Supersaurus — a diplodocid that lives up to its name
May 12, 2008
Long-time SV-POW! readers will have detected a brachiosaurid bias in our writings, and this is for a good reason: it is because brachiosaurids are best. They just are. But there are a few diplodocids that really get our juices flowing — not just Amphicoelias fragillimus (which we would post on, except that Darren did such a good job on it a while back on Tet Zoo [part 1, part 2]) but also the enigmatic Supersaurus. Until very very recently, this genus was known just from a single specimen, recovered by “Dino” Jim Jensen, and formally described by him in 1985, twelve long years after the name first appeared in print, in that well-known international journal Reader’s Digest. (No, I am not kidding.)
I don’t want to say too much about Jensen’s specimen right now, because I plan to cover it in more detail soon, but I do need to say that Jensen suffered from two debilitating conditions. The first was a congenital inability to see a scapulocoracoid without lying down in the dirt next to it for a photographer. Exhibit A is Jensen (1985:fig 4A), which also appears as Jensen (1987:fig 6A):
And Exhibit B is figure 6B from the very same paper:
(“What is this?”, I hear you cry. “Sauropod Scapulocoracoid Picture of The Week?” Sorry for the appendicularity, we’ll be getting you back to your regularly scheduled programme of vertebrae RSN, but the plain fact is that vertebrae are just not as good for lying down next to as scaps and humeri — though heaven knows we’ve done our best.)
Jensen’s second handicap was a tendency to figure fossils the way he thought they ought to be rather than how they actually were. For example, Jensen (1985:fig 2A) shows the very same Supersaurus cervical that Matt covered last week. Jensen’s version is influenced, we might charitably conclude, by a certain amount of imagination:
The taxonomic history of the various Supersaurus elements in Jensen’s specimen is baroque and Byzantine even by the standards of sauropod taxonomy, and I won’t go into it just now (again, stay tuned), but the result of all the to-ing and fro-ing is that a fair sample of elements is available (the cervical, two dorsals, a crushed sacrum, a handful of caudals, two scaps and pelvic elements). But many aspects of its anatomy remain obscure, and the most that can be said about its affinities is that it seems to be similar to, but distinct from, the diplodocine diplodocid Barosaurus.
No longer! As of six days ago, a new paper by Lovelace, Hartman and Wahl is — finally — out. It’s no exaggeration to describe this as one of the most eagerly awaited sauropod papers of the last decade, because it describes a brand new specimen of Supersaurus, WDC DMJ-021, from a quarry in Wyoming. It’s a little smaller than Jensen’s specimen, but very much in the same size class, that is, bigger than “Seismosaurus” and a lot bigger than any of the common-or-garden diplodocids you might see in museums, such as the Carnegie Diplodocus that seems to follow me around the museums of Europe. The paper contains some nice skeletal reconstructions (figs. 7-8 ) which show this well.
The new specimen consists of nine cervicals (in various conditions), six dorsals (ditto), nine or so caudals including the two most proximal, ribs, pelvic fragments and tibiae and fibulae. And here — tah-dah! — is the proximal caudal (lacking neural spine) in right lateral view, courtesy of Scott Hartman:
Take a moment to look at those scale bars, by the way. On my screen, if I blow the image up to full size, it’s roughly life-sized. Scroll around a bit and take in the topography. You may gulp a little if you wish. A certain amount of gasping may also be in order.
Lovelace et al. do a convincing job of showing that, while Supersaurus does resemble Barosaurus in gross proportions, it is in fact more closely related to Apatosaurus — a big surprise given that Apato is freakishly robust, and really stands alone among non-titanosaurian sauropods in terms of being absurdly over-engineered. As pointed out in the paper, however, this is more true in Apatosaurus excelsus and Apatosaurus louisae than in the type species, Apatosaurus ajax — and, indeed, if you check out the reconstruction of A. ajax in the frontispiece to Upchurch et al.’s (2005) monograph on a specimen of that species, you’ll notice that it’s not so crazy-fat as the Greg Paul apatosaur reconstruction we’ve all grown used to.
