Its not only in trex, majority of theropods had small arms, famous of which being carnotaurus that had such useless arms but vibrant in color and their hands were used in mating rituals, every group carcharodontasaurids, tyrranosaurids and lot more, only two specific group of theropods had useful arms, spinosaurids and dromeosaurids, spinosaurids like baryonyx, suchomimus and famous spinosaurus had useful arms for grabbing fish and dromeosaurids with famous ones like trodon, utharaptor, pyroraptor and velociraptor had armsto hold onto the prey
@prehistoric-fan I'll need to find the source, but I read that the front limbs were put in the wrong position when assembled and really meant to lay against the body and were flightless wings used for mating or dominance displays
Nahh, GOD made them like that...I believe they have small arms from the beginning. Even though a small arm would be advantageous they can't do shit if they had big long arms