Chacaicosaurus is an extinct genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of long-snouted thunnosaur ichthyosaur

Ichthyosaur

Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...

 known from northwestern Patagonia

Patagonia

Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

 of Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

.

Description

Chacaicosaurus is known only from the holotype

Holotype

A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...

 MOZ 5803, articulate

Articulate

Articulate may refer to:*Articulate *Articulate sign*Articulate sound*Joint, also known as articulation*Word modified by an article...

d partial skeleton

Skeleton

The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 which preserved partial skull

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 and almost complete forefin

Forelimb

A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....

. 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

Bajocian

In the geologic timescale, the Bajocian is an age or stage in the Middle Jurassic. It lasted from approximately 171.6 Ma to around 167.7 Ma . The Bajocian age succeeds the Aalenian age and precedes the Bathonian age....

 stage of the Middle Jurassic

Middle Jurassic

The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....

, about 171.6-170 million years ago. It was found near Zapala

Zapala

Zapala is a city in the Patagonian province of Neuquén, Argentina with about 32,000 inhabitants according to the .The city is located at the geographic center of the province at the confluence of national and provincial roads, on a route to the Andes and Chile...

 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

Aalenian

The Aalenian is a subdivision of the Middle Jurassic epoch/series of the geologic timescale that extends from about 175.6 Ma to about 171.6 Ma . It was preceded by the Toarcian and succeeded by the Bajocian.-Stratigraphic definitions:...

-Bathonian

Bathonian

In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...

 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

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 thunnosaur which is placed outside both Stenopterygiidae and Ophthalmosauridae

Ophthalmosauridae

Ophthalmosauridae is an extinct family of thunnosaur ichthyosaurs from the Middle Jurassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America. Currently, the oldest known ophthalmosaurid is Mollesaurus from the early Bajocian of Argentina. Named by George H...

.

Etymology

Chacaicosaurus was named by Marta S. Fernández in 1994

1994 in paleontology

-Dinosaurs:* Aff. Rebbachisaurus gastroliths documented.* The "Talkeetna Mountains Hadrosaur" specimen was discovered in a quarry being excavated for road material. The quarry is near the Glenn Highway, approximately 150 miles northeast of Anchorage...

 and the type species

Type species

In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the 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

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

for "lizard".