Metriorhynchid

Description

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 viviparousKeichousaurus, 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.