Askeptosaurus is known from several well-preserved fossils found in Middle Triassic marine strata in what is now Italy and Switzerland.

Askeptosaurus is known from several disarticulated and articulated skeletons preserved at the MSNM (Museo Civico di Storia Naturale di Milano) in Milan, Italy, and the PIMUZ (Paläontologisches Institut und Museum der Universität Zürich, Paleontological Institute and Museum [de; fr] of the University of Zurich) in Zurich, Switzerland.

These specimens were discovered in the Grenzbitumenzone of Monte San Giorgio, a UNESCO World Heritage Site on the Swiss-Italian border.

Also known as the Besano Formation in Italy, the Grenzbitumenzone has produced many well-preserved fossils from the Anisian-Ladinian boundary within the Middle Triassic.

They are implanted in a pleurothecodont manner, meaning that they lie in shallow sockets along a groove which has a lowered edge on the lingual (tongue) side of the tooth row.

Unlike thalattosauroids, the teeth have the same general shape and there is no diastema (gap) between the premaxillary and maxillary tooth rows.

Further back, each side has a bulbous basipterygoid process (which connects to the palate), deep grooves for arteries and nerves, and finally a sharp posterolateral (rear-outer) tip.

The parabasisphenoid is followed by a broad and fairly simple basioccipital (lower rear plate), which forms the occipital condyle.

The dentary sheaths over the entire splenial and much of the angular, making it by far the longest bone when the jaw is seen in lateral view.

The cervical ribs in the latter half of the neck increased in length and acquired small additional projections between the joints with the vertebrae.

The two sacral (hip) vertebrae are also obscure, but their fan-shaped, single-headed ribs were known to connect to the lower part of the centrum.

The rest of the tail had prominent chevrons and neural spines which were shorter and more sharply inclined backwards.

The ilium was deep around the acetabulum (hip) socket, and sends back a long rectangular process at its posterodorsal (upper-rear) corner.

There is some uncertainty over the presence of a thyroid fenestra (a gap between the pubes and ischia) based on specimen preservation, but it was probably small or absent.

The tibia is stout and semi-cylindrical while the fibula is fan-shaped and flattened, expanded near the ankle and narrow near the knee.

Recent analyses corroborate Renesto's (1992)[4] argument that Endennasaurus is more closely related to Askeptosaurus than to other thalattosaurs.

Within Askeptosauridae, Askeptosaurus is considered to be the sister taxon of Anshunsaurus, from Middle Triassic deposits in Guizhou, China.

[5] At present, the Askeptosauroidea is only known from the Alpine Triassic and southern China, and Askeptosaurus represents the oldest record for this clade.

It lived alongside many other fish and marine reptiles, including two other genera of thalattosaurs: Clarazia and Hescheleria.