Scientists Found a New Giant Feathered Dinosaur With Nightmare Claws. But It Ate Plants

The new dinosaur reveals how several bizarre plant-eaters shared the same Cretaceous ecosystem.

About 125 million years ago, in what is now northeastern China, a young dinosaur died before it made it into adulthood. It was already more than 3.6 meters long, carried enormous curved claws on its front limbs and had a mouth crowded with dozens of tiny teeth.

Even as not yet an adult, it was a giant among its immediate relatives.

The ancient creature, newly named Kayrasaurus changliensis, is now the largest therizinosaur known from the famous Jehol fossil beds despite being only a subadult.

These dinosaurs were basically giant, feathered “scythe lizards” with three-foot hand claws. Despite looking like a nightmare predator with the longest claws in history, they were actually peaceful, tree-browsing plant-eaters.

But size is not the most revealing thing about Kayrasaurus. In a new study, paleontologists argue that it adds to evidence that several kinds of therizinosaurs living in the Jehol ecosystem were much more diverse than we were led to believe. They appear to have divided up their environment, evolving different ways of moving and feeding.

“The integration of morphological, biomechanical and body size data suggests the presence of at least two distinct ecological strategies among Jehol therizinosaurians: one favoring oral processing and robust body builds, and the other emphasizing agility,” the authors wrote.

Meet the Kayra Lizard

The new fossil comes from the Yixian Formation in Liaoning, part of the extraordinarily fossil-rich Jehol Biota of northeastern China. Researchers recovered an almost complete skeleton, including much of the skull and both forelimbs. The hind limbs and the very end of the tail are missing.

The animal measured more than 3.6 meters from head to tail, making it at least 1.6 times longer than the former largest Jehol therizinosaur, Beipiaosaurus. The researchers estimate its mass at somewhere between 117 and 274 kilograms, though the missing legs make that figure uncertain.

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Several bones in its spine and skull had not yet completely fused, which can only mean the animal was a subadult. A fully mature individual may therefore have been larger.

Its skull was nearly complete, although burial flattened it to just a few millimeters thick in places. Even from the pancake skull, the researchers could identify a striking collection of features that distinguish Kayrasaurus from its relatives.

Its mouth held an unusually dense battery of teeth—over 32 in the upper jaw and more than 45 in the lower. Researchers say the latter is the highest count yet reported for a therizinosaur. The front portion of the snout lacked teeth, while the nasal bones stretched unusually far backward and ended in a distinctive two-pronged structure.

The dinosaur had a conspicuously elongated neck. Even the shortest cervical vertebra was more than 30% longer than the vertebrae of its back. And one of its hand claws carried a peculiar upward-projecting “lip” along its upper edge, a feature the authors have not identified in any other therizinosaur.

Taken together, those traits convinced the researchers that the specimen represented an entirely new genus and species. The researchers named the dinosaur Kayrasaurus changliensis. The genus name combines Kayra, the creator god in Turkic shamanism, with the Greek sauros, meaning “lizard.” In Turkic mythology, Kayra is the father of Erlik, a reference to the new dinosaur’s evolutionary connection with Erlikosaurus. The species name, changliensis, comes from Changli, a historical place name associated with the region where the fossil was discovered.

A Complicated Family

Therizinosaurs were among evolution’s stranger experiments. They belonged to the theropod lineage that also produced T. rex and birds. However, many abandoned the meat-heavy lifestyle associated with their predatory relatives. Later species developed small heads, long necks, broad bodies, and extraordinary hand claws while eating mostly plants.

Their anatomy was so confusing that early fossils were variously classified as belonging to turtles and other dinosaur groups. The picture became clearer after scientists described Beipiaosaurus in 1999. That fossil not only helped establish therizinosaurs as theropods but also preserved filamentous body covering, suggesting that at least some members of the group were feathered or fuzzy.

With the addition of the new species, Jehol has now produced four early therizinosaurs: Kayrasaurus, Beipiaosaurus, Jianchangosaurus and Lingyuanosaurus. The latter three were much closer in length, yet their skeletons already hinted that they lived quite differently.

The 2013 description of Jianchangosaurus revealed an unusual arrangement of the teeth and jaws that researchers interpreted as evidence of a distinctive feeding style. The 2019 description of Lingyuanosaurus includes long lower legs, which suggests it was better suited to running than Beipiaosaurus, whose shorter tibia points to a slower, more robust build.

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Kayrasaurus now gives researchers a much larger-bodied dinosaur to the roster.

Niche Partitioning

Jianchangosaurus and Lingyuanosaurus had relatively slender bodies, which made them more agile and reliant on their forelimbs while feeding.

Beipiaosaurus, however, was heavier and apparently less suited to fast movement. A 2017 biomechanical study of therizinosaur jaws found that its lower jaw experienced relatively little stress and strain under simulated feeding loads compared with Jianchangosaurus. That does not simply translate into a more powerful bite—the same models actually produced relatively low bite-force estimates for Beipiaosaurus—but it suggests a jaw better able to withstand certain feeding stresses.

Kayrasaurus appears to fall closer to that robust end of the spectrum. Its anatomy shares several features with Beipiaosaurus, and the researchers suggest both may have relied more heavily on processing food with their mouths while carrying slower and sturdier bodies.

Because Kayrasaurus stood substantially larger than the other Jehol therizinosaurs, it may have been able to browse vegetation at heights its smaller relatives could not easily reach.

This kind of niche partitioning could help explain how several closely related dinosaurs occupied the same ecosystem without competing for the same resources.

But the authors stress that the idea remains provisional. Kayrasaurus has no preserved hind limbs, which makes its locomotion difficult to reconstruct, and researchers have yet to carry out direct biomechanical modeling of the new species.

For now, Kayrasaurus adds a new body plan to an already peculiar family—and suggests that long before therizinosaurs evolved into the giant, pot-bellied forms of the Late Cretaceous, their earlier relatives were already experimenting with markedly different ways of being a herbivorous theropod.

The study was published in the Journal of Systematic Palaeontology.