Stereospondyli
The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecological niche to modern crocodilians prior to the diversification of pseudosuchian archosaurs.
Classification and anatomy
The group was first defined by Zittel on the recognition of the distinctive vertebral anatomy of the best known stereospondyls of the time, such as Mastodonsaurus and Metoposaurus. The term 'stereospondylous' as a descriptor of vertebral anatomy was coined the following year by Fraas, referring to a vertebral position consisting largely or entirely of the intercentrum in addition to the neural arch. While the name 'Stereospondyli' is derived from the stereospondylous vertebral condition, there is a diversity of vertebral morphologies among stereospondyls, including the diplospondylous condition, where the arch sits between the corresponding intercentrum and pleurocentrum, and the plagiosaurid condition, where a single large centrum ossification is present, and the arch sits between subsequent vertebral positions. The concept of Stereospondyli has thus undergone repeated and frequent revisions by different workers. Defining features include a tight articulation between the parasphenoid and the pterygoid and a stapedial groove.Evolutionary history
Stereospondyls first definitively appeared during the early Permian, as represented by fragmentary remains of a rhinesuchid from the Pedra de Fogo Formation of Brazil. Rhinesuchids are one of the earliest groups of stereospondyls to appear in the fossil record and are predominantly a late Permian clade, with only one species, Broomistega putterilli, from the Early Triassic of South Africa. However, almost all other groups of stereospondyls are not known from any Paleozoic deposits, which remained dominated by non-stereospondyl stereospondylomorphs. The taxonomically unresolved Peltobatrachus pustulatus, which has historically been regarded as a stereospondyl, is also known from the late Permian of Tanzania. Several more fragmentary records are known from horizons spanning the Permo-Triassic boundary in South America, such as the rhinesuchid-like Arachana nigra from Uruguay and an indeterminate mastodonsaurid from Uruguay.Following the Permo-Triassic mass extinction, stereospondyls are abundantly represented in the fossil record, particularly from Russia, South Africa, and Australia. This led Yates & Warren to propose that stereospondyls had sheltered in a high-latitude refugium that would have been somewhat shielded from the global effects of the extinction, and that they subsequently radiated from present-day Australia or Antarctica. Recent discoveries of a diverse rhinesuchid community in South America alongside non-stereospondyl stereospondylomorphs have led to an alternative hypothesis for a radiation from western Gondwana in South America. By the end of the Early Triassic, virtually all major clades of stereospondyls had appeared in the fossil record, although some were more geographically localized than those with cosmopolitan distributions.
Stereospondyls were the latest-surviving temnospondyl group. With the diversification of crocodile-like archosaurs and an extinction event at the end of the Triassic, most other temnospondyls disappeared. Chigutisaurid brachyopoids persisted into the Jurassic in Asia and Australia, including Koolasuchus, the youngest known stereospondyl from what is now Australia. There is also sparse evidence for the persistence of some trematosauroids into the Jurassic of Asia. If the recent hypothesis that Chinlestegophis, a Late Triassic stereospondyl from North America, is indeed a stem caecilian is correct, then stereospondyls would survive to the present day.