Science

For dinos like T. rex, puny arms may have been the price of a giant head

In the rolling hills of Argentina’s Patagonian Desert, Juan Canale struck paleontological gold. Within half the length of a soccer pitch, his team discovered five dinosaur skeletons, including a new species that’s a Tyrannosaurus rex doppelgänger—the third known giant dinosaur to evolve stubby arms and cartoonishly large heads. In a new study, Canale’s team suggests the forelimbs shrank as a consequence of skull growth, not for any function of their own.

“The fact that you see it over and over again in parallel does suggest that there might be a common driver,” says Matthew Carrano, curator of dinosaurs at the Smithsonian’s National Museum of Natural History, who wasn’t involved with the study.

The new species is a carcharodontosaurid—an 11-meter-long dinosaur that went extinct 20 million years before T. rex lived and is only distantly related to it. Despite the evolutionary distance, scientists suspected they shared a similar body plan, but couldn’t know for sure because the preserved fossils were fragmentary; small forelimb segments are rare in the fossil record. The new discovery was a “lucky strike,” says Canale of the Ernesto Bachmann Paleontological Museum, and lead author of the study.

In Las Campanas Canyon, Canale first found half of a carcharodontosaurid vertebra. To excavate the rest of the skeleton, the researchers chiseled and hammered into a thick layer of 95-million-year-old sandstone, using brute strength and bare hands under a sweltering summer Sun. Finally they resorted to a jackhammer. They eventually excavated a half-complete skeleton with an intact skull and a full set of limbs. They later realized it was a new species, and named it Meraxes gigas, after a Targaryen dragon from Game of Thrones.

“It’s not that common to find half a dinosaur,” says Carrano, who says the find is “by far the best” skeleton of a carcharodontosaurid discovered to date.

The intact M. gigas fossil allowed the researchers to be certain that the species’ small arms went with its massive head, and to trace the evolution of the carcharodontosaurid body plan from 150 million to 90 million years ago by comparing the new fossil with ones described previously. As time inched forward, carcharodontosaurid species evolved larger heads and shorter arms, the researchers found. Two other giant carnivorous dinosaur families, tyrannosaurids and abelisaurids, exhibit similar trends.

Big heads are packed with jaw-closing muscles and would have helped these carnivores capture large prey, says Emily Rayfield, a paleobiologist at the University of Bristol who wasn’t involved with the study. “We would predict that as skulls gets larger, they also get stronger.”

Heavy-headed, bipedal creatures might require small forelimbs in order to keep their balance. Or, as powerful heads replaced predatory arms, the forelimbs may have shrunk because they weren’t needed, the authors say. “Once you stop using something for function, evolution will start to get rid of it because it’s costly to build and maintain any structure,” Carrano says.

But dinosaurs didn’t lose their arms completely. In all three lineages, the arms shrank no further than a forelimb-to-femur ratio of 0.4. And M. gigas had the hallmarks of a functional arm: muscles, attachment points, and functional joints. Carrano and Canale agree the arms must have had some function other than feeding that prevented additional shrinkage, but just what that might have been is still a mystery. Researchers speculate that the big dinos used their stubby arms to push up from a prone position or to grasp mates. But with current fossil evidence, Carrano says “we have no idea.”

Researchers continue to seek more specimens. “You always want more,” Carrano says. But for now, “this is a nice piece of the puzzle.”

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