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New Fossil Dubbed ‘Giant Thunderclap at Dawn’ Shows How Big Dinos Went From Two Legs to Four

A new species discovered in South Africa shows how dinosaurs went from bipedal beasts to four-legged giants like brontosaurus

(Viktor Radermacher, University of the Witwatersrand)
smithsonian.com

“Giant thunderclap at dawn”— that’s what the latest addition to the dinosaur family’s scientific name means in the local South African language, Sesotho. And the new kid on the prehistoric block, Ledumahadi mafube, certainly lives up to the title, as researchers described today in the journal Current Biology.

The largest dinosaurs to ever rumble across the earth were the titanosaurs, which hit their peak of giganticness about 145 million years ago. But those mega-lizards had to evolve from somewhere. The newly revealed dino from South Africa shows that the road to getting huge was not always straightforward, reports Michael Greshko at National Geographic.

Unlike the sauropod dinosaurs, including the titanosaurs and the classic brontosaurus, all of which had four straight tree-trunk legs and long necks, Ledumahadi is an earlier relative called a sauropodomorph. Weighing the equivalent of two elephants, the beast had limbs that flexed more than its later cousins, as well as flexible mobile forelimbs, giving it a cat-like crouch and a unique gait.

Early dinosaurs were all bipeds, and moved around on two hind legs using their forelimbs to snatch eggs out of nests or catch prey. But as the size of dinosaurs grew, the need for more stability and weight distribution also grew, eventually leading to stout four-legged beasts like triceratops and diplodocus in the Jurassic period. Ledumahadi is one of the species that appeared during the transition period from two to four legs.

“This was the animal that wanted to have everything,” the study’s lead author, Blair McPhee of the University of São Paulo, tells Greshko. “It wanted to be really big, like a sauropod, and wanted to walk predominantly quadrupedally, like a sauropod. But when it came to relinquishing that primitive mobile forelimb, it didn’t want to do that.”

According to a press release, it wasn’t apparent at first whether Ledumahadi actually walked on all fours or primarily used its hind legs. To figure that out the researchers measured the size of the animal’s limbs and compared them to the amount of weight that other dinosaurs and modern animals can carry. The results suggested that the animal did walk on all fours in order to bear its weight and that massive size was possible in the ancestors and cousins of the later giant sauropods. It also shows that the path to the titanosaurs was messy.

“This tells us that different groups of early dinosaurs were experimenting with different ways of becoming large … before ultimately the true sauropods hit on their column-limbed design that was perfectly suited for supporting monstrous size,” paleontologist Stephen Brusatte of the University of Edinburgh, not involved in the study, tells Hannah Osborne at Newsweek. “And this is what enabled them to grow into the largest animals to ever live on land in the history of the Earth—some of them bigger than Boeing 737s.”

It turns out that the sauropodomorphs evolved four-legged postures at least twice before developing the straight upright limbs of brontosaurus familiar to us today.

“It means that walking on all fours came first, before truly gigantic body size, and that it took a while to ‘perfect’ quadrupedal locomotion,” co-author Jonah Choiniere of the University of the Witwatersrand in South Africa tells Osborne.

Ledumahadi’s path from the ground in South Africa to the halls of science was complex, too. Greshko at National Geographic reports that the fossil was first discovered around 1990 by a paleontologist working with the Lesotho Highland Waters Project. He collected the bones sticking out of a cliff, but was more interested in ancient mammals. So the fossils sat undisturbed at the University of the Witswatersrand until the mid-2000s when paleontologist Adam Yates recognized their potential importance. Yates and his colleagues tracked down the spot where the fossils were originally excavated and dug out more fossils between 2012 and 2017.

In the release, Choiniere says this and other recent discoveries show South Africa was once a thriving dinosaur ecosystem, and is worthy of more consideration by paleontologists.

“Africa, and particularly South Africa, is known for its big game,” he says. “I think we should be just as famous for our big game of the early Mesozoic, 200 million years ago.

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