There is something hellish in this creek

Very few people seem to think the Hell Creek Formation has quite the number of freshwater beasties that would probably make it one of the most dangerous places in the world to swim. Several species of crocodilian (one getting to a size much larger then modern crocs and possibly rivaling some of the largest ones ever), sawfish that could give you a nasty prick when they swing their long snout, Didelphodon (A otter-like metatherian mammal that possesses the most powerful bite of any mammal for its size), the giant champsosaurs, the gigantic soft-shell turtles that are as long as a man, and lets not forget that the largest land predator to have ever lived was probably patrolling the riverbank, eating anything on land and in the water if it needs to. Not to mention all the other possible dangers that could have occurred in Hell Creek, like water bound parasites and disease-carrying bugs which abound rivers everywhere, as well as the non-animal threats like floods. If those aren't enough reasons for you to enter into the water, then I might as well address yet another thing that Hell Creek has to throw at you, and its something you wouldn't expect at all from the region.

Meet Hell Creek's Freshwater Mosasaur.

Yes, that's right, there were freshwater mosasaurs at this time. Only known from a handful of bones (parts of the skull and vertebra), the remains are enough to tell this is a mosasaur. It was found in the southern region of North Dakota (quite confusing I know), and the interesting thing was where it was deposited. The bones were found in what was once a freshwater ecosystem, not a marine one, and to add on to the animal was living in rivers, the bones were found alongside those of land based animals, like hadrosaurs. 

This is a very interesting find, as the only other mosasaurs found in freshwater deposits were from earlier rocks, closer to the origin of the mosasaurs. It seems that this species is secondarily freshwater-dwelling, descending from marine ancestors who decided to enter into narrower channels. This is also seen in plesiosaurs, which have entered freshwater multiple times over the course of their reign (leptocleidids, elasmosaurs). The transition from marine to freshwater environments has also shrunk some plesiosaurs to barely half the length of their ancestors (Marine elasmosaurs are usually 8-10 meters in length, but the new species from Dinosaur Park is roughly half that length). The same goes for the mosasaur. The remains suggest an animal around 5-6 meters, half the size of the 10-12 meter giants that roam the oceans at the time. However, at half the length, it is still as big as some of the largest crocodilians alive, and we all know that mosasaurs were not only faster then crocs in the water, but also had a more elevated metabolism, so if we take that into consideration, a Hell Creekian freshwater mosasaur would potentially be a bigger threat to you then any croc.

ORRRRRR it wasn't a full time freshwater beast and it lived like a bull shark, able to transition to both.

Thank you to for telling me about this animal. I wish I could tell more about this amazing creature, though most of the information is unpublished and under embargo, so Tristan told me the parts that weren't under an embargo.

Since the remains are fragmentary and the classification is hard to pin down, I decided to base it off of a mosasaurine, as they are the only mosasaurs in the region. I modified PWNZ3R-Dragon's skeletal and looked at some modern monitor lizards for the patterning.

Edit: Turns out there is also a plioplatecarpine that was found in freshwater deposits.