What would Vancleavea be, if thalattosaurs were not known?

This is lesson 5 in taxon exclusion…
to see where select clades would nest in the absence of their proximal taxa.

Figure 1. Vancleavea surrounded by purported sister taxa as figured by Nesbitt and Wikipedia. None of these taxa share more traits with Vancleavea than does Helveticosaurus (Fig. 2), a taxon ignored before and since it was proposed here.

Traditionally,
(Nesbitt et al. 2009) aquatic Vancleavea (Figs. 1, 2) was nested with Doswellia and other archosauriforms (Fig. 1), but only in the absence of a long list of taxa including thalattosaurs.

By contrast,
as we learned seven years earlier, in the large reptile tree (LRT, 1242 taxa) Vancleavea nests with Helveticosaurus, as a derived thalattosaur. Vancleavea is so different from archosauriforms, that I wondered where it would nest in the absence of thalattosaurs? Would it nest with archosauriforms?

Figure 2. Vancleavea with its sister, Helveticosaurus. This connection might have been missed previously because the skull of Helveticosaurus is so badly smashed. These two share more traits in the LRT with each other than either does with any other taxon. Taxon exclusion prevented earlier workers from seeing this.

Having a wide gamut taxon list
permits us to ‘play’ with hypothetical scenarios like this.

If we restrict potential candidates to the Archosauromorpha
(in order to keep calculation time down) then Vancleavea nests with a wide variety of basal sauropterygians. The absence of thalattosaurs (except Vancleavea) creates this loss of resolution. However, the absence of Vancleavea, too, returns complete resolution to basal sauropterygians, as in the complete analysis.

If we restrict potential candidates to Arcosauromorpha sans Enaliosauria (ichthyosaurs + plesiosaurs)
then Vancleava nests with the first outgroup taxon to the Enaliosauria, Anarosaurus.

Figure 3. Anarosaurus nests at the base of the Enaliosauria (= Sauropterygia + Ichthyosauria)

If we restrict potential candidates to the Lepidosauromorpha
(sans Archosauromorpha) then Vancleavea nests with Lanthanosuchus (Fig. 3).

Figure 4. The skull of Lanthanosuchus in several views and colorized.

In none  of these tests
does Vancleavea nest with any Archosauriformes (Fig. 1). Once again, a biased, cherry-picked and restricted taxon list missed several opportunities/possibilities that the LRT includes due to its wide gamut of tested taxa. In 2018 many workers are still including Vancleavea in archosauriform studies. Why? That ‘by default‘ nesting due to cherry-picked taxa needs to stop immediately. There are many better candidates out there for sister taxa. Sprinkle a few thalattosaurs in there and see where Vancleavea nests when given the opportunity.

Taxon exclusion
has been the number one problem in traditional paleontology. That’s why this blogpost exists. That’s why the LRT includes such a wide gamut of taxa. The result is a minimizing of taxon exclusion and the problems that attend it.

References
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean Society 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.
Parker WG and Barton B 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica 11 (3): 20p.

wiki/Vancleavea