The Washington Post

Discovery shows another dino with short arms


Close your eyes and imagine a Tyrannosau­rus rex. Can you see its huge head, sharp teeth and puny arms? The late- Cretaceous Period predator of the Northern Hemisphere wasn’t the only dinosaur that had this weird body type. So did abelisauri­ds of the Southern Hemisphere, such as Carnotauru­s. And so did a carcharodo­ntosaurid called Meraxes gigas, also from the world’s southern half but nearly 20 million years earlier, during the early Cretaceous.

M. gigas is a newly discovered species found in Patagonia in Argentina. Peter Makovicky of the University of Minnesota was one of a team of paleontolo­gists who dug it up. (Paleontolo­gists are scientists who study ancient life.) Makovicky says finding M. gigas caused him and other researcher­s to think about what these meat-eating theropods had in common and why.

They all had “really short arms and very massive skulls,” he says. M. gigas weighed nearly 41/ tons, and


T. rex and Carcharodo­ntosaurus weighed about 71/ to 81/ tons. Their

2 2

heads grew to about 4 or 5 feet long, and their arms were about the length of an adult human’s.

“The fact that these lineages are so similar in their body plan just seemed to us like it couldn’t be a coincidenc­e,” Makovicky says.

In the past, many researcher­s focused on what theropods used their tiny arms for. Makovicky says M. gigas shows that while their arms might have had purposes as they evolved to be smaller and smaller, such as helping the dinos lie on their stomachs, they probably became less important — at least for catching prey.

These different theropod lineages evolved over millions of years, and on different continents, “to look like that for some reason, and something about that [theropod] body plan [means] you have to make some trade-offs,” Makovicky says. “As their skulls get disproport­ionately bigger, their arms get disproport­ionately shorter, and they’re transferri­ng whatever the predatory function of the forelimbs is to the head.”

Patagonia has been a dinosaur hot spot for a long time, not just for giant theropods but also for rare small fossils and dinosaur tracks. Makovicky and his team found not just M. gigas but at least two other sauropod fossils at a site in the Neuquén Basin. “You just had this huge pile of bones that as you kept digging, more bones would show up,” he says. “At one point our meat-eating dinosaur started running into the skeleton of a big sauropod, so it became like a game of pickup sticks.”

Adding to the challenge was figuring out how to excavate things upside down. Says Makovicky: “Some of the bones had stuck to the rock layer above and formed a ledge. You actually had to crawl under and figure out a way to pop the bones out of the roof of this little space, which normally you don’t do. Normally you dig down, and you lift up.”

Some of those discoverie­s still have to be studied and described, and there might be more new species in the mix. But Makovicky says he wants to investigat­e the theropod arms more. “The arm length does not get shorter than a certain proportion. Why is that?” he asks.

Meat-eating theropods had “really short arms and very massive skulls.” Peter Makovicky, paleontolo­gist

 ?? PHOTO By JORGE Gonzalez/illustrati­on By CARLOS PAPOLIO ?? This is an illustrati­on of Meraxes gigas, a newly discovered dinosaur species found in Patagonia in Argentina.
PHOTO By JORGE Gonzalez/illustrati­on By CARLOS PAPOLIO This is an illustrati­on of Meraxes gigas, a newly discovered dinosaur species found in Patagonia in Argentina.

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