Imagine the Western Plains of North America, from Texas to Alaska, submerged under a broad, shallow seaway. To the west lay the ancestral Rocky Mountains. To the east were low-lying plains traversed by large, sluggish rivers.
This was the landscape of North America in the mid-Cretaceous period — a geological age with a time span of 105 to 75 million years ago — when the global sea level was around 100 metres higher than it is now.
