Abelisaurid arms

[Rexy]

Why did Abelisaurids have such short, immobile arms when every other Theropod had arms that were able to be used? I've heard a theory that they lost the function of two genes that regulate the forelimbs' development, but is there any evidence to back up this theory?

[Dinoxels]

I don't have the how, but I have the why. Similar to Tyrannosaurs Abliesaurids used their mouths in combat rather then their arms. So the arms slowly became less of an importance. Plus, it helps streamline the animal.

[Rexy]

Tyrannosaur arms were short compared to what's seen on other Theropods, but they were still functional. I believe the arms shrank because the head got bigger so it was more practical to have short, light-weight arms rather than arms that were long and heavy. Abelisaurids, however, had smaller heads than what you'd see on a Tyrannosaur, yet their arms were a lot less developed.