Why Did Carnotaurus Have Such Wimpy Arms?

Tyrannosaurus gets a lot of guff for having relatively small, two-fingered hands, but that isn’t really fair. Though small, the arms of Tyrannosaurus and other big tyrannosaurs were robust and heavily muscled, hinting that the dinosaurs may have used their arms like meat hooks while tangling with struggling prey. So let’s have no more of this “Tyrannosaurus had sissy arms” nonsense. If we’re going to poke fun at any dinosaur for having wimpy forelimbs, it should probably be Carnotaurus.

While tyrannosaurus were among the most formidable predators in North America and Asia during the Late Cretaceous, in South America the same roles were often played by a different breed of theropod dinosaur known as abelisaurids. Of these, Carnotaurus is probably the most famous—the fact that this “meat-eating bull” had two horns sticking out of its short, deep skull gave it an instant appeal. As fearsome as Carnotaurus looked, though, it’s hard not chuckle at the dinosaur’s arms—the hand and lower part of the forelimb were so reduced in size that some paleontologists have viewed them as vestigial structures that have almost entirely lost their ability to function in acquiring prey. In a new paper published in Palaeontology, researcher Javier Ruiz and colleagues reexamine the strange arms of this dinosaur and how they compare to those of other abelisaurid predators such as Majungasaurus and Aucasaurus.

As pointed out by Ruiz and co-authors, the arms of Carnotaurus have a robust lower portion, made up of the radius and ulna, that is about a quarter of the length of the upper arm bone (the humerus). The hand itself has four fingers, and unlike in the other abelisaurids considered in the paper, the fourth metacarpal bone is the biggest bone in the hand. This small and peculiar difference helps set Carnotaurus apart, but the comparisons among this dinosaur, Majungasaurus and Aucasaurus may also add some new information about how the arms of these dinosaurs got to be so wimpy.

In the big picture of theropod evolution, the abelisaurid dinosaurs belong to an even larger group called ceratosaurs. Earlier representatives of this group such as Limusaurus and Ceratosaurus already had relatively short and stubby hands in the Jurassic, and it appears that the hands of abelisaurids followed this evolutionary trend. The question is why this reduction in limb size happened. We can come up with “just so” stories in an attempt to explain the trend, but testing the idea is another matter entirely and something that is not touched on in the paper by Ruiz and collaborators. Equally perplexing is why the hand of Carnotaurus was so small while the other arm bones were thick and powerful-looking, even compared to other abelisaurids. We don’t yet have a good answer for why this should be so.  For now, Ruiz and colleagues conclude that the hands of the odd abelisaurids were as odd and diverse as the different arrangements of crests, horns and bumps which adorned their skulls. How the structures related to the lives of the animals themselves will require further study.

References:

RUIZ, J., TORICES, A., SERRANO, H., & LÓPEZ, V. (2011). The hand structure of Carnotaurus sastrei (Theropoda, Abelisauridae): implications for hand diversity and evolution in abelisaurids Palaeontology DOI: 10.1111/j.1475-4983.2011.01091.x

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.