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.