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Eonatator is a genus of halisaurine mosasaur from the Upper Cretaceous of North America and Europe. Originally, this taxon was included within Halisaurus, but was placed in its own genus. Eonatator is presently known from the Smoky Hill Chalk Member of the Niobrara Formation (Upper Coniacian - Lower Campanian) of Kansas, from the Eutaw Formation (Santonian) and Mooreville Chalk Formation (Selma Group; Santonian-Lower Campanian) of Alabama, and from the Kristianstad Basin of southern Sweden (late early Campanian).

The name Eonatator means "dawn swimmer". At this time it contains only a single species, E. sternbergii. The species is named in honor of Charles H. Sternberg and his son, Levi, who discovered the type specimen in the Niobrara Formation during the summer of 1918.

Like many mosasaurs, this genus has a complicated taxonomic history. The type specimen (UPI R 163, Uppsala University Palaeontological Institute, Uppsala, Sweden), a nearly complete skeleton, was originally referred to the genus Clidastes by Wiman and then to Halisaurus by Russell. Hence, Clidastes sternbergii became Halisaurus sternbergii. However, by the late 1980s, some paleontologists began to suggest that H. sternbergii belonged in its own genus and that Halisaurus was polyphyletic.

Bardet et al. (2005, p. 465) diagnose Eonatator sternbergii as follows: "Ambiguous characters: premaxilla-maxilla lateral suture ending posterior to 9th maxillary teeth; tail about 40% of the head and trunk length (convergent in mosasaurines); caudal vertebra length greater than width; fewer than four pygal vertebrae; femur length about twice distal width (convergent in Clidastes). Autapomorphies: parietal with smooth triangular table extending very far posteriorly, bearing medium-sized circular foramen, located at distance twice its diameter from the frontal-parietal suture, and surrounded anteriorly and posteriorly by two parallel ridges; rounded quadrate with regularly convex tympanic ala; vertebral formula: seven cervicals, 24 dorsals, four pygals, 28 median caudals and at least 41 terminal caudals; humerus length approximately 2.5x distal width." A cladistic analysis of the Halisaurinae has indicated that Eonatator is the sister group to the clade Halisaurus and that it represents one of the most basal of known mosasaur taxa.

Eonatator was among the smaller mosasaurs. The length of the type skeleton, which represents an adult, is only 2.65 meters in length. Like many mosasaurs, it likely fed primarily on fish and smaller marine reptiles.

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