Laramidia

Laramidia is a paleogeographic region that existed during the Late Cretaceous when a shallow seaway split the North American continent into two landmasses. The western landmass, Laramidia, lay to the west of the Western Interior Seaway, stretching from the modern Arctic regions in the north to the tropical regions at the south and from the Rocky Mountains to the Pacific coast. The eastern landmass, Appalachia, lay to the east.

Timeframe: It existed roughly between 93 and 70 million years ago, as sea levels rose and fell, creating the seaway that separated Laramidia from Appalachia. The term derives from the Laramie Formation in Wyoming, part of the region’s Mesozoic sedimentary record.

Geology and climate: The seaway produced a chain of inland seas; Laramidia was largely coastal and mountainous, with river systems and floodplains. Climate was warm and humid by modern standards, supporting lush vegetation and diverse vertebrate faunas.

Fauna: Laramidia is renowned for its diverse Late Cretaceous dinosaur assemblages, including vast hadrosaurid, ceratopsian, tyrannosaurid, and ankylosaurid communities. Many taxa show endemism to Laramidia, differing from contemporaneous faunas in Appalachia. Fossil sites across Montana, Alberta, Utah, Wyoming, and Texas have yielded rich remains.

Significance: The contrast between Laramidia and Appalachia provides key insights into dinosaur evolution, biogeography, and the effects of changing sea levels on continental faunas.