It is wildly strange to imagine that the air above Denver’s skyscrapers was once a warm, shallow sea packed with gigantic, sharp‑toothed reptiles. Yet for millions of years during the Late Cretaceous, roughly the time of the last dinosaurs on land, much of what is now Colorado lay beneath the Western Interior Seaway, an ocean that sliced North America in two. If you could peel back the highways and ski towns and drop yourself into that water, you would find yourself eye to eye with some of the most formidable marine predators Earth has ever produced.
These were not just big fish. We are talking about animals with skulls as long as a person is tall, jaws lined with conical teeth the size of your fingers, and bodies built for speed in open water. Some looked like nightmare dolphins, others like dragon‑sized crocodiles with flippers, and a few were basically biological torpedoes. What makes it better is that their bones have literally turned up in Colorado rock, sometimes shockingly close to places people now hike, drive, and build homes. Let’s dive into eight of the most impressive sea monsters that once patrolled the ancient seas above modern Colorado.
Mosasaurus: The Apex Reptile That Ruled Colorado’s Ancient Sea
Mosasaurus is the celebrity of the Western Interior Seaway, the massive, torpedo‑shaped marine reptile that dominated the food chain near the very end of the Cretaceous. Imagine a komodo dragon stretched out to the size of a bus, then streamlined with flippers, a powerful tail, and jaws lined with heavy, cone‑shaped teeth made for gripping and tearing. While complete Mosasaurus fossils from Colorado are rare, related large mosasaurs from the same group are well documented in nearby states, and Colorado preserves the same seaway rocks from that time window, making their presence here almost certain.
These predators were not picky eaters. They fed on fish, sharks, turtles, and even other marine reptiles, and some skulls from their relatives show bite marks that look suspiciously like mosasaur on mosasaur violence. If you stood on today’s Front Range around 70 million years ago, the shoreline would have been nearby, and off that coast an adult Mosasaurus might have been cruising the deeper water, hunting with a combination of speed and ambush tactics. Picture it twisting its body to rocket forward, jaws opening just enough to snap around an unlucky animal – one bite, a violent shake, and the meal would be gone in a cloud of shredded flesh.
Tylosaurus: The Spear‑Nosed Missile of the Western Interior Seaway
Tylosaurus was another giant mosasaur, and to me it is even more terrifying in design than Mosasaurus itself. Instead of a mouth full of evenly spaced teeth right up to the snout, Tylosaurus had an elongated, bony “ramming” snout with the teeth set slightly back. That gave it a built‑in battering ram, which many paleontologists think it used to stun prey before delivering the killing bite. Fossils of Tylosaurus and close relatives have been found in seaway rocks extending into the broader region, and the same formations that yield them appear in parts of eastern and northeastern Colorado…