Thecodontosaurus was bipedal. So were most dinos in the Late Triassic.

After 22 years professor Michael Benton
once again returns his attention to the pre-sauropod, Thecodontosaurus antiquus (Fig. 1), one of the first dinosaurs described (Riley and Stutchberry 1836, Morris 1843).

Figure 1. Thecodontosaurus traced from Benton et al. 2000 and filled in.
Figure 1. Thecodontosaurus traced from Benton et al. 2000 and filled in here. the tail is unknown.

Benton et al. 2000 described
the anatomy (Fig. 1) and systematics (Fig. 2) of Thecodontosaurus.

Ballell, Rayfield and Benton 2022 described
the myology (muscle attachment points) of Thecodontosaurus.

Thecodontosaurus was found in Bristol, England,
so these University of Bristol researchers (Ballell, Rayfield and Benton 2022) and the original discoverers and describers (Riley and Stutchberry 1836, Morris 1843 did not have to travel far.

Figure 2. Cladograms from Benton et al. 2000 (above) and Ballell, Rayfield and Benton 2022 (below) both featuring Thecodontosaurus. Only a few taxa are tested in both studies. Color added to the top graphic. Bone shapes removed from the bottom graphic.

Benton et al. 2000 wrote:
“Although much of the original topotype material found in the 1830s in Bristol, England, has now been lost, some 245 specimens remain A cladistic analysis indicates that Prosauropoda is probably a clade, rather than a series of outgroups to Sauropoda, but support for this conclusion is weak.”

Their 2000 and 2022 cladograms are both shown here (Fig. 2).
Some taxa were omitted from the earlier study, according to the 2022 cladogram.
Others were added.

Ballell, Rayfield and Benton 2022 wrote:
“The skeletal anatomy and arrangement of forelimb muscles indicate that elbow flexors and extensors were well-developed in Thecodontosaurus, and the range of motion at the shoulder would have been limited, suggesting that the forelimbs were not used in habitual locomotion but in manipulation, as in other early sauropodomorphs.”

The general proportions of Thecodontosaurus present a bipedal configuration.
Moreover, quadrupedal dinosaurs are rare in the Late Triassic (Fig. 4).

Figure 3. Subset of the LRT focusing on phytodinosauria. Here Ornithischia is a clade within Sauropodomorpha and within Phytodinosauria. Pampadromaeus is a last common ancestor for both clades.

At some point sauropods reverted to a quadrupedal configuration.
The question is: did that reversion happen just once? Or several different times by convergence?

According to the LRT
all basal phytodinosaur taxa (Fig. 4), other than Saturnalia (Fig. 4), had a bipedal configuration with short forelimbs or hands inappropriate for placing on the ground. By contrast, in the LRT Early Jurassic Yizhousaurus nests basal to the large quadrupedal sauropods. The Yizhoursaurus hand had robust short fingers with flat unguals. These are traits expected if bearing weight. Yizhousaurus is another close relative to the Thecodontosaurus clade with the present short list of tested taxa. Given the present taxon list, quadrupedalism in sauropomorphs appeared twice.

Figure 4. Phytodinosaur taxa from the LRT to scale. Colors match those in figure 3. It is worthwhile noting the comings and goings of a short vs long neck. All but one or two are in the same size range here.

Ballell, Rayfield and Benton examined every muscle scar on every bone
of Thecodontosaurus to conclude that it was bipedal. That’s excellent, but it is something that was described and illustrated 22 years earlier by Benton et al. 2000.

Recent publicity
(see below) makes news of bipedalism in Thecodontosaurus by comparing it to more popularly known giant quadrupedal sauropods. Readers, if you want to grab headlines, follow this example. Ballell, Rayfield and Benton know how to do it, and so should you.

Ballell A, Rayfield EJ and Benton MJ 2022. Walking with early dinosaurs: appendicular myology of the Late Triassic sauropodomorph Thecodontosaurus antiquus, Royal Society Open Science (2022). DOI: 10.1098/rsos.211356
Benton MJ, Ruul L, Storrs GW and Galton PM 2000. Anatomy and systematics of the prosauropod dinosaur Thecodontosaurus antiquus from the Upper Triassic of Southwest England. Journal of Vertebrate Paleontology 20(1):77–108.
Morris J 1843. A Catalogue of British Fossils. British Museum, London, 222 pp.
Riley H and Stutchbury S 1836. A description of various fossil remains of three distinct saurian animals discovered in the autumn of 1834, in the Magnesian Conglomerate on Durdham Down, near Bristol. Proceedings of the Geological Society of London 2:397-399.



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