The gigantic and slow sauropod dinosaurs like Diplodocus had small two-legged ancestors – and one, Thecodontosaurus, was quick and nimble
19 January 2022
An early ancestor of large, long-necked, four-legged dinosaurs like Brachiosaurus and Diplodocus was a quick, nimble biped that probably used its forelimbs to grasp its food, which included leaves, branches – and meat.
Thecodontosaurus antiquus, a 30-centimetre-high dinosaur that lived more than 200 million years ago during the late Triassic, was a ‘sauropodomorph’ – which means it belonged to the same group as the gigantic herbivorous sauropods that lived later, in the Jurassic and Cretaceous.
T. antiquus had muscles in its back legs that favoured speed over force, whereas its forelimb muscles would have been more apt for grasping than weight-bearing. By the early Jurassic about 20 million years later, however, its descendants had shifted into slow-moving quadrupeds with muscles capable of supporting much more weight, says Antonio Ballell at the University of Bristol, UK.
Ballell and his colleagues examined the muscle insertion points, grooves, protrusions, crests, and scars related to muscle morphology in limb and body bones of T. antiquus – one of the first dinosaurs ever studied and the first Triassic dinosaur species to be named. The fossils, which come from an ancient fissure in the Triassic land surface of what is now southwest England, are exceptionally well-preserved.
“This is not very common,” says Ballell. “Usually the surfaces of the bone are weathered away so you can’t see the fine detail.” The researchers compared the fossil bone surfaces with the surfaces of modern crocodile and bird bones, looking in particular at sites on the bones where muscle and other soft tissues were once attached. They also looked at bones from modern-day lizards and examined data previously acquired about four-legged sauropodomorphs from the Triassic and Jurassic.
They found that T. antiquus’s hindlimb muscles would have contracted quickly, thus allowing for fast, agile movement, Ballell says. “There’s a trade-off [because] muscles that generates a lot of force contract slowly, and muscles that contract fast usually produce less force,” he says. “So the position and orientation of the muscles of T. antiquus indicate that it was towards the fast side of the spectrum, meaning that it could move faster … and probably do fast turns.”
By contrast, the giant Jurassic sauropodomorphs were “megaherbivores”, which means they needed four pillar-like legs to help hold their “huge digestive tracts” necessary for digesting all the plant matter they were consuming, Ballell says.
As for the forelimbs of T. antiquus, a “deep and scarred” groove and other marks in the elbow bone and shoulder blade suggests they had great elbow flexibility and extension. The long, slender shape of the arm and shoulder bones hints at smaller muscles and less shoulder extension, making them unlikely to serve as weight-bearing limbs. The hip bones also reflect the presence of muscles that would have allowed the dinosaur – which the researchers say was likely omnivorous – to stand up and walk on its two hindlimbs.
“They were probably using their hands – which were very well-developed with a claw in the [thumb] – to cut branches and help them procure food,” Ballell says.
The findings contribute to a better understanding of how and when dinosaurs – and the sauropodomorph line in particular – evolved from walking on two legs to walking on all fours, says Ballell. “The evolution of posture in dinosaurs is quite complicated,” he says.
Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.211356
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