Speculative Marine Reptiles I

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The Triassic was full of all kinds of strange and wonderful reptiles, most of which unfortunately went exitinct at the end of that period. Phytosaurs are one of those groups. Phylogenetically, they have gone back and forth between being true croc-line archosaurs or being more archaic archosauriforms that merely resembled modern crocodilians through convergent evolution. Although quite successful throughout the whole Triassic, they left no descendants into the Jurassic. Or did they?

Presented here is Iniasaurus, a speculative phytosaur that survived into the Early Jurassic. Some real life phytosaurs, such as Mystriosuchus or Diadongosuchus had already adapted towards a mostly marine life and, perhaps, these were not failed experiments as we would like to think. Descending from a Mystriosuchus-like ancestor, I imagine the line leading towards Iniasaurus surviving the mass extinction (as well as potential competition with pseudosuchians) by giving up the freshwater environments and increasingly adapting towards fully marine life, where the extinction of archaic sauropterygians and large ichthyosaurs has opened up new niches. Its tail has developed into a proper fluke and the hindlegs have become vestigial, only used during mating. Mostly unable to walk on land, it gives live birth, much like other marine archosauromorphs such as Dinocephalosaurus or the metriorhynchids.

Calling Iniasaurus a marine reptile is however no longer entirely accurate, as it has begun advancing back into the freshwater habitats from which its ancestors came. I imagine it like the Mesozoic equivalent of a river dolphin, swimming through murky rainforest rivers and estuaries in search of fish, which it catches with its gharialous jaws. Instead of echolocation it perhaps uses electroreceptors at the snout-tip to detect its prey. The clawed flippers come in handy when clambering over sunken treelogs, while the ancestral dermal armour makes for good protection against the theropods which now dominate the land.

Iniasaurus is accompanied by an (admittedly more fanciful) creature, another Triassic survivor. Neohescheleria (or alternatively Nothocadborosaurus if you want to be cheeky) is a rainforest thalattosaur. Its bizarre-looking skull is actually quite archaic and still close to that of its clam-eating predecessors such as Nectosaurus or Clarazia. Yes, their heads really were just that weird, which makes them all the more underappreciated. The derivation is instead observed in its body: Taking anguiliform swimming to a whole new degree, Neohescheleria has lost its forelimbs and swims much more like a sea serpent. Similar to the Cretaceous pachyophiid marine snakes, the hindlimbs still remain and are used in steering or giving quick escape-bursts. The serpentine trunk greatly helps in navigating the cluttered river environment and probing deep into crevasses and burrows where the animal can pick up and crush freshwater clams and crustaceans.

Like many bursts of my creativity, Iniasaurus is inspired by a new book I just read, Darren Naish's brand new Ancient Sea Reptiles. Particularly this short passage from page 80:

"On that note, it is surprising that phytosaurs only took to marine life twice, since they look ideally suited for it. Maybe the existence of during the Triassic of ichthyosaurs, sauropterygians and other groups prevented their movement into the marine realm."

What Naish is probably referring to here is the fact that phytosaurs had retracted their nostrils so far up their skull that they sat right in front of the eyes. Not too dissimilar from the blowhole of cetaceans, this would have been a perfect pre-adaptation for a fully marine life. Compare that with the Jurassic-Cretaceous marine crocs, which always had their nostrils at the snout-tip, which some have even singled out as a reason for their eventual extinction. It really makes you wonder why something like Iniasaurus never existed... or maybe it did. Rainforest environments are usually quite bad at preserving vertebrate fossils.

My fascination with thalattosaurs goes back a fair bit longer, in part because in the museum I work at we actually have some original fossils from Monte San Giorgio. If you want to learn more about them and other marine reptiles from the Ticino region, I can greatly recommend Mesozoic Sea Dragons by Olivier Rieppel.

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References:

• Butler, Richard; Jones, Andrew; Buffetaut, Eric, et al.: Description and phylogenetic placement of a new marine species of phytosaur (Archosauriformes: Phytosauria) from the Late Triassic of Austria, in: Zoological Journal of the Linnean Society, 187, 2019, p.198–228.
• Naish, Darren: Ancient Sea Reptiles. Plesiosaurs, Ichthyosaurs, Mosasaurs and More, Washington 2023.
• Rieppel, Olivier: Mesozoic Sea Dragons. Triassic Marine Life from the Ancient Tropical Lagoon of Monte San Giorgio, Bloomington 2019.