|Woodruff & Fowler 2012|
WOODRUFF, D.C., & FOWLER, D.W. (in press) Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology. [earlyview pdf] for a copy Email me.
Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of the three subclades Rebbachisauridae, Dicraeosauridae, and Diplodocidae is strongly influenced by a relatively small number of characters. Neural spine bifurcation, especially within the cervical vertebrae, is considered to be a derived character, with taxa that lack this feature regarded as relatively basal. Our analysis of dorsal and cervical vertebrae from small-sized diplodocoids (representing at least 18 individuals) reveals that neural spine bifurcation is less well developed or absent in smaller specimens. New preparation of the roughly 200-cm long diplodocid juvenile Sauriermuseum Aathal 0009 reveals simple nonbifurcated cervical neural spines, strongly reminiscent of more basal sauropods such as Omeisaurus. An identical pattern of ontogenetically linked bifurcation has also been observed in several specimens of the basal macronarian Camarasaurus, suggesting that this is characteristic of several clades of Sauropoda. We suggest that neural spine bifurcation performs a biomechanical function related to horizontal positioning of the neck that may become significant only at the onset of a larger body size, hence, its apparent absence or weaker development in smaller specimens. These results have significant implications for the taxonomy and phylogenetic position of taxa described from specimens of small body size. 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. Our findings emphasize the view that nonmature dinosaurs often exhibit morphologies more similar to their ancestral state and may therefore occupy a more basal position in phylogenetic analyses than would mature specimens of the same species. In light of this, we stress the need for phylogenetic reanalysis of sauropod clades where vital characters may be ontogenetically variable, particularly when data is derived from small individuals.
POST-PUBLICATION AUTHOR NOTES
Some corrections to the discussion / conclusions: In our conclusions we state:
"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."
Post-publication, online discussion has proposed that this is incorrect, and I acknowledge this. Our original wording was sloppy and does not convey our / my meaning, thus some clarification is required. Contra our phrasing, the phylogenetically unstable taxon Haplocanthosaurus is unlikely to be sunk into Apatosaurus or Diplodocus. Our meaning is that ontogenetic and statigraphic analysis of the characters that diagnose particular taxa within dipodocoidea (particularly those described based on immature holotypes) may significantly alter the structure of the phylogenetic tree. Further, isolated specimens currently attributed to a given taxon may instead turn out to be ontogenetic stages of a different taxon (without sinking the original designated taxon), the misassignment due to heterochronic shifts that are only recognizable with stratigraphic and ontogenetic analysis.
This admission does not change the original finding that birfucation proceeds through ontogeny. This hypothesis is easily falsifiable by the discovery of a non-mature diplodocoid (or camarasaurid) that exhibits full adult-level development of bifurcation. Despite hundreds of known immature diplodocoid / camarasaurid vertebrae being found, none studied by us show this condition.
We welcome the opportunity to respond to criticisms via a peer-reviewed comment/reply on the subject, but do not see non peer-reviewed (and often incorrect) blog postings as an appropriate venue.
Additional image: An image of MOR592 is available on FlickR. One of the key specimens from the study is an immature diplodocoid from the Morrison Fm, that shows less-developed bifurcation than seen in adults. The specimen comprises 4 dorsal vertebrae and 11 cervical vertebrae (it is missing 4 cervical vertebrae, and 6 dorsals).