize this wasn't exceptional at all—it was normal.