Amphibian August #16 -- Metoposaurus
Known from the Late Triassic of Europe and North America (~228-216 mya), Metoposaurus was a typical member of the metoposaurid temnospondyls. Although they were closely related to the marine trematosaurids, these amphibians had flatter bodies, broader snouts, and eyes set much further forward on their heads, and inhabited freshwater ecosystems such as rivers and lakes.
Reaching up to 3m long (9′10″), Metoposaurus was probably entirely aquatic, with weak limbs that wouldn’t have been able to support its own weight on land. It had a particularly stiff spine and couldn’t undulate its body to swim, instead using flipper-like motions of its limbs to move around underwater.
Large numbers of metoposaurids have been found in several mass grave sites, with fossils representing hundreds of animals in dense jumbled bone beds. These have often been interpreted as mass stranding deaths from droughts, but they may actually be the result of river currents carrying remains to a spot where they all accumulated over longer periods of time -- with their actual origin being occasional individual deaths at large breeding gatherings further upriver.
Growth rings in juvenile Metoposaurus bones from Poland also suggest that they were capable of aestivating during long dry seasons, burrowing underground until more favorable conditions returned.