Nesbitt (2011) wrote,
“Vancleavea stands as one of the most bizarre archosauriforms recorded to date.”
Figure 1. Vancleavea skeleton, sans osteoderms. Closeup of skull, manus and what little is known of the pes. Covered by osteoderms, the number of dorsal vertebrae is a guess based on sister taxa.
Vancleavea campi (Long and Murray 1995, Parker and Barton 2008, Nesbitt et al. 2009) Late Triassic,~210 mya, ~1.2 meters in length, was originally considered a very weird archosauriform close to Doswellia, Turfanosuchus, Chanaresuchus and Erythrosuchus. Never mind that some of these taxa are not even sisters (see chart below). And never mind that Vancleavea lacked the hallmarks of the clade: supratemporal, antorbital and mandibular fenestrae (see above). Nesbitt et al. (2009) made one mistake by assuming Vancleavea was an archosaur. Given an opportunity to nest elsewhere, it did.
In a Larger Study Vancleavea Nests as a Thalattosaur Close to Helveticosaurus
Unfortunately Nesbitt et al. (2009) did not even attempt to compare Vancleavea to Helveticosaurus, with which it now nests. The robust posterior process of the jugal does resemble that of Erythrosuchus and Chanaresuchus, but it’s the same in Helveticosaurus. The huge teeth had proportions similar to those of Erythrosuchus, but it’s the same in Helveticosaurus. The elevated naris nested Vancleavea with Chanaresuchus. Even so, we find a better match here and elsewhere in the overall morphology with Helveticosaurus. And parsimony rules. All it needs is a level playing field without exclusions (a tip of the cap to Branch Rickey and Mike Taylor).
Figure 2. Helveticosaurus, a Vancleavea sister.
So Then, What Is Helveticosaurus?
Wikipedia reports that Helveticosaurus was classified as a placodont, but it lacks traits common to that clade. The large study (Figure 3) nested Helveticosaurus in the thalattosaurs, a clade of widely varying morphology, especially in the skull. So Vancleavea turns out to be a very distinct, yet kind of a weird thalattosaur. The nesting of Vancleavea is also a prime example of what happens when the gamut of the inclusion set is made to sit on the bus while the other players get to play. As in pterosaurs. As in mesosaurs. As in caseasaurs. As in Tetraceratops, etc. etc.
The Value of First-Hand Observation Was Tested Here
I have not studied the fossil first hand, but this is also an example of what can be done with the photographs and descriptions that appear in a paper. Sometimes you don’t have to have the specimen in front of you in order to make a contribution. What Nesbitt et al. (2009) lacked was a comprehensive dataset for the family tree of the Reptilia to nest their enigma. What they had was a preconceived inclusion set. The lesson here is: If “Vancleavea stands as one of the most bizarre archosauriforms recorded to date,” then maybe, just maybe it’s not an archosauriform.
Vancleavea is clearly distinct, even among the very weird thalattosaurs. Then again, every known thalattosaur is distinct. The gamut of morphologies within this clade hints at a much broader range yet to come. Learn more here.
Figure 3. Click to enlarge. Complete resolution in a 238 taxon Reptile family tree, with Vancleavea nesting with Helveticosaurus and Miodentosaurus in the Thalattosauriformes.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References:
Long RA and Murry PA 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4: 1–254.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships ofVancleavea 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.
The Pterosaur Heresies 