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.
Montanoceratops
The first fossil remains of what we now know as "Montanoceratops" were discovered on the Blackfeet Indian Reservation, west of Buffalo Lake, Montana in the St. Mary River Formation. It was collected in 1916 by Barnum Brown and Peter C. Kaisen of the American Museum of Natural History from terrestrial sediments that were deposited during the Maastrichtian stages of the Cretaceous period, approximately 70 million years ago. After having mounted a reconstruction in 1935, Brown and his assistant Erich Maren Schlaikjer named it in 1942 as a new species of "Leptoceratops": "Leptoceratops cerorhynchos"; this is the type species of "Montanoceratops". The genus name "Montanoceratops", means "Montana horned face", and is derived from the state of Montana, which is the site of the discovery of the holotype specimen, and the Greek words "keras" (κερας) meaning "horn", and "ops" (ωψ) meaning "face". The type and only valid species known today is "Montanoceratops cerorhynchos".