"Potamornis (River Bird) is a more advanced member of the Hesperornithes, and more similar in form to Hesperornis. It is an adept swimmer, but is completely flightless."
―Saurian: A Field Guide to Hell Creek
Potamornis was a hesperornithean avialan known to have lived in the Hell Creek Formation and in the Lance Formation. It was small, weighing around 1,5 to 2 kg, meaning it had the perfect weight to be a flying animal. However, analysis show Potamornis share more similarities with hesperornithids, a group of hesperornitheans known to have been flightless and dedicated to a purely aquatic life. If that's true, then Potamornis could have been a small flightless avialan that crawled awkwardly on land, or at least had few flying abbilities, but had a good set of swimming gracile techniques. Potamornis had a shape simillar to a loon, grebe or cormorant, meaning it was a diver, using its back legs for underwater propulsion. In this way, it was able to catch its favourite food: small fish and aquatic invertebrates.
In Saurian[]
Potamornis swimming
There are no known plans for the inclusion of the Potamornis in the final game, but it is currently in the official game's artbook.
Concept art of Potamornis swimming with Champsosaurus and Axestemys