Neural spine bifurcation in sauropods, Part 4: is Suuwassea a juvenile of a known diplodocid?
April 12, 2012
I don’t intend to write a comprehensive treatise on the morphology and phylogeny of Suuwassea. Jerry Harris has already done that, several times over (Harris 2006a, b, c, 2007, Whitlock and Harris 2010). Rather, I want to address the contention of Woodruff and Fowler (2012) that Suuwassea is a juvenile of a known diplodocid, building on the information presented in the first three posts in this series (Part 1, Part 2, Part 3).
In the abstract, Woodruff and Fowler (2012:1) wrote:
On the basis of shallow bifurcation of its cervical and dorsal neural spines, the small diplodocid Suuwassea is more parsimoniously interpreted as an immature specimen of an already recognized diplodocid taxon.
First of all, that’s not what ‘parsimoniously’ means. It’s just not. In a phylogenetic analysis using unweighted characters, there is no such thing as a ‘key’ character — which by the way means that the subtitle of the paper, “a critical phylogenetic character”, is wrong. All characters are equal. Even if the characters were weighted, neural spine bifurcation would have to be weighted pretty darned heavily for it to outweigh all the other characters combined, which is what the sentence quoted above suggests.
Comparisons with known diplodocids
Next problem: if Suuwassea is a juvenile of an already recognized diplodocid, it shouldn’t take long to figure out which one. There aren’t all that many candidates, and we can consider them in turn. There are loads of characters, especially cranial and appendicular, separating Suuwassea from Apatosaurus, Diplodocus and the rest, and anyone who wants to keep track of all of them is welcome to do so. I care about vertebrae, and I’m prepared to argue that Suuwassea is a distinct taxon based on cervical morphology alone.
Here are the sixth cervical vertebrae of Suuwassea emiliae ANS 21122 (Harris 2006c:Text-fig. 7B) and Diplodocus carnegii CM 84/94 (Hatcher 1901:pl. 3, flipped left-to-right for ease of comparison). They are not to scale–I made the images the same cotyle diameter for ease of comparison.
Elongation first. C6 of S. emilieae has a centrum length of 257 mm, a cotyle diameter of 75 mm, and so an EI of 3.4. C6 of D. carnegii has a centrum length of 442 mm, a cotyle diameter of 99 mm, and an EI of 4.5. So Diplodocus is one third more elongate than Suuwassea. It is true that sauropod cervicals elongate through ontogeny, but the Suuwassea holotype is a decent-sized animal, and would be expected to have attained adult proportions even if it was not fully adult (also, ANS 21122 has more cervical ribs fused than CM 84/94). We know from the juvenile ?Sauroposeidon vertebra YPM 5294 (Wedel et al. 2000:372) that subadult sauropod cervicals attained great elongation: that element is from an animal young enough to have had an unfused neural arch but it has an EI exceeding 5.0.
Then there’s neural spine shape. Yes, it is variable in sauropods, but this is ridiculous. I strongly doubt that any non-pathological Diplodocus cervical anywhere ever has had a neural spine shaped like that of the Suuwassea vertebra.
Also note that the prezygapophyses of the D. carnegii C6 strongly overhang the condyle but are only slightly elevated, whereas those of S. emilieae are right above the condyle but strongly elevated, so that the prezygapophyseal rami might fairly be called pedestals. Such pedestaling of the prezygapophyses is present in some cervicals of Apatosaurus, although perhaps not to the same extreme. Some Apatosaurus cervicals have pretty funky, smokestack-looking neural spines, although–again–not to the same extreme as in Suuwassea. Still, from the mid-centrum on up, S. emiliae looks a bit apatosaur-ish. So let’s try that next.
Here we have C6 of Suuwassea as before, this time with Apatosaurus louisae CM 3018 (Gilmore 1936:pl. 24), again scaled to the same cotyle diameter.
C6 of A. louisae has a centrum length of 440 mm, a cotyle diameter of 150 mm, and an EI of 2.9 (I know it doesn’t look that short, but I’m going off Gilmore’s data, and I trust the measuring tape more than the drafting pen, no matter how skillfully the latter is wielded.) So this is not a bad match with the value of 3.4 for Suuwassea.
Of course, the glaring problem with suggesting that Suuwassea is a juvenile Apatosaurus is that it has normal-sized cervical ribs, not the insane scythes of doom that hang below the centrum of every post-axial Apatosaurus cervical (see these posts [#1, #2, #3] for some crazy examples, and this post for more pictures and discussion). The giant cervical ribs are present even in very juvenile Apatosaurus cervicals, such as the large collection of juvenile apatosaurs in the BYU collection from Cactus Park (albeit unfused; the immense parapophyses still point the way even if the ribs themselves are missing).