So what does this mean? For one thing, it means that Taylor and Naish’s (2005) rather obvious phylogenetic definition of Apatosaurinae as (Apatosaurus not Diplodocus) now has some substance to it, as the clade includes not only Apatosaurus but also Supersaurus and — it turns out, according to Lovelace et al’s analysis — Jerry Harris’s Suuwassea. The latter result is not wholly unexpected, as Jerry’s (2006a) abominable(*) paper on Suuwassea‘s axial skeleton did point out similarities to Apatosaurus, but this is the first time such a topology has been recovered by a published phylogenetic analysis, Jerry’s own (2006b) analysis having found Suuwassea as an unresolved basal flagellicaudatan in a trichotomy with Diplodocidae and Dicraeosauridae.
[(*) Why is the Jerry's axial osteology paper "abominable"? Because it uses a nomenclatural system unique to a small group of workers consisting of, uh, Jerry, and is therefore near-incomprehensible to everyone who's grown used to the standard Wilson (1999) nomenclature for vertebral laminae. If I had time to burn, I'd do a translation of Harris (2006a) and submit it to the Polyglot Paleontologist. Let me clearly say that, in other areas, I have nothing but respect for Jerry, whose work is both comprehensive and readable -- a glorious combination -- and whose reviews are the best and most useful I have ever seen. But this is one of those sad occasions when a very clever person has done a very dumb thing. I now await a rebuttal in the comments :-) ]
Another interesting consequence is that Apatosaurus‘s characteristically robust morphology now seems to be autapomorphic for the genus Apatosaurus itself, so that the ancestor of apatosaurines had a slender build, similar to that of Diplodocus, which was inherited by basal apatosaurines. It’ll be interesting to know what Jerry makes of this, and specifically how well Suuwassea fits this model.
One odd side-effect in the phylogeny of Lovelace et al. (2008:fig. 14) is that (Barapasaurus + Patagosaurus + (Mamenchisaurus + Losillasaurus)) form a clade, which is the outgroup to (Jobaria + Neosauropoda). Omeisaurus falls outside this group, which is a surprise as it has sometimes been thought congeneric with, or at least mixed up with, Mamenchisaurus. (Both Mamenchi and Omei have multiple species, all based on lovely material but described in only the most cursory fashion, and it’s thought that some species of each genus may belong in the other.) It’s not obvious why adding a deeply nested apatosaurine diplodocid diplodocoid should have such a dramatic effect on so much more basal a part of the tree, but that’s what happens. I’d like to hear thoughts on this.
Well, that’s enough for now, in what has become a much longer post than I intended. I finish by noting that Lovelace et al. also added “Seismosaurus” to their matrix, but found that not only did it clade with Diplodocus, it was scored identically! Accordingly, they concur with another recent paper that “Seismosaurus” is a junior subjective synonym of Diplodocus. In fact, they go further than that other paper did, and argue that poor old Sam is not just congeneric but conspecific with Diplodocus longus. That’s sort of sad — “Seismosaurus” was a pretty cool name. On the other hand, it does mean that NMMNH 3690 is the world’s biggest Diplodocus, and by some distance.
And finally, there are a couple of neat photos of a Supersaurus mount, based largely on the WDC specimen, over on Scott Hartman’s site. I recommend them. Strongly.
- Harris, Jerald D. 2006a. The axial skeleton of the dinosaur Suuwassea emilieae (Sauropoda: Flagellicaudata) from the Upper Jurassic Morrison Formation of Montana, USA. Palaeontology 49 (5): 1091-1121.
- Harris, Jerald D. (2006). The significance of Suuwassea emiliae (Dinosauria: Sauropoda) for flagellicaudatan intrarelationships and evolution. Journal of Systematic Palaeontology 4: 185-198.
- Jensen, James A. 1985. Three new sauropod dinosaurs from the Upper Jurassic of Colorado. Great Basin Naturalist 45 (4): 697-709.
- Jensen, James A. 1987. New brachiosaur material from the Late Jurassic of Utah and Colorado. Great Basin Naturalist 47 (4): 592-608.
- Lovelace, David M., Scott A. Hartman and William R. Wahl. 2008. Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, Rio de Janeiro, 65 (4): 527-544.
- Taylor, Michael P. and Darren Naish. 2005. The phylogenetic taxonomy of Diplodocoidea (Dinosauria: Sauropoda). PaleoBios 25 (2): 1-7.
- Upchurch, Paul, Yukimitsu Tomida, and Paul M. Barrett. 2005. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs No.26. Tokyo.
- Wilson, Jeffrey A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19 (4): 639-653.