During the Cretaceous Period (between 145 to 65 million years ago), a huge inland sea split the North American continent in two.
This Western Interior Seaway happened to run directly through what is now eastern Montana. As landscapes have folded and shifted over time, fossils from the Cretaceous have been brought to the surface, including those of fauna that once dwelt in Montana’s prehistoric sea.

Sharks and shark-like creatures have been discovered in Montana’s Bear Gulch Limestone (near Lewistown), which was deposited around 320 million years ago. Remains of the large Cretaceous shark Cretoxyrhina (sometimes called the “Ginsu- shark” as its teeth have been compared to Ginsu knives) have been found in Montana.

Though different from modern-day sharks, these animals at least bare passing resemblance to the aquatic predators of today. Mosasaurs, on the other hand, are far removed from species you’ll find in modern-day Montana. These prehistoric marine reptiles could grow longer than forty feet (depending on the species) and had a double-hinged jaw and flexible skull (like a snake) for devouring prey.

Similarly, a plesiosaur fossil is surreal to behold. Just imagine: a massive, long-necked reptile paddling along the sea shore—right here in Montana! It’s difficult to comprehend the image. And that’s partly why paleontology is so fascinating.

The Montana Dinosaur Trail offers a glimpse into the lives of sea creatures that inhabited our state. When you view these remains, remember: these are not fossils brought in from some distant location; they are remnants of our own prehistoric history.