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.
Prehistory of the United States
The Late Triassic also saw the origin of the dinosaurs. Dinosaurs left behind abundant fossils in the Four Corners region and this area is now known as one of the best sources of Late Triassic dinosaur fossils in the United States. Texas is also good source of dinosaur remains from this time. The oldest dinosaur remains in the eastern US are about 225 million years old. So, dinosaurs had reached the east coast of the United States not long after they evolved in the first place. Fossil footprints are the most common kind of early dinosaur fossil in the eastern United States.