Wed. May 25th, 2022

Limb muscles from the Bristol dinosaur, Thecodontosaurus antiquus
Artwork by Gabriel Ugueto

New research led by the University of Bristol has revealed how giant 50-ton sauropod dinosaurs, like Diplodocus, evolved from much smaller ancestors, like Thecodontosaurus the size of a wolf.

In a new study published today in the journal Royal Society Open Science, researchers present a reconstruction of the limb muscles in Thecodontosaurus that describes the anatomy of the major muscles involved in movement.

Thecodontosaurus was a small to medium-sized bipedal dinosaur that roamed what is today Britain during the Triassic period (about 205 million years ago).

This dinosaur was one of the first ever discovered and named by scientists in 1836, but it still surprises scientists with new information about how the earliest dinosaurs lived and evolved.

Antonio Ballell, a PhD student at the Bristol School of Earth Sciences and lead author of the study, said: “The University of Bristol houses a huge collection of beautifully preserved Thecodontosaurus fossils discovered around Bristol. The amazing thing about these fossilized bones is “that many retain the scars and rugosities that the limb muscles left on them with its attachment.”

These features are extremely valuable in scientific terms for deriving the shape and direction of the limb muscles. Muscle reconstruction in extinct species requires this kind of exceptional fossil conservation, but also a good understanding of the muscle anatomy of living, closely related species.

Antonio Ballell added: “In the case of dinosaurs, we need to look at modern crocodiles and birds, which form a group that we call archosaurs, which means ‘ruling reptiles’. Dinosaurs are extinct members of this genus, and because of evolutionary similarity, we can compare the muscle anatomy of crocodiles and birds and study the scars they leave on the bones to identify and reconstruct the position of these muscles in dinosaurs. “

Professor Emily Rayfield, co-author of the study, said: “This kind of muscle reconstruction is fundamental to understanding functional aspects of the life of extinct organisms. We can use this information to simulate how these animals walked and ran with calculation tools.”

Based on the size and orientation of its limb muscles, the authors claim that Thecodontosaurus was quite agile and probably used its limbs to grab objects instead of walking.

This is in contrast to its later relatives, the giant sauropods, who partially achieved these enormous body sizes by switching to a four-legged position. The muscular anatomy of Thecodontosaurus seems to indicate that key features of later sauropod-line dinosaurs had already evolved in this early species.

Professor Mike Benton, another co-author, said: “From an evolutionary perspective, our study adds several pieces to the puzzle of how movement and posture changed during dinosaur evolution and in the queue for the giant sauropods.

“How did the limb muscles change in the development of multi-toned four-legged from small bipedal? Reconstruction of the limb muscles in Thecodontosaurus gives us new information about the early stages of the important evolutionary transition.”

This research was funded by the Natural Environment Research Council (NERC).


‘Walking with early dinosaurs: appendicular myology of the Late Triassic sauropodomorph Thecodontosaurus antiquus’ by A. Ballell, EJ Rayfield and MJ Benton in Royal Society Open Science.

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