Xenopermian Biota of the Ural Sea: Chronoperifronius thassalicus, a trematosaurine temnospondyl

The Xenopermian is a collaborative effort between Scott, Raven, Zach and myself to outline a very different, speculative world. In some ways this is not all that different than the exercises of Dougal Dixon, After Man and The New Dinosaurs. Rather than speculating on what the dinosaurs would be like if they had not gone extinct, much like his New Dinosaurs or the Spec World Project, or project into the future with After Man or The Future is Wild, our team asked the question of ‘what if the Permian Extinction did not happen?

This is the next post about the fauna of the Xenopermian in the Ural Sea region. We have talked about a ‘fossil’ and a faux controversy associated it with. We have talked about the geological staging differences in the XenoPermian timeline, and have even talked about the differences in the world in general under such a different period. We have generalized about the fauna, but now we want to get into specifics. In our first post, we talked about the first faunal member of the Xenopermian, Graviloricanasus roma, a pseudochelonid and very derived pareiasaur. Then we talked about Elyardia hensonii, a very derived anomodont. Then we talked about the alternate pterosaur, Maralae whittoni.

Today we leap from the amniotes to the nonamniotes.  Generally, the amniotes dominate the terrestrial ecology of the Xenopermian, but there are a number of interesting and important nonamniotes which participate.  Because there was no mass extinction, but rather a great evolutionary churn from the extended and muted Siberian Traps eruption.

Our first foray into nonamniote territory is Chronoperifronius thassalicus, a Xenopermian temnospondyl trematosaur.  What is a trematosaur?

Trematosaurs: the Marine Temnospondyls of the Permian, Triassic & Jurassic

The trematosaurs were a clade of temnospondyli.  Temnospondyls were one of the clades of what could be loosely called amphibians.  The two other are lissamphibians and lepospondyls.  The exact relationship between them is very contentious. 

Trematosaurs were a clade which originated in the Permian.  They were one of two clades of temnospondyls to survive the Permian Triassic Extinction in our time line.  They would diversify and even do something which is quite remarkable: they invaded the marine environment.  That's something not very common for tetrapods prior to reptiles.  Trematosaurs would become increasingly adapted to their new environment and would be excellent examples of parallel evolution relative to the crocodilians. 

It was thought the trematosaurs declined over the course of the Triassic and to be wiped out finally in the Triassic Jurassic extinction.  However, it was proven incorrect.  It seems the extinction failed to do the deed: postcranial skeletal fossils from a trematosaur were found in China from the late Jurassic.  If anything, its quite possible they may have survived longer still: the chigutisaurs (Koolasuchus) made it to the Aptian Cretaceous.

Trematosaurs of the XenoPermian

The XenoPermian didn't have the PT Extinction and this would cause no small ripple effects when it came to biota.  The increased numbers of temnospondyls is one of them.  In our case, in the Ural Sea Biota, Chronoperifronius thassalicus.

Chronoperifronius thassalicus - time despiser of the sea - is a marine, or rather beach dwelling, trematosaur.  The trematosaurs of our time line were piscovores.  C. thassalicus has started to return to the land or at least raiding the beach and the near shore, becoming a true carnivore.  With the increase of beach dwelling critters, such as the marululateans and the walrodonts, the edibles have increased from merely the washed ashore or whatnot.

C thassalicus takes two different approaches.  When it encounters a colony of beach dwelling therapsids, like those mentioned above, it attempt to lie in wait just past the wave break, not unlike how crocodiles would in freshwater of our time line.  When the colony would take to the sea, one or more would act in ambush to take a warm blooded therapsid.  However, during breeding seasons, when the therpasids have either pupped or laid eggs, C thassalicus will often charge the beach, hoping to scatter much of the colony to grab a pup or eggs.  

C thassalicus is starting to have competition though.  The archosaur, Xenosuchus prognathus, has moved east from what would be Russia and has successfully crossed to both sides of the Ural Sea.  Not as amphibious, but perfectly capable of swimming, has also taken to primarily predating the therapsid beach colonies.  This increases the pressure on the trematosaurs.  It will not, however, cause them to become extinct.  They largely go under during the X-J Extinction with some relic populations in the Arctic and Antarctic.  There they will survive until the K-T or rather K-Pg Extinction at 55.8 Million Years.

In the mean time, C thassalicus and her relatives will dispute the water ways with the crododilians.  The archosaurs and their relatives will be held at bay until the XJ Extinction, largely excluded from the waterways, but not totally.  One great casualty of the temnospondyl strength will be the 'fact' phytosaurs will never grade the waterways of the allohistorical Earth we describe.

One of the reasons is that C thassalicus and her marine relatives produced an interesting reproductive strategy: they birthed live young, fully formed, not unlike what frogs do with undersized juveniles, the "froglets." This allows the trematosaurs to no longer require returning to freshwater to breed.  Other trematosaurs would become thoroughly adapted to marine life, becoming limbless and become obligatory aquatic.    Producing greater sea monsters for a time, but eventually the purely aquatic forms losing out to hovasaurs and others with better respiratory systems at the XJ Extinction.

In the end, they will have made their mark and have given a good run, from the Mid XenoPermian until the end of the Mesozoic even if they do despise the passage of time.