When it died, an elasmosaur (ee-LAZ-moh-sawr) sunk to the bottom of the ocean. There, dirt covered up its skeleton. Fast forward many years. The dirt turned to rock. Little by little, ocean water made the skeleton dissolve. Then the rock held a cavity—or hole—shaped just like the elasmosaur.

But the cavity didn’t stay a cavity. Ocean water seeped inside it. Minerals in the water created something called a cast. The cast looks like the elasmosaur’s bones. But really it is just a copy. It shows the bones’ size and shape. We call the cast a fossil. Years later, the fossil turned up on Earth’s surface. The rock around it had worn away. This June, scientists identified the fossil in Anchorage, Alaska.

Scientists looked hard at the fossil. They guessed that elasmosaurs had very long necks, with arms and legs that acted just like paddles. One scientist says the elasmosaur looked just like the Loch Ness monster, a creature some people believe lives in Scotland. And even though the elasmosaur has “saur” at the end of its name, scientists don’t call it a dino. They call it a marine reptile.

It takes a lot of water pressure to make a fossil. But not all fossilized creatures lived in the sea. The remains of some land animals washed into the ocean. Then they spent years turning into fossils. This year, scientists found the fossil of an ancient beaver in Oregon. The creature measured about half the size of today’s beavers. Scientists believe it roamed the Earth long after the dinosaurs. They believe three-toed horses, two-horned rhinos, giant pigs, saber-tooths, rabbits, and several species of dogs were hanging around at the same time. Oregon gets lots of rain and snow. That wet weather made the rock that held the fossil shrink and swell. Then, pop! The fossil pushed out of the rock.

In order for a fossil to form, conditions have to be just right. Just think of how many animals and plants die without leaving such a lasting record! Think of how many such records we still might discover!