I know, I know, I just said that there is no such thing as a key character. But all of the known species of Apatosaurus have giant cervical ribs, and indeed are often identified in the field as Apatosaurus on that basis alone. I suppose it’s not impossible that Suuwassea is nested within the other Apatosaurus species, based on some bizarre combination of as-yet undiscovered characters and intermediate specimens, and lost the giant cervical ribs along the way, but now we’re into angels dancing on the heads of non-existent pins. If Suuwassea is an apatosaurine but outside the clade of giant-cervical-rib-bearing Apatosaurus, then whether we call it a species of Apatosaurus or a separate genus–say, Suuwassea–is more a matter of taste than anything else. Note that Lovelace et al. (2008) recovered Suuwassea as an apatosaurine, but not as Apatosaurus.
Lest anyone without access to the paper think I’m cheating by hiding serial variation, here are the other well-preserved cervicals of Suuwassea, to scale:
So, if Suuwassea is a juvenile of a known diplodocid but it’s not Diplodocus or Apatosaurus, what’s left?
Okay, now I’m just messing with you.
Is Suuwassea even a juvenile?
By now it is probably obvious, even from cervical morphology alone, that if ANS 21122 is a juvenile of anything, it’s a juvenile Suuwassea. But is it in fact a juvenile?
We-ell. The cervical neural arches are all fused, but not all of the cervical ribs are. Jerry did a fine job of describing exactly what was going on at each serial position (Harris 2006c). In C3, the left cervical rib is not attached, and the right one is attached at the parapophysis but not fused. In C5, the ribs are attached, not fused at the parapophyses, and fused at the diapophyses*. In C6, the ribs are fused at both attachment points. C7 lacks the ribs, but their absence appears to be caused by breakage rather than lack of fusion. One fragmentary posterior cervical of uncertain position is missing the diapophyses but has one rib fused at the parapophysis.
* This is cool because it is the first time that I know of that anyone has documented which of the two attachment points fused first within a single cervical rib. I wonder if other sauropods did it the same way?
So based on cervicals alone, we would infer that Suuwassea was not fully mature. However–and this is absolutely crucial for the synonymization hypothesis–the Suuwassea holotype ANS 21122 already has a greater degree of cervical element fusion than Diplodocus carnegii holotype CM 84/94 (which has unfused ribs back to C5) and Apatosaurus CM 555 (which has unfused arches back to C8 and unfused ribs throughout), both of which have attained essentially ‘adult’ morphology. So if Woodruff and Fowler (2012) are correct, the ontogenetic clock has to run forward from CM 555 and CM 84/94, through a Suuwassea-like stage, and then back to normal Apatosaurus or Diplodocus morphology.
But we don’t have to rely on cervicals alone, because ANS 21122 also includes some dorsals and caudals. And the caudals are very interesting in that the neural arches are not fused through most of the series. Harris (2006c:1107):
Of all the caudal vertebrae preserved in ANS 21122, only the distal, ‘whiplash’ caudals are complete. All the remaining vertebrae consist only of vertebral bodies that lack all phylogenetically informative portions of their respective arches. On the proximal and middle caudals, this absence is due to lack of fusion as evidenced by the deeply fluted articular surfaces for the arches on the bodies. In contrast, the arches on the most distal vertebrae that retain them are seamlessly fused, but everything dorsal to the bases of the corporozygapophyseal laminae are broken.
Now this is pretty darned interesting, because it shows that neural arch fusion in Suuwassea was not a simple zipper that ran from back to front (as in crocs [Brochu 1996] and phytosaurs [Irmis 2007*]) or front to back. We can’t really say, based on this one specimen, what the sequence was, but we can say for certain that the anterior and middle caudals came last. Oh, and for what it’s worth, the scap-coracoid joint is also unfused (Harris 2007), but we know that that’s often the case for substantially “adult” sauropods such as the mounted Berlin Giraffatitan.
* Relevant to this entire post series are the wise words of my homeboy and former Padian labmate Randy Irmis, who wrote in the abstract of his 2007 neurocentral fusion paper:
A preliminary survey indicates that there is considerable variation of both the sequence and timing of neurocentral suture closure in other archosaur clades. Therefore, it is unwise to apply a priori the crocodylian pattern to other archosaur groups to determine ontogenetic stage. Currently, apart from histological data, there are few if any reliable independent criteria for determining ontogenetic stage. I propose that histology be integrated with independent ontogenetic criteria (such as neurocentral suture closure) and morphometric data to provide a better understanding of archosaur ontogeny.
The unfused arches in the Suuwassea caudals are especially interesting because, for the first time that I know of, we have a sauropod with cervical neural arches and at least some cervical ribs fused, but with unfused neural arches elsewhere in the body. This is in contrast to D. carnegii CM 84/94, in which all the neural arches are fused but the anterior cervical ribs are not. So the developmental timing in Suuwassea is dramatically different than in D. carnegii, at least, which is one more problem for the synonymization hypothesis. Two more problems, actually, in that (1) Suuwassea probably isn’t Diplodocus, and (2) it doesn’t belong in the same ontogenetic series as Diplodocus, contra Woodruff and Fowler (2012:Figs. 3 and 9)–if the timing of the various fusions differs between the taxa, there is no basis for assuming that the hypothetical ontogenetic bifurcation would follow the same rules.
