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The largest ‘prosauropods’. Part 2

February 16, 2011 Leave a comment

OK time to get the countdown started. The list is ordered by femoral length, or extrapoloated femoral length, mostly because good skeletal reconstructions from which volumetric estimations could be made are lacking. Nevertheless the strong similarity of the body plan of basal sauropodomorphs means that femoral length is a pretty good proxy for body size. We’ll count down the top ten taxa.

Coming in at number ten we have:

Melanorosaurus readi with a maximum recorded femoral length of 63 cm.

Yes, that’s right. The archetypal ‘big prosauropod’ is actually quite a tiddler and barely scrapes into my top ten list. Nevertheless the individual elements of this guy are quite heavy and robust which may have fostered the myth that this near-sauropod was generally bigger than Plateosaurus and its ilk. It probably only tipped the scales at 800 kg.

Melanorosaurus reconstruction by 'steveoc'. Image from Wikimedia commons.

9. Lufengosaurus’ magnus. 74 cm

I put commas around the genus name because no-one has proposed any good characters that might link L. magnus to L. huenei even though they are fairly universally treated as synonyms. They could be but I doubt it, there are some odd features of L. magnus that are not present in L. huenei and vice versa.

A partial skeleton of what might be 'Lufengosaurus magnus' left in situ on Dawa Hill, near Lufeng, Yunnan.

8. Jingshanosaurus xinwaensis. 75 cm

Not the largest basal sauropodomorph by any means but it does include the largest nearly complete mounted skeleton of a basal sauropodomorph.

7. Plateosauravus cullingworthi. Estimated  83 cm

This is the taxon that has been farly consistently confused with Euskelosaurus since the 1970’s. It isn’t. The known femora of Plateosauravus are not so big, but there are other bones included in the topotype collection that indicate that some members of this species got quite large. Scaling up from these bones we arrive at an estimated femoral length of 83 cm for the biggest individuals in the topotype bone bed. Based on other basal sauropodomorphs we can estimate that an individual with an 83 cm femur would have weighed in at 1.1 to 1.2  tons.

6. Euskelosaurus browni. Estimated 85 cm

Regardless of whether or not this is a nomen dubium the individual upon which the name was bestowed did exist, and was pretty large. Further it demonstrably didn’t belong to any of its well-known contemporaries such as Antetonitrus, Melanorosaurus, Eucnemesaurus or Plateosauravus so provisionally, I’m treating this as a valid taxon.

The rather busted-up and incomplete type femur of Euskelosaurus browni

So oddly enough places 10-6 consist entirely of taxa either from the Early Jurassic of China or the Late Triassic of South Africa. Another take home point is that a lot of these measurements are based on very limited sample sizes. Given the rather small differences between many of them it would be silly to treat this list as accurately reflecting the mean sizes of each of these species, or the top sizes that they reached. Nonetheless the very biggest on the list were in a real biological sense large animals than those from the bottom of the list. What were the top five? You’ll have to wait for part 3.

A note on missing taxa. I could get only get skull measurements for Yimenosaurus, so it is not included. Chinshakiangosaurus, Antetonitrus and Lessemsaurus are arbitrarily excluded on suspicion of being sauropods.

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Categories: sauropodomorphs

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

February 4, 2011 6 comments

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.

Categories: sauropodomorphs