Metriorhynchidae

Metriorhynchids is an extinct clade of fully-aquatic crocodyliforms that lived in seas of the Jurassic and Cretaceous periods. Their fore-legs were reduced and paddle-like, and unlike living crocodilians, they lost their osteoderms ("armour scutes"). Their body shape maximised hydrodynamy (swimming efficiency), as did having a shark-like tail fin.

Metriorhynchids were the only group of archosaurs to become fully adapted to marine life, becoming pelagic in lifestyle.

The name Metriorhynchidae was coined by the Austrian zoologist Leopold Fitzinger in 1843.

Geographical distribution
The family has a wide geographic distribution, with material found in Argentina, Chile, Cuba, England, France, Germany, Italy, Mexico, Poland, Russia and Switzerland.

Genera
The type genus for Metiorhynchidae is Metriorhynchus from the Middle to Late Jurassic. Other genera included within this family are Teleidosaurus, Geosaurus, Dakosaurus and Enaliosuchus. Genera considered nomina dubia ("doubtful names") within this family are Aggiosaurus and Neustosaurus.

The species classified as Purranisaurus and Suchodus are currently considered to belong within Metriorhynchus; if correct, this makes Purranisaurus and Suchodus junior synonyms of Metriorhynchus. Various palaeontologists have regarded the genus Cricosaurus as a junior synoynm of either Metriorhynchus, Geosaurus or Dakosaurus.

The genus Capelliniosuchus, was once thought to be a metriorhynchid similar to Dakosaurus. However, Sirotti demonstrated that it is a junior synonym of Mosasaurus.

However, recent phylogenetic analyses cast doubt on the idea that many genera form a natural group (i.e. evolved from a common ancestor). The monophyly of Metriorhynchus,  Teleidosaurus, Geosaurus,   and Dakosaurus has been questioned. The monophyly of the poorly known genus Cricosaurus is also not supported, while Purranisaurus may be a distinct genus of metriorhynchid.