Carnotaurus was a weirdo. Not only did this 26-foot predator of Argentina’s Late Cretaceous have prominent horns jutting from its short, deep skull, but, since the time of the dinosaur’s discovery in 1985, paleontologists have been puzzled by the strange arms of the theropod. Despite having absolutely huge shoulder bones, Carnotaurus had wimpy arms that were even stubbier than those on the oft-ridiculed tyrannosaurs. Stubby forelimbs go all the way back to the beginning of the lineage that Carnotaurus belonged to–the abelisaurids–but this ancient South American predator took the reduction to extremes.
Among the relatively short-armed tyrannosaurs, at least, the evolution of small arms is often associated with developing big, well-muscled heads. As tyrannosaur heads became larger and heftier, their arms became smaller to compensate. The idea is that it’s all about balance–if you have a huge head and beefy arms, you’re going to fall on your face. (Sorry, Trogdor.) So far as I know, no one has actually tracked these evolutionary trends, but it remains the prevailing hypothesis. An in-press Acta Palaeontologica Polonica paper about the neck of Carnotaurus forwards a similar explanation for the puny arms of abelisaurids.
The study, written by paleontologist Ariel Méndez, compares the neck vertebrae of Carnotaurus with the same bones in the dinosaur’s close cousin from Cretaceous Madagascar, Majungasaurus. Both were big, short-snouted predators with strange head ornaments, but, as Méndez points out, the neck of Carnotaurus is much more heavily built. For example, the neck vertebrae of Carnotaurus are much wider, with the last bone in the series being as wide as the dinosaur’s skull. In Majungasaurus, the last neck vertebra is only about half the width of the skull (although it should be noted that the Majungasaurus neck vertebrae were inflated in size by about 20 percent to match the neck of a subadult to an adult skull).
So what do these differences mean? Unfortunately, Méndez does not include a full muscular reconstruction in the study but notes that the bony differences almost certainly indicate different muscle arrangements. In general, it seems that Carnotaurus was a more robust animal than Majungasaurus, although increased power may have come with a cost of reduced flexibility between the base of the neck and the tail. Méndez, referring to previous research, also points out that having more heavily-built skulls and necks may be associated with smaller forelimbs. Indeed, while skulls are often the focus of feeding studies, recent research on a variety of carnivores–such as Tyrannosaurus, the sabercat Smilodon and the modern Komodo dragon–have affirmed the importance of neck muscles to feeding. Even carnivores with relatively weak bites, such as sabercats and Komodo dragons, receive a great deal of extra power from their neck muscles while feeding. Perhaps the same was true of Carnotaurus.
Yet the stouter neck of Carnotaurus doesn’t actually explain why this dinosaur had tiny arms. After all, Majungasaurus also had the robust shoulder girdle-vestigial arm combination, yet its neck is clearly not as heavily built as in Carnotaurus. More than that, big shoulders and smallish arms seem to go all the way back to early abelisaurids, such as the recently-described Eoabelisaurus. Although the hefty head and neck-small arms idea makes sense, the idea has yet to be rigorously tested against the actual history of dinosaurs such as abelisaurids and tyrannosaurs. Why huge, powerful carnivores had puny arms remains an evolutionary puzzle.
Méndez, A. (2012). The cervical vertebrae of the Late Cretaceous abelisaurid dinosaur Carnotaurus sastrei Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0129