Is there any evolutionary reason that the T-Rex dinosaur had such short, small arms?
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It seems to me that it was a balance issue, because you only see those tiny arms in very big theropods.
T. rex was about as big as it could possibly be and still move pretty fast. Any heavier and it would need thicker legs, and you’re getting into diminishing returns here. And it was a biped, but not like humans are: its body was balanced horizontally at the hips.
It had a really big head with strong jaws, and a big heavy tail to balance the head. If it had big heavy arms as well, it would either have had to have a smaller head, making its bite less effective, or it would have to have had an even heavier tail to balance the extra weight of the arms, which would stress its hips and make it too heavy to run fast.
Also, we think it used those arms to help itself get up from lying down, and to hang on to a mate. It also went after things like ankylosaurs and sauropods which had club or whip tails. With its big jaw it could grab onto an ankylosaur and hold it far enough away that it probably couldn’t whack the rex on the shins, but long arms would be sticking out in the way of the tail-club, without adding much to the animal’s effectiveness as a predator. A broken arm would be disproportionately disabling because it would make it hard to stand in the morning, and meanwhile jaws and clawed feet were quite enough to kill with. Think if it as a really enormous secretary bird.
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