Patagonia, Argentina The Tyrannosaurus rex is notorious for its disproportionally small arms, but recent paleontological discoveries in Patagonia suggest it may face stiff competition in the realm of puny limbs. A newly identified species of abelisaurid, named Koleken inakayali, showcases even more diminutive appendages than the famed T. rex.
Koleken inakayali, a carnivorous, bipedal dinosaur whose remains were uncovered in Patagonia, may have had even weaker upper limbs than T. rex. Although the fossilized remains of K. inakayali did not include arm bones, researchers believe its arms were similar to those of its relative, Carnotaurus sastrei, another Patagonian abelisaurid.
Like Carnotaurus, K. inakayali likely had immobile elbows and rudimentary wrist joints, rendering its arms nearly useless. These appendages, unbendable and unable to grasp, would have been mere stubs against its chest during movement.
At the end of the Late Cretaceous, roughly 90 to 66 million years ago, abelisaurids were the dominant dinosaurs on Earth. While T. rex roamed North America and Asia, South America has yielded the most comprehensive fossil record of their relatives, revealing a diverse and widespread clade.
Despite their anatomical peculiarities, the evolutionary reasons behind abelisaurids’ small arms remain elusive. Some theories propose that T. rex’s claws might have been used for slashing or holding onto mates, while others suggest these limbs were evolutionary remnants from distant ancestors. It is hypothesized that as the skulls of abelisaurids grew in size, they relied more on their powerful jaws than their arms for hunting, potentially evolving shorter arms to avoid being bitten off during group feeding.
The discovery of K. inakayali, along with other recent finds, highlights the rich diversity of abelisaurids in Patagonia just before the mass extinction event. These findings suggest multiple evolutionary paths leading to reduced arm size, as evidenced by the 2022 discovery of a South American abelisaurid with minuscule arms and one of the smallest brains among its relatives.
“This finding sheds light on the diversity of abelisaurid theropods in Patagonia right before the mass extinction event,” said National Geographic explorer Diego Pol, who led the discovery with an international team from Argentina, the USA, and China. “Our study also analyzes the evolution of abelisaurids and their relatives through time and identifies pulses of accelerated rates of skull evolution in the Early Cretaceous. It expands what we know about abelisaurids living in this area during the Cretaceous Period and shows that they were more diverse than previously understood.”
