Neural spine bifurcation in sauropods, Part 5: is Haplocanthosaurus a juvenile of a known diplodocid?
April 14, 2012
Last time around, Matt walked through a lot of the detailed cervical morphology of Suuwassea and known diplodocids to show that, contra the suggestion of Woodruff and Fowler (2012), Suuwassea is distinct and can’t be explained away as an ontogenomorph of a previously known genus.
Although Suuwassea is singled out for special treatment in this paper, other genera do not escape unscathed. From the Conclusions section on page 9:
Just as particularly large diplodocid specimens (e.g., Seismosaurus; Gillette, 1991) have been more recently recognized as large and potentially older individuals of already recognized taxa (Diplodocus; Lucas et al., 2006; Lovelace et al., 2007), taxa deﬁned on small specimens (such as Suuwassea, but also potentially Barosaurus, Haplocanthosaurus, and ‘‘Brontodiplodocus’’), might represent immature forms of Diplodocus or Apatosaurus.
I have to admit I more or less fell out of my chair when I saw the suggestion that poor old Haplocanthosaurus might be Diplodocus or Apatosaurus. I think this idea comes from a misstatement in the very first sentence of the abstract:
Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of the three subclades Rebbachisauridae, Dicraeosauridae, and Diplodocidae is strongly influenced by a relatively small number of characters.
As a statement of fact, this is simply the opposite of the truth: in all the major phylogenetic analyses, the arrangement of subclades with Diplodocoidea is the most stable part of the tree, supported by more characters than all the other clades.
For example, in the analysis of Upchurch et al. (2004) in The Dinosauria II, fig. 13.18 shows that the nodes with the highest bootstrap percentages are Diplodocinae (96%), Dicraeosauridae (95%) and Diplodocidae (93%).
Or consider the analysis of Wilson (2002). While it’s getting on a bit, it still scores highly by being the most explicit published sauropod analysis, with comprehensive lists of apomorphies. Table 12 lists the decay indexes for the 24 nodes in the strict consensus tree. Apart from the three very basal nodes separating sauropods from their outgroups, the two highest-scoring clades are Diplodocidae and Diplodocinae (DI=7), followed by four clades all with DI=5 of which two are Dicraeosauridae and Flagellicaudata (which Wilson just called “Dicraeosauridae + Diplodocidae” as it had not yet been named). (It’s well worth reading Wilson’s Appendix 3 to see the synapomorphies supporting these nodes in the MPTs: he lists 14 separating Diplodocimorpha from the node it shares with Haplocanthosaurus, 18 separating Flagellicaudata from the node it shares with Rebbachisauridae, 16 separating Diplodocidae from the node it shares with Dicareosauridae, and seven separating Diplodocinae from the node it shares with Apatosaurus).*
* Why are the lists of apomorphies longer than the decay indexes? Because they list the apomorphies as they occur in the specific topology of the consensus tree. Nodes within that tree can be made to collapse without wiping out all the apomorphies by rejuggling other parts of the tree to move character-state transitions around. So although (for example) 26 characters separate Flagellicaudata from Rebbachisauridae (18 + 8 synapomorphies respectively) you can rejuggle the whole tree to break the monophyly of Flagellicaudata while making the entire tree only five steps longer.
Anyway, for whatever reason, Woodruff and Fowler felt that the stability of the diplodocoid clades was in question, and this presumably influenced their hypothesis that Haplocanthosaurus could be easily moved down into one of the diplodocid genera.
Next time we’ll be considering the implications for the tree. But today, let’s take a moment to do this the old-fashioned way, by looking at …
Hatcher (1903), ever helpful, included a comparative plate in his monograph which should help us to evaluate the idea that Haplo is a known diplodocid:
Based on this, the pelvis of Haplocanthosaurus differs from those of the diplodocids in having a proportionally lower ilium, in the absence of the laterally facing rugosity on the posterodorsal margin of the ilium, in the very small distal expansion of the pubis and in the almost non-existent distal expansion of the ischium. These are all characters of the limb-girdle elements, which do not change greatly through ontogeny in sauropods.
But the evidence from the sacral vertebrae is just as significant: the neural spines in the sacral area are less than half as tall as in the diplodocids — and this in an animal whose dorsal neural spines are conspicuously tall. The spines are also more anteroposteriorly elongate and plate-like. What’s more, sacral spines 1, 2 and 3 have fused into a single plate in Haplocanthosaurus, while the spine of S1 remains well separated from 2 and 3 in the diplodocids. So the ontogenetic hypothesis would have to say that the spine of S1 unfuses through ontogeny. Which is not something I’ve heard of happening in any sauropod, or indeed any animal.
So the pelvis and sacrum seem distinct. But Woodruff and Fowler’s (2012) notion of ontogenetic synonymy is built on the idea that the differences in the cervical and dorsal vertebrae are ontogenetic. So let’s take a look at them.