And speaking of ontogenetic bifurcation, a final point about the ‘bifurcations’ in Suuwassea.
The first line of the caption is misleading. Two of these vertebrae have weakly bifurcated neural spines because they are sixth cervicals (Suuwassea in B, Apatosaurus in D), and that’s what you expect in C6 in adult diplodocids. One of them, the C5 of Suuwassea in C, isn’t bifurcated at all: it’s broken. Harris (2006c:1099):
The spinous process expands mediolaterally toward its apex, attaining maximal width just proximal to its terminus. A long, narrow crack at the distal end gives the appearance of bifurcation, but the collinear dorsal margin indicates that no true split was present.
As for the final vertebra, MOR 592 in A, who knows? Woodruff and Fowler (2012) do not say what serial position it is from. Based on the shallow notch in the spine, I’ll bet it’s either a C6 or very close to it–and if so, no deeper split is expected.
So the entire rationale for the taxonomic side of Woodruff and Fowler (2012)–that Suuwassea has incompletely bifurcated neural spines because it is a juvenile –turns out be an illusion caused by not taking serial variation into account. Suuwassea ANS 21122 probably is a subadult, based on the unfused caudal neural arches, but its cervical vertebrae already show the expected adult morphology in neural arch fusion, cervical rib fusion, and–most importantly–neural spine bifurcation.
The evidence that Suuwassea is not a juvenile of a known diplodocid is not in this post. It’s in the hard work, comprehensive descriptions, and detailed, thoughtful comparisons by Harris and Dodson (2004), Harris (2006a, b, c, 2007), Lovelace et al. (2008), Whitlock and Harris (2010), and Whitlock (2011). This post is just an arrow scratched in the dirt. Please, go read those papers. And then read all the monographs I cited in the first post in this series (and am too lazy to cite again here). Give those people their due by taking their work seriously and learning from it.
The rest of the series
Links to all of the posts in this series:
- Part 1: what we knew a month ago
- Part 2: why serial position matters
- Part 3: the evidence from ontogenetic series
- Part 4: is Suuwassea a juvenile of a known diplodocid?
- Part 5: is Haplocanthosaurus a juvenile of a known diplodocid?
- Part 6: more reasons why Haplocanthosaurus is not a juvenile of a known diplodocid
and the post that started it all:
- Brochu, C.A. 1996. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. Journal of Vertebrate Paleontology 16:49-62.
- Galiano, H., and Albersdörfer, R. 2010. A new basal diplodocid species, Amphicoelias brontodiplodocus from the Morrison Formation, Big Horn Basin, Wyoming, with taxonomic reevaluation of Diplodocus, Apatosaurus and other genera. Dinosauria International, LLC, Wyoming. 50 pages.
- Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
- Harris, J.D. 2006a. Cranial osteology of Suuwassea emilieae (Sauropoda: Flagellicaudata) from the Upper Jurassic Morrison Formation of Montana, U.S.A. Journal of Vertebrate Paleontology 26:88–102.
- Harris, Jerald D. 2006b. The significance of Suuwassea emiliae (Dinosauria: Sauropoda) for flagellicaudatan intrarelationships and evolution. Journal of Systematic Palaeontology 4: 185-198.
- Harris, J.D. 2006c. The axial skeleton of the dinosaur Suuwassea emilieae (Sauropoda: Flagellicaudata) from the Upper Jurassic Morrison Formation of Montana, USA. Palaeontology 49:1091-1121.
- Harris, Jerald D. 2007. The appendicular skeleton of Suuwassea emilieae (Sauropoda: Flagellicaudata) from the Upper Jurassic Morrison Formation of Montana (USA). Geobios 40:501-522. doi:10.1016/j.geobios.2006.02.002
- Harris, J.D., and Dodson, P. 2004. A new diplodocoid sauropod dinosaur from the Upper Jurassic Morrison Formation of Montana, USA. Acta Palaeontologica Polonica 49:197–210.
- Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
- Irmis, R.B. 2007. Axial skeleton ontogeny in the Parasuchia (Archosauria: Pseudosuchia) and its implications for ontogenetic determination in archosaurs. Journal of Vertebrate Paleontology 27:350-361.
- Lovelace, D.M., Hartman, S.A., and Wahl, W.R. 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.
- Wedel, M.J., Cifelli, R.L., and Sanders, R.K. 2000. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45(4):343-388.
- Whitlock, J.A. 2011. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society 161:872-915.
- Whitlock, J.A., and Harris, J.D. 2010. The dentary of Suuwassea emilieae (Sauropoda: Diplodocoidea). Journal of Vertebrate Paleontology 30(5):1637-1641.
- Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.