A BITE SO BIG YOU LOSE YOUR ARMS

Researchers have landed on a pretty convincing new proposal for why T.rex (and friends) had such stubby arms. For a long time we’ve assumed their little limbs were vestigial, losing all their original function over time and having little to no practical use. But the reason for this transition was not so clear as it appears now. It looks less like a weird mutation that stuck, or that entire species were passionate accordion players, and more like convergent evolution where an increasingly large head and powerful jaws took over for grasping limbs as their prey increased in size to outperform these predators. 

In their paper, Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs, Scherer, Steell, and Upchurch offer that in T.rex and 82 species from five other carnivorous theropod lineages, reduction in forelimb size appears to be not merely correlated with their trend toward gigantism but “causally associated with increases in body size and increasing cranial robusticity.” Interestingly too, the team found a stronger correlation between smaller forelimbs and skull strength than with increased skull or body size. Spelling this out, they observe how several smaller theropods had equally robust heads and tiny arms paired with bodies a fraction the size of T.rex. So, it’s kind of like how in the military arms race between nations, the weight and physical size of a bomb or missile is far less relevant to all concerned than the yield of the explosive therein. In the case of T.rex and friends, the more crushing power in its jaws the more redundant its forelimbs; and then, of course, they all but lost those limbs as they stopped using them.

...of course, I'm still waiting for all of this research to be instantly overturned when someone finds a cache of accordions in the Cretaceous stratigraphy.