For a dinosaur whose scientific name literally translates to tyrant lizard king, Tyrannosaurus rex gets a whole lot of shit for its hilariously puny arms. From the safety of their computer screens, and with the confidence that can come only from 66 million years of evolutionary separation, online artists have documented, time and again, all the things that this vicious predator probably couldn't do—high five, apply sunscreen, put on a cardigan, do push-ups, etc.
Now, a newly discovered dinosaur with equally comical arms adds to a growing body of evidence that this was—for some reason—a pretty desirable trait totally undeserving of our long-arm-biases and snark.
Here's the thing: The new dinosaur, discovered in Argentina and described in a paper published Wednesday in the journal PLOS ONE, is much more primitive than the T. rex. The new find, given the name Gualicho shinyae, lies, evolutionarily speaking, at the base of a clade of dinosaurs called tetanurans—a group so broad it includes both the tyrannosaurs and the velociraptors. That means this new dino, a medium-sized, slender carnivore with, as co-author Nathan Smith, a curator at the Los Angeles Museum of Natural History Dinosaur Institute, described them, "odd-looking" small arms, must have evolved its stubs independently of T. rex's evolutionary lineage.
The utility of puny arms, the scientific term for which is reduced forelimbs, has been a long-standing mystery in paleontology since they were first discovered on the T. rex. As early as 1905, pioneering paleontologist Henry Osborn speculated that Tyrannosaurus' "absurdly reduced" arms might be used to latch on to a mate when things started getting hot and heavy (this was not his terminology).
"They must have still been doing something, or else evolution would probably have just gotten rid of them"
Others have suggested that the arms were used to latch onto prey in a kind-of "bear hug" maneuver after it had been captured by the beast's giant jaws. Other ideas that have been floated, Smith told Motherboard, include that the smaller arms helped to counterbalance the weight of its enormous head, or that the arms simply didn't catch up with the the evolution of its large skull.
The issue with these hypotheses is that they are specifically geared toward explaining only one type of animal—Tyrannosaurids. Over time, a number of other theropod dinosaurs have been found that also have reduced forelimbs. These include dinosaurs that range from the massive Abelisaurus to the tiny Mononykus. "As we get more and more instances of this happening, there's not a really good answer as to why these reductions are taking place," Smith said. Gualicho is the latest dino to throw a wrench into the works.
"The more dinosaurs we find, the more it is becoming clear that many theropods reduced their forelimbs. It is a recurring pattern," said Steve Brusatte, a paleontologist at University of Edinburgh who was not involved in the study. In an email, he told Motherboard that the reasons these dinosaurs got to that point seem to have been different depending on the dinosaur.
"A lot of the Abelisaurs," Smith explained, had "really stocky, robustly built arms with a lot of the fingers still present." Gualicho, on the other hand, has a much more slender arm and lost all but two of its digits, he said. "Although we can say in a broad stroke that they're removing some digits and/or reducing the forelimb overall, they probably weren't doing the same things so they might not have been responding to the same selective pressures."
"I think there is good evidence that the arms got smaller as the head got larger, so the head was taking over many of the duties that the arms once had, like procuring and processing food," said Brusatte. "But they must have still been doing something, or else evolution would probably have just gotten rid of them, the same way snakes lost their legs when their legs no longer served a purpose."
Both scientists agree, however, that any guess as to their purpose wouldn't be much more than speculation at this point. "I don't think we have a really good idea [about the reasons for reduced forelimbs] yet", Brusatte said, "the more fossils we find, the more we will learn."
So, internet, maybe hold off on the Gaulicho shinyae comics until we have all the facts this time?