Ichthyosaurus (derived from Greek ἰχθύς (ichthys) meaning 'fish' and σαῦρος (sauros) meaning 'lizard') is a genus of ichthyosaurs from the late Triassic and early Jurassic (Rhaetian - Pliensbachian[2]) of Europe (Belgium, England, Germany, Switzerland, and Portugal[3]) and Asia (Indonesia). It is among the best known ichthyosaur genera, as it is the type genus of the order Ichthyosauria.[4][5][6]
Ichthyosaurus was the first complete fossil to be discovered in the early 19th century by Mary Anning in England.[7] The name Ichthyosaurus was first used by Charles König 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 De la Beche and William Conybeare in 1821 considered Ichthyosaurus to have taxonomic priority over Proteosaurus and named the species I. communis based on a now lost specimen. 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.[8]
I. anningae, described in 2015 from a fossil found in the early 1980s in Dorset, England, was named after Anning.[9][10][8] 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.[9]
Ichthyosaurus ancestors were land-dwellers that had went down from an ancient fish and evolved to walk on land. It is intriguing to contemplate why the ichthyosaur ancestor chose to return to the sea. Scientist haven't found the definite answer, but there are a couple of reasonable explanations. On land, there was a fierce competition, so exploring a new environmental niche would have been the best way to avoid predators whilst exploiting the rich sources of marine food. Thus, it seemed proper to re-enter and take advantage of the watery habitat.[11]
Ichthyosaurus was smaller than most of its relatives, with individuals measuring up to 3.3 metres (11 ft) in length.[12] Many Ichthyosaurus fossils are well-preserved and fully articulated. Some fossils still had baby specimens inside them, indicating that Ichthyosaurus was viviparous. Similar finds in the related Stenopterygius also show this.[13][14] 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.[8]
This cladogram below follows the topology from a 2010 analysis by Patrick S. Druckenmiller and Erin E. Maxwell.[15]