| Besanosaurus Temporal range: Middle Triassic, latest
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| Cast of BES SC 999, the holotype (top) and interpretative drawing (bottom) | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Shastasauridae |
| Genus: | †Besanosaurus Dal Sasso & Pinna, 1996 |
| Type species | |
| †Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996
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| Synonyms | |
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Besanosaurus (meaning 'reptile from Besano') is a extinct genus of Middle Triassic ichthyosaur from Monte San Giorgio of Italy and Switzerland, containing the single species B. leptorhynchus. Besanosaurus was named by Cristiano Dal Sasso and Giovanni Pinna in 1996, based on the nearly complete flattened skeleton BES SC 999, the holotype specimen. This skeleton is preserved across multiple thin rock slabs spanning 3.5 by 4 metres (11 by 13 ft) when assembled and took thousands of hours to prepare. Additional specimens from Monte San Giorgio that have previously been considered separate genera, including a partial skull named Mikadocephalus and a well-preserved, largely complete skeleton, have been reinterpreted as additional specimens of Besanosaurus. Putative specimens of Besanosaurus have been discovered in the Norwegian archipelago of Svalbard and Germany, although their attribution to this genus remains disputed.
As an ichthyosaur, Besanosaurus had flippers for limbs and a fin on the tail. Besanosaurus is a large ichthyosaur, with the largest known specimen estimated to measure about 8 metres (26 ft) long. It has a long, slender body with a small head and long tail. The snout of Besanosaurus is long and thin, and contains numerous small pointed teeth. In the upper jaw, the teeth are mostly set into sockets but the rearmost teeth are implanted in a groove. The lower jaw bears enlarged coronoid processes for the anchorage of jaw muscles. There are 61 vertebrae in front of the hips, two in the hip region, and at least 138 in the tail. The tail made up more than half the animal's length and displays a downward bend. The forelimbs are longer than the hindlimbs, and the humeri are round and squat. The phalanges (finger and toe bones) are elliptical in the forelimbs but constricted in the hindlimbs.
While it is understood to be a shastasaurid-type ichthyosaur, how exactly Besanosaurus and other members of this group are related to each other is unclear. 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. Ichthyosaurs gave birth to live young, and the holotype may contain the remains of an embryo in its chest cavity. All definite specimens of Besanosaurus come from the Besano Formation. During the Anisian, this region was a lagoon populated by a wide variety of marine life, including a variety of other ichthyosaurs. These different ichthyosaurs are thought to have used different feeding strategies to avoid competition.