January 24, 2012
A few months ago, I wrote about a big, carnivorous dinosaur with what may have been the wimpiest arms of all time. No, not Tyrannosaurus, but a very distantly related predatory dinosaur from Cretaceous South America called Carnotaurus. Despite this dinosaur’s massive, beefy shoulderblade, the arm of Carnotaurus was little more than a nub that would have barely stuck out from the body. And, according to a recent fossil find from Madagascar, Carnotaurus wasn’t alone in having ridiculously tiny forelimbs.
Carnotaurus belonged to a group of theropods called abelisaurids. Among them were large predators that spread through the southern portion of the Cretaceous world, including Majungasaurus from Madagascar. (This dinosaur got a brief publicity boost thanks to the first episode of the sensationalistic show Jurassic Fight Club.) This was another hefty carnivore with bizarre head ornamentation. As demonstrated in a new Journal of Vertebrate Paleontology paper by researchers Sara Burch and Matthew Carrano, Majungasaurus also had truly vestigial arms.
Tiny arms are a common abelisaurid feature. Majungasaurus was expected to share this feature with other closely related dinosaurs, but a lack of fossil evidence prevented paleontologists from seeing what the forelimb of this animal really looked like. That changed in 2005, when paleontologists discovered a nearly complete and mostly articulated skeleton of Majungasaurus, including elements from the entire forelimb and shoulder girdle. (Among the lot was a furcula, or the equivalent of a wishbone, which is the first time this bone has been found in an abelisaurid.)
When viewed together, the forelimbs of this animal look like an evolutionary joke. A large humerus connects to a broad shoulder girdle, but the lower part of the arm—from the radius and ulna down to the dinosaur’s four fingers—is composed of short, stout bones that altogether make up less than a third of the length of the upper arm bone. And the fingers were short, stubby, and lacked sharp claws.
But what may be even stranger is that the arms of Majungasaurus were probably capable of a relatively wide range of motion. The connection between the humerus and the shoulder girdle was more flexible than in many other theropod dinosaurs, and Burch and Carrano suggest that the wrist of Majungasaurus, too, could probably be extended quite far. Conversely, though, the paleontologists note that the fingers were probably relatively stiff and the dinosaur lacked the ability to move them very much, so perhaps the dinosaur used its hand as a single unit—like a dinosaurian mitten. That’s assuming that Majungasaurus was doing anything with its arms at all. This dinosaur’s arms and hands had become so reduced that it is difficult to imagine what they could have possibly done with them other than impotently flap them around. We may never know for sure.
Burch, S., & Carrano, M. (2012). An articulated pectoral girdle and forelimb of the abelisaurid theropod Majungasaurus crenatissimus from the Late Cretaceous of Madagascar Journal of Vertebrate Paleontology, 32 (1), 1-16 DOI: 10.1080/02724634.2012.622027
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