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When sharks swam the Great Plains


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If you’ve ever been to, lived in, or even flown over the central swath of North America, then you’ve seen the remnants of what was a uniquely fascinating environment. Scientists call it the Western Interior Seaway, and at its greatest extent, it ran from the Caribbean Sea to the Canadian Arctic.

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Sharks are the archetypal ocean predators but they are far from the only ones to have existed. Besides arthropods such as anomalocarids and sea scorpions, the first big predators to swarm the ocean were placoderms, the first fish to have jaws to crush their prey, but no teeth. Most apex predators to follow actually returned to the sea after an evolutionarily brief stint on land. These groups gave rise to the largest animals to have ever lived and include the ichthyosaurs, plesiosaurs, metriorhynchids, mosasaurs, whales, seals and penguins. Nostalgia for the sea also overcame grazing animals including sloths, turtles and iguanas.

These later seaward migrations of large animals are what drove sharks to get bigger and resulted in megalodon. The sheer number of whales was so great that they affected the biogeochemistry of the deep ocean. The megafauna boom didn’t last and megalodon eventually went extinct, being outcompeted by the great white shark. To this day, sharks continue their legacy as highly effective predators and stewards of marine forests, notably kelp and mangrove forests, but they are under threat of disappearingin the Athropocene Extinction.

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