Why South America’s Inland Seas and Mild Climates Could Have Made It a Civilization Hotspot

Geological reconstructions of the Late Cretaceous period indicate that North America 92 million years ago was characterized by a fragmented landmass divided by a vast interior seaway. This Western Interior Seaway split the continent into two distinct landmasses, creating a geography dominated by shallow marine environments and coastal plains, according to paleogeographic data discussed in geography-focused research forums.

The environment of this era was markedly different from the current continental structure. The Western Interior Seaway stretched from the Arctic Ocean in the north to the Gulf of Mexico in the south, effectively isolating the eastern and western portions of the continent. This marine corridor influenced global climate patterns and created extensive shoreline habitats that supported diverse prehistoric ecosystems.

The Impact of the Western Interior Seaway on North American Geography

The presence of the interior seaway meant that much of the central United States and Canada were submerged under shallow saltwater. This geological arrangement created a “maritime” effect, where the proximity to large bodies of water moderated temperatures across the landmass. According to geological records, this resulted in a generally warmer and more humid climate than is found in the same regions today.

The landmass was divided into two primary regions: Laramidia to the west and Appalachia to the east. Laramidia consisted of a narrow strip of land along the western coast, characterized by rising mountain ranges and volcanic activity. Appalachia, located to the east, was a broader landmass that remained more stable but was increasingly isolated from its western counterpart.

Comparative Environmental Conditions of the Cretaceous Period

The climate of the Late Cretaceous was significantly warmer than the modern era, with higher atmospheric carbon dioxide levels and no polar ice caps. This global warmth, combined with the inland seas, created “pleasant climates” and protected waters that provided ideal conditions for the proliferation of life. While North America was split, other regions like South America also featured protected inland seas and favorable climates that served as hotspots for biological evolution.

These conditions supported a wide array of marine reptiles, such as mosasaurs and plesiosaurs, which thrived in the shallow waters of the Western Interior Seaway. On land, the separation of Laramidia and Appalachia led to divergent evolutionary paths for dinosaur populations, as the two groups were unable to migrate across the saltwater barrier.

Geological Transitions and the Closing of the Seaway

The geography of 92 million years ago was not static. The Western Interior Seaway eventually began to shrink and disappear due to a combination of tectonic uplift and falling sea levels. The rise of the Rocky Mountains in the west played a critical role in pushing the waters out of the continental interior, eventually reconnecting the eastern and western landmasses.

This transition shifted the environment from a coastal, marine-dominated landscape to the terrestrial, interior-heavy geography seen in the Cenozoic era. The sedimentary deposits left behind by the retreating seaway created the vast layers of shale and limestone currently found in the Great Plains, which provide modern geologists with a record of the marine life and climate of the Cretaceous period.