Ichthyosaurus (derived from Greek () meaning 'fish' and () meaning 'lizard') is an extinct genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian) of Europe (Belgium, England, Germany and Portugal). Some specimens of the ichthyosaurid Protoichthyosaurus from England and Switzerland have been erroneously referred to this genus in the past. It is among the best known ichthyosaur genera, as it is the type genus of the order Ichthyosauria.
History of discovery
thumb|left|Cast of the first known complete Ichthyosaurus specimen (originally referred to as Proteosaurus), which was destroyed during [[World War II|World War 2]]
Ichthyosaurus was the first complete fossil to be discovered in the early 19th century by Mary Anning in England; the holotype of I. communis, no coll. number given, was a fairly complete specimen discovered by Mary and Joseph Anning around 1814 in Lyme Regis but was reported as lost by McGowan (1974) in his review of the latipinnate ichthyosaurs of England. The name Ichthyosaurus was first used by Charles Konig in 1818, but it was not used in a formal scientific description, with the earliest described ichthyosaur being Proteosaurus by James Everard Home in 1819 for a skeleton which is now attributed to Temnodontosaurus platyodon. Henry Beche and William Conybeare in 1821 considered Ichthyosaurus to have taxonomic priority over Proteosaurus and named the species I. communis based on BMNH 2149 (now NHMUK PV R1158), a now partially lost specimen now assigned to Temnodontosaurus that was discovered and collected between 1811 and 1812. One specimen that Home had assigned to Proteosaurus was the first complete ichthyosaur skeleton known, but it was destroyed during World War 2. Two casts were rediscovered in 2022, showing that the specimen belonged to Ichthyosaurus, but of uncertain species. During the 19th century, almost all fossil ichthyosaurs were attributed to Ichthyosaurus, resulting in the genus having over 50 species by 1900. These species were subsequently moved to separate genera or synonymised with other species. The fossil was acquired by Doncaster Museum and Art Gallery, where it was misidentified as a plaster cast. In 2008, Dean Lomax, from the University of Manchester, recognised it as genuine and worked with Judy Massare, of the State University of New York, to establish it as a new species. In comparison, other species were much smaller, with the I. communis reaching up to in length, I. larkini probably up to , I. anningae up to , I. breviceps up to , and I. conybeari up to . Jurassic ichthyosaurs had a fleshy dorsal fin on their back as well as a large caudal fin. Icthyosaurus is distinguished from other ichthyosaurs by having a wide forefin with 5 or more digits with an anterior digital bifurcation, but the morphology of the humerus and coracoids are also distinct from that of other Lower Jurassic ichthyosaurs, as is the arrangement of the dermal bones, though the suture lines used to diagnose these are not always visible.
- Ichthyosaurus somersetensis (Lomax and Massare, 2017)