| Brachauchenius Temporal range: Late Cretaceous,
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| Holotype specimen of B. lucasi (USNM 4989), as view in its original position | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Pliosauridae |
| Subfamily: | †Brachaucheninae |
| Genus: | †Brachauchenius Williston, 1903 |
| Type species | |
| †Brachauchenius lucasi Williston, 1903
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Brachauchenius (meaning 'short neck') is an extinct genus of pliosaurs that lived during the Late Cretaceous in what are now North America and North Africa. Only one species is known, B. lucasi, initially described by Samuel Wendell Williston in 1903 from a partial fossil skeleton discovered in a quarry in Kansas, United States. Many other fossil specimens attributed to the species were subsequently discovered, including an individual from Morocco whose presence was made official in 2016. Many contemporary pliosaur specimens were formerly attributed to Brachauchenius, but have since been reidentified as belonging to other genera or are recognized as indeterminate.