Inconsistent fusion in Haplocanthosaurus sacra
April 29, 2012
Matt and I have been looking in more detail at indications of maturity in sauropod skeletons, as we prepare the submission of the paper arising from our response to Woodruff and Fowler (2012) [part 1, part 2, part 3, part 4, part 5, part 6]. Here is an oddity.
Sacra of Haplocanthosaurus. Top, H. utterbacki holotype CM 879 in right lateral view, from Hatcher (1903:fig. 15). Bottom, H. priscus holotype CM 572 in left lateral view (reversed), from Hatcher (1903:pl. IV, part 3). To the same scale.
H. priscus is the type species of Haplocanthosaurus; H. utterbacki is the second species, named by Hatcher in the 1903 monograph that described the original material in detail. As previously noted, the type species is based on adult material, and the referred specimen on subadult material. This is shown by their different stages of neurocentral fusion, and corroborated by the size of the specimens as indicated in the composite illustration above.
There is a lot of fusion going on in the sacra of dinosaurs:
- sacral neural arches fused to their centra
- consecutive sacral centra fused together
- consecutive sacral neural spines fused together
- sacral lateral processes fused to ilia
As we would expect, the less mature of the two Haplocanthosaurus individuals is less fused in most respects: none of the centra were fused either to each other or their respective neural arches, and the ilium was not fused to any of the lateral processes, whereas in the adult all neural arches are fused to their centra, the five sacral centra are all fused together, and the ilium is fused to the lateral processes.
How strange, then, that the consecutive neural spines are more fused in the juvenile! Not only are spines 1, 2 and 3 fused along their entire dorsolateral length, as in the adult, but spine 4 is similarly fused. And more: the neurapophysis of spine 5 is fused to that of 4, even though the spines are not fused more ventrally.
What does this mean? Hatcher (1903:27-28) took it as indicative of species-level separation. After briefly noting that the posterior dorsal centra of H. utterbacki are more opisthocoelous than those of H. priscus, and speculating that the adult of the referred species was probably larger than that of the type, he continued:
But the most distinctive character is to be found in the sacrum which, in the present species, has the five neural spines normally coössified. The first four are cocoössified throughout their entire length, forming a long bony plate. The union between the fourth and fifth is limited to the extremities while medially [sic, presumably meaning half way up the spines] they are separated by an elongated foramen. In H. priscus only the spines of the three anterior sacrals are coössified, those of the first and second [sic, presumably intending fourth and fifth] sacrals remaining free. This difference exists notwithstanding that the type of the present species was scarcely adult, the sacral centra being neither coössified with one another nor with their neural arches. By some this character might be considered as of generic importance although I prefer to consider it as of only specific value since in all other parts of the skeleton preserved, there are no distinguishing characters which could be considered as of generic value.
At present, however, the synonymy of H. utterbacki with the type species, proposed by McIntosh and Williams (1988:22), seems to be universally accepted. If they truly belong to the same taxon then the only realistic possibility is that we are seeing individual variation in the timing of fusion. That certainly seems to have been the opinion of McIntosh and Williams (1988:14), writing about the sacrum of their own specimen, the H. delfsi holotype CMNH 10380:
As in CM 572 the short to moderately long spines of sacrals one through three are firmly united throughout, and those of sacrals four and five are firmly united to midheight. In CM 572 spines four and five are free, but this is probably an individual character because in the even younger CM 879 all five spines are united.
All of which means: we need to be really careful when drawing conclusions about taxonomy or ontogeny from individual observations of skeletal fusion.
Bonus Pneumaticity Observation: In the image at top, you’ll see that the centrum of sacral 4 in CM 879 has a couple of pneumatic fossae. For more than you probably wanted to know about those specific holes in that specific bone, see this post and the linked paper.
References
- 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.
- McIntosh, J.S., and Williams, M. E. 1988. A new species of sauropod dinosaur, Haplocanthosaurus delfsi sp. nov., form the Upper Jurassic Morrison Fm. of Colorado. Kirtlandia 43:3-26.
