What evolutional benefit is that?
There was an evolutionary trend among carnivorous theropods that the larger they grew the less they used their arms to grab the prey: If you can either inflict a deadly bite or grab the whole neck in your jaw, grabbing with claws becomes unnecessary. Two lineages took it particularly far: in abelisaurids, such as carnitaurus, the arms became vestigial and in some cases completely disappeared.
In a separate theropod lineage, T-Rexes, while much bigger than any abelisaurid and with a bigger head, did not have vestigial arms. Their arms, tho tiny, still had bones that locked into each-other and seemed to have muscle attachments. There’s been several theories as to why: This certainly meant that T-Rexes still used their arms, but how? The main theory is that juvenile T-Rexes had a different morphology as their adult counterpart. They already hunted, but much smaller preys, and with a different technique, so they would’ve still used their arms. On top of that, there’s the idea that adult T-Rexes might’ve slept on their bellies and their arms would’ve been useful to bush themselves back up.
Another reason for T-Rex’s large heads, in addition to their big jaws being terribly efficient, is that it may help with placing their center of gravity over the knees, which is useful to keep balance while walking or running.
Whaaat? As if a trex could push itself anywhere with it’s arms, that’s ridiculous!
I know how unlikely this sounds, but I didn’t make it up, it’s an actual theory.
https://zenodo.org/record/3674749
https://www.scientificamerican.com/article/if-t-rex-fell-how-did-it/
