Paleontology and geology
In the Early Cretaceous, Manitoba was above sea level, and erosion continued to strip away any sediments that were deposited. But by the Middle Cretaceous, marine waters spread across the center of the continent, forming the Western Interior Seaway. The limestones, sandstones, and shales exposed across southwestern Manitoba were deposited in these shallow waters and preserve fossils of many animals that lived here. The seas were home to many invertebrates, including foraminifera, bivalves, and crustaceans. Of the cephalopods from this period, those from Manitoba are some of the largest ever known. A great variety of sharks and other fish, as well as huge marine turtles like Archelon, which grew to 4 meters, plied the waters. But the rulers of the seas were giant marine reptiles like plesiosaurs and mosasaurs, which reached over 15 meters in length. Crocodiles lived near the shore and in shallow areas, and early birds flew over the land. Although the borders of the seaway shifted over time, it lasted through the end of the period.