Besanosaurus (meaning 'reptile from Besano') is an extinct genus of Middle Triassic ichthyosaur from Monte San Giorgio of Italy and Switzerland, containing the single species B. leptorhynchus.

The skull bones of Besanosaurus indicate that it would have possessed strong jaw muscles, but its delicate snout suggests it would have fed on small fish and coleoid cephalopods, which it could have caught with rapid, snapping bites.

Two shastasaurid specimens from Switzerland were deposited in the collections of Paläontologisches Institut und Museum der Universität Zürich in the 1920s, both of them being mentioned in passing in the literature of the century.

The other specimen, hailing from layer 116 of the Cava Tre Fontane mine, is cataloged as PIMUZ T 4847 and is very large,[9] and poorly preserved and disarticulated, missing the tail and limbs.

[8] Dal Sasso and Pinna considered this specimen similar to Californosaurus and proportionally distinct from Besanosaurus in their description of the latter genus, following discussion with Robert Appleby.

He also considered it possible that Wimanius, another fragmentary ichthyosaur named in 1998 by Maisch and Matzke, was a juvenile Besanosaurus rather than its own genus, though he also noted that it may instead belong to the same species as the unnamed medium-sized shastasaurid.

[3] In a 2003 book, Christopher McGowan and Motani listed both Mikadocephalus gracilirostris and Wimanius odontopalatus as a species inquirendae, and also considered the high diversity of ichthyosaurs reported from Monte San Giorgio suspicious.

While Pessosaurus polaris had previously been considered nondiagnostic and thus a nomen dubium, McGowan and Motani suggested that due to its wide historical recognition the name could be revived for some distinctive referred material, noting its similarity to that of the Swiss medium-sized shastasaurid.

[13] Another study published earlier that year, led by Christian Klug, considered Wimanius to probably be a distinct genus from Besanosaurus, though noted that more research would be needed to confim this.

[6]: 85–86  While Motani had previously agreed with Pessosaurus being a nomen dubium, he took a different stance in his 2003 book with McGowan, noting that the morphology of the humerus was unique, as was that of the bones associated with it.

Like Maisch and Matzke, he recognized its similarity with the specimen in Zürich, but instead argued that Pessosaurus should be reinstated as the name for the material due to its historical significance, in which case it could be the senior synonym of Mikadocephalus.

Since PIMUZ T 4376 had a femur similar to those historically assigned to Pessosaurus, Motani argued that Wiman was probably right that only one large ichthyosaur was present in the Upper Saurian Niveau.

[7]: 128 In their 2013 reassessment of Triassic ichthyosaurs of Svalbard, Erin Maxwell and Benjamin Kear argued in favor of Sander and Faber's assessment of Pessosaurus as a nomen dubium, since the eight centra (vertebral bodies, some of which were subsequently lost) making up its holotype were nondiagnostic.

While they acknowledged the distinctiveness of the forelimb and shoulder material, they argued it was too poorly known to make any definite taxonomic statement on it, and that there was insufficient evidence to assume only one was present in the Upper Saurian Niveau.

[13] In 1916, Friedrich von Huene erected a second species of Pessosaurus, P. suevicus, on the basis of a single vertebra discovered in the Muschelkalk of the Black Forest in the state of Baden-Württemberg, Germany.

[18]: 33  Although this taxon has been widely recognized as a nomen dubium since the late 20th century,[4][7]: 136  Maisch and Matzke noted in 2000 the possibility that the fossil material came from Besanosaurus or a similar ichthyosaur, but agreed that it could not be distinguished from other representatives of the group.

[6]: 85 Another large ichthyosaur from Svalbard, Pessopteryx nisseri, comes from the Early Triassic Vendomdalen Member of the Vikinghøgda Formation, was named based on various specimens by Wiman in 1910.

[3] While initially interpreted as not forming part of the narial border in the holotype,[2] further specimens revealed that the nasals, which run along the top of the snout, do participate in the upper back corners of the external nares.

While Dal Sasso and Pinna noted that Besanosaurus superficially resembled Cymbospondylus in overall body shape, they found it to compare more favorably to the shastasaurines in the fine points of its anatomy, and thus tentatively assigned it to Shastasaurinae.

Unlike Motani, Sander found the three shastasaurids he included in his analysis to form a natural group, with Besanosaurus being the sister taxon of Cymbospondylus and Shonisaurus being basal to the two.

Concerns were also raised about the reproducibility of his results and the extremely high amount of homoplasy (convergent evolution of features by unrelated groups) implied by his cladogram.

Within the grade, Besanosaurus was found to be the earliest diverging member, while within the clade it was either in a similar position or in a smaller nested group with Guizhouichthyosaurus and "Callawayia" wolonggangense.

Bindellini listed many potential advantages of large size, including it allowing various feeding techniques, being an anti-predator adaptation, and resulting in more efficient thermoregulation.

Bindellini and colleagues also noted that the large size of Besanosaurus could have helped with prey capture as well by providing more inertia, with the head being able to have broad movement while the body stayed in place.

[13] All known definite specimens of Besanosaurus come from the Besano Formation (alternatively called the Grenzbitumenzone), a unit composed of oil shale, laminated dolomite, and tuff.

This formation is one of a series of Middle Triassic units atop a carbonate platform at Monte San Giorgio, and measures 5–16 metres (16–52 ft) thick.

Noting that other Middle Triassic ichthyosaur genera were very widespread, Bindellini and colleagues considered it likely that Besanosaurus too was a wide-ranging genus, despite all definite specimens being known from this unit in the Alps.

[3] In the Triassic, when the Besano Formation was being deposited, the region where Monte San Giorgio is would have been a marine lagoon, located in a basin on the western side of the Tethys Ocean.

[37] Many bony fish have been recorded in this formation, with actinopterygians being quite diverse, including abundant small species as well as larger representatives like Saurichthys, though rarer sarcopterygian coelacanths were also present.

[3] Besides ichthyosaurs and sauropterygians, marine reptiles in the Besano Formation are represented by the thalattosaurs Askeptosaurus, Clarazia, and Hescheleria[45] in addition to the semiaquatic, long-necked Tanystropheus.