Vancleavea occiput and comments

The original Vancleavea paper (Nesbitt et al. 2009) illustrated the occiput of Vancleavea (Fig. 1), but from oblique angles of the in situ scattered bones. The skull was removed from the matrix, but the occiput was not shown in straight-on, reconstructed occipital view. Here, trying to remedy that, I attempt to reconstruct the Vancleavea occiput from the published data.

Vancleavea is a derived thalattosaur, close to Helveticosaurus and Eusaurosphargis which, due to their crushed and scattered fossils, provide few clues to guide the reconstruction. In fact thalattosaurs as a group provide few clues as to how the occiput blueprint comes together. So, even if this reconstruction is correct, there is little to nothing to compare it to.

Figure 1. Vancleavea occiput gleaned from several oblique views of cranial elements.

Figure 1. Vancleavea occiput gleaned from several oblique views of cranial elements. If you have better data, please send it along.

The Vancleavea occiput
is dominated by a large supraoccipital plate bordered by post parietals fused to the roofing parietals. It’s fairly solid with a single opening for the spinal chord above the single-headed occipital condyle and paired openings on either side of the condyle, set not much higher than the ventral quadrates.

Not (m)any thalattosaur occiputs are known to compare this to.

This is also a great time
to respond to David Marjanovic’s notes on why Vancleavea is not a thalattosaur published in the Dinosaur Mailing List here.

One key to understanding the several arguments offered by Marjanovic is the nesting of Vancleavea with the odd thalattosaur sister to Vancleavea, Helveticosaurus and Eusaurosphargis, which are not even considered by Marjanovic. He compares Vancleavea to Askeptosaurus and Miodentosaurus among others.

The other key to understanding is the identification of several breaks in the skull of Vancleavea that look like sutures and vice versa. Remember, Vancleavea was described as the oddest of all archosauriformes in that it lacked an antorbital fenestra, upper temporal fenestrae and mandibular fenestra, among several other noteworthy traits otherwise common to archosauriforms. So the cracks that fit that interpretation were identified as sutures. David Marjanovic’s comments are identified as DM. I am DP.

DM: Judging *Vancleavea* from the illustrations here [link no longer active]: Thalattosaurs retain the supratemporal. *Vancleavea* lacks it.

DP: The supratemporal of Askeptosaurus is a vestige and that bone is also indistinct or lacking in Eusaurosphargis and Helveticosaurus. So indeed Vancleavea lacks the supratemporal, as does its closest sister taxa.

DM: – Thalattosaurs have an open lower temporal arch (the caudal process of the jugal doesn’t reach very far back, so the lower temporal fenestra is open ventrally); I’m not even sure if they have a quadratojugal at all (none is in any case preserved in *Miodentosaurus*). It’s closed in *Vancleavea*, the qj is large, and the caudal process of the j is very broad dorsoventrally, much unlike the very delicate T-shaped jugals of thalattosaurs.

DP: Helveticosaurus likewise does not have an open lower temporal arch, the quadratojugal is large and the caudal process of the jugal is very broad dorsoventrally. Eusaurosphargis is too scattered to determine but my guess, based on the brevity of the jugal posterior process, is the quadratojugal is a trait shared by only Helveticosaurus and Vancleavea.

DM: – The maxilla contacts the naris in thalattosaurs. In *V.*, they are separated by a huge caudodorsal process of the premaxilla.

DP: This is a misinterpretation based on a bone break, coincidentally on both side of the skull. These are not sutures. The actual sutures are visible at the bases of this purported process on both sides. On the same subject, Nesbitt nested Vancleavea between Erythrosuchus, which has these caudodorsal process of the premaxilla, and Chanaresuchus + Tropidosuchus, which do not.

DM: – Thalattosaurs have a very large pineal foramen. *V.* lacks any trace of it.

DP: The size of the pineal foramen varies within thalattosaurs, larger in Anshunsaurus and Miodentosaurus, smaller in Endennasaurus. Nesbitt et al. 2009 overlooked the pineal foramen in Vancleavea, perhaps because the parietals are split apart.

