| Aristonectes Temporal range: Late Cretaceous, Maastrichtian, ~
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| Life restoration of A. parvidens | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Elasmosauridae |
| Subfamily: | †Aristonectinae |
| Genus: | †Aristonectes Cabrera, 1941 |
| Type species | |
| Aristonectes parvidens Cabrera, 1941
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| Other species | |
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| Synonyms | |
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List
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Aristonectes (meaning "best swimmer") is an extinct genus of large elasmosaurid plesiosaurs that lived during the Maastrichtian stage of the Late Cretaceous. Two species are known, A. parvidens and A. quiriquinensis, whose fossil remains were discovered in what are now Patagonia and Antarctica. Throughout the 20th century, Aristonectes was a difficult animal for scientists to analyze due to poor fossil preparation, its relationships to other genera were uncertain. After subsequent revisions and discoveries carried out from the beginning of the 21st century, Aristonectes is now recognised as the type genus of the subfamily Aristonectinae, a lineage of elasmosaurids characterized by an enlarged skull and a reduced length of the neck.
Measuring more than 10 m (33 ft) long, Aristonectes is a notably imposing plesiosaur. A referred specimen discovered in Antarctica has an estimated size of more than 11 m (36 ft) long, which would make this genus one of the largest known plesiosaurs. The paddle-like limbs of Aristonectes are unusually large for an elasmosaurid, reaching almost 3 m (9.8 ft) in length, suggesting a total wingspan of around 7 m (23 ft) for the animal. The skull is ogival-shaped and is flattened, possessing sharp forward-facing teeth. According to estimates, 13 teeth would have been present in each premaxilla, 50 teeth in the maxilla and 50 to 63 or more teeth in the lower jaw.
According to its morphology, mainly cranial, Aristonectes fed by mixing prey and sediment in the benthic zones, like the modern gray whale. As in other plesiosaurs, gastroliths (stomach stones) would have been used by Aristonectes either to help digest its food or to aid in buoyancy, although there is little support for the first hypothesis. According to its geographical distribution in the fossil record, Aristonectes would have regularly migrated between Patagonia and Antarctica.