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Newly discovered dinosaur shows pattern of huge animals with tiny 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 4½ tons, and T. rex and Carcharodontosaurus weighed about 7½ to 8½ tons. Their 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.

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