Incredible Sea Monsters That Once Ruled The Oceans

(Source: list25)

Have you ever wondered what swam in Earth’s oceans millions of years ago? Things were much different then. Humans weren’t around, so we didn’t have to worry about being devoured by gargantuan monsters swimming in the deep. A big part of North America was covered in water called the Western Interior Seaway, brimming with predatory reptiles. These sea creatures were gigantic, fierce, and had razor sharp teeth, but there’s still a lot we don’t know about them. Curious to find out what we do know? These are some incredible sea monsters that once ruled the oceans.


Meaning “Fish Lizard,” Ichthyosaurs shared the sea with several large dinosaurs at the time. They originally swam like eels but advanced into looking and swimming like a modern day fish or dolphin. Like modern whales, they breathed air and lacked gills but could give birth in the water.


Located in California, British Columbia, and China, the Shastasaurus was a massive sea creature, measuring up to 21 meters long. In fact, it’s considered to be the largest marine reptile to ever live. In spite of it’s size, Shastasaurus primarily lived off of small fish, cephalopods, and squids.



Built for speed, the Pliosaur had a short neck and could swim up to 10 km/h. They hunted fish, cephalopods, and other marine reptiles but have been found to also have land dinosaur remains in their stomachs.



Found in Colorado and Montanna, the Thalassomedon has a long neck and was only known in the Late Cretaceous due to other predators taking over.


Resembling a modern day Great White Shark, the squalicorax has been found in North America, Europe, and North Africa. Remarkably, it’s been discovered that the squalicorax ate dinosaurs. With teeth measuring up to 21 millimeters wide we’re sure this creature had no problems crunching through dinosaur bones.


Xiphactinus is considered one of the fiercest and largest bony fish of the Late Cretaceous. At 13-feet-long it swam around the Western Interior Seaway and was capable of eating a 6-foot fish whole.


Nicknamed “Godzilla” due to its skull shape, the Dakosaurus’s appearance to the famed beast stops there. It swam around in the sea and shared much of the same waters of modern-day crocodiles.


One of the best-known plesiosaurs in the world, the Elasmosaurus is renowned for its incredibly long neck. While not a fast swimmer, it would use stalking tactics to prey on its food.

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