M. gigas is a newly discovered species found in Patagonia in Argentina. Peter Makovicky of the University of Minnesota was part of a team of paleontologists who unearthed it. (Paleontologists are scientists who study ancient life.) Makovicky says the discovery of M. gigas got him and other researchers thinking about what these meat-eating theropods had in common and why.
They all had “very short arms and very massive skulls,” he says. M. gigas weighed almost 4.5 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.
“The fact that these bloodlines are so similar in body plan struck us as it couldn’t be a coincidence,” Makovicky says.
In the past, many researchers have focused on why theropods use their small arms. Makovicky says Mr gigas shows that while their arms may have had purposes as they evolved to get smaller and smaller, like helping dinos lie on their stomachs, they probably became less important – at least to catch prey.
These different lineages of theropods have evolved over millions of years, and on different continents, “to look like this for some reason, and something about it [theropod] body map [means] you have to compromise,” says Makovicky. “While their skulls grow disproportionately larger, their arms shorten disproportionately, and they transfer the predatory function of the forelimbs to the head.”
Patagonia has long been a dinosaur hotspot, not only for giant theropods, but also for rare small fossils and dinosaur tracks. Makovicky and his team found not only 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 appeared,” he says. “At some point, our carnivorous dinosaur started bumping into the skeleton of a large sauropod, so it became like a game of pick-up sticks.”
Adding to the challenge was figuring out how to dig things upside down. According to Makovicky, “Some of the bones were stuck to the rock layer above and formed a ledge. You actually had to crawl under and find a way to get the roof bones out of that little gap, which you normally don’t do. Normally you dig and heave.
Some of these discoveries have yet to be studied and described, and there could be more new species in the mix. But Makovicky says he wants to investigate the theropod’s arms further. “The length of the arm does not become shorter than a certain proportion. Why so?” he asks.
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