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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 during the late Triassic Period, who in turn would be replaced by crocodilians during the Jurassic Period.
The Hell Creek Formation of the north-central United States is famous for its dinosaur fossils, notably those of Tyrannosaurus, Triceratops, and others whose names are well-known to children and adults. However, this fossilized environment was home to more than just dinosaurs. The Hell Creek Formation was home to a wide range of fish, amphibians, reptiles, and mammals. One of the animals that called this landscape home during the late Cretaceous was Habrosaurus.
Despite its name, Habrosaurus was not a dinosaur, and it wasn’t even a reptile. It was, in fact, an amphibian, and a large one at that. Habrosaurus dilatus was a three-foot long siren, a type of salamander that bears more of a resemblance to an eel than the lizard-like forms that we associate salamanders with. Unlike most salamanders, sirens are fully-aquatic amphibians that retain gills throughout their whole lives, unlike other amphibian species that possess gills only in the early development stages of their lives. Sirens also possess small rudimentary lungs, and are able to breath air. There are four species of sirens that are alive today, and all of them are found within North America. Depending upon the species, they can have one to three gill slits on each side of the head. They have completely lost their hind limbs, and their front limbs have shrunk considerably, with three or four short stubby fingers on each hand. Sirens have tiny eyes and no eyelids, and possess a long tail reminiscent of an eel or a sea snake – ideal for swimming. Sirens prefer to live in slow-moving or static bodies of water with lots of underwater vegetation and muddy bottoms. They might occasionally come onto land during the night if the ground is wet or if it’s raining.
Habrosaurus is, to date, the oldest-known siren genus. So far, there are two species known: H. prodilatus, which was found in Alberta, Canada in rocks dating to the Campanian Phase (83-70 MYA) of the late Cretaceous, and H. dilatus, which is much more widespread in the western United States, being found in Montana and Wyoming (with more specimens being found in Wyoming) and dating to the Maastrichtian Stage (70-65 MYA) of the late Cretaceous, as well as being found in the early Paleocene Epoch of the Tertiary Period. This means that H. dilatus was one of several species to survive the K-T Extinction, if only for a short while. It may be possible that H. dilatus is simply the evolved form of H. prodilatus.
Habrosaurus dilatus was named by the eminent paleontologist Charles W. Gilmore in 1928. To my knowledge, six specimens have been found of this animal, and all of them have been found in stream channel deposits. The presence of this type of animal, as well as its impressive size of three feet in length, indicates the presence of large bodies of fresh water, such as slow-moving rivers or ponds. However, the possibility of a dry year was ever-present, and for a fully-aquatic or mostly-aquatic animal like Habrosaurus, that could spell doom. During dry periods or droughts, modern-day sirens are able to dig burrows into the mud and encase themselves in a cocoon, like a lungfish, and Habrosaurus might have adopted the same strategy.
Habrosaurus had rows of blunt teeth arranged in the roof of its upper jaw, which indicates that these jaws were designed for crushing rather than grabbing. Presumably, it fed upon tiny mollusks and arthropods, such as snails and shrimp. Modern-day sirens feed mainly upon worms, aquatic snails, shrimp, and occasionally algae. Like fish, sirens possess lateral lines to find prey by indicating differences in water pressure and underwater vibrations.
An appropriate modern-day analog for the three-foot long Habrosaurus dilatus is the Greater Siren (Siren lacertina), which also grows to three feet long and is the largest siren species in the world today.
Below is a simple drawing of a Habrosaurus that I made with a felt-tipped marker. This style is a considerable departure from my usual style of highly-detailed pencil drawings, but I wanted to do some artistic experimenting.