Over at Fish Feet Sarda has been contemplating the actual function of Tyrannosaurus rex forearms (and by extension, those of its kin like Daspletosaurus, Gorgosaurus, Albertasaurus, etc.). In mulling over the problem of arms that may have been powerful but had a small range of movement, I thought I had arrived at the answer; they were used by males to help anchor themselves during mating, and if we had skin from males and females of mating age this could be inferred by females having thicker skin on the back that would be heavily scarred (but then they’d be alive and we’d know the answer). Little did I know that Henry Fairfield Osborn already came up with this idea over 100 years ago. It’s important to remember that we didn’t actually know what a Tyrannosaurus forelimb looked like until the 1980’s (previously it was inferred from Gorgosaurus limbs), but during Osoborn’s time we did have the humerus, and that bone did belie relatively small forelimbs. In his discussion of a skeleton that was further excavated in 1905 in Laramie, Hell Creek, Montana, Osoborn makes the following comments about what remained of the dinosaur’s arms;
Humerus. -The humerus is so small that grave doubts were
entertained as to its association with this animal. These were finally
set aside for three reasons: (i) the humerus is hollow, proving that
it belonged to one of the Theropoda; (2) the head of the humerus
fits into the glenoid cavity of the scapula; (3) while absurdly reduced
as -compared with the femur it nevertheless is provided
with very stout muscular attachments, a powerful deltoid ridge,
which proves that it served some function, possibly that of a grasping
organ in copulation.
The communication included to-scale drawings of the humerus and femur of the Tyrannosaurus rex as well, which are reproduced below.
The truth of the matter is we still don’t know what the real function of the limbs were other than they had some function, and I think the “copulation hypothesis” is as good as any (or at least better than the idea that tyrannosaurs used their limbs to push themselves up off the ground after sleeping). While Tyrannosaurus gets plenty of attention, I wonder if biomechanical studies have been done for its relatives as well. Indeed, I’d be interested to see how muscle attachment, limb growth, and potential strength varied in other members of the tyrannosaur family, perhaps giving us some kind of clue as to how much (if any) the little forelimbs had changed. As with anything, perhaps there is no one answer; perhaps they aided in copulation but also worked as “meat hooks” allowing the dinosaurs to anchor their prey before taking out a bite (although this might have been better achieved with its large feet). Regardless of current hypotheses, I just find it interesting that the more I learn about paleontology, the more I realize that past scientists have already beaten me to the punch.