Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Paleontology in Montana
For thirty million years following the deposition of the Morrison Formation sediments in Montana were being eroded rather than deposited. Seawater was still present in Montana, now extending as an embayment down from the Arctic Circle. Rivers were still flowing into the local sea, but the local dinosaur fauna was very different. Carnivores living here included "Deinonychus" and "Microvenator". Herbivores included "Sauropelta" and "Tenontosaurus". Around this time flowering plants would make their first appearance in the state. During the middle part of the Cretaceous Montana was partially covered by the Western Interior Seaway. Significant volcanic eruptions occurred in Montana during the time of the Western Interior Seaway. This sea was home to cephalopods, clams, and plesiosaurs. The sediments deposited by this sea compose what is now called the Colorado Group. No dinosaur fossils are known to have been preserved in the Colorado group in Montana, but outside the state occasional discoveries have been made. Southwestern Montana was dry land, but none of those deposits have yielded dinosaur fossils yet. 85 million years ago the Western Interior Seaway withdrew slightly leaving western Montana as a coastal plain. Dinosaurs were present but rarely left fossilized remains. The sea was home to animals such as cephalopods, gastropods, mosasaurs, pelecypods, and plesiosaurs.