Temporal range: Middle Jurassic, 171.6–170Ma
|Species:||† C. cayi|
Chacaicosaurus is known only from the holotype MOZ 5803, articulated partial skeleton which preserved partial skull and almost complete forefin. It was collected in the Chacaico Sur locality from the Emileia giebeli ammonoid zone of the Los Molles Formation, Cuyo Group, dating to the early Bajocian stage of the Middle Jurassic, about 171.6-170 million years ago. It was found near Zapala city of the Neuquén Basin. Chacaicosaurus, alongside with Mollesaurus periallus which was found at the same locality, are the only diagnostic ichthyosaur specimens from the Aalenian-Bathonian interval.
In the original description Fernández didn't assign Chacaicosaurus to a particular family. Maisch and Matzke (2000) and Maisch (2010) regarded it to be an advanced stenopterygiid. However, a recent cladistic analysis found that Chacaicosaurus is a basal thunnosaur which is placed outside both Stenopterygiidae and Ophthalmosauridae.
Chacaicosaurus was named by Marta S. Fernández in 1994 and the type species is Chacaicosaurus cayi. The generic name is derived from the name of the Chacaico Sur locality, in which the holotype was collected, and sauros, Greek for "lizard".
- Marta S. Fernández (1994). "A new long-snouted ichthyosaur from the Early Bajocian of Neuquén Basin, Argentina". Ameghiniana 31 (3): 291–297.
- Marta S. Fernández (1999). "A new ichthyosaur from the Los Molles Formation (Early Bajocian), Neuquen Basin, Argentina". Journal of Paleontology 73 (4): 677–681. JSTOR 1306766.
- Michael W. Maisch and Andreas T. Matzke (2000). "The Ichthyosauria". Stuttgarter Beiträge zur Naturkunde: Serie B 298: 1–159.
- Michael W. Maisch (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art". Palaeodiversity 3: 151–214.
- Fischer, V.; Masure, E.; Arkhangelsky, M.S.; and Godefroit, P. (2011). "A new Barremian (Early Cretaceous) ichthyosaur from western Russia". Journal of Vertebrate Paleontology 31 (5): 1010–1025. doi:10.1080/02724634.2011.595464.