- 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.
Sauropod Vertebra Picture of the Week 
April 29, 2012 at 10:05 pm
Could this be a result of sexual dimorphism, where the females needed to reinforce the sacrum earlier than males?
April 30, 2012 at 1:26 am
It could be, but we have no reason to prefer that hypothesis to regular non-sexual-dimorphism intraspecific variation. Sacral fusion is variable in Apatosaurus (Riggs, 1904) and Camarasaurus–there’s a nice table on this in one of the papers in the Thunder-Lizards volume, IIRC the one by Virginia Tidwell,
Takehito Ikejiri, andKen Stadtman, and Allen Shaw on an old Cam pelvis*–so there’s no reason to assume it’s not variable in Haplocanthosaurus also. Seriously, compare any two sacra of a given genus and you’re likely to come up with some variation that doesn’t make sense. And this is just my gut impression, but from looking at Apatosaurus and Diplodocus the fusion of adjacent sacral spines seems to be especially variable.So sexual dimorphism might be the answer, but (1) I can’t think of any way to test that, and (2) this isn’t an anomaly that’s crying out for an explanation; the vanilla variation we see in other Morrison sauropods will do (although some of that might also be caused by sexual dimorphism).
* I had my wires crossed when I originally wrote this, and had confused the following two papers. Both papers have fairly alarming tables of characters that vary among Camarasaurus specimens, but the one that focuses on sacral variation is Tidwell et al., not Ikejiri et al. (although the latter does discuss some sacral characters).
Ikejiri, T., Tidwell, V., and Trexler, D.L. 2005. New adult specimens of Camarasaurus lentus highlight ontogenetic variation within the species; pp. 154-179 in Tidwell, V., and Carpenter, K. (eds.), Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press, Bloomington, 495 pp.
Tidwell, V., Stadtman, K., and Shaw, A. 2005. Age-related characteristics found in a partial pelvis of Camarasaurus; pp. 180-186 in Tidwell, V., and Carpenter, K. (eds.), Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press, Bloomington, 495 pp.
May 2, 2012 at 4:11 pm
Both specimens have a very brachiosaur-like configuration to the sacral spines. The type looks very similar to Giraffatitan in the angles and proportions as well as the fusion sequence. Then we have the very short ischial peduncle and the much longer pubic peduncle, again a feature which compares very closely to brachiosaurs. And the ilium being far more expanded in front of the hip socket than behind, is another brachiosaur-like trait.
I suspect that, far from being juveniles of some diplodocid (as Woodruff and Fowler suggest), haplocanthosaurs are basal brachiosaur cousins. Of course they can not be true brachiosaurs, but they do appear more brachiosaur-like than camarasaurs based on neck and dorsal morphology. The single neural spines on the neck also point in that direction, although in profile they speak of something more primitive than a brachiosaur. I’ve been critical of Fowler’s lumpery in the past when it was just limited to Torosaurus… but here it seems to stretch the limits of credulity.
May 2, 2012 at 4:38 pm
Well, I will agree that the Haplocanthosaurus holotype’s sacrum/ilium is less unlike that of a brachiosaur than that of a diplodocid. But I’m not convinced that the resemblance is particularly strong. Remember Janensch never illustrated an adult Giraffatitan sacrum, only some rather fragmentary juveniles. It’s interesting that in the sacrum of the Brachiosaurus holotype FMNH P25107, sacral spines 2 and 3 are fused along their length, but those of S1, S4 and S5 are all free. This, remember, in an animal that we think of as adult, certainly as regards neurocentral fusion in the dorsals.
