Distinctly Montana Magazine
Issue link: https://digital.distinctlymontana.com/i/82275
MOST OF MONTANA COVERED BY THE WATERS OF INTERCONTINENTAL CRETACEOUS SEAWAY Late Cretaceous Period 100,000,000 YEARS AGO two seas met, they divided North America into two landmasses. Named the INTERCONTINENTAL CRETACEOUS SEAWAY, this body of water helped maintain a subtropical environment in Mon- tana and Alberta from about 80 million years ago until around 65 million years ago. It was about this time when the Rocky Mountains began to uplift to the west and push the inland sea out of western Montana, limiting its extent in the eastern part of the state. With the Rock- ies to the west and an inland sea to the east, a broad coastal plain formed in much of the central and western part of eastern Montana. The landscape was flat and vegetated with plants similar to many of the plants now grow- ing in Louisiana, like palms and cypress trees. There were no grasses and the open plains were abundant with ferns and bushes. Hardwood trees lined the rivers and the conifers intermin- gled. This broad, subtropical coastal plain is represented by several rock formations, each of which produce important dinosaur faunas. The dinosaur site named Egg Mountain, located just west of Choteau, revealed the first evidence that dinosaurs cared for their young and lived in giant nesting grounds. It was in this area that the first dinosaur embryos were discovered, and evidence of giant herds of dinosaurs were found. The Two Medicine Formation has yielded the remains of dinosaurs such as Montana's state fossil, the duck-bill Maiasaura, and the horned dinosaurs Einiosaurus and Achelousaurus, as well as the carnivorous Daspletosaurus, Troodon, and Bambiraptor. Dinosaur specimens from Egg Mountain and nearby areas can be seen at the Museum of the Rockies in Bozeman, The Old Trail Museum in Choteau, the Two Medicine Dinosaur Center in Bynum, and the Depot Museum in Rudyard. The fossil remains from the Two Medicine Formation represent animals that lived near the Rocky Mountains around about 75 million years ago. At the same time there were other dinosaurs living closer to the seaway, and these dinosaurs are found in a rock unit called the Judith River Formation. Common to the Judith River Formation were dinosaurs such as the duck-bills Brachylophosaurus and Gry- posaurus, the horned dinosaur Avaceratops, and meat-eaters such as Albertosaurus. These and other dinosaur remains from the Judith River Formation can be seen in the Depot Museum in Rudyard, the H. Earl Clack Memorial Museum in Havre, the Great Plains Dinosaur Museum and Phillips County Museum in Malta, the Blaine County Museum in Chinook, and the Upper Musselshell Museum in Harlowton. From approximately 73 million years ago until about 69 million years ago, the in- land sea expanded covering most of Montana east of the Rockies with an ocean. At this time the Bearpaw Formation was deposited, containing many marine creatures like Plesiosaurs and Mososaurs, some of which can be seen in the Museum of the Rockies, in Bozeman, and the Fort Peck Interpretive Center in Fort Peck. The late Cretaceous Period contains the best dinosaur fossils, as there are many places in eastern Montana where the rocks from this time are exposed at the surface of the ground. 54 DISTINCTLY MONTANA • FALL 2012