Impressions on validity of Suuwassea

In a paper discussing ontogeny in the neck vertebrae of diplodocids, Woodruff and Fowler (2012) questioned the validity of Suuwassea emilieae and its placement in Dicraeosauridae. For example, they noted that several characters used to place the genus in Dicraeosauridae (tall cervical neural spines and an anterior prominence at the dentary symphysis) are either symplesiomorphic or also found in the juvenile specimen MOR 592 (referred to Amphicoelias sp. by Wilson and Smith 1996). Moreover, the lack of scapulocoracoidal fusion, the slight bifurcation of the cervical neural spines, and the elongation of the foot bones are used by the authors to point to the immature growth status of ANSP 21122. Therefore, Woodruff and Fowler concluded that Suuwassea might be a possible juvenile form of one of other diplodocoids from the Late Jurassic Morrison Formation.

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 Diplodocus skull CM 11255 lacks such a foramen and Suuwassea and Dicraeosaurus have a postparietal foramen smaller than that of Kaatedocus and MB.R.2387.

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 (e.g. sharp sagittal crest on the supraoccipital) are also found in the diplodocid Galeamopus, but agrees with Whitlock (2011) that Suuwassea is a dicraeosaurid. Whitlock and Wilson Mantilla (2020) also report a postparietal foramen for the newly redescribed Morrison diplodocoid taxon Smitanosaurus (formerly "Morosaurus" agilis) and recover Kaatedocus as a dicraeosaurid rather than a diplodocid. Therefore, a postparietal foramen is clearly a non-ontogenetic trait exclusive to Dicraeosauridae.

Remes, K., 2009. Taxonomy of Late Jurassic diplodocid sauropods from Tendaguru (Tanzania). Fossil Record 12: 23–46.

Tschopp, E. & Mateus, O., 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.

 

Whitlock, C. & Wilson Mantilla, J., 2020. The Late Jurassic sauropod dinosaur 'Morosaurus’ agilis Marsh, 1889 reexamined and reinterpreted as a dicraeosaurid. Journal of Vertebrate Paleontology  DOI: 10.1080/02724634.2020.1780600