Large Carnivorous Dinosaurs with Tiny Arms: Why Tyrannosaurus rex had Relatively Short Forelimbs

Tyrannosaurus rex (T. rex) is perhaps the most famous non-avian dinosaur due to its large size, ferocity, and portrayal in popular culture. T. rex was the apex predator of its time and dominated parts of North America during the Late Cretaceous around 66 million years ago. This theropod ranged from 12–13 meters in length and weighed around 7 tonnes. It had long, serrated banana-shaped teeth adept at crushing bone, a skull length of around 1.2 meters, and the most powerful jaws of any land animal ever to have lived. Curiously, T. rex had highly reduced forelimbs approximately 28% of its hindlimb length. The purpose of this paper is to discuss why T. rex had such proportionally short forelimbs, and what, if anything, they were used for. Proposed functional hypotheses range from signaling to holding prey to slashing. Hypotheses aiming to describe the mechanism behind reduction have included a compensatory relationship, where forelimb reduction is linked to the increase in skull size for more effective predation. Additionally, selective pressures from group feeding may have favoured reduction, as shorter forelimbs would reduce vulnerability to injury. These hypotheses and conclusions are discussed in terms of phylogenetics, behavioural ecology, and a literature-based assessment.