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When scientists stopped looking at size alone and studied what Tyrannosaurus arms could tell us about the dinosaur's muscular anatomy, however, it was immediately apparent that its forelimbs were not useless vestiges after all. Though small, the forelimbs of Tyrannosaurus were actually quite beefy and probably acted like meathooks in securing live prey. As reconstructed by paleontologist Ken Carpenter, Tyrannosaurus was a "clutcher" that held struggling prey close with its claws while its enormous head took care of the dirty work. It's about time that we cut Tyrannosaurus a break. But there is another group of dinosaurs that truly did have amusingly stunted arms.
Last year paleontologist Phil Senter conducted a review of vestigial structures in dinosaurs. Such structures were scattered across a wide array of genera, but Senter stressed that "vestigial" does not mean the same as "useless." Instead he looked for structures that became so reduced in size that they could no longer carry out their original function even if they still retained some other secondary function. For example, despite having only two fingers, Tyrannosaurus rex retained a single bone from its third finger—the metacarpal—which was enclosed within its hand. Since many other theropods, including some early tyrannosauroids, had three fingers, this splint of bone in Tyrannosaurus rex fits the definition of a vestigial structure.
With this framework in place, Senter recognized that the entire group of predatory dinosaurs called the abelisaurids had partially vestigial arms. Represented by dinosaurs such as Carnotaurus and the recently described Skorpiovenator, the abelisaurids had stout upper arm bones followed by much shorter lower arm bones (the radius and ulna) held together by an immobile elbow joint. They also had a reduced number of stubby, fused fingers, which could not grasp and lacked claws, making their arms useless for prey capture. Whereas Tyrannosaurus had functional forelimbs that played a role in stabilizing struggling prey, Carnotaurus and its kin had only tiny forelimbs that probably just hung there.
Why abelisaurids like Carnotaurus, Aucasaurus and Majungasaurus had vestigial forelimbs is unclear. It is difficult to imagine what these dinosaurs could have been doing with their arms, and it is possible that their forelimbs did not have any function at all. (As we learned from the debates about the arms of Tyrannosaurus, it is easy to come up with stories about the potential function of a trait but difficult to test those ideas.) In terms of how their arms got that way, however, in 2002 Alexander Vargas proposed that changes during the early development of these dinosaurs may have been involved. According to Vargas, the stumpy, fused, vestigial forelimbs of the abelisaurids may have been caused by a loss of function in two genes that regulate the development of the forelimb, HOXA11 and HOXD11. This is a plausible explanation, but it only gives us the proximal trigger for the change in these dinosaurs. Determining why such stubby arms were widespread among these dinosaurs—and how they hunted without the use of their forelimbs—is another matter.
Agnolin, F., & Chiarelli, P. (2009). The position of the claws in Noasauridae (Dinosauria: Abelisauroidea) and its implications for abelisauroid manus evolution Paläontologische Zeitschrift, 84 (2), 293-300 DOI: 10.1007/s12542-009-0044-2
Senter, P. (2010). Vestigial skeletal structures in dinosaurs Journal of Zoology, 280 (1), 60-71 DOI: 10.1111/j.1469-7998.2009.00640.x