Thalattosaurus (pronounced: , tha-la-to-SORE-us from Ancient Greek: θάλαττα, romanized: thálatta, lit. 'sea' and σαῦρος, saûros, 'lizard') is an extinct genus of marine reptile in the family Thalattosauroidea. Known exclusively from the Triassic period, it was a 2–3 metres (6.6–9.8 ft) long shellfish-eating diapsid with paddle-like limbs and a down-turned rostrum. Fossils were recovered in the Lower and Middle Triassic Sulphur Mountain Formation of British Columbia as well as the Upper Triassic Hosselkus Limestone of California. It has gained notoriety as a result of studies on general diapsid phylogeny. Although originally described as four distinct species by Merriam in 1905, one was proven to be T. alexandrae upon further inspection and another has a missing type specimen. Currently it is believed to include two known species; Thalattosaurus alexandrae and T. borealis.
Discovery and naming
In summer 1903, Annie Alexander led an expedition with Miss Edna Wemple, Eustace Furlong, Merriam John C, W.B. Esterly, and Mr. F.S. Ray to Shasta County where they discovered what they initially thought was Shastasaurus. One of the fossils were found in the North Fork of Squaw Creek (Triassic of the United States) in Shasta County, California. The environment it was found in was a shelly/skeletal limestone with a horizon composed largely of broken shells. It was estimated to be from the Hosselkus Limestone Formation from the Carnian period, dating from 235 to 221.5 million years ago. It was later collected by the University of California. The fossil found by Annie Alexander in 1903 had much of original bone in preorbital area gone, vomer was exposed, an incomplete mandible, two dorsal ribs and centra, and three articulated caudal vertebrae pressed against the vomer. After Merriam's further studies, it was categorized as a new species named Thalattosarus alexandrae. The first thalattosaurs to be described were Thalattosaurus and Nectosaurus from the Upper Triassic of California by Merriam in 1904, 1905 and 1906. Thalattosaurus alexandrae was named by Merriam in 1904. 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. This material was later reviewed by Nicholls in 1999. Originally four subtaxa of Thalattosaurus were classified; Thalattosaurus alexandrae, Thalattosaurus perrini, and Thalattosaurus shastensis by Merriam but later additional examination of the type of T. shastensis suggested that it does not belong in the genus Thalattosaurus. It is still currently under study. The type skull of T. perrini has not been located, but the vomer figured by Merriam in 1905 did not differ from the vomer of T. alexandrae. In 1993, another species named Thalattosarus borealis was found in a talus slope near Wapiti Lake, British Columbia, in the Sulphur Mountain Formation. The environment, similar to the discovery of T. alexandrae, was also marine shale, and marl. This specimen was discovered and collected by the Royal Tyrrell Museum of Paleontology field crew. Thalattosaurus borealis was named after its Northern discovery location, with Thalattosaurus meaning "ocean lizard" and borealis coming from boreas (Greek word, βορέας) meaning "Northern." Discovered elements include the anterior part of skull, incomplete mandible, centra, isolated ribs, and left pterygoid.
Description
As noted by Merriam in 1905, the skull of the holotype, referred to as UCMP 9085, was preserved in four pieces. They were originally connected by calcite vein-filled cracks, but were separated during preparation. Three of these four pieces found made up the rostrum. When aligned, the rostrum shape suggested dorsal curvature of the anterior end of the maxilla but ventral deflection in the anterior end of premaxilla. The prefrontal, however, came down ventrally under the margin of the maxilla and contacts the anterior tip of the suborbital process A line drawn from the preserved anterior alveolar margin of the maxilla to the lower edge of the prefrontal showed that the ventral margin of the maxilla was straight, very similar to the rostral structure known of Clarazia. Upon further examination, the other characteristics found true of this fossil were a striated external surface of bone with smooth bone resembling "pseudodont teeth". The first of which was blunt, procumbent and short, the second was pointed and thinner, with the third (although the tip was broken) the thick base implied a blunt tooth. There was an additional broken stump which may suggest a fourth tooth.
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