One of our planet's most stereotypically ferocious beasts also had one of the animal kingdom's silliest features. Yes, we're talking about the comically short arms of Tyrannosaurus rex. According to a study published Wednesday in PLOS One, T. rex wasn't the only beast to develop these arrested appendages: A newly discovered dino dubbed Gualicho shinyae had similarly stunted arms. But these two dinosaurs weren't related in a way that suggests they got their unusual limbs from a common ancestor. Apparently, something about these short arms made them evolve multiple times.
Like T. rex, Gaulicho's arms appear to have been useless. Researchers believe that the creature grew to be about the size of a polar bear but had arms the size of a human child's. It was an upright, birdlike theropod like the T. rex, but the two were extremely distant cousins. Researchers say the new genus was fairly unique — carrying a "mosaic" of features previously seen in other lineages — but it likely most closely resembled African genera like Deltadromeus.
Based on its features and place in time and space, Gaulicho's last common ancestor with T. rex had arms of a normal length. That makes it the third dinosaur thought to evolve puny digits, after T. rex and a creature called Carnotaurus.
When multiple lineages develop similar features, scientists call it convergent evolution. Generally, when a trait evolves multiple times throughout the animal kingdom, we assume it must be a pretty desirable trait. If something happened twice, it must have been pretty nice. Right? Right. But why would a dinosaur benefit from having such tiny arms?
It could be that these dinosaurs simply didn't need powerful arms. Instead, they hunted with their heads.
“A pretty straightforward explanation, you would think, is that the limbs are being reduced in these lineages that probably are relying more on the jaws for prey capture,” study author Nathan Smith of the Natural History Museum of Los Angeles County told The Guardian.
With snapping jaws taking the place of slashing claws, smaller limbs might have made these ferocious dinosaurs more nimble hunters.
“Reducing the arms was probably ‘beneficial’ in that they got them out of the way of the more powerful jaws,” University of Maryland paleontologist Thomas Holtz, Jr., who wasn't involved in the new research, told Smithsonian Magazine. It would also have kept them from wasting body energy on limbs they didn't need, and could even have made room for more robust musculature in the torso and neck.
As scientists uncover more theropods that bore puny arms, they might be able to pinpoint minute differences in limb and hand formation that show the evolutionary pressures behind each individual case of the adaptation.
"By learning more about how reduced forelimbs evolved, we may be able to figure out why they evolved," study author Peter Makovicky, The Field Museum's Curator of Dinosaurs, said in a statement.