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Excalibosaurus (meaning "Excalibur's lizard") is a monotypic genus of marine prehistoric fish-like reptiles (ichthyosaurs) that lived during the Sinemurian stage of the Early Jurassic period in what is now England. It is characterized by the extreme elongation of the rostrum, with the lower jaw about three fourth of the length of the upper jaw, giving the animal a swordfish-like look. The only known species is Excalibosaurus costini.

Description
This relatively rare animal is known from two skeletons. The holotype discovered in 1984 near a beach on the Somerset coast, consists of the skull, forefin, part of the pectoral girdle ans some vertebrae and ribs. It has been described in 1986 by McGowan. The fossil is hosted in the City of Bristol Museum and Art Gallery. The second specimen is an almost complete skeleton collected in the same area in 1996 and was purchased by the Royal Ontario Museum. It was described again by McGowan in 2003

Excalibosaurus is related to two other genera of ichthyosaurs, Leptonectes from the Rhaetian (Late Triassic) to the Sinemurian (Early Jurassic) of England and Eurhinosaurus from the Toarcian (Early Jurassic) of Germany. The three genera are grouped in the family Leptonectidae and the suborder Eurhinosauria. It was once thought that Excalibosaurus was a junior synonym of Eurhinosaurus but the description of the 1996 specimen show many morphological differences such as the shape of the forefin (much shorter and broader in Excalibosaurus), the slender shape of the body, that clearly differenciate the two genera. The estimate body length of the 1996 specimen is 7 m with a 1.54 m long skull. The holotype specimen was much smaller with a skull length of 0.785 m and an estimated body length of 4 m, indicating that it was a juvenile.