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.
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.
- 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.