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This is Promastodonsaurus, literally meaning “before Mastodonsaurus”. Despite its saurian name, it was not a dinosaur, or even a reptile. It was actually a large amphibian. Fossils of Promastodonsaurus were found in Argentina within the rocks of the Ischigualasto Formation, dated to the middle Triassic Period approximately 230 million years ago. The species was officially named in 1963 by the famed South American paleontologist José Bonaparté, in reference to another large amphibian named Mastodonsaurus which lived in Europe during a slightly later time.
Cladistically-speaking, this animal belonged to a large group of amphibians called the “labyrinthodonts”, so-named because a cross-section of their teeth looked like a maze. Within this broad group is a sub-division called the “temnospondyls”, “the cut vertebrae” because each of their backbones is divided into several parts. The temnospondyls were a diverse group of labyrinthodont amphibians which first appeared during the Carboniferous Period and lasted into the Cretaceous Period – a span of nearly 200 million years. Within the order Temnospondyli is the sub-order “Stereospondyli”, and within this is a division called the capitosaurians, “the head lizards”, so-named due to their freakishly huge heads. Promastodonsaurus was a member of this group. It was essentially a giant meat-eating salamander with the head of an alligator.
The only evidence that we have of this animal is a single partial skull. Based upon its similarity to the skulls of other temnospondyl amphibians within its family, it is believed that the animal’s head measured 45 centimeters long (Hans-Dieter Sues and Nicholas C. Fraser, Triassic Life on Land: The Great Transition. New York: Columbia University Press, 2010. Page 69). This in turn would make the entire animal somewhere in the vicinity of 6 feet long, as big as a medium-sized alligator.
Promastodonsaurus bellmani. © Jason R. Abdale. February 9, 2021.
During the middle Triassic Period, crocodilians did not exist, so the capitosaurians like Mastodonsaurus and Promastodonsaurus essentially filled in that ecological niche as crocodilian analogs. Large amphibians like these would continue to dominate freshwater environments until they were replaced by the phytosaurus, who in turn would be replaced by crocodilians.