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Temporal range: Middle Jurassic, 171.6–170 Ma
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Ichthyosauria
Clade: Baracromia
Genus: Chacaicosaurus
Fernández, 1994
Species: C. cayi
Binomial name
Chacaicosaurus cayi
Fernández, 1994

Chacaicosaurus is an extinct genus of long-snouted thunnosaur ichthyosaur known from the northwestern Patagonia area of Argentina.[1]


Chacaicosaurus is known only from the holotype MOZ 5803, an articulated partial skeleton which preserved a 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.[1] Chacaicosaurus, along with Mollesaurus periallus, which was found at the same locality, are the only diagnostic ichthyosaur specimens from the Aalenian-Bathonian interval.[2]

In the original description Fernández did not assign Chacaicosaurus to a particular family.[1] Maisch and Matzke (2000) and Maisch (2010) regarded it to be an advanced stenopterygiid.[3][4] However, a recent cladistic analysis found that Chacaicosaurus is a basal thunnosaur which is placed outside both Stenopterygiidae and Ophthalmosauridae.[5]


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".[1]

See also[edit]


  1. ^ a b c d Marta S. Fernández (1994). "A new long-snouted ichthyosaur from the Early Bajocian of Neuquén Basin, Argentina". Ameghiniana. 31 (3): 291–297. 
  2. ^ 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. 
  3. ^ Michael W. Maisch; Andreas T. Matzke (2000). "The Ichthyosauria" (PDF). Stuttgarter Beiträge zur Naturkunde. Serie B (Geologie und Paläontologie). 298: 1–159. 
  4. ^ Michael W. Maisch (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art" (PDF). Palaeodiversity. 3: 151–214. 
  5. ^ Fischer, V.; Masure, E.; Arkhangelsky, M.S.; 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.