Aquadontosaura

Aquadontosaura was a species of Hadrosaur that lived in what is now Montana. It was among a few species of Hadrosaur that was adapted for swimming, it had no rods in its tail and the tail itself was strong so it could swim easily. Unlike most other Hadrosaur species, it was an upright biped much like outdated Hadrosaurs of 1950s and 1940s, so no one knows why they had that posture, but it is possible that it was due to its tail as well as its back becoming less stiff than its fully terrestrial ancestors, so it had a more kangaroo-like posture (although it walked and run rather than hop since it was too big to hop and didn't have any signs of muscles to hop). It could have defended itself against T-Rex by going into the watery depths. It fed on leaves, ferns, cycads, and club mosses, but it mainly fed on underwater plants. It could have died out due to a mass extinction brought on by an asteroid impact and its effects.