The Reinaut Formation in the south of the continent called Guralta is one of, if not the most extensive fossil site from the late Bobossic. Dated to have been formed between 125 and 122 million Eryobian years ago, the Reinaut Formation offers a glimpse of what life was like in the Kikilian, the latest stage of the Bobossic right before the devastating mass extinction dubbed “The World Scarring” happened.
Back when it was formed, the Reinaut Formation was likely a subtropical or temperate open woodland subject to seasonal rains. The fossils discovered in the Reinaut Formation so far, have mostly been megafaunal animals which tended to fossilise better than smaller animals in the conditions that were present in Reinaut 122 million years ago.
Reinaut does not feature many animals with recognisable modern relatives, so when a fossil of a an apparently three-jawed creature was discovered, we were quite surprised. Reinauspathosteus grandis is one of the oldest Trapezostomatan Anisospondyls we have found so far and is by far the largest pre-Thyellian Trapezostome. Not only that, but its skull anatomy and possible position of the nares suggests it might even have been a Liomedactylomorph, rather closely related to the ancestor of the majority of modern Trapezostomes.
The third “jaw” is actually a bony extension of the caecal operculum and was quite well developed in Reinauspathosteus. The tip was shaped like a spade and likely was used to uproot and to scrape the bark off plants.
Growth patterns in its bones hint at an elevated metabolism already being present in Reinauspathosteus, suggesting that mesothermy was ancestral to Liomedactylomorpha as a whole before the World Scarring occurred. While no filaments have been discovered in Reinaut, the fact that many Trapezostomes immediately following the World Scarring sported coats of filaments could mean that Reinauspathosteus itself was also, at least partially, covered in hair.
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