What's So Great About Having T. rex Arms?

Dinosaurs evolved tiny arms more than once.

Tyrannosaurus rex and its closest relatives might not have been able to put on a hat or shake hands, but a new study published in PLOS ONE shows that they weren’t the only ones.

A newly discovered dinosaur from Patagonia called Gualicho shinyae also had absurdly short forelimbs — about the size of a human child’s — along with short, two-fingered claws, similar to the T. rex.

Both Tyrannosaurus and Gualicho were theropods, a group of two-legged, bird-like dinosaurs. But they were distant relatives, sitting on separate branches of the family tree, which indicates that short arms evolved independently in each lineage rather than arising from a common short-armed ancestor.

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It’s a clear case of convergent evolution — where the same trait evolves on separate occasions in organisms occupying similar environments. Another classic example is the ability to fly in bats, which are distantly related to birds. Each of their ancestors was limited to walking on land, but both groups evolved flight independently through adaptations of their forelimbs.

When a trait evolves again and again, it becomes likely that the trait is of high adaptive value to the organism, helping it survive or reproduce in a given environment

So what would have been the advantage for Tyrannosaurus and Gualicho of having tiny arms and hands with only two claws?

Researchers aren’t quite sure. “We don’t actually know what would have triggered a reduction in the forelimb in each individual lineage,” senior author Peter Makovicky, from the Field Museum of Natural History in Chicago, tells The New York Times. “But obviously there was some adaptive advantage because we see it multiple times in different lineages of theropods.”

More research — and more digging — may yield some answers.

"By learning more about how reduced forelimbs evolved, we may be able to figure out why they evolved," he says in a press release.

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