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.Fitzinger LJFJ. 1843. Systema Reptilium. Wien: Braumüller et Seidel, 106 pp. The group contains two subfamilies, the Metriorhynchinae and the Geosaurinae. They represent the most marine adapted of all archosaurs.
Description 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. However, unlike those marine reptiles, they were poikilothermic with little to no endothermic ability.
Metriorhynchids were the only group of archosaurs to become fully adapted to the marine realm, becoming pelagic in lifestyle.Steel R. 1973. Crocodylia. Handbuch der Paläoherpetologie, Teil 16. Stuttgart: Gustav Fischer Verlag,116 pp. 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.
Classification
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, Dakosaurus and Cricosaurus 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. The monophyly of Metriorhynchus and Teleidosaurus is also unsupported, and the species of these genera are pending reclassification.
The classification presented by Young and Andrade in 2009 was approved in later studies of the Metriorhynchidae. Metriorhynchidae is a node-based taxon defined in the PhyloCode by Mark T. Young and colleagues in 2024 as "the smallest clade within Metriorhynchoidea containing Thalattosuchus superciliosus, Gracilineustes leedsi, Metriorhynchus brevirostris, Rhacheosaurus gracilis, and Geosaurus giganteus" The cladogram below follows the topology from the 2020 analyses by Young et al. and reduced to genera only.
List of genera The type genus of the family Metriorhynchidae is Metriorhynchus from the Middle to Late Jurassic. 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 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.DakosaurusQuenstedt FA. 1856. Sonst und Jetzt: Populäre Vortäge über Geologie. Tübingen: Laupp, 131. 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.GeosaurusCuvier G. 1824. Sur les ossements fossiles de crocodiles, 5. In: Dufour & D'Occagne, eds. Recherches sur les ossements fossiles, 2nd édition. Paris: 143-160 Valid Cuvier, 1824 upper Kimmeridgian to upper ValanginianFrance, Germany, Italy and Switzerland.Brachytaenius HalilimnosaurusGracilineustes Young et al., 2010 middle Callovian to lower KimmeridgianEngland and France.Maledictosuchus Parrilla-Bel et al., 2013 middle Callovian to KimmeridgianSpain and Mexico.Metriorhynchus von Meyer, 1832 lower Kimmeridgian.France.Metiorychus (lapsus calami)Neptunidraco Cau & Fanti, 2011 earliest BathonianItaly.Neustosaurus nomen dubium Valanginian — potential senior synonym of EnaliosuchusPlesiosuchus Valid Owen, 1884 late Kimmeridgian to early TithonianEngland and possibly Spain.Purranisaurus Rusconi, 1948 upper TithonianArgentina and Chile.Rhacheosaurus von Meyer, 1831Germany.Suchodus Lydekker, 1890 middle Callovian to lower OxfordianEngland and France.Thalattosuchus Young et al., 2020 lower Callovian to lower OxfordianTorvoneustes Andrade et al., 2010 Kimmeridgian to TithonianEngland.Tyrannoneustes Young et al., 2013 Callovian
Subclades
Summary of the phylogenetic definitions of metriorhynchid subclades shown in the classification section. These are based on Young and colleagues 2024.
Name Named by Definition Dakosaurina Young et al., 2024 The largest clade within Metriorhynchidae containing Dakosaurus maximus, but not Metriorhynchus brevirostris, Geosaurus giganteus, Plesiosuchus manselii, and Torvoneustes carpenteri. Geosaurina Bonaparte, 1845 The largest clade within Metriorhynchidae containing Geosaurus giganteus, but not Metriorhynchus brevirostris, Plesiosuchus manselii, Torvoneustes carpenteri, and Dakosaurus maximus. Geosaurinae The largest clade within Metriorhynchidae containing Geosaurus giganteus, but not Rhacheosaurus gracilis, Metriorhynchus brevirostris, and Gracilineustes leedsi. Geosaurini The smallest clade within Metriorhynchidae containing Tyrannoneustes lythrodectikos, Geosaurus giganteus, Torvoneustes carpenteri, and Dakosaurus maximus. Metriorhynchidae Fitzinger, 1843 The smallest clade within Metriorhynchoidea containing Thalattosuchus superciliosus, Gracilineustes leedsi, Metriorhynchus brevirostris, Rhacheosaurus gracilis, and Geosaurus giganteus. Metriorhynchinae Lydekker, 1888 The smallest clade within Metriorhynchidae containing Rhacheosaurus gracilis, Metriorhynchus brevirostris, and Gracilineustes leedsi. Plesiosuchina Young et al.,2014 The largest clade within Metriorhynchidae containing Plesiosuchus manselii, but not Metriorhynchus brevirostris, Geosaurus giganteus, Torvoneustes carpenteri, and Dakosaurus maximus. Rhacheosaurini Young et al., 2011 The largest clade within Metriorhynchidae containing Rhacheosaurus gracilis, but not Metriorhynchus brevirostris and Gracilineustes leedsi.
See also
List of marine reptiles
References
External links