Researchers reporting in Current Biology on September 27 have described a new species of sauropodomorph dinosaur named Ledumahadi mafube, which means "a giant thunderclap at dawn" in the African language Sesotho. The researchers say that the Ledumahadi specimen unearthed in South Africa is a close relative of the famed sauropod Brontosaurus and walked predominantly on all fours, anticipating the style of locomotion perfected by Brontosaurus and its ilk. But, they note, Ledumahadi goes back much farther into the prehistoric past to the earliest Jurassic.
"It shows us that even as far back as 200 million years ago, these animals had already become the largest vertebrates to ever walk the Earth," says Jonah Choiniere of the University of the Witwatersrand in Johannesburg, South Africa.
Choiniere says that he first saw the specimen in the field after his graduate student and the study's first author, Blair McPhee, showed him the bones back in 2012, soon after Choiniere moved to South Africa.
"Blair told me how important he thought it was, and even showed me that some of its bones were still sticking out of the rocks in the field!" Choiniere recalls. Together, they and their colleagues began excavating the skeleton over a period of years.
It was obvious from the beginning that Ledumahadi was quite big. The researchers now estimate that the specimen represented a full-grown adult, about 14 years old. It most likely weighed about 12 metric tons or 12,000 kilograms.
Choiniere notes that understanding the biology of these ancient animals represented only in the fossil record is extremely difficult. To determine whether Ledumahadi walked on two legs, like its ancestors, or on four, the researchers developed a method using measurements of today's animals. The method involved taking measurements of the thickness of Ledumahadi's limbs to infer their weight and how many limbs that weight must have been carried on.
Their findings suggest not only that Ledumahadi was a quadruped, but also that many other early sauropodomorph dinosaurs (often called "prosauropods") were "experimenting" with walking on all fours. The discovery shows that gigantic body sizes were possible in early four-legged "prosauropods," which arose from earlier two-legged species. However, they say, the early quadrupeds lacked the columnar, elephant-like limb postures of later sauropod species such as Brontosaurus.
"The evolution of sauropods isn't quite as straightforward as we once thought," Choiniere said. "In fact, it appears that sauropodomorphs evolved four-legged postures at least twice before they gained the ability to walk with upright limbs, which undoubtedly helped make them so successful in an evolutionary sense."
More broadly, the findings show that millions of years before Tyrannosaurus or Velociraptor came on the scene in the northern hemisphere, "there was a thriving dinosaur ecosystem here in South Africa, at the bottom of the world, featuring 12 ton giants like Ledumahadi, tiny carnivores like Megapnosaurus, the earliest mammals, some of the earliest turtles, and many, many others. Africa, and particularly South Africa, is known for its big game," Choinere says. "I think we should be just as famous for our big game of the early Mesozoic, 200 million years ago."
To that end, his team is keeping themselves busy collecting more fossils of dinosaurs and other extinct animals of South Africa's Triassic and Jurassic periods.
The work was primarily funded by: The National Research Foundation (NRF) Competitive Programme for Rated Researchers; the Palaeontological Scientific Trust (PAST) and its Scatterlings of Africa programs; the DST-NRF Centre of Excellence in Palaeosciences (CoE-Pal); the Friedel Sellschop Award administered by University of the Witwatersrand; and the Royal Society of London.
Current Biology, McPhee et al.: "A Giant Dinosaur from the Earliest Jurassic of South Africa and the Transition to Quadrupedality in Early Sauropodomorphs" https://www.cell.com/current-biology/fulltext/S0960-9822(18)30993-X
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