| Summary: | Redescription of Hainosaurus bernardi Dollo, 1885a, from the early Maastrichtian of the Ciply Phosphatic Chalk of Belgium, results in a reassignment of the taxon to the genus Tylosaurus Marsh, 1872, because the genus Hainosaurus cannot be diagnosed independent of Tylosaurus. The diagnosis of Hainosaurus bernardi by Dollo, 1885a, is reviewed, and the two incomplete and poorly preserved specimens assigned to the taxon are compared with recognized species of Tylosaurus. Hainosaurus was originally diagnosed from characters of the jugal, quadrate, maxilla, premaxilla, frontal, parietal, and teeth. Here, we show that most of the characters of these elements are shared with the genus Tylosaurus, and that those that are not shared, but that are purported key diagnostic characters for Hainosaurus, are simply not preserved, or are too poorly preserved to support a differential diagnosis of H. bernardi at the generic level. The available data support the conclusion that Hainosaurus is a junior synonym of Tylosaurus because no anatomical features distinguish the former from the latter. The genus Tylosaurus occupied a wider geographic and temporal distribution than has been previously suggested, inhabiting the North Atlantic Circle Basin from the Turonian to the Maastrichtian. There are species-level features that support T. bernardi as distinct from other described species of Tylosaurus.
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