January 24, 2012
Fearsome Dinosaur Had Ridiculously Short Arms
The reconstructed shoulder and arm of Majungasaurus. From Burch and Carrano, 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.
References:
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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“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.”
Scratch its belly after dinner??
The hypothesis that tyrannosaurs’ forelimbs were reduced due the enlargement of the skull doesn’t seem to apply to abelisaurs. Their skulls aren’t particularly massive but actually the contrary and their forelimbs aren’t just ridiculous short but atrophic too.
Great article! What could be the purpose of Majungasaurus having such disproportionate limb bone sizes? More specifically, what use could such a big shoulder blade have if the muscles don’t need to support big arms?
And regarding Henrique Niza’s question: we know tyrannosaurids had powerful crushing skulls, has any work been done on abelisaurid biting strength?
I recall reading quite a bit about how supposed weak abelisaurs’ jaws were, specially Paul Sereno thoughts about how Rugops was primarily scavenger due its supposed weak skull. It seems feasible since their jaws look less robust than even allosaurs’ jaws and these didn’t have that strong bite.
Is there evidence for feather attachment in the surrounding matrix? The range of motion in combination with feathers could argue for the some sort display function, be it sexual or defensive.
I suspect that by flapping its stubby little arms wildly the Majungasaurus could use them to signal panic, just like in humans.
Or maybe they were a sort of distraction device. While prey animals were doubled over in laughter at the Majungasaurus’s pathetic little arms, the Majungasaurus would casually stroll over and eat them.
Perhaps whatever evolutionary forces caused Majungasaurus arms to become so small simply reached a tipping point: once the arms became that tiny, perhaps there was no longer any advantage to them becoming any smaller, so natural selection simply ceased to have any impact on the feature. I’m not an evolutionary biologist, so I honestly don’t know if that makes sense, but many vestigial features fail to vanish altogether. I’m thinking of some whale’s “feet” and the kiwi’s wings, as well as perhaps the human appendix.
I must say, though, that the image of it waving around its comical arms in panic made me giggle for a long time.
With a wide range of movement in the arms, but little to no actual manual dexterity… I think the arms may have been used like the ‘spurs’ on male boas and pythons (actually the remains of rear limbs, with the spur itself being a claw that is visible outside the body on either side of the cloaca).
Male boas and pythons use these spurs during mating, and move them back and forth to stroke or tickle the female. While this just helps to get the female ‘in the mood’ in boids, with large theropod dinosaurs, this ‘tickling’ could literally be a way of saying ‘I’m your mate – not your dinner!’
I’ll have to agree with Lisa here. The arms of abelisaurs are useless as weapons but make quite effective as sexual seduction tickle tools, and perhaps also as spurs for fighting males. The flexibility of the arms joints would have prevented serious injury, and the bizarrely huge shoulder blades may have been so big to protect the chest and upper torso against heavy impacts from sideswiping rivals. During these more heavy impacts the arm could be tucked down and back.