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.
Back in 2013, I posted a picture of the 25 foot long pliosaur known as Megalneusaurus, which swam in a shallow sea that once covered the central part of North America during the middle and late Jurassic Period. The illustration that I put up was based heavily upon a skeletal drawing of Liopleurodon, it had hardly any detail, and the skull was shockingly shrink-wrapped. Not my best work.
I’m happy to say that I have recently done a revised drawing of this 25-foot long marine reptile, and I think that it’s a substantial improvement over the earlier illustration. Here is the “new and improved” version…
This drawing was made in 1/20 scale with No. 2 and No. 3 pencils on roll paper, with some touch-up tweaking on my computer. From the tip of its nose to the tip of its tail, this animal measures 15 inches long, which would make it 25 feet long in real life. As for the fish swimming around it for protection, they’re just generic fish, I suppose. I tried to find images of fish fossils from the Sundance Sea, but I couldn’t find anything worthwhile.
Hope you enjoy it. Please like and comment.