Researchers have discovered fossils from South Africa’s largest dinosaur yet — a find that they say changes their understanding of how four-legged walking evolved in the lingeage that includes some the biggest animals ever to walk the planet.
The newly described species, called Ledumahadi mafube, would have weighed about 12 tonnes and is a type of sauropodomorph, a large group of dinosaurs with long necks and tails. When L. mafube lived around 200 million years ago during the early Jurassic period, it would have been the largest animal walking on Earth.
In a study describing the find, published on 27 September in Current Biology1, the researchers argue that this species walked on four legs, which upsets the current understanding of how and when this behaviour evolved in the lineage.
Palaeontologist James Kitching first found fossils of L. mafube in 1988 near South Africa’s border with Lesotho. But the bones were left on a shelf for more than a decade and ‘rediscovered’ only in the 2000s in the collections of the University of the Witwatersrand in Johannesburg, South Africa. Palaeontologists returned to the site in 2010 and completed the excavation last year.
In the Southern Sotho language, ledumahadi means giant thunder clap, and mafube means dawn, indicating the species’ relatively early position in their evolutionary lineage.
A giant emerges
Although South Africa is known for its large living animals, such as the African elephant, most of its dinosaur discoveries have been of animals that would have weighed about five tonnes, says study co-author Jonah Choiniere, a palaeoscientist at Witwatersrand.
The discovery of such a heavy creature shows “we don’t know the dinosaurs of South Africa as well as we thought”, says Choiniere.
Other researchers agree that L. mafube was probably the largest animal of the early Jurassic. “South Africa and Argentina have provided the most important fossils for understanding how dinosaurs became gigantic animals about 200 million years ago,” says Diego Pol, a palaeontologist at the Egidio Feruglio palaeontology museum in Trelew, Argentina. Although today both countries are separated by the Atlantic Ocean, they were once neighbours in the supercontinent Pangaea, which once spanned Earth, explaining why researchers have unearthed fossils of similarly gigantic animals on the continents.
But the find is even more significant because it seems to show that quadrupedalism — walking on four legs — emerged in this lineage of dinosaurs at least 10 million years earlier than thought — and then disappeared before returning again.
“We thought [quadrupedalism] might be a one-time evolution: a quadruped walks once, is successful, and it sticks in that lineage,” says Choiniere.
A walking experiment
Other, later sauropodomorphs, such as Brontosaurus, had straight, ‘columnar’ limbs. This stature could support their huge mass, often in the region of 80 tonnes.
Palaeontologists also know of sauropodomorphs that came before Brontosaurus and its kind, but after the time L. mafube would have lived, that walked on two legs.
But the researchers now argue that the much earlier L. mafube also walked on four legs. They detail a method for determining how dinosaurs walked, based on a ratio of the circumferences of thigh and arm bones, calculated from dinosaur specimens and hundreds of animals alive today. The method led the researchers to deduce that L. mafube walked on four legs, even though later relatives in this group are known to have walked on two legs before evolving to walk on all fours once more.
This suggests that there was evolutionary experimentation — some sauropodomorphs had quadrupedalism and then the group lost it, says Choiniere.
That claim is controversial, says Michael Benton, a palaeontologist at the University of Bristol, UK. Unlike later sauropods, L. mafube’s legs flexed out to the sides, a stance that is typically able to carry less mass than columnar limbs, which could support truly massive weights.
“What’s needed next is a true biomechanics test of whether 12 tonnes is the maximum size an animal can reach without having columnar limbs,” he says.
Nature 562, 17 (2018)