The Largest ‘Prosauropods’. Part 1: The megaplateosaur that wasn’t

Recently I’ve been looking into the size of basal sauropodomorphs. Some of these guys reached really impressive sizes, and it wasn’t just the so-called melanorosaurids. Indeed two of the top three on my list are probably going to be a surprise to some.

But before we get to the list I have to deal with a non-starter in the competition for title of ‘largest prosauropod’

Pachysaurus? giganteus: the megaplateosaur that never was.

Don’t be surprised if you haven’t heard of this guy. There is precious little written about it and precious little of it in the first place. The species was first coined by Friedrich von Huene (1932), the doyen of European plateosaur studies, for a cluster of long bones that he took to be part of an articulated metatarsus (the part of the foot that fits between the toes and the ankle). He tentatively placed it in the now defunct genus Pachysaurus (a synonym of Plateosaurus). They were found in the Trossingen Plateosaurus quarry, well known for its abundance of articulated Plateosaurus fossils including articulated skeletons as well as many dispersed isolated bones. The longest of these supposed metatarsals was a massive 52 cm long. To put that into some kind of perspective: the longest metatarsal of the larger individual of Aardonyx (itself a big basal sauropodomorph) is less than half as long. Indeed we can extrapolate from this a range of femur lengths for P. giganteus from 1.5 to 2 meters (depending on wether we give it a stumpy Aardonyx-like foot or a more elongate Plateosaurus-like foot). The upper end of that range is nosing into brachiosaur size territory. As appealing as the notion of Brachiosaurus-sized plateosaurs cruising around the Late Triassic might be, I’m afraid that it almost certainly wasn’t so. The specimens were re-examined by David Weishampel who found that they were not metatarsals at all, rather a cluster of three fibulae (Weishampel, vide Galton, 2001). Presumably they would have been brought together by current action, long after their respective carcasses had decayed and their skeletons dispersed.  Such current sorting is not unusual. Here is a picture of me next to a little minicluster of tibiae in one of our Spion Kop quarries (no they are not Aardonyx bones).

Now of course this kind of reassessment of bone ID makes me a little uncomfortable. Von Huene was no fool and to suppose that he mistook fibulae for metatarsals does strain credibility a little. On the other hand Weishampel is no fool either and the preservation of the P. giganteus bones is truly awful, obscuring their true identity. See for yourself.

The original bones of 'Pachysaurus? giganteus'. Scale bar equals 5 cm. Image from Galton (2001, fig. 10).

Admittedly the figure from the paper is not the best (anyone who has photos, or access to the collections in Tubingen want to send some images in?) but it should be immediately apparent that they are way slender for metatarsals, especially metatarsals that exceed half a meter in length. First compare them to known metatarsals of Plateosaurus (on the right) and a fibula of Plateosaurus (on the left).

Image from Galton (1990).

Not only do the bones of P. giganteus fit the size of an ordinary  Plateosaurus fibula, the shape seems to match well.

References

HUENE, F. v. (1932) – Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte. Monogr Geol. Palaeont., (1) 4: 1-361.

GALTON, P. M. 1990 – Basal Sauropodomorpha–Prosauropoda. 320–344. In WEISHAMPEL, D. B., DODSON, P. and OSMÓLSKA, H. (eds). The Dinosauria. University of California Press, Berkeley, 733 pp.

GALTON, P. M. 2001 – The prosauropod dinosaur Plateosaurus MEYER, 1837 (Saurischia: Sauropodomorpha; Upper Triassic). II. Notes on the referred species. Revue de Paléobiologie, Genève, 20:  435–502.