Figure 1. Vancleavea pineal foramen.

Figure 2. Vancleavea pineal foramen.

DM: – Thalattosaurs retain caudal ribs — real free mobile ribs on the first few tail vertebrae. Can’t see such a thing in the low-resolution reconstruction of *V.*

DP: In like fashion Helveticosaurus and Eusaurosphargis also lack caudal ribs. Askeptosaurus has four. Miodentosaurus has two. So these taxa show this trait gradually disappearing.

DM: *V.* shares all these character states with all other archosauriforms, except for the thin jugal of… maniraptoriforms or something. Both, however, have a single bone called the “postorbital” in *V.* and the “postfrontal” in thalattosaurs…

DP: Indeed this bone convergently results from the fusion of the postorbital and postfrontal, not the disappearance of one or the other, which remain separate in Askeptosaurus, fused in Helveticosaurus and Vancleavea, separate in Erythrosuchus, fused in Tropidosuchus.

DM: The quadrate looks similar, too, differing from the pillar dinosaurs have, but I’m not familiar with enough diapsid quadrates to tell what that means.

DP: The quadrate leans anteriorly like most thalattosaurs and archosauriforms, like Erythrosuchus.

DM: – In thalattosaurs the rostral margin of the orbit is formed by the maxilla. In *Vancleavea*, the prefrontal does that, and (perhaps because the orbit is so remarkably small) it even contacts the jugal, excluding the maxilla from the orbit altogether. Or is the “prefrontal” of *V.* actually the lacrimal, which is altogether missing in thalattosaurs (or at least *Miodentosaurus*)?

DP: It appears the lacrimal is tiny, at the base of the prefrontal in Vancleavea. Similar structure in Helveticosaurus, reduced from the larger bones in Eusaurosphargis and Miodentosaurus.

DM: – *V.* has very long hemal spines. Thalattosaurs never.

DP: These spines are longer in Anshunsaurus and even longer in Helveticosaurus.

DM: – *V.* has considerably smaller limbs, especially hands & feet, than thalattosaurs.

DP: Vancleavea does have smaller limbs than other related thalattosaurs. Askeptosaurus, Concavispina and Xinpusaurus, not closely related to Vancleavea, also have smaller limbs.

DM: Finally, *V.* is much smaller than *Miodentosaurus*. This means that things like the relatively much smaller orbit of *V.* (complete with taller jugal and maxilla) or its relatively much shorter extension of the jaw joints caudal to the occiput cannot be size-related — the opposite relationship would be expected.

Figure 3. The closest kin of Vancleavea recovered by the large reptile tree. Askeptosaurus, Vancleavea and Helveticosaurus, all thalattosaurs unrelated to archosauriforms.

Figure 3. Askeptosaurus, Vancleavea and Helveticosaurus for comparison and to illustrate points brought out in the text above.

DP: Helveticosaurus also has a similar small orbit.

Okay,
so Vancleavea is not like the majority of thalattosaurs. It’s like Helveticosaurus, which is also distinct from the majority of thalattosaurs, but even more unlike any of the 359 other reptiles tested in that matrix.

And selecting even several distinct traits from a list of 228 is like pulling a Larry Martin (famous for challenging birds-are-dinosaurs people to name one, just one trait, that links birds to dinosaurs). Everyone who deals with phylogenetic analysis knows there can be massive convergence and it is the suite of traits that nests taxa, not just a few or even a dozen traits.

Phylogenetic analysis
Vancleavea nests with Helveticosaurus. Removing Helveticosaurus and all the thalattosaurs does not change the remaining tree topology. Removing all the enaliosaurs does shift Vancleavea and also the proterosuchid Fugusuchus next to the choristodere Doswellia, not far from Erythrosuchus. But that takes a lot of deletion to make this work.

References
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.

wiki/Vancleavea

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