As with my earlier post on the scavenger/hunter debate, this entry aims to inspire you to think rather than to provide you with exhaustively researched facts. Therefore, please bear in mind that the facts mentioned here might be outdated; it might be sensible to double-check with a more reliable, well-referenced source for pure information.
Tyrannosaurus rex is famous for its characteristically short forelimbs with only two fingers, and their use has puzzled paleontologists for a century. It is known that they had fairly powerful muscles, so they clearly must have served some purpose – otherwise, those muscles would have been a waste of tissue, a pointless investment of muscle growth, which most likely should have been done away with through natural selection.
So, the scientists ask themselves: what were they used for? Several sugestions have been made, including holding a mate in position during copulation; (somehow) holding struggling prey while killing it with massive jaws; assisting in getting up from sleeping position (proposedly sitting on its belly).
However, I feel that these proposals fall short in explaining the next logical question: why did the arms become short? and why did they lose one finger? If they were used for grabbing things or for pushing the body up from the ground, why would they atrophy?
An explanation for the selection pressures that may have caused the reduction of T. rex's arms focuses on balance: the arms became shorter as a trade-off for growing a larger head, in order to avoid getting front-heavy.
Now, there is another way of maintaining balance while letting the front of the body grow heavier: growing a longer and/or thicker tail. But, a point to note is that the more weight you place far away from the centre of the body, the more effort and time it will take to turn the body, especially while running! So, if you were to do this, you would compromise agility instead of arm range. It seems T. rex preferred to let its arms dwindle than become awkwardly clumsy. Could this 'choice' tell us something about the underlying purpose of the shortened arms?
The enlargened skull is most likely an adaptation for hunting and/or feeding, and the choice to reduce the forelimbs rather than extending the tail suggests that (some measure of) agility was desired, probably in order to take or hunt down prey. The fact that the arms were atrophied in favour of hunting-focused adaptations might hint that the arms were not used much for hunting, perhaps not for grabbing at all. What else, then?
Another result of shortening the arms and losing a finger, rather than enlarging the tail, is saved tissue investment. This is an acceptable justification for shortening them if they are longer than necessary to do whatever it is they were used for, regardless of having balance difficulties or not.
Considering the above, the idea of the arms being used to help T. rex getting up from a sitting position seems the more plausible, in my view. The arms may have been just long enough to assist lifting the bulk of the body just high enough to allow the powerful legs to take over comfortably and raise the animal to full standing position, and two fingers might have been sufficient to give a decent push-grip on the surface.
Having in mind what I wrote about the need for every animal to reach ground level with their heads in order to drink, which is essential for survival, as we all know but tend to forget, I also propose that T. rex's arms may have been used to help getting up and/or down to the lake or river bank for a drink, not just for sleep. Powerful arms to boost the efficiency of getting up could potentially reduce T. rex's vulnerability to ambush while drinking.
Although I personally find that the experts may be focusing their efforts on hypotheses that are too narrow or even missing the point, maybe not even asking the right questions will give us a clear answer to the mystery of T. rex's stubby arms.
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