| Tatanectes Temporal range: Late Jurassic,
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| Front paddle of the holotype | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Cryptoclididae |
| Genus: | †Tatenectes O'Keefe and Wahl, 2003 |
| Species: | †T. laramiensis
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| Binomial name | |
| †Tatenectes laramiensis (Knight, 1900)
[originally Cimoliosaurus] | |
| Synonyms | |
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Tatenectes is a genus of cryptoclidid plesiosaur known from the Upper Jurassic of Wyoming. Its remains were recovered from the Redwater Shale Member of the Sundance Formation, and initially described as a new species of Cimoliosaurus by Wilbur Clinton Knight in 1900. It was reassigned to Tricleidus by Maurice G. Mehl in 1912 before being given its own genus by O'Keefe and Wahl in 2003. Tatenectes laramiensis is the type and only species of Tatenectes. While the original specimen was lost, subsequent discoveries have revealed that Tatenectes was a very unusual plesiosaur. Its torso had a flattened, boxy cross-section and its gastralia (belly ribs) exhibit pachyostosis (thickening). The total length of Tatenectes has been estimated at 2–3 meters (6.6–9.8 ft).
Tatenectes is related to Kimmerosaurus, although their taxonomic placement has varied. They were once considered to be close relatives of Aristonectes in the family Cimoliasauridae or Aristonectidae, but later assigned to Cryptoclididae. The unusual body shape and pachyostotic gastralia of Tatenectes would have helped to make it more stable and resistant to turbulence. Based on stomach contents, Tatenectes fed on cephalopods and fish. It would have lived in the shallow waters of the Sundance Sea, an epicontinental sea covering much of North America during part of the Jurassic. Tatenectes shared its habitat with invertebrates, fish, ichthyosaurs, and other plesiosaurs, including another cryptoclidid, Pantosaurus, and the large pliosaurid Megalneusaurus.