Metriorhynchidae

Extinct family of reptiles

Metriorhynchidae is an extinct family of specialized, aquatic metriorhynchoid crocodyliforms from the Middle Jurassic to the Early Cretaceous period (Bajocian to early Aptian) of Europe, North America and South America. The name Metriorhynchidae was coined by the Austrian zoologist Leopold Fitzinger in 1843. The group contains two subfamilies, the Metriorhynchinae and the Geosaurinae. They represent the most marine adapted of all archosaurs.

Metriorhynchids are fully aquatic crocodyliforms. Their forelimbs were small and paddle like, and unlike living crocodylians, they lost their osteoderms ("armour scutes"). Their body shape maximised hydrodynamy (swimming efficiency), as they did have a shark like tail fluke. Like ichthyosaurs and plesiosaurs, metriorhynchids developed smooth, scaleless skin. Metriorhynchids were the only group of archosaurs to become fully adapted to the marine realm, becoming pelagic in lifestyle. 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, these characters suggest that metriorhynchids gave live birth. A fossil of a pregnant Dakosaurus female recovered from the Late Jurassic plattenkalk, Bavaria, preserves the complete skeleton of a neonate with small, paddle like forelimbs unsuited for walking on land, similar to those of adults, further supporting live birth in metriorhynchids. Recent research posits that despite their successful adaptation to a pelagic lifestyle, basal metriorhynchids were uniquely disadvantaged among aquatic tetrapods in evolving into sustained swimmers due to little to no posterodorsal retraction of the external nares (unlike other reptilian groups such as mesosaurs, phytosaurs, thalattosaurians, saurosphargids, ichthyosauriforms, sauropterygians, pleurosaurids or mosasauroids, as well as mammalian cetaceans or sirenians). The family has a wide geographic distribution, with material found in Argentina, Chile, Cuba, England, France, Germany, Italy, Mexico, Poland, Russia, Switzerland and Czech Republic.

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, and Teleidosaurus is also unsupported, and the species of these genera are pending reclassification. Metriorhynchidae is a node based taxon defined in 2009 as the least inclusive clade consisting of Metriorhynchus geoffroyii and Geosaurus giganteus.

The type genus of the family Metriorhynchidae is Metriorhynchus from the Middle to Late Jurassic. Within this family, the genus Neustosaurus and Enaliosuchus are considered nomen dubium ("doubtful name"). The genus Capelliniosuchus was once thought to be a metriorhynchid similar to Dakosaurus. However, it was later found to be a mosasaur. Genus Status Author Age Location Description Synonyms ImagesAggiosaurus Ambayrac, 1913 late OxfordianFrance. Sometimes considered to be a nomen dubium. Cricosaurus Valid Wagner, 1858 middle Oxfordian to upper ValanginianArgentina, Chile, Cuba, England, France, Germany, Mexico, Russia and Switzerland. Dakosaurus Valid von Quenstedt, 1856 upper Kimmeridgian to lower BerriasianArgentina, England, France, Germany, Mexico, Poland, Switzerland and possibly Russia. Dacosaurus (lapsus calami)Enaliosuchus nomen dubium Koken, 1883 ValanginianGermany. Geosaurus Valid Cuvier, 1824 upper Kimmeridgian to upper ValanginianFrance, Germany, Italy and Switzerland. Brachytaenius
HalilimnosaurusGracilineustes Valid Young et al., 2010 middle Callovian to lower KimmeridgianEngland and France. Maledictosuchus Valid Parrilla Bel et al., 2013 middle Callovian to KimmeridgianSpain and Mexico. Metriorhynchus Valid von Meyer, 1832 lower Kimmeridgian. France. Metiorychus (lapsus calami)Neptunidraco Valid Andrade et al., 2010 Kimmeridgian to TithonianEngland. Tyrannoneustes Valid Young et al., 2013 CallovianEngland.