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The Western Interior Seaway was a large sea that covered much of interior North America during the Early and Middle Cretaceous, famous for its skeletons of sea monsters such as mosasaurs, ichthyosaurs, and plesiosaurs. It was created by a transgression event caused by seabed widening and seafloor spreading in the Atlantic Ocean. The sea was relatively shallow and covered most of the southeastern United States.
The Western Interior Seaway, also called the Cretaceous Seaway, was a massive stream that covered much of interior North America during the Early and Middle Cretaceous, about 100 to 70 million years ago. The Western Interior Seaway was among the largest continental seas of all time, extending from Utah in the west to the western Appalachians in the east, totaling approximately 1000 km (621 mi) in width. Its deepest point was only about 800 or 900 meters (about half a mile) under water, relatively shallow for a sea. The Western Interior Seaway also covered most of the shallow southeastern United States, including all states adjacent to today’s Gulf of Mexico.
The Western Interior Seaway was created during one of the greatest trespassing events of all time. In paleogeography, a transgression event is the rise in world sea levels. This may be caused by melting ice sheets, but in this case the ice sheets were already melted and the transgression was caused by seabed widening, building mountains underground. Here, seafloor spreading occurred in the Atlantic Ocean, creating so much new rock that it lowered the overall capacity of this basin, raising sea levels worldwide for millions of years.
The Western Interior Seaway is famous for its skeletons of sea monsters: mosasaurs, ichthyosaurs and plesiosaurs, ancient marine reptiles that lived during the Age of Dinosaurs but were later wiped out during the Late Cretaceous Extinction. Some of the mosasaur fossils discovered in places like present-day Kansas were up to 18 m (60 ft) long, representing some of the largest marine predators of all time. Mosasaurs resembled giant marine crocodiles with fins.
Ichthyosaur fossils are also found in the dried up basin of the Western Interior Seaway. Ichthyosaurs are marine reptiles that superficially resemble fish or dolphins. The world’s largest ichthyosaur fossil to date was unearthed by Canadian paleontologist Dr. Elizabeth Nicholls of Pink Mountain in British Columbia. It measured 23m (75ft), truly pushing the size limits of marine animals. These animals had a teardrop shape for swimming and huge eyes for hunting in dark water.
The Western Interior Seaway was also inhabited by the iconic plesiosaurs, long-necked marine reptiles. The Loch Ness Monster is sometimes said to be a surviving plesiosaur, although this is highly unlikely. True plesiosaur fossils are being unearthed all the time in the central United States, with some species up to 20 m (65 ft) long. One species, Thalassomedon, had a neck longer than its body, in one of the most extreme examples of neck extension among plesiosaurs ever found. This would have allowed it to poke its head into delicious schools of fish for snacks, without frightening them with the massive bulk of its body.
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