| Chacaicosaurus Temporal range: Middle Jurassic, Early
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| Illustration of the partial forelimb articulated as preserved | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Node: | †Neoichthyosauria |
| Genus: | †Chacaicosaurus Fernández, 1994 |
| Species: | †C. cayi
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| Binomial name | |
| †Chacaicosaurus cayi Fernández, 1994
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| Synonyms | |
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Chacaicosaurus is a genus of neoichthyosaurian ichthyosaur known from the Middle Jurassic of Argentina. The single known specimen of this genus was excavated from the Los Molles Formation in Neuquén Province, and is housed at the Museo Olsacher under the specimen number MOZ 5803. This specimen consists of a skull, forelimb, some vertebrae, and some additional postcranial elements. The genus was named by Marta Fernández in 1994, and contains a single species, Chacaicosaurus cayi, making it the first named distinctive ichthyosaur from the Bajocian stage. It is a medium-sized ichthyosaur with a very long snout, which bears a ridge running along each side. The forelimbs of Chacaicosaurus are small and contain four main digits.
Different authors have classified Chacaicosaurus in different ways; some consider it a thunnosaur closely related to the ophthalmosaurids, others instead place it outside of Thunnosauria, often near Hauffiopteryx. However, as it is very similar to Stenopterygius, some researchers instead classify it within that genus as S. cayi, a placement originally suggested by Fernández in 2007. The only known specimen of Chacaicosaurus appears to be an adult based on the shape of its limb bones. Chacaicosaurus inhabited open marine waters which it shared with the ophthalmosaurid ichthyosaur Mollesaurus as well as a plesiosaur, a thalattosuchian, and various invertebrates.