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Approximately 77 million years ago, in the Late Cretaceous, North America was a very different continent from today. Rather than one large landmass, there lay a huge inland sea called the Western Interior Seaway (WIS) that extended from the Arctic Ocean to the Gulf of Mexico. This body of water split the continent into two distinct landmasses: Laramidia to the west and Appalachia to the east. These regions each had their own unique ecosystems, filled with an array of prehistoric life.
The Western Interior Seaway: A Prehistoric Ocean Highway
The Western Interior Seaway was a shallow sea corridor that existed from the early Late Cretaceous to the earliest Paleocene, covering tens of millions of years. It linked the warm waters of the ancient Gulf of Mexico with the cold Arctic Ocean to create a large aquatic environment that had a diverse marine life. The shifting of Earth’s tectonic plates, particularly the subduction of the Farallon and Kula Plates beneath the North American Plate, caused this basin to form and fill with ocean water.
This seaway played a crucial role in shaping North America’s geography and ecosystem. Over time, the rising and falling sea levels caused fluctuations in the size and depth of the WIS, eventually leading to its disappearance by the end of the Cretaceous period due to regional uplift and mountain formation.
Laramidia: The Dinosaur Paradise
To the west of the Western Interior Seaway lay Laramidia, a narrow strip of land along what is now the western coast of North America. This region was home to some of the most famous dinosaurs of all time, including:
- Tyrannosaurus rex – The apex predator of its time, known for its massive size, powerful jaws, and keen hunting abilities.
- Triceratops – A large herbivorous dinosaur with its iconic three-horned face and frilled skull.
- Hadrosaurs – Also called “duck-billed dinosaurs,” these plant-eaters were among the most common herbivores of the region.
The landscape of Laramidia was balmy and luxuriant with dense forest, river valley, and floodplains. These habitats provided the perfect setting for an abundance of dinosaur species to thrive.
Appalachia: A Land of Isolation
On the eastern side of the Western Interior Seaway lay Appalachia, a landmass separated from Laramidia. Unlike its western counterpart, Appalachia’s dinosaurs evolved in relative isolation. While fossils from this region are rarer due to geological factors, scientists believe that unique species of hadrosaurs, nodosaurs, and dromaeosaurs roamed these lands.
Appalachia’s landscape was dominated by dense forests, swamps, and a humid climate. Flowering plants were beginning to flourish, creating a new food source for herbivorous dinosaurs. Without large predatory dinosaurs like T. rex, smaller theropods likely filled the role of apex predators.
The Marine Giants of the Western Interior Seaway
While dinosaurs dominated the land, the Western Interior Seaway was teeming with life beneath its waves. Some of the most remarkable marine creatures included:
- Mosasaurs – Giant marine reptiles, sometimes exceeding 50 feet in length, that ruled the ocean as top predators.
- Plesiosaurs – Long-necked marine reptiles that hunted fish and squid-like creatures.
- Xiphactinus (X-Fish) – A massive, aggressive fish that could reach up to 20 feet long, known for swallowing prey whole.
- Ammonites – Spiral-shelled mollusks, distant relatives of squid and octopuses, that were common in the seaway.
- Pterosaurs – While not marine creatures, these flying reptiles soared over the waves, hunting for fish and carrion.
At times, the bottom of the Western Interior Seaway became anoxic (lacking oxygen), which slowed decomposition and created excellent conditions for fossil preservation. Many fossils of marine life from this time remain remarkably well-preserved in today’s sedimentary rock layers.
Modern Traces of the Western Interior Seaway
Though the seaway vanished millions of years ago, its traces can still be seen in the rock formations found across the central United States. Some of the most notable remnants include:
- Monument Rocks, Kansas – Large chalk formations made from the remains of tiny marine organisms called coccolithophores.
- Pierre Shale, South Dakota – A fossil-rich deposit that has yielded mosasaur and ammonite specimens.
- Austin Chalk, Texas – A famous marine deposit containing well-preserved fossils of ancient sea life.
These deposits serve as a time capsule, allowing scientists to study the ancient ecosystems that once thrived in the Western Interior Seaway.
The Disappearance of the Western Interior Seaway
As the Late Cretaceous period came to a close, tectonic activity caused the land to rise, draining the Western Interior Seaway. This change resulted in major reshaping of the geography of North America, preparing the continent for its current form. Slowly, the great inland sea gave way to prairies, forests, and the nascent beginnings of mountain ranges.
The end-Cretaceous extinction event, probably resulting from a huge asteroid impact, then remodeled the planet’s environments. The Cretaceous epoch of dinosaurs closed, opening up the way to the emergence of mammals and later human society.
