Shonisaurus popularis vs. ‘Shonisaurus’ sikanniensis

Earlier we looked at the mistaken renaming of ‘Shonisaurus sikanniensis’ by Sander et al. 2011 to Shasatasaurus sikanniensis. S. sikanniensis and Shastsaurus don’t nest together, and share relatively few traits, so they can’t be the same genus.

Nicholls and Manabe (2004) described Shonisaurus’ sikanniensis (Fig. 1) as a 21m monster, the largest known ichthyosaur.

Figure 7. The giant sixth putative Shastasaurus, S. sikanniensis.

Figure 7. The giant sixth putative Shastasaurus, S. sikanniensis.

Unfortunately
their scale bars (Fig. 1) don’t confirm that length, but suggest one closer to 18 meters. That includes the 1 meter of missing distal tail they presume.

Worse yet
‘Shonisaurus sikanniensis’ shares very few traits with Shonisaurus popularis (Camp 1976, 1080, Kosch 1990; Fig. 2), the holotype for the genus. S. popularis nests with Guizhouichthyosaurus. S. sikanniensis nests with Cymbospondylus and YGMR SPC V3107, a specimen formerly attributed to Shastasaurus linagae by Sander et al. 2011. Like   S. sikanniensis, the 3107 specimen has a skull twice as wide as tall and a large orbit.

Figure 2. Shonisaurus populars compared to 'Shonisaurus' sikanniensis to scale.  Note the distinct skull and pectoral girdle morphologies.

Figure 2. Shonisaurus populars compared to ‘Shonisaurus’ sikanniensis to scale. Note the distinct skull and pectoral girdle morphologies. Click to enlarge. The torso is not so deep in S. popular is when they are angled back, as shown in most skeletons.

Interestingly,
no teeth are found in adult Shonisaurus popularis, only juveniles. Both Shonisaurus have expanded rib tips. Both are giants. Both may be toothless as adults.

Figure 3. Two ichthyosaurs once considered Shastasaurus suction feeders. The 3107 specimen nests with S. sikanniensis and both taxa need a new genus name. The 3108 specimen is very primitive and nests with Mikadocephalus.

Figure 3. Two ichthyosaurs once considered Shastasaurus suction feeders. The 3107 specimen nests with S. sikanniensis and both taxa need a new genus name. The 3108 specimen is very primitive and nests with Mikadocephalus.

I’m not sure how
and why my trees differ in detail from previously published work, but during the course of this study I’ve found prior data that did not agree with one another. So, evidently the data can be interpreted more than one way. And too often, I’m stuck with using published tracings as data without a photo to confirm. On the other hand, we’re in close agreement on many taxa and sister taxa recovered by the large reptile tree do resemble one another and make sense with regards to evolutionary patterns. Putting the reconstructions together, side-by-side, continues to be an important way to uncover prior and current mistakes.

Figure 4. Cladogram with Shonisaurus popular is added. Bootstrap scores shown.

Figure 4. Cladogram with Shonisaurus popular added. Bootstrap scores shown. Note the two Shonisaurus specimens do not nest together, nor do they share many traits.

References
Camp CL 1976. Vorläufige Mitteilungüber grosse Ichthyosaurier aus der oberen Trias von Nevada. Österreichische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Sitzungsberichte, Abteilung I 185:125-134.
Camp CL 1981. Child of the rocks – The story of the Berlin-Ichthyosaur State Park. Nevada Bureau of Mines and Geology, Special Publication 5, 36 pp.
Kosch, BF 1990. A revision of the skeletal reconstruction of Shonisaurus popularis (Reptilia: Ichthyosauria). Journal of Vertebrate Paleontology 10 (4): 512.
Nicholls EL, Manabe M 2004. Giant ichthyosaurs of the Triassic – a new species of Shonisaurus from the Pardonet Formation (Norian: Late Triassic) of British Columbia. Journal of Vertebrate Paleontology 24 (3): 838–849.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480

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