It should be immediately apparent that the Haplocanthosaurus cervicals have less extensive pneumatic features than those of the diplodocids, but that is one feature which we know does vary ontogenetically. There are other differences: for example, the cervical ribs in Haplocanthosaurus are level with the bottom centrum rather than hanging below. Still, if you kind of squint a bit, you could probably persuade yourself that the Haplocanthus vertebrae look like possible juveniles of Diplodocus.
Unless you look at them from behind:
(Unfortunately, these are the only Haplocanthosaurus cervical vertebrae that Hatcher had illustrated in posterior view, so we can’t compare more anterior ones.)
From this perspective, we can immediately significant differences:
- First, that unsplit spine. Yes, we know that Woodruff and Fowler (2012) have argued that it could be ontogenetic, but these are vertebrae from the most deeply bifurcated region of a diplodocid neck, in a decent sized animal, and there is nothing that so much as hints at bifurcation.
- That whacking great ligament scar running right down the back (and also the front, not pictured) of the neural spine. There is nothing like this in any diplodocid — neither on the metapophyses nor running though the trough. And remember, scars like these tend to become more prominent through ontogeny.
- The neural arch (i.e. the region between the postzygapophyses and the centrum) is taller in Haplocanthosaurus — much taller in the case of C15.
- The plates running out to the diapophyses are less dorsoventrally expanded in Haplocanthosaurus.
- The centrum is smaller as a proportion of total height — especially, much smaller than in Diplodocus.
- The parapophyses extend directly laterally rather than ventrolaterally (hence the position of the cervical ribs level with the bottom of the centrum).
So it doesn’t look good for the juvenile-diplodocid hypothesis. But let’s take a look at the …
Here we see that Haplocanthosaurus has dorsolaterally inclined diapophyses (which we’ll see more clearly in a minute), a prominent spinodiapohyseal lamina in posterior dorsals, and no infraparapophyseal lamination. Also, the dorsal vertebrae have reached their full height by the middle of the series (in fact the last nine dorsals are startlingly similar in proportions), whereas in diplodocids, total height continues to increase posteriorly.
Now let’s see those vertebrae in posterior view:
Here is where it all falls apart. The Haplocanthosaurus dorsals differ from those of the diplodocids in almost every respect:
- Of course we have the non-bifid spine in again, in the anterior dorsal, but let’s not keep flogging that dead horse.
- In the mid and posterior dorsals, the neurapophysis is rounded in posterior view rather than square.
- In the posterior dorsal, the neural spine has laterally directed triangular processes near the top.
- All three Haplocanthosaurus neural spines have broad, rugose ligament scars, whereas those of the diplodocids have narrow postspinal laminae.
- The neural spines (measured from the diapophyses upwards) are much shorter than in the diplodocids; but
- The neural arches (measured from the centrum up to the diapophyses) are much taller.
- The diapophyses have distinct club-like rugosities at their tips.
- the diapophyses of the mid and posterior dorsals are inclined strongly upwards
- The hyposphenes of mid and posterior dorsals have very long centropostzygapophyseal laminae curving up in a gentle arch.
- The centra are smaller than those of Apatosaurus, and much smaller than those of Diplodocus.
(By the way, it’s interesting how very different the D5s of Apatosaurus and Diplodocus are. Since both are from uncontroversially adult specimens, bifurcation was evidently very different between these genera.)
So based on the vertebrae alone, the case of Haplocanthosaurus as an immature form of Diplodocus or Apatosaurus is blown right out of the water. And this is without even looking at the appendicular material — for example, the scapula and coracoid illustrated by Hatcher (1903:figs 17-19), which are so completely different from those of diplodocids.
But there’s more. Tune in next time for the rest.
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:
- Gillette, D.D. 1991. Seismosaurus halli, gen. et sp. nov., a new sauropod dinosaur from the Morrison Formation (Upper Jurassic/Lower Cretaceous) of New Mexico, USA. Journal of Vertebrate Paleontology 11(4):417-433.
- Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
- 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.
- Hatcher, J.B. 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds; additional remarks on Diplodocus. Memoirs of the Carnegie Museum 2:1-75.
- Lovelace, D.M., Hartman, S.A., 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.
- Lucas, S.G., Spielmann, J.A., Rinehart, L.F., Heckert, A.B., Herne, M.C., Hunt, A.P., Foster, J.R., Sullivan, R.M. 2006, Taxonomic status of Seismosaurus hallorum, a Late Jurassic sauropod dinosaur from New Mexico. New Mexico Museum of Natural History and Science Bulletin 36:149-162.
- Upchurch, P. Barrett, P.M., Dodson, P. 2004. Sauropoda. pp. 259-322 in D.B. Weishampel, P. Dodson and H. Osmólska (eds.), The Dinosauria, 2nd edition. University of California Press, Berkeley and Los Angeles. 861 pp.
- Wilson, J.A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136:217-276.
- 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.
Special bonus illustrations
I composited the cervical and dorsal series above into the following compound illustrations. As always, click through for full resolution.