Although ontogeny could explain some of the non-diplodocid characters in Suuwassea, it is important to note several things. First, the postparietal foramen used to place Suuwassea in Dicraeosauridae is also found in the diplodocine diplodocid Kaatedocus siberi (Tschopp and Mateus 2013) and the indeterminate flagellicaudatan braincase MB.R.2387 (Remes 2009). Other putative dicraeosaurid synapomorphies of Suuwassea listed by Whitlock (2011) (sharp sagittal crest on supraoccipital) are also present in Kaatedocus. Given the diplodocid placement of Kaatedocus and the uncertain status of MB.R.2387 within Flagellicaudata, the presence of a postparietal foramen in both Suuwassea and Kaatedocus appears to be a case of convergent evolution because, as pointed by Tschopp and Mateus, the diplodocid
Even if the holotype of Suuwassea were subadult, it would still be a distinct species judging from available evidence above. It may take future discoveries to confirm or refute the hypothesis by Woodruff and Fowler (2012) regarding the validity of Suuwassea.
Update: The landmark revision of Morrison diplodocid alpha-taxonomy by Tschopp et al. (2015) indicates that some putative dicraeosaurid characters of Suuwassea (postparietal foramen, sharp sagittal crest on supraoccipital) are also found in the diplodocid Galeamopus, but recovers Suuwassea as dicraeosaurid.
Remes, K., 2009. Taxonomy of Late Jurassic diplodocid sauropods from Tendaguru (Tanzania). Fossil Record 12: 23–46.
Emanuel Tschopp & Octávio Mateus, 2013. The skull and neck of a new flagellicaudatan sauropod from the Morrison Formation and its implication for the evolution and ontogeny of diplodocid dinosaurs. Journal of Systematic Palaeontology 11 (7): 853-888, DOI: 10.1080/14772019.2012.746589
Whitlock, J. A. 2011. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society 161: 872–915.
Wilson, J.A., and Smith, M., 1996. New remains of Amphicoelias Cope (Dinosauria: Sauropoda) from the Upper Jurassic of Montana and diplodocoid phylogeny. Journal of Vertebrate Paleontology 16 (3 Suppl.): 73A.
Woodruff, C. & Fowler, D. W. 2012. Ontogenetic influence on neural spine bifurcation in diplodocoidea (Dinosauria: Sauropoda): A critical phylogenetic character. Journal of Morphology 273: 754–764.