Thalattoarchon is a genus of large, Middle Triassic predatory ichthyosaur from North America, containing the single species T. saurophagis.

Thalattoarchon would have been an apex predator, its sharp and cutting teeth suggesting that it would have mainly preyed on contemporary marine reptiles.

The only specimen of Thalattoarchon was discovered in 1997 by the paleontologist Jim Holstein during a field expedition in the Augusta Mountains, located in Nevada, United States.

[3] The specific name also comes from Ancient Greek and is derived from the words σαῦρος (saûros, "lizard") and φᾰγεῖν (phageîn, "to eat"),[1] meaning "lizard-eater", in reference to its carnivorous diet.

The frontal bones are smaller, forming the anteromedial edge of the superior temporal fenestrae and surrounding the pineal foramen.

In the lower jaw, the dentary bone extends to the level of the maxillae but apparently lacks teeth on its rearmost part.

The teeth represent one of the main autapomorphies of Thalattoarchon, being large and thin, having two sharp edges each and a fairly smooth crown.

However, the latter are characterized for having a flattened ventral surface and articular facets for the double-headed ribs, a trait also observed in some basal ichthyosaurs and in neoichthyosaurians dating from the Late Triassic to the Cretaceous.

The hind limbs and the pelvic girdle are proportionally small in relation to the body of the animal, with just the length of the femur being twice the height of the caudal vertebrae.

[1] In the phylogenetic analyses published by Fröbisch and colleagues in 2013, Thalattoarchon was classified as a basal representative of the Merriamosauria clade, being seen as more derived than Cymbospondylus and the Mixosauridae.

[1] However, in 2016, a phylogenetic revision carried out on the ichthyopterygians by Cheng Ji and his colleagues moved Thalattoarchon within the Cymbospondylidae, i.e. in a more basal position than the Merriamosauria as proposed three years earlier.

These positions varied, indicating that Thalattoarchon might be more closely related to later-evolving ichthyosaurs, situated earlier in the lineage than Cymbospondylus, or classified as the sister taxon to C.

Its position as an apex predator is comparable to that of current orcas, because its cutting teeth would have been adapted to attack prey of considerable size, possibly even larger than Thalattoarchon itself, in the same way as killer whales.

[1][2][3][5] However, large ichthyosaurs like Cymbospondylus would not have been Thalattoarchon's primary target; the generally preferred prey would be medium-sized marine reptiles or juveniles instead.

[1] In 2021, Sander and his colleagues suggested that if Thalattoarchon would have fed on Cymbospondylus, it would have preyed on it using a pack-hunting strategy, a behavior that is not at all reported in the fossil record.

The authors of this study also suggest that Thalattoarchon would have only fed on smaller animals, both alive and dead, a feeding strategy comparable to that of the great white shark.

[4] All of these characteristics combined with the datatation of this taxon suggested that Thalattoarchon would have been the oldest aquatic megapredator tetrapod known in the fossil record.

However, it is possible that the diversification of large ichthyosaurs similar to Thalattoarchon took place during contemporary periods, when marine biodiversity was almost completely recovering from the Permian–Triassic extinction event.

[4] During this period, the Fossil Hill Member represented the eastern part of the Panthalassan Ocean, and the proven presence of archosaurs like Benggwigwishingasuchus shows that the region would have been coastal.