Scientists just dug up a new dinosaur—with tinier arms than a T.Rex

It all started with a claw.

While scouring Argentina’s La Colonia Formation for new dinosaur fossils, paleontologists noticed the single toe bone sticking out of the ancient rock. When they dug in, they found a new dinosaur—a carnivore that roamed prehistoric Patagonia several million years before an asteroid impact brought the Cretaceous to a fiery close.

The new dinosaur turned out to be a short-snouted meat-eater known to experts as an abelisaurid. Diego Pol, a paleontologist at Argentina's Egidio Feruglio Paleontology Museum and colleagues have named it Koleken inakayali, a tribute to Inakayal, a deceased chief from the Tehuelche Indigenous people of eastern Patagonia, and a name from their Teushen language meaning “coming from clay and water.”

Pol, a National Geographic Explorer, and colleagues uncovered Koleken as part of an ongoing effort to understand dinosaur evolution prior to the mass extinction that forever changed Earth’s biodiversity 66 million years ago. To date, most of what we know about the last days of the dinosaurs comes from a relatively small area of North America. Finds like Koleken in Patagonia are revealing how dinosaurs were continuing to evolve and thrive in the several million years before that mass extinction. (How the world’s deadliest mass extinction gave rise to dinosaurs.)

Digging in after the discovery of the first claw, Pol and colleagues returned to the site in the hopes of finding more. “We found there was a concretion just below the surface where all these bones were coming from,” says Pol, whose study was published this week in Cladistics. The team carefully collected skull bones that had already eroded from the rock and were scattered around the concretion, as well as various parts of the theropod’s spine, hips, and limbs.

“When we prepared the concretion at the lab, we found the entire back end of Koleken was preserved in perfect articulation,” Pol says. Part of the spine, the hips, and complete legs were all encased together, hinting that the dinosaur was buried and preserved before the body decayed and parts scattered.

Finding dinosaur bones still in articulation with each other is relatively rare, especially considering how much sediment was needed to bury and preserve an animal that could get to be more than 20 feet in length.

It wasn’t immediately apparent that Koleken was a new dinosaur, however. Decades earlier, in 1985, paleontologists named a famous carnivore from the same geologic formation called Carnotaurus.

The “meat-eating bull” quickly became a dinosaur celebrity, characterized by the triangular horns jutting above each eye. On close inspection, however, Pol and colleagues did not find any sign of horns or other telltale Carnotaurus traits, indicating that the bones of the carnivore they’d recovered belonged to a species no one had seen before.

Tiny arms and a deadly bite

“I find the arguments of the authors very compelling,” says Mauricio Cerroni, a paleontologist at the Bernardino Rivadavia Natural Science Argentine Museum who was not involved in the new study.

That’s because the species’ differences are clear: The nasal bones of Koleken and Carnotaurus, for example, are easily distinguished from each other, and the lack of broad eyebrow horns in Koleken is another point that differentiates the two. (See stunning photos of dinosaur fossils.)

Whether Koleken and Carnotaurus lived side-by-side is unknown. The La Colonia Formation spans about five million years, and so the two carnivores could have been contemporaries or separated by millions of years. Nevertheless, the discovery of Koleken has provided Pol and colleagues with more information about how the short-snouted abelisaurids fared late into the Cretaceous.

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These carnivores thrived through much of the Southern Hemisphere, from Carnotaurus in Argentina and Rugops in Niger to Majungasaurus in Madagascar, among a few other spots on the globe. While tyrannosaurs were living large in North America and Eurasia, the abelisaurids were among the most widespread and diverse carnivores of the southern continents. Abelisaurids thrived alongside large sauropod dinosaurs: The long-necked herbivore Titanomachya, named by Pol and colleagues from the same formation earlier this year, may have been prey for Koleken.

“Abelisaurids are among the most notable and fascinating carnivores of the Cretaceous, with a unique variability in skull ornamentation like crests, domes, and horns,” Pol says.

The carnivores had a somewhat stubby appearance compared to the more familiar tyrannosaurs. In life the arms of abelisaurids would have barely protruded from the body, says Federico Agnolin, a paleontologist at the Argentine museum also not involved in the new research.

Abelisaurids had short, deep skulls, thick necks, and stout arms, an unmistakable combination of features that make the carnivores instantly recognizable.

“Abelisaurids also had ridiculous arms even compared to tyrannosaurs. Abelisaurids had massive shoulder bones but extremely short arms with multiple small fingers, clearly useless in any form of prey capture and yet somehow quite flexible. “We still don’t know what they were using their forelimbs for,” Pol says.

Family ties

Stranger still, abelisaurids were closely related to another group of Southern Hemisphere dinosaurs called noasaurids. Where abelisaurids were bulky, muscular ambush predators, noasaurids were often smaller and lanky, preferring small prey or even plants.

“They are so different from each other that it’s funny they are close relatives,” Pol says.

The discovery of Koleken offered Pol and colleagues a prompt to examine the evolutionary history of both lineages and work out how these dinosaurs became so different after splitting from a common ancestor in the Jurassic. (Read more how scientists are reimagining dinosaurs.)

Pol and colleagues found that abelisaurids like Koleken and noasaurids began to rapidly evolve different body plans between the late part of the Jurassic and the early days of the Cretaceous. The importance of the work, Cerroni says, is that it makes sense of when the two dinosaur groups began to diverge despite being such close relatives.

Noasaurids experienced changes to their hind limbs and trunk, while abelisaurid skulls were evolving rapidly into the short, ornamented, highly carnivorous forms like those of Carnotaurus and Koleken. Abelisaurids also evolved a diverse array of skull shapes, Agnolin notes, while the rest of their bodies remained relatively similar to each other.

Fossils from this window of time are rare as yet, but the discovery of Koleken has helped experts refine ideas about where to go looking for these key transitional fossils. “We certainly need to keep looking for theropods during these periods,” Pol says, “to find out more about these key moments.”