Did dinosaurs end up walking on all fours because they got too heavy to walk on two legs? Or did walking on all fours let them grow big?
An international team of scientists say they’ve shed light on this question with their study of a newly described species of dinosaur.
Meet Ledumahadi mafube, which means “a giant thunderclap at dawn” in the African Sesotho language.
It weighed 12 tonnes and lived 200 million years ago in what is now southern Africa.
The animal is an early member of the group that includes the giant sauropod dinosaurs — the most famous of which is the 60-odd-tonne Brontosaurus, which literally means “thunder lizard”.
Jonah Choiniere, a palaeontologist at the University of Witwatersrand in Johannesburg and leader of the study published on Saturday in Current Biology, said sauropods were the largest animals ever to walk the Earth.
“The idea is that if they were walking next to you, the ground would be thundering with their footsteps,” Professor Choiniere said.
Along with a discovery earlier this year, Ledumahadi pushes back the date of gigantism in dinosaurs.
“Ledumahadi mafube was the largest land animal to have ever existed at the time it lived in the earliest Jurassic,” the researchers reported.
But, Professor Choiniere said, the beast’s real contribution is to our understanding of when and why this group of dinosaurs started walking on four legs.
How did Ledumahadi walk?
Sauropods were the dominant plant-eating animal on Earth for 135 million years, and it is generally believed their success was due to their enormous size and ability to walk on all fours.
They were aided by specialised column-like legs, making them very efficient at carrying heavy weights — think of an elephant’s legs.
But when dinosaurs first evolved 240 million years ago, they were bipedal, meaning they walked on two legs.
Palaeontologists don’t know when and why some started walking on four legs — or, became quadrupedal — like the giant sauropods.
The researchers thought Ledumahadi could offer some clues because, while it was the size of a true sauropod, it came from an earlier lineage, generally assumed to be bipedal.
Instead of elephant-like limbs, Ledumahadi had partially flexed forelimbs, which means it had more of a crouched habit.
So, was it the biggest-ever bipedal dinosaur — or was it an unusual quadruped?
Analysis of arm and leg bone diameter
To tackle this question, the researchers examined the ratio between the diameter of Ledumahadi’s upper ‘arm’ bone and its upper leg bone.
They then compared this with the same ratios from 81 dinosaur specimens, hundreds of mammals and several large-bodied reptiles that are known to walk on either two legs or four.
Professor Choiniere explained that thicker forelimbs develop when you put weight on them. The smaller the ratio, the more likely the animal walked on all fours.
“That ratio tells us with great certainty whether or not things walk on four legs or walk on two legs,” he said.
The method could be applied to all dinosaurs, Professor Choiniere said, and indeed, “every extinct animal that walked on land”.
He said the measurements suggested Ledumahadi walked on all fours, but it would have been in a crouched position — more like a cat than an elephant.
The researchers also showed that quadrupedalism evolved a number of times in the ancestors of sauropods, suggesting “evolutionary experimentation”.
Importantly, Professor Choiniere said, this suggested that walking on four legs evolved before dinosaurs grew into giants, as evidenced by quadrupedalism in dinosaurs weighing just 2 tonnes.
In fact, walking on all fours didn’t just evolve before gigantism, the researchers argue, it was a necessary precursor to them getting bigger.
And it’s all down to their digestive processes.
Advantages of walking on all fours
Unlike rhinos and hippos, which have a giant head full of chewing teeth, sauropod-like dinosaurs had no munching teeth to speak of.
They likely ripped leaves off trees and gulped them down whole, extracting nutrients by “composting” the leaves in their gut via fermentation, Professor Choiniere said.
He said walking on all fours would have enabled dinosaurs like Ledumahadi to support a bigger gut size by distributing its weight over two sets of limbs.
And a bigger gut size, in turn, supports a bigger body.
“If you evolve the ability to walk on four legs, it actually facilitates the evolution of large body mass,” Professor Choiniere said.
While some dinosaur lineages that developed quadrupedalism died out, others evolved to have the elephant-like legs of the true sauropods.
The true sauropods grew bigger and eventually replaced other lineages because their specialised legs offered a more efficient way to carry a heavy body around.
A ‘neat method’, but caution urged
Steve Salisbury of the University of Queensland supported the idea that walking on all fours would have helped dinosaurs grow bigger.
“Once you start walking on all fours it puts you in a better position to get bigger, and that’s where we see the emergence of the true sauropods,” he said.
But he said getting bigger doesn’t necessarily lead to walking on four legs.
He pointed to the bipedal Muttaburrasaurus, which was 10 metres long, 3 metres high. Tyrannosaurus rex was even larger, but also walked on two legs.
And Dr Salisbury was not convinced by the conclusions of Professor Choiniere and team.
While he said comparing the bone radius ratios was a “neat method” for investigation, he said he would be “cautious” of relying on it to make conclusions about whether an animal walked on two legs or four.
“There’s not enough of the skeleton to be really sure about that,” he said.
He said the shift to gigantism in dinosaurs would have involved modifications to the skeleton, which aren’t necessarily linked to size: “There’s more to it than that.”
Dr Salisbury was also wary of comparing across vast groups of animals.
He said different animals had different ways of transmitting forces through the body to support their weight — and dinosaurs were unique in evolutionary history.
“When you’re comparing sauropods and elephants, it’s like comparing apples and oranges … We can’t use just one set of rules,” he said.
Still, no matter how the animal walked, Dr Salisbury was impressed by Ledumahadi’s size.
“It is a big dinosaur for its day,” he said.