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If you went swimming 100 million years ago, you might have bumped into a plesiosaur (PLEEZ-ee-oh-sawr). They were as common in the oceans as dinosaurs were on land. But they were NOT dinosaurs. These animals were swimming reptiles! Some were as small as people. Others were as big as buses. They ate fish, reptiles, and other ocean animals.

Most reptiles lay eggs that later hatch into babies. But some reptiles give birth to live young. Which group did plesiosaurs belong to?

Scientists know that at least one group of plesiosaurs gave birth to large, live babies. This makes sense for a few reasons. Plesiosaurs would have had trouble pulling their huge bodies out of the water to lay eggs on the sand. Giving birth to live young also makes it easier for babies to survive. They come out like tiny copies of their parents, ready to go!