Metriorhynchidae

Metriorhynchids are fully aquatic crocodyliforms. Their forelimbs were small and paddle-like, and unlike living crocodilians, they lost their osteoderms ("armour scutes"). Their body shape maximised hydrodynamy (swimming efficiency), as they did have a shark-like tail fluke.[6]

Metriorhynchids were the only group of archosaurs to become fully adapted to the marine realm, becoming pelagic in lifestyle.[7] With tail flukes, reduced limb musculature, and long bones histologically comparable to other obligately aquatic animals, they were almost certainly incapable of terrestrial locomotion; combined with an unusually tall hip opening, as also seen in other obligately aquatic reptiles including the viviparous Keichousaurus, this has lent credence to the suggestion that metriorhynchids gave live birth.[8] Recent research posits that despite their successful adaptation to a pelagic lifestyle, basal metriorhynchids were uniquely disadvantaged in evolving into sustained swimmers due to little-to-no posterodorsal retraction of the external nares.[9]

The family has a wide geographic distribution, with material found in Argentina, Chile, Cuba, England, France, Germany, Italy, Mexico, Poland, Russia and Switzerland.[7][5]

Phylogenetic analyses published during the 2000s cast doubt on the idea that many traditional metriorhynchid genera formed natural groups (i.e., include all descendants of a common ancestor). The traditional species of Geosaurus,[10][11][12] Dakosaurus[10] and Cricosaurus[10] were found to represent unnatural groups, and the species traditionally classified in these genera were reshuffled in a study published in November 2009 by Mark T. Young and Marco Brandalise de Andrade.[4] The monophyly of Metriorhynchus [10][11][13] and Teleidosaurus[10][12] is also unsupported, and the species of these genera are pending reclassification.[4]

The classification presented by Young and Andrade in 2009 was approved in later studies of the Metriorhynchidae.[5][14][15] Metriorhynchidae is a node-based taxon defined in 2009 as the least inclusive clade consisting of Metriorhynchus geoffroyii and Geosaurus giganteus.[4] The cladogram below follows the topology from a 2011 analysis by Andrea Cau and Federico Fanti with clade names from Young et al. 2011 and reduced to genera only.[5][14]

List of genera

The type genus of the family Metriorhynchidae is Metriorhynchus from the Middle to Late Jurassic.[7] Other genera included within this family are Cricosaurus, Geosaurus, and Dakosaurus. Though once considered a metriorhynchid, Teleidosaurus has since been found to be slightly more distantly related to these animals within the superfamily Metriorhynchoidea.

Within this family, the genus Neustosaurus is considered nomen dubium ("doubtful name").[16]

The genus Capelliniosuchus was once thought to be a metriorhynchid similar to Dakosaurus.[17] However, Sirotti demonstrated that it is a junior synonym of Mosasaurus.[18]

Genus	Status	Author	Age	Location	Description	Synonyms	Images
Aggiosaurus		Ambayrac, 1913	late Oxfordian	France.	Sometimes considered to be a nomen dubium.		Cricosaurus suevicus Dakosaurus andiniensis Metriorhynchus superciliosus Neptunidraco ammoniticus Purranisaurus casamiquelai Suchodus durobrivensis
Cricosaurus	Valid	Wagner, 1858	middle Oxfordian to upper Valanginian	Argentina, Chile, Cuba, England, France, Germany, Mexico, Russia and Switzerland.		Enaliosuchus[19]
Dakosaurus[20]	Valid	von Quenstedt, 1856	lower Oxfordian to lower Berriasian	Argentina, England, France, Germany, Mexico, Poland, Switzerland and possibly Russia.		Dacosaurus (lapsus calami)
Geosaurus[21]	Valid	Cuvier, 1824	upper Kimmeridgian to upper Valanginian	France, Germany, Italy and Switzerland.		Brachytaenius Halilimnosaurus
Gracilineustes[22]	Valid	Young et al., 2010	lower Kimmeridgian	England and France.
Maledictosuchus	Valid	Parrilla-Bel et al., 2013		Spain.
Metriorhynchus[23]	Valid	von Meyer, 1830	lower Callovian to upper Kimmeridgian.	England, France, Germany and Switzerland.		Metiorychus (lapsus calami)
Neptunidraco[14]	Valid	Cau & Fanti, 2011	late Bajocian to earliest Bathonian	Italy.
Neustosaurus	nomen dubium				— potential senior synonym of Cricosaurus
Plesiosuchus	Valid	Owen, 1884	late Kimmeridgian to early Tithonian	England and possibly Spain.
Purranisaurus	Valid	Rusconi, 1948	lower Callovian to lower Tithonian	Argentina and Chile.
Rhacheosaurus	Valid	von Meyer, 1831	lower Tithonian	Germany.
Suchodus	Valid	Lydekker, 1890	lower Callovian to lower Oxfordian	England and France.
Torvoneustes[24]	Valid	Andrade et al., 2010	upper Kimmeridgian	England.
Tyrannoneustes	Valid	Young et al., 2013	Callovian	England.

• List of marine reptiles