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Did the dinosaurs eventually walk on all fours because they were too heavy to walk on two legs? Or did walking on all fours let them grow?
An international team of scientists said they have studied this issue by studying a new species of dinosaur.
Meet Ledumahadi mafube, which means "a giant thunderbolt at dawn" in African language, the sesotho.
It weighed 12 tons and lived 200 million years ago in what is now southern Africa.
The animal is one of the first members of the group, which includes the giant sauropod dinosaurs, the most famous of which is the brontosaurus of about sixty tons, which literally means "thunder lizard".
Jonah Choiniere, paleontologist at the University of the Witwatersrand in Johannesburg and head of the study published Saturday in Current Biology, said sauropods were the greatest animals ever to walk on the earth.
"The idea is that if they walked next to you, the ground would thunder their footsteps," said Professor Choiniere.
With a discovery earlier this year, Ledumahadi is pushing back the date of gigantism among dinosaurs.
"Ledumahadi mafube was the largest land animal ever created at the time when he lived in the oldest Jurassic," said the researchers.
But Professor Choiniere said that the true contribution of the beast to our understanding of when and why this group of dinosaurs began to walk on all fours.
How did Ledumahadi work?
Sauropods have been the dominant phytopharmaceutical animal on Earth for 135 million years, and it is generally believed that their success is due to their enormous size and ability to crawl.
They have been helped by special column-shaped legs, which makes them very effective for carrying heavy weight – think of an elephant's legs.
But when dinosaurs evolved 240 million years ago, they were bipedal, which meant they walked on two legs.
Palaeontologists do not know when and why some people started walking on four legs – or became quadrupeds – like giant sauropods.
The researchers thought that Ledumahadi could offer some clues because, although he was the size of a true sauropod, he came from an earlier lineage, usually considered a bipedal.
Instead of elephant-like members, Ledumahadi had partially flexed the forelegs, which meant that he had more of a crouching habit.
So, was it the largest bipedal dinosaur – or was it an unusual quadruped?
Analysis of the diameter of the bone of the arm and the leg
To address this issue, researchers examined the relationship between the diameter of the upper arm bone of Ledumahadi and that of the leg.
They then compared this result to the same ratios of 81 dinosaur specimens, hundreds of mammals and several large reptiles known to walk on two or four legs.
Professor Choiniere explained that thicker forelimbs grow when you put weight on them. The smaller the ratio, the more the animal tends to crawl.
"This ratio tells us for sure if things are going on all fours or walking on two legs," he said.
The method could be applied to all dinosaurs, said Professor Choiniere, and even to "any extinct animal that was walking on dry land".
He said that the measures suggested Ledumahadi to walk on all fours, but that it would have been in a squatting position – more to a cat than to an elephant.
Researchers have also shown that quadrupedalism has evolved several times in the ancestors of sauropods, suggesting an "evolutionary experimentation".
According to Professor Choiniere, it is important to note that walking on four legs has evolved before the dinosaurs become giants, as evidenced by the quadruped dinosaurs weighing barely 2 tons.
In fact, crawling has not only evolved before gigantism, say the researchers, it was a necessary precursor for them to grow.
And everything depends on their digestive process.
Benefits of walking on all fours
Unlike rhinoceros and hippopotamuses, whose heads are gigantic, dinosaurs resembling sauropods do not have to chew their teeth.
They probably tore the leaves off the trees and swallowed them whole, extracting the nutrients by "composting" the leaves in their gut by fermentation, Professor Choiniere said.
He said that crawling would have allowed dinosaurs like Ledumahadi to support a larger digestive tract by spreading his weight over two pairs of limbs.
And a larger gut size, in turn, supports a larger body.
"If you develop the ability to walk on four legs, this will actually facilitate the evolution of a significant body mass," said Professor Choiniere.
While some dinosaur lineages developing quadrupeds have become extinct, others have evolved to adopt elephant – shaped legs of true sauropods.
True sauropods grew up and eventually replaced other lineages, as their specialized legs offered a more efficient way to carry a heavy body.
A "careful method", but caution is required
Steve Salisbury of the University of Queensland supported the idea that crawling would have helped the dinosaurs to get fatter.
"Once you start to crawl, you're in a better position to grow and that's where we see the emergence of true sauropods," he said.
But he said that getting fat does not necessarily lead to crawling on all fours.
He said the biped Muttaburrasaurus, which was 10 meters long, 3 meters high. Tyrannosaurus rex was even bigger, but also walked on two legs.
And Dr. Salisbury was not convinced by the conclusions of Professor Choiniere and his team.
While he stated that the comparison of bone radius ratios was a "neat method" of investigation, he said that it would be "cautious" to be there. proud to determine if an animal was walking on two or four legs.
"There is not enough skeleton to be really sure of that," he said.
He added that the move to gigantism among dinosaurs would have involved skeletal changes, which are not necessarily related to size: "It's not just that."
Dr. Salisbury was also wary of comparing large groups of animals.
He said that different animals had different ways of transmitting forces through the body to support their weight – and that dinosaurs were unique in the history of evolution.
"When you compare sauropods and elephants, it's like comparing apples and oranges … We can not use a single set of rules," he said.
Nevertheless, no matter how the animal walked, Dr. Salisbury was impressed by Ledumahadi's size.
"It's a big dinosaur for his day," he said.
-ABC
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