Prionosuchus plummeri

Prionosuchus is an extinct genus of large temnospondyl. A single species, P. plummeri, is recognized from the early Permian period (some time between 299 and 272 million years ago). Its fossils have been found in what is now northeastern Brazil.

The fragmentary remains of this animal have been found in the Pedra do Fogo Formation in the Parnaíba Basin of Northeastern Brazil, and it was described by L.I. Price in 1948. The incomplete skull of the holotype specimen has been estimated to be 50 centimetres (20 in) long. Several more fragmentary specimens have been found. One very fragmentary but very large specimen (BMNH R12005) appears to have come from an individual nearly three times the size of most other specimens, and may have had a skull that measured up to 1.6 metres (5.2 ft) long. Based on related species, the total body length of this specimen has been estimated at about 9 metres (30 ft), making it the largest known species of temnospondyl. The average mass of an adult Prionosuchus is estimated at over 2,000 kilograms (4,400 lb).

With an elongated and tapered snout, numerous sharp teeth, long body, short legs, and a tail adapted for swimming, its general appearance was very similar to a modern gharial or gar, and it probably had a similar lifestyle as an ambush aquatic predator feeding on fish and other aquatic animals. A study on the closely related Archegosaurus shows that it had a heat balance, gas exchange, osmoregulation, and digestion more similar to that of fish than modern aquatic amphibians; the same probably applied to Prionosuchus as well.

Prionosuchus has been classified as an archegosaurian by Carroll. The genus is monotypic with P. plummeri being the only species described. The archegosaurs were a group of temnospondyli that occupied the ecological niche of crocodiles and alligators during the Permian, and of which the European genus Archegosaurus is typical. The group became extinct at the end of the Permian and the niche was subsequently filled by other, new temnospondyls, later joined by reptiles such as the phytosaurs in the Triassic period.

Cox and Hutchinson re-evaluated Prionosuchus in 1991 and synonymized it with the genus Platyoposaurus from Russia. On the basis of this study, the Pedra do Fogo Formation was reevaluated to be of Middle to Late Permian age. However, studies based on plants and pollens indicate that this formation is actually early Permian in age, making Prionosuchus not contemporary with Platyoposaurus. More recent research in 2015 rejects the later Permian date.

Prionosuchus lived in a humid and tropical environment as indicated by the petrified forest of the Pedra do Fogo formation in which the fossil has been found. The strata composed of siltstones, shales and limestones were deposited in lagoonal and fluvial environments. Other animals discovered in the same rocks include fish (primitive sharks, palaeoniscids, and lungfishes) and amphibians.