In the summer of 1903 Annie Alexander led an expedition with Miss Edna Wemple, Eustace Furlong, Merriam John C, W.B.

One of the fossils were found in the North Fork of Squaw Creek (Triassic of the United States) in Shasta County, California.

It was estimated to be from the Hosselkus Limestone Formation from the Carnian period, dating from 235 to 221.5 million years ago.

Its name is Thalattosaurus meaning "sea lizard" and alexandrae in honor of Annie Alexander, an amateur paleontologist and patron to the University of California Museum of Paleontology.

[9] In 1993, another species named Thalattosarus borealis was found in a talus slope near Wapiti Lake, British Columbia, in the Sulphur Mountain Formation.

Discovered elements include the anterior part of skull, incomplete mandible, centra, isolated ribs, and left pterygoid.

The bone is split sagittally and it is possible that an additional row of teeth was present but broke away during preservation.

The posterior mandibular teeth of T. borealis differ from these genera, however, being set flush with the margin of the jaw.

In both T. alexandrae and Clarazia the posterior, bulbous teeth are set slightly ventral and medial to the jaw margin.

In all thalattosaurs found, the posterior end of the dentary bifurcates into two diverging processes (upper and lower).

[1] Originally illustrated by Merriam in 1905, his reconstruction of the skull of T. alexandrae portrayed the rostrum as being straight and showed six conical, striated teeth on the premaxilla.

[10] Thalattosauroidea (which contains Clarazia and Thalattosaurus) have a relatively short rostrum, distinct from the elongate primitive condition, with convergent lateral margins that terminate in a pointed tip.

It is also characteristic of their supratemporal to contact the frontal bone, having a heavy postorbital bar, diastema present that separates the premaxillary from the maxillary teeth, and a deep lower jaw.

[5] The Sulphur Mountain Formation, where remains of Thalattosaurus have been found, consists of a series of marine siltstones, silty limestones, and fine gained sandstones.