Biochronology of Late Triassic Metoposauridae (Amphibia, Temnospondyli) and the Carnian pluvial episode

During the Late Triassic, metoposaurids were one of the last significant families of temnospondyl amphibians, and they have long been used in the correlation of Late Triassic tetrapod assemblages. Their fossil record extends across much of Late Triassic Pangea, including important metoposaurid fossils from the USA, Canada, Portugal, Germany, Poland, Morocco, India and Madagascar. Six genera of metoposarids are recognized: 1) Adamanian-Apachean Apachesaurus, endemic to the western USA, 2) Otischalkian Arganasaurus and Dutuitosaurus, endemic to Morocco, 3) Otischalkian-Revueltian Koskinonodon, endemic to the western USA, 4) Panthasaurus from the Otischalkian of India, and 5) Otischalkian-Adamanian Metoposaurus, known from the western USA, eastern Canada, Portugal, Italy, Germany, and Poland. Of the metoposaurid genera, only Metoposaurus has a broad enough palaeogeographic distribution and relatively restricted temporal range to be of biochronological value; its biochron identifies the Otischalkian–Adamanian (middle–late Carnian). The oldest metoposaurids are of middle Carnian age, the age of the Carnian pluvial episode of global climate. The middle Carnian broad palaeogeographic distribution, diversity and cosmopolitanism of the metoposaurids, followed by reduced diversity and relative endemism, likely indicates that climate changes were an important factor in their evolution.