The arms of a T-rex are iconic for several reasons. First, they are comically small. T. rex itself is a superstar of the dinosaur world – perhaps the most famous extinct predator. Its jaws are massive and terrifying. Yet just behind those killer teeth there are these tiny arms that seem out of proportion, and scientists struggle to figure out what they are for and why they are so small. In fact, when the first T. rex skeleton was discovered by Barnum Brown in 1902 he did not think the arms were part of the same skeleton, they were just too small. The mystery of the T. rex’s arms remains an enduring scientific question.

There are, in fact, three groups of dinosaurs that are typified by very large heads and jaws and tiny forelimbs, the tyrannosaurids, the carcharodontosaurids, and the abelisaurids. Also, the ancestors of the tyrannosaurids had longer arms, and it appears that these three groups of theropod dinosaurs did not share a common short-armed predecessor. Therefore this feature of tiny arms seems to have evolved independently in the three groups. This deepens the mystery. T. rex is not some quirky evolutionary one-off. This was a trend in the large theropods, which strengthens the conclusion that there was a clear evolutionary pressure for this morphology.

The debate over T. rex’s tiny arms comes up every time a new relevant discovery is made, and a recent discovery of a carcharodontosaurid is no exception. The fossil is of a new species, named Meraxes gigas (yes, after one of the dragons in Game of Thrones). Most importantly, it has small forearms, confirming that this lineage also had this strange feature. So what, then is the reason for the tiny arms?

The short answer is that we don’t know, mostly because we cannot observe the behavior of these animals to see how they use them. Also, evolution can be tricky, and we cannot always determine a single use for a feature or cause for its form. Often there is a complex web of reinforcing factors. But here are the main contenders for possibly relevant factors. Evolutionary pressure can come in several forms. One is to avoid a detriment, another is because the feature has a specific use, and a third is sexual selection (a product of the choice of mates). In the case of the reduced forelimbs, focus has primarily been on the first two factors.

The limbs, for example, may be small for balancing reasons. These dinosaurs fight and eat primarily with their large heads and jaws. These massive heads have to be balanced by their large tails, and they are obligate bipeds, so the heads and tails have to balance on these legs. If the heads became too big for the rest of their bodies, they would be front-heavy and off balance. So perhaps the arms shrunk simply to allow the heads to be maximally large without adversely affecting their balance. This also assumes that the utility of a larger head was greater than the loss of function of shrinking arms.

Also, when an anatomical feature is not used at all they do tend to atrophy away over evolutionary time, because there is not benefit to offset the risk of injury or disease. In fact, recently a scientist proposed that perhaps the forelimbs in these dinosaurs are small to keep out of the way of fellow feeding predators. If T. rex hunted and fed in groups, arms could get injured in the frenzy. Best to keep them small and out of the way.

These are all negative reasons for small limbs – to avoid a detriment. But if these are the only evolutionary factors, why do they retain limbs at all? There are snakes and legless lizards that show that limbs can disappear entirely in reptiles if they are not useful or in the way. Did the reduced arms, then, serve some positive function? They are too short to reach the mouth, so no useful for feeding. They are too small to be effective as hunting weapons or for defense. So what’s left?

As evidence that they likely did serve some purpose is the fact that these relatively small forelimbs (in T. rex they were about three feet long) hard strong muscles and even long claws. They were not withered and useless. Right from the beginning, Henry Fairfield Osborn, who described the first T. rex, hypothesized that they may have been used during mating, to hold onto their mate. This remains a viable hypothesis, although it is still debated.

A few years ago another hypothesis emerged – perhaps they were used to slash prey. What else would that claw be for? How would that work? It’s possible, it is hypothesized, that during fighting an enemy may get inside such a dinosaur’s biting range. But any creature getting right up against the predator in order to avoid those teeth could find themselves viscously slashed by those strong and sharp (if short) arms. In fact some argue this may have been the primary reason for the reduced arm length, for close-in combat, because longer arms would not be useful for this specific purpose. Longer arms would be redundant in a fight to the more powerful and effective jaws, but if smaller could be useful at even closer range, too close for the jaws. As anyone who has sparred with a weapon that has long reach knows, the enemy getting in too close means defeat. So this explanation makes sense to me.

I will emphasize that we should not be thinking of a specific reason for the small arms. It is likely that some combination of these and perhaps other factors were at work. The arms may have shrunk for one reason, then found a new purpose which reinforced the reduction in size, or stabilized them at the optimal size for some new function. Evolution often is not a response to one factors, but rather reaches equilibrium points balancing multiple factors.

We may never know the final answer to this fascinating question. Perhaps we will be able to have simulated T. rexes in virtual world where we can test some of these behavioral question. That would be an interesting new line of evidence. Meanwhile, we have to make due with the fossil evidence, which is helpful but will likely never be definitive.