Aristonectes (meaning "best swimmer") is an extinct genus of large elasmosaurid plesiosaurs that lived during the Maastrichtian stage of the Late Cretaceous.

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.

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.

According to its morphology, mainly cranial, Aristonectes fed by mixing prey and sediment in the benthic zones, like the modern gray whale.

[1] In 1848, the Franco-Chilean naturalist Claude Gay described the first known plesiosaur from South America, Plesiosaurus chilensis, on the basis of a single caudal vertebra discovered in Quiriquina Island, in Concepción Province, Chile.

[6] In 1949, Edwin H. Colbert found that the holotype specimen of "P." chilensis (the caudal vertebra originally described by Gay) belongs to a pliosauroid, but also was uncertain about its generic placement.

[7][8] In an analysis published in 2013 by José P. O'Gorman and colleagues, the holotype specimen of "P." chilensis was recognized as potentially belonging to A. parvidens.

[1] The first specimen formally identified as A. parvidens was collected by Cristian S. Petersen in collaboration with local resident Victor Saldivia, in the Cañadón de los Loros [ceb], near the municipality of Paso del Sapo in Chubut Province, Argentina.

A vertebra and incomplete phalanges from the same region had previously been donated to the Museo de La Plata by Mario Reguiló, and were subsequently referred to the holotype, catalogued as MLP 40-XI-14-6, as they most likely came from the same specimen.

[13] Since the unknown parts of the original specimen were heavily reconstructed, the precise anatomy of Aristonectes remained uncertain throughout the 20th century.

[14][15][16] In 2003, Argentine paleontologist Zulma Gasparini and colleagues re-prepared the holotype skull along with the atlas and axis, in an attempt to clarify its anatomy.

[11] The first discovery of the second known species, A. quiriquinensis, dates back to the late 1950s, when Argentinian paleontologist Rodolfo Casamiquela discovered a partial skeleton in Las Tablas Bay, located north of Quiriquina Island, Chile.

The specimen, cataloged as SGO.PV.260, was found in Maastrichtian beds of the Quiriquina Formation; initially, five vertebrae, a humerus fragment, and the distal end of a limb were visible.

The preparation uncovered additional body parts, with the specimen including all four limbs, a posterior portion of the neck, most of the trunk, and a complete tail.

[18] In 2001, Chilean paleontologist Mario E. Suárez collected a partial skull, mandibular fragments and 12 anterior cervical vertebrae from a beach near Cocholgüe [es], located north of Tomé.

This excavation recovered 119 blocks of sandstone, most of them with bone material, but some were degraded by the periodic immersion of sea water that caused brittle surfaces on the most delicate parts.

These are composed of a complete left femur and a proximal part of a humerus, cataloged respectively as SGO.PV.135 and SGO.PV.169, which were previously referred to the now dubious genus Mauisaurus.

[15][16] Its relationships remained unclear until more recent research carried out on A. parvidens and the discovery of the second species A. quiriquinensis have made it possible to redescribe its anatomy and classify it among the elasmosaurids.

The posterior extensions of the pterygoid bones are surrounding the axis and the atlas, which reduces the space between the skull and the vertebrae, and would therefore limit lateral movement at their joints.

[12][11] Among the unique characters of the skull of A. quiriquinensis is the presence of a mental boss (a ridge along the mandibular symphysis), a feature not observed in the holotype of A.

[9][12][23][11][b] The mandibular symphysis of the dentary bones in A. quiriquinensis is comparatively thicker and lacks the deep groove seen in ventral view in A. parvidens.

In A. parvidens, the labial contact between the angular bone and the surangular, anterior to the glenoid fossa, bears a deep cavity which is not present in A.

[12] The teeth of A. quiriquinensis have an oval cross-section, being very similar to elasmosaurids of the Northern Hemisphere and the Weddellian Province (a geographical area that appeared after the isolation of Antarctica), as in Kaiwhekea.

In aquatic tetrapods, such limb proportions are only observable in the rhomaleosaurid Meyerasaurus from the Early Jurassic and in the modern humpback whale.

This group is characterized by a very enlarged skull compared to the width of the body, a moderately short neck, and more than 25 teeth in the maxilla,[17] a higher number than in other elasmosaurids.

[24][25][29][30] The following cladogram is modified from Otero & Acuña, (2020):[30] Speeton Clay plesiosaurian Wapuskanectes Callawayasaurus Libonectes Hydrotherosaurus Futabasaurus Thalassomedon Tuarangisaurus Wunyelfia Kaiwhekea Alexandronectes Morturneria Aristonectes Kawanectes Vegasaurus Terminonatator Elasmosaurus Albertonectes

In a study published in 2015 by Otero and his colleagues, it was noted that during their growth, the humerus and femur change from a flat capitulum in juveniles to hemispherical shaped heads at adulthood.

[38][22] The presence of Aristonectes within these formations shows that it would have been endemic in the Weddellian Province, a geographical area that appeared after the isolation of Antarctica.

These include Kaikaifilu, Moanasaurus, Mosasaurus, Liodon and Plioplatecarpus, although the validity of some of these genera is disputed as they are primarily based on isolated teeth.