| Thalassomedon Temporal range: Late Cretaceous,
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| Mounted cast of the type specimen, American Museum of Natural History | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Elasmosauridae |
| Genus: | †Thalassomedon Welles, 1943 |
| Species: | †T. haningtoni
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| Binomial name | |
| †Thalassomedon haningtoni Welles, 1943
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| Synonyms | |
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Thalassomedon (from Greek, thalassa, "sea" and Greek, medon, "lord" or "ruler", meaning "sea lord") is a genus of plesiosaur, named by Welles in 1943.