The largest ‘prosauropods’ – part 3.

So last time the count-down had reached number 6.  Now lets finish this little series off with the top five largest. Once again the measurements are femur lengths.

5. Aardonyx celestae. Estimated  85.1 cm

Our newly described taxon was not small. We only recovered the femur of the smaller individual but scaling to the larger of the two individuals gives an estimated femur length close to that of the type specimen of Euskelosaurus. Assuming a more nearly circular abdomen than in Plateosaurus leads to a mass estimate of somewhere between 1300 kg and 1575 kg, with a mass of 1450 kg seeming quite reasonable. Thats about the mass of an average white rhino; not supergiant by dinosaur standards but respectable enough. However bone histology showed clearly that it was still an actively growing juvenile despite its size. Who knows how big an adult Aardonyx got?

A cameraman and photographer record images of fossilized bones of a new dinosaur species, Aardonyx celestae, from the early Jurassic period (about 200 million years old) during an announcement of the discovery at the University of the Witwatersrand, in Johannesburg, Wednesday, Nov. 11, 2009. The fossils, displayed on the table, were found in the town of Senekal, near Bethlehem in the Northern Free State, in South Africa. (AP Photo/Denis Farrell)

4. The unnamed postcranium referred to Sinosaurus triassicus. 88cm

The type specimen of Sinosaurus is a small fragment of maxilla bearing large, blade like teeth that are typical of carnivorous archosaurs. Given the early Jurassic age for the lower Lufeng Formation, this maxillary fragment more than likely came from a large theropod. Whether or not it is diagnostic, I don’t know. Anyway the typically sauropodomorph postcranium attributed to it clearly does not belong to Sinosaurus. Just what it is is uncertain, it has been referred to as an unnamed melanorosaurid, or another specimen of Jingshanosaurus, but it may belong to a number of large, poorly known basal anchisaurian taxa present in the Early Jurassic of China (Yimenosaurus, Chinshakiangosaurus – if it isn’t a sauropod, or ‘Yunnanosaurus’ robustus). Whatever it is it is big.

So now we get to the top three.

3. Plateosaurus engelhardti. 98 cm.

This is surprising. The classic, medium-sized, dull-as-dishwater ‘prosauropod’ is actually one of the very largest known members of the group. The largest known specimen is the 98 cm long femur from the holotype of Pachysaurus wetzelianus which was found in the famous Trossingen quarry alongside lots of other Plateosaurus specimens. Its distinctiveness as a different genus and species is not well-founded and I’m happy to continue treating it as a synonym of Plateosaurus engelhardti. It is interesting to note that Plateosaurus is the most well-represented basal sauropodomorph with dozens of referred specimens. An average Plateosaurus is much smaller than the giant ‘P. wetzelianus’ specimen, so is its discovery related to the much larger sample-size available for this taxon? I would that yes, this is almost certainly so. Because all of the other taxa in this list are based on much smaller sample sizes (that in some cases include only juveniles) the biological meaning of this list is rather limited.

2. Camelotia borealis. 100 cm

Since this list is supposed to be arbitrarily restricted to non-sauropods, there is a good chance that this taxon does not belong in it. However there are some features, such as the unusually abbreviated mediolateral length of the femoral head, that could potentially indicate a closer relationship with Melanorosaurus than with Sauropoda.

and the winner is……

1. ‘Yunnanosaurus‘ youngi. Estimated 110 cm

This is yet another surprising entry. Although no femur exists we have a pelvis and vertebral column. These bones indicate two things: 1) ‘Y.’ youngi wasn’t proportionately different from other, smaller basal sauropodomorphs and 2) It was exceptionally large with an estimated femur length of 110 cm in length (definately up there in small sauropod territory). Scaling up from Heinrich Mallinson’s model of Plateosaurus gives a rough estimate of about 3.5 tons for this beast, that is about the size of Shunosaurus or female african elephant. Certain things about ‘Y’ youngi are rather odd. Despite its size it seems to be less closely related to sauropods than other basal sauropodomorphs like Camelotia and Melanorosaurus. Indeed it may well have had the classic bipedal prosauropod body plan. It is also the only non-sauropod sauropodomorph that apparently survived into the Middle Jurassic.