Askeptosauroids were endemic to the Tethys Ocean, their fossils have been found in Europe and China, and they were likely semiaquatic fish eaters with straight snouts and decent terrestrial abilities.

[2] The largest species of thalattosaurs grew to over 4 meters (13 feet) in length, including a long, flattened tail utilized in underwater propulsion.

[8][9] Thalattosaurs have moderate adaptations to marine lifestyles, including long, paddle-like tails and slender bodies with more than 20 dorsal vertebrae.

Thalattosauroids also have tall neural spines on their neck, back, and especially the tail vertebrae, increasing the surface area for swimming via lateral undulation.

Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a short tibia and fibula, with the latter expanding near the ankle.

This leads to an unusual trait that is characteristic of thalattosaurs, where the left and right nasal bones are separated from each other and restricted to a small portion of the snout near the nares.

Thalattosauroids seemingly spent all of their time in the water, with short, wide limbs, poorly developed wrist and ankle bones, and tall vertebrae adapted for swimming via lateral undulation.

Unlike these other marine reptiles, there is no evidence that thalattosaurs fully adapted to a pelagic life out in the open ocean, and instead they probably all lived in warm waters close to the coast.

[15][18] Gunakadeit's slender teeth correlated with the "Pierce II" guild of Massare (1987), indicating it likely fed on soft, fast-moving fish and squid.

The eastern portion of Pangaea was incised by a massive tropical inland sea, the Tethys Ocean, which extended all the way from China to Western Europe.

[20] Although the sister group to Thalattosauria is still debated, one possibility, the icthyosauromorphs, seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier.

[9] The oldest known thalattosauroids (Thalattosaurus, Paralonectes, and Agkistrognathus of British Columbia's Sulphur Mountain Formation) lived in eastern Panthalassa, along what is now the western coast of North America.

[1][21] However, this is based on the hypothesis that Nectosaurus (from California), Xinpusaurus (from China), and an unnamed species from Austria formed a clade basal to other thalattosaurs, a classification scheme which contrasts with many other studies.

[11] Evidently thalattosaurs were capable of dispersing throughout major marine regions multiple times before the group's extinction, with thalattosauroids likely more prolific at spreading than askeptosauroids due to their greater aquatic adaptations.

Based primarily on the overall skull shape, it was hypothesized to have been close to the reptile order Rhynchocephalia, which includes Sphenodon (the living tuatara).

[24] Further discussion by Merriam (1905) considered a relationship with ichthyosaurs due to their similar ecology, but questioned why their skull and vertebral anatomy would diverge so widely if they had a close common ancestor.

[5] Rieppel (1998)'s re-evaluation of the thalattosaur-like pachypleurosaur Hanosaurus argued that thalattosaurs have affinities with the aquatic reptile order Sauropterygia, which itself is aligned with turtles within an expansive interpretation of Lepidosauromorpha.

However, cladograms generated by these analyses change in unpredictable ways through alterations to their methodology (such as including or excluding aquatic adaptations or switching between parsimony and bayesian inference), leading some to have concerns over the validity of the "marine superclade".

[8][9][25][26][27] While thalattosaurs are almost certainly diapsids, the large degree of uncertainty surrounding their outgroup relations has led most modern paleontologists to classify them as Diapsida incertae sedis.

She used a restricted definition of Thalattosauria which referred to a clade including all reptiles more closely related to Nectosaurus and Hescheleria than to Endennasaurus or Askeptosaurus.

[15][1][21] However, most studies focusing on the group have preferred to retain a broader definition of Thalattosauria equivalent to Nicholls' Thalattosauriformes clade, including reptiles close to both Askeptosaurus and Thalattosaurus.

After Müller et al. (2005) re-affirmed that Endennasaurus was closely related to Askeptosaurus,[13] all thalattosaurs known at the time were finally combined into phylogenetic analyses.

[2] The internal relationships of thalattosaurs is still considered tentative and inconclusive, although the fundamental structure of the group (a monophyletic Thalattosauria clade split into askeptosauroids and thalattosauroids) is very stable.