The Tyrannosaurus rex was one of the most notorious predators this planet has ever seen. Adults measured around 40 feet long and had teeth the size of large knives. But one aspect of the T. Rex is less frightening than the rest: its little stubby arms. Why are T. rex's arms so puny? Minute Earth tries to answer that question in the above video.
The simple truth is that scientists aren't sure exactly why T. rex's arms are so short, but there's a number of possible explanations. Perhaps the most likely is that the dino's arms just weren't very useful. Carrying around giant useless arms is pretty disadvantageous, so over many generations, tiny arms won out via natural selection. This hypothesis is supported by looking at distant relatives of the T. rex, almost all of which have tiny arms too.
Another possibility is that small arms evolved on the T. rex entirely by chance. Sometimes, traits that are neither helpful nor harmful can become more common in a population simply by chance. This is unlikely to have happened in a larger population like the T. rex, but it's not impossible.
A third possibility is that the tiny arms were actually useful in some way that we haven't found yet. Researchers are analyzing the arms of one particular T. rex specimen to find out how exactly it used them, in the hope of discovering some unknown activity where tiny arms are beneficial.
Source: Minute Earth