It’s a question that has haunted paleontologists for years: why did the mighty T. rex those ridiculously short arms? And an American researcher now comes up with an interesting hypothesis.

The Tyrannosaurus Rex impresses; with its huge head that could tower more than three meters above the ground and a body that could reach more than 13 meters in length. Not to mention the sharp teeth and powerful jaws with which this carnivorous dinosaur made life miserable for other dinosaurs more than 66 million years ago. But what always detracts from all that display of power, are the forearms of the carnivore. Or rather: the forearms. They are in fact completely out of proportion; so had a little more than 13 meters long T. rex a head 1.5 meters long, but forearms only 90 centimeters long. Just to show how strange that is: in the same proportions, a human 182 centimeters tall would have arms only 12.7 centimeters long.

Theories

The mystery of those short forearms has been much discussed by paleontologists. And that has led to some interesting hypotheses about the function of those forearms. This is how some researchers think T. rex the short forearms used during mating or to restrain prey or knock over a Triceratops. But in the leaf Acta Palaeontologia Polonica Paleontologist Kevin Padian now states that we may have been approaching those forearms the wrong way all along and he comes up with a new, interesting hypothesis.

different view

Previously, researchers mainly wondered: in view of which functions does the T. rex these short forearms obtained through evolution? But according to Padian, we might be asking the wrong question. Instead, we may have to ask ourselves what advantage the short arms do? T. rex yielded. Padian already sets a good example in his paper and – based on that line of thought – comes up with a whole new hypothesis about the evolution of those strange forearms. The originally much longer arms of the T. rex would have shrunk to avoid being bitten off accidentally or on purpose by conspecifics.

That may sound a bit far-fetched, but just imagine a carcass of a large herbivorous dinosaur. Soon several tyrannosaurs gather around that carcass and they begin to feast on the flesh of the dead dinosaur with their huge heads with razor sharp teeth and very powerful jaws. Things get wild: people bite, bones are broken, meat is pulled loose. In such a place, long forearms are quite disadvantageous; Before you know it, your neighbor will bite them off – accidentally or on purpose, because you’re getting a little too close. And that can have serious consequences for a T. rex: He can bleed to death or contract a deadly infection a short time later. “So it can be beneficial to develop shorter forearms, as you don’t need them while hunting prey anyway,” Padian says.

About the earlier hypotheses

As mentioned, the forearms of the ancestors of T. rex much longer. There must have been a reason that these arms have shrunk over time. “All the ideas that have been vented about that are either untested or impossible, because it just doesn’t work.” For example, the arms are too short and not strong enough to embrace an (unwilling) partner during mating. Pushing or picking up prey must also have been difficult, because the much larger head was constantly in the way. “They just can’t get close enough to things to pick them up (…) The arms are simply too short. They cannot touch each other, they cannot reach the mouth and they have such limited mobility that they cannot reach far. The huge head and neck stick out far out and are basically the death machine you saw in ‘Jurassic Park’.” Moreover, none of the previous hypotheses explain why the arms would become shorter. “At best they explain why the arms should remain so short. And in all cases, the proposed functions – such as holding mates or prey – would have been much more effective if the arms had been longer.”

Hunt in groups

Also the hypothesis Padian now comes up with is – he must admit – 66 million years after the extinction of the T. rex difficult to test. But it is a hypothesis in which the short forearms were clearly advantageous for the T. rex† Padian discovered his alternative explanation for the evolution of short forearms when he learned that other paleontologists had found tentative evidence that T. rex hunted prey in groups. “Several sites uncovered in the last 20 years harbor adult and juvenile tyrannosaurs. We can’t really assume they lived or even died together. All we know is that they are buried together. But when you find the same animals in different places, that’s a strong signal. And it is possible that they hunted in groups.” And pulling those mighty tyrannosaurs together while hunting and feeding might have been the driving force behind those forearms shrinking, Padian thought.

Follow-up research

It may never be possible to determine with certainty whether this really happened. But researchers can explore the hypothesis more closely. For example, Padian suggests, taking another look at fossil remains of tyrannosaurs for bite marks. “Bites on the skull and other parts of the skeleton are common in tyrannosaurs and other carnivorous dinosaurs. If fewer bite marks are found on the shrunken arms, it may indicate that the arm shrinkage worked.”

Incidentally, Padian is not immediately out to prove his point. Above all, he hopes that his research article will make others think. “We tell a lot of stories about the possible functions of T. rexbecause that’s an interesting problem. But are we really looking at that problem in the right way?” Padian clearly thinks not and grabs the bull by the horns in his investigation. “First of all, I wanted to establish that the prevailing functional ideas (about the forearms, ed.) don’t work. With that we are back to square one. We then have to use an integrated approach, in which, in addition to purely mechanical considerations, we also consider social organisation, eating behavior and ecological factors.” Incidentally, in future research, it is also necessary to look further than T. rex alone. There are more carnivorous dinosaurs that – independently of each other and probably for other reasons – developed shorter forearms. “The size and proportions of the limbs in these groups are different, but other aspects of their skeleton are also different. We should not expect them (the forearms, ed.) to have shrunk in the same way.” And so there is still plenty of work to be done for paleontologists.