Sauropods as neotenous prosauropods

In the course of dinosaur evolution
sauropods reverted to quadrupedal locomotion, a trait found in embryo prosauropods, like Massospondylus, Fig. 1), but not in adult prosauropods or their dinosaurian ancestors.

FIgure 1. Massospondylus embryo in situ and reconstructed.

FIgure 1. Massospondylus embryo in situ and reconstructed.

This topic came to mind after seeing the new paper
on the Early Jurassic basal saurpodiform, Yizhousaurus (Zhang et al. 2018, which appears to remain bipedal as an adult; Fig. 2).

Notably, and despite it’s bipedal appearance,
in the large reptile tree (LRT, 1286 taxa), Yizhousaurus nests with the embryo Massospondylus (Fig. 1), not the adult (Fig. 4). Hence the title of this blogpost.

Figure 1. Yizhousaurus is an early Jurassic basal sauropod.

Figure 2. Yizhousaurus the early Jurassic basal sauropod that currently nests with the embryo Massospondylus.

Yes, the skeleton of Yizhousaurus
has much longer hind limbs than front limbs, which shows that the transition to a quadrupedal locomotion was gradual in adults, but the skull has several sauropod traits and the manual digit 1 ungual is no longer a big hook, but a stub, like the other manual unguals.

Figure 2. Skull of Yizhousaurus in several views.

Figure 3. Skull of Yizhousaurus in several views.

Sauropods like Yizhousaurus had their genesis
in the Early Jurassic and their greatest radiation in the Late Jurassic. Some clades extended to the Late Cretaceous.

FIgure 4. Massospondylus adult in situ.

FIgure 4. Massospondylus adult in situ.

Did sauropods have several or a single origin?
I have no idea, but the idea is already floating around out there.

References
Barrett PM 2009. A new basal sauropodomorph dinosaur from the upper Elliot formation (Lower Jurassic) of South Africa. Journal of Vertebrate Paleontology 29(4):1032-1045.
Carrano MT 2005.The evolution of sauropod locomotion: morphological diversity of a secondarily quadrupedal radiation.” in The Sauropods: Evolution and Paleobiology, edited by Curry Rogers, K. A. and Wilson, J. A., 229–251. University of California Press.
Morris J 1843. A Catalogue of British Fossils. British Museum, London, 222 pp.
Reisz RR, Scott D; Sues H-D, Evans DC and Raath MA 2005. Embryos of an Early Jurassic prosauropod dinosaur and their evolutionary significance. Science. 309(5735): 761–764.
Reisz RR, Evans DC, Roberts EM, Sues H-D and Yates AM 2012. Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus PDF. Proceedings of the National Academy of Sciences of the United States of America. 109(7): 2428–2433.
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.
Zhang Q-N, You H-K, Wang T and Chatterjee S 2018. A new sauropodiform dinosaur with a ‘sauropodan’ skull from the Lower Jurassic Lufeng Formation of Yunnan Province, China. Nature.com/scientificreports 8:13464 | DOI:10.1038/s41598-018-31874-9

wiki/Massospondylus
wiki/Yizhousaurus

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