Riggs (1904) in the big descriptive Brachiosaurus monograph referred Haplo to his new family Brachiosauridae; but that was on the basis of humeral length, number of dorsals, and unsplit neural spines, not the sacra or ilia. As we showed a couple of weeks ago it takes six more steps in the phylogenetic analysis of Taylor (2009) for Haplo to resolve as the sister taxon to Brachiosaurus, compared with 24-34 for the diplodocids. So, yes, that hypothesis is less unparsimonious than that of Woodruff and Fowler. Better still would be Camarasaurus which needs only two more steps.
But remember: Haplo is Haplo, not any of these other animals. It’s founded on a good, adult specimen that is pretty complete and very distinctive.
May 3, 2012 at 12:50 am
Some pedantry on Haplocanthosaurus morphology:
Then we have the very short ischial peduncle and the much longer pubic peduncle, again a feature which compares very closely to brachiosaurs.
Um, is there a sauropod for which this is not true? Hatcher’s comparative illustration does not show Haplo as having a longer public peduncle than the diplodocids–indeed, that of Diplodocus might be slighter longer, proportionally.
Of course [haplocanthosaurs] can not be true brachiosaurs, but they do appear more brachiosaur-like than camarasaurs based on neck and dorsal morphology.
In having unsplit neural spines, and fairly short, broad dorsal neural spines I agree. On overall form of the cervical vertebrae, I disagree. Camarasaurus cervicals are proportionally longer, not as long as brachiosaur cervicals but still longer than Haplo cervicals. Cam has long overlapping cervical ribs (at least in some specimens), like brachiosaurs but unlike Haplo. And, most interestingly to me, Haplo has crazy tall neural spines with deeply kinked spinoprezygapophyseal laminae in the posterior cervicals–see this image–which are very similar to the tall, kinked posterior cervical neural spines of diplodocids, other than not being bifid. In phylogenetic analyses, Haplo bounces around the base of Neosauropoda, coming out sometimes as a basal diplodocoid and sometimes as a basal macronarian. If I had to bet one way or the other, given the posterior cervical neural spine shape and the short cervical ribs, I’d say it’s a diplodocoid, but it will probably require more and better material to say what it is with any more confidence.
And on Brachiosaurus:
It’s interesting that in the sacrum of the Brachiosaurus holotype FMNH P25107, sacral spines 2 and 3 are fused along their length, but those of S1, S4 and S5 are all free. This, remember, in an animal that we think of as adult, certainly as regards neurocentral fusion in the dorsals.
True as far as it goes, but don’t forget that it also has an unfused scap-coracoid joint, just like the mounted Giraffatitan. That individual is 13% smaller than the biggest known Giraffatitan individuals–I wonder if the same is true of the Brachiosaurus altithorax holotype relative to fully grown brachiosaurs?
May 3, 2012 at 2:15 am
[…] Mike will straight-up murder me if I post from Chicago without it, here’s your friendly local not-quite-fully-mature mounted holotype specimen of […]
May 3, 2012 at 7:08 am
Matt points out:
Or indeed as a camarasauromorph macronarian (Upchurch et al. 2004) or a non-neosauropod (Upchurch 1999).
(If it seems to bounce wildly around the tree, that’s an illusion. Although it can be recovered as a member of different “major clades”, all it’s really doing is wandering around close to the diplodocoid/macronarian divergence point.)
May 3, 2012 at 7:14 am
Of course. That’s why I wrote “… that we think of as adult”.
Dude, it’s like you only read Taylor 2009 once and hardly memorized any of it. From page 804:
May 3, 2012 at 4:43 pm
These are all excellent points. Still I can’t escape the fact that the Haplo holotype sacrum looks an awful lot like B. altithorax in the spinal configuration, and H. utterbacki’s sacrum looks more camarasaur-like. Haplo is just one of those weird animals that I wish were known from more ontogenic stages.
May 3, 2012 at 5:25 pm
Haplo is known from more ontogenetic stages than the great majority of sauropods!
May 3, 2012 at 9:57 pm
And there is the Haplocanthosaurus specimen in the utah Field House etc. museum still needing description…
So there are at least four specimens of this genus.
LeeB.