Xenosuchus prognathus is NOT an Archosaur

Writer's Note: This is another post in the alternate timeline we have been calling the Xenopermian. Zach Miller of When Pigs Fly Returns and I have been hashing this out for some time. Our previous posts are located here, here and here. The Xenopermian is an alternate timeline where the Permian Extinction, more specifically the Permian-Triassic Mass Extinction, does not happen. What would have life been like through an extended Permian with a changing and evolving Paleozoic ecology.

That said, Zach and I are now exploring a a faux scientific controversy. This creature does not exist. It is not real. it is purely a product of our imaginations. Please do not start citing us as scientific literature for something doesn't exist. ok?

That said, we've been refering to the 'pseudocroc' for some time. The working name inspired how we'd introduce it to the TL and the public. Zach's post is here.

The scientific establishment has taken the view that the amniotes are a singular evolutionary event: all amniotes are evolved from a single common ancestor that developed the amniotic sack for protection against drying out the embryos prior to hatching. Our view is an alternate to this misguided belief. Our collective point of view is best summarized by its title: Paraphylitic Amniote Origination Theory or PAOT[1]. Our theory holds that the amniotic groups are all independently derived clades from basal reptilomorphs. Synapsida, Diapsida, Euryapsida, and Anapsida all independently evolved from reptilomorphs and that Amniota is a paraphyletic and not a natural group.

Finally, a fossil has arisen that supports our theory: Xenosuchus prognathus.

(figure a, Miller et al 2009)

Xenosuchus prognathus is from the Late Xenopermian of the Ural Sea/East European Sea deposits of Russia, Xenosuchus is a moderate to large sized semi-aquatic post-reptilomorph para-amniote. It's preferred environment seems to have been in the brackish to salt water swamps where competition with the reigning freshwater clades of large labrynthodonts, other temnospondyls, and reptilomorphs were less common, if not absent.

Like most fossil vertebrates, its postcranial material is extremely sparse and undiagnostic. However, occurrence through various strata of very similar remains of postcranial Xenosuchus like remains helps identify their transition from freshwater to moderate to highly saline and their differentiation from other reptilomorphs to paraamniotic characteristics.

The cranial materials are very good for their unique and easily-identifiable characters. They demonstrate that the "amniotes" are very much paraphyletic in their origin. There is one primary characteristic of the Xenosuchus skull: there are traces ridges that adorn the skull. Very worn and residual while the organism was alive, these ridges are only viewable under close scrutiny and only on portions of the skull. They are most evident on the premaxiilla, but can also be seen on the fragment of the lower dentary.

Observed Xenosuchus ridging.
(Figure a, Baird et al 2010)

These ridges are a derived trait of the temnospondyl clade, which are not considered by the mainstream to be ancestral to Amniota. Because this is a unique trait for the temnospondyls, this leads us to believe that the Xenosuchus lineage is derived directly from temnospondyls and only shares characteristics with Archosauria through parallel evolution. This lends overwhelming credibility to the idea that Xenosuchus fits our scenario of the “amniotes.” Xenosuchus is not a traditional archosaur nor a temnospondyl, but a derived postreptilomorph para-amniote.

Since our argument for Xenosuchus rests largely on the temnospondyl derived skull ridging, we must address why the ridging is only present on small parts of the cranial remains. Here we argue that this is due to two separate processes that are completely unrelated.

The first process is an evolutionary one. The xenosuchine lineage obviously loses the ridges completely by the beginning of the Paleozoic/Allozoic boundary, 200 mya. Our hypothesis is that the transition onto land reduced the necessity of the ridging which may have been used for nerves sensitive to water pressure differences to help with the capture of aquatic prey. Without the selection bias being for this sensory adaptation, eventually as the xenosuchids moved more and more terrestrial, this was lost. It should be noted that the Xenosuchus ridging differs from the standard temnospondyl pattern in that it is very fine grained and small such that if it was not being looked for, it would be overlooked. Xenosuchus is a rare find, a transitional fossil that it exhibits the more basal characteristics of its ancestry while generally being very derived.

The second process that reduced the ridging on the Xenosuchus cranial remains is preservational. Xenosuchus was discovered in sandstone and weathering and tumbling prior to the deposition would explain why the cranial ridging is present only in select, barely detectable locales in the fossil. Obviously, the skull tumbled in sand, possibly even the surf, prior to being laid to rest. However, we argue that the ridging is actually not a depositional artifact and that the deposition removed living organism’s skull ridging.

To test this hypothesis, we acquired multiple alligator skulls. We acknowledge that the alligators are a poor substitute, but there are few large paraamniote skulls that are of the appropriate shape. The gavial is a better approximation based on general morphology. However, our attempts to acquire sufficient crania were unsuccessful. Furthermore, the single skull we did acquire was simply too delicate for the experiment to proceed.

Meticulously, we used a laser engraver to etch in a pattern not unlike what we have detected in Xenosuchus. This took several attempts and ruined multiple skulls. Our success rate was 30%. Laser engravers are not generally meant to etch bone. The pattern covered the whole skull and was no more than 1mm deep and ¼ mm wide for the ridges and valleys.

Once we did get six successfully engraved skulls, we placed them in a tumbler with sufficient quantities of sand. The tumbler was made from a standard 55 gallon (214 liter) oil barrel. The tumbler was hooked up to an electric motor that provided 60 rpm for the tumbler chamber. One skull was placed with twenty-two pounds (ten kilograms) of sand and filled with water to capacity of water. The skull was left running in tumbler for 96 hours.

While not producing the exact same wear patterns as seen on Xenosuchus, our experiment produced not completely dissimilar results. The ridging was largely removed from the skull. We also found that the ridging that did remain was fairly similar to the cranial remains. We repeated the experiment five times with five alligator craniums and found that the results were very similar within an order of magnitude: sometimes the results were more thorough in removing the ridging. This removal was as much or more than the fossil. Other times it was less. We suspect that the differences are largely due to the variations in ridge depth and skull composition. No two skulls are exactly the same. Variations in size, shape, bone density and composition would all account for differing results.

One skull was placed in the tumbler until the ridging was completely and undetectably removed. The experiment did not run continuously. We interrupted the process every 24 hours to measure the removal of the ridging. The ridging was completely removed after 168 hours (7 days). This means that sometime between six and seven days the ridging was removed. In multiple instances, the skulls themselves were ruined. Often the jaws were smashed or the cranial remains were in a disarticulated state. This matched Miller et al’s observed state for the fossil cranial deposition. This constrains the time frame that the Xenosuchus remains were tumbled prior to deposition.

This experiment raises some interesting implications that support our hypothesis. The loss of ridging through depositional processes would indicate that more fossils may be of the para-amniote line than previous thought. More remains that have been cladisticly identified as archosaurs may in fact be of the Xenosuchine lineage. We feel that this deserves further, careful investigation. It could be that archosaurs as a whole are a paraphylitic, unnatural group and that this is not just constrained to the amniotes as a whole.

In any case, the discovery of the skull ridging on the Xenosuchus cranial fossil has uncovered a diagnostic feature that is constrained to the temnospondyls alone. This demonstrates that Xenosuchus was not an archosaur. This in turn demonstrates that “amniote” characteristics were acquired by multiple lineages independently and that amniotes are an unnatural group as such.

1. This is unfortunately pronounced as 'pout' by our detractors and adherents as 'Pouters'. We do not approve of this derogatory comment.

Note: this is a faux post as a work of science fiction.