Newfound Dinosaur Had Tiny Arms Like T. Rex

But what were they for?

Jorge González and Pablo Lara

By William Herkewitz

Jul 13, 2016

The newly discovered dinosaur was an entire ton of carnivorous, flesh-ripping terror when it stalked the Earth millions of years ago. Also, it had silly little baby arms.

A team of paleontologists led by Pete Makovicky, the head dinosaur curator at the Field Museum in Chicago, has just revealed a new species of two-legged carnivorous dinosaur. It is Gualicho shinyae, a distant cousin to the better-known Velociraptor and T. rex. According to the scientists, Gualicho is an especially interesting find because like the T. rex, it evolved forelimbs that are absurdly small—each had just two clawed, dainty fingers. The fossil find is described today in the journal PLOS ONE.

What's especially strange, the researchers says, is that Gualicho evolved its embarrassing arms independently—they didn't come from a common ancestor of this dino and the T. rex. That means Gualicho is the "third instance of this interesting forelimb reduction" found in dinosaurs, says Makovicky. Yet whatever evolutionary benefit these tiny arms confer remains a mystery.

Downsized Carnivore

Makovicky's team found Gualicho in the Argentinan part of Patagonia in 2007, and has been carefully preparing and classifying the fossil ever since. To be sure, the paleontologists didn't find the whole fossilized skeleton, which is an exceedingly rare prospect for beasts of its size. Rather, they found "about 40 percent of it," says Makovicky. Mostly they got the middle bits: parts of the legs, hips, forearms, ribs, and vertebrae. "Unfortunately, not the skull, which would have been very helpful towards understanding this creature's lifestyle and more exact placement in the evolutionary tree," he says.

Jorge González and Pablo Lara

Luckily, that 40 percent was enough to confidently infer the beast's size and weight—and to notice the dinosaur's strange arms. Makovicky says Gualicho would have weighed "about a ton or a bit under, and been 6 foot tall at the hips and maybe 25 foot long," as it stalked the continental island of South America 90 million years ago, during the late cretaceous period. That's much smaller than T. rex, which could weigh in at around 10 tons. It's even smaller thanother carnivorous dinosaurs that are known to have coexisted alongside Gualicho in what is now Argentina, such as the thundering Mapusaurus. "We're talking more of a mid-sized predatory dinosaur which, I mean, is still huge by human standards," says Makovicky.

Yet Gualicho's arms "are about the size of a human child's," and are an excellent example of something biologists call convergent evolution. That's where specific traits or features evolve independently in separate, sometimes vastly unrelated species. Take, for example, the similarly shaped tail flukes in dugongs and whales, or the body armor in pangolins and armadillos. Evolving independently of each other, Gualicho got arms almost identical to T. rex's. Both lost all but the thumb and index digit.

The Why of Wee Arms

Usually, convergent evolution happens because some trait is so beneficial that it happens in multiple species. You'd be hard-pressed to design a more efficient fin than the one whales and dugongs evolved separately. The thing is, it's not entirely clear to paleontologists what the T. rex-style mini-limbs bring to the table, as there are plenty of examples of powerful carnivorous dinosaurs with longer, stronger arms.

Paleontologists once floated the idea that the T. rex could have used its arms like meat-hooks, latching onto and holding prey close as it crunched down with its banana-sized teeth. "The problem with that theory is that the arms don't get whatever its holding anywhere near the mouth," says Makovicky.

Makovicky suspects the answer may turn out to be that the arms don't offer any benefit. The arms of the T. rex and Gualicho could have been shrinking toward an eventual total disappearance, like the rear legs of early whales. It's possible, had the dinosaurs survived the great extinction event 65 million years ago, the descendants of these terrifying beasts would have been all teeth, no arms, Makovicky says. "We simply don't know."