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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 Epoch, was a sauropodomorph – which means it belonged to the same group as the gigantic herbivorous sauropods that lived later, in the Jurassic and Cretaceous periods.

T. antiquus had muscles in its back legs that favoured speed over force, whereas its forelimb muscles would have been more appropriate 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 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 the limb and body bones of T. antiquus, which was��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 south-west 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, says Ballell. “There’s a trade-off [because] muscles that generate 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 the “huge digestive tracts” necessary for digesting all the plant matter they were consuming, says Ballell.

As for the forelimbs of T. antiquus, a “deep and scarred” groove and other marks in the elbow bone and shoulder blade suggest that they had great elbow flexibility and extension, says Ballell. The long, slender shapes of the arm and shoulder bones hint that they had 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 probably 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,” says Ballell.

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.