The margins of the long jaws were equipped with numerous sharp outward-pointing teeth, indicating a diet of fish and squid.

The Nothosauroidea has been suggested to consist of two suborders, the Pachypleurosauria, which are small primitive forms, and the Nothosauria (including two families Nothosauridae and Simosauridae), which may have evolved from pachypleurosaurs.

[4][5] Nothosaur-like reptiles were in turn ancestral to the more completely marine plesiosaurs, which replaced them at the end of the Triassic period.

The results of their phylogenetic analyses are shown in the cladogram below:[5] Placodus Pistosauridae Majiashanosaurus Hanosaurus Nothosauroidea Pachypleurosauria A 2024 description of a fossil nothosaur vertebra from the Anisian of New Zealand indicates that nothosaurs dispersed worldwide from their region of origin in the northern Tethys much earlier than presumed, eventually reaching the southern polar region of Panthalassa by the Middle Triassic.

This vertebra is the oldest sauropterygian fossil from the Southern Hemisphere, and appears to be from a taxon related to Nothosaurus and Lariosaurus.