Jurassic refers to a period in Earth's history that lasted from about 201 to 145 million years ago, when dinosaurs ruled the planet. This was the middle chapter of the age of dinosaurs, between the Triassic and Cretaceous periods.

During the Jurassic, some of the most famous dinosaurs lived: the enormous long-necked Brachiosaurus, the plated Stegosaurus, and the fierce predator Allosaurus. The first birds evolved during this time from small feathered dinosaurs. The continents were still connected differently than today, and the climate was warmer and wetter, allowing lush forests to grow across much of the world.

When scientists find fossils in Jurassic rock layers, they know those creatures lived during this specific stretch of prehistoric time.

You might recognize the word from Jurassic Park, though that movie actually featured dinosaurs from multiple periods, not just the Jurassic. When paleontologists describe a fossil as Jurassic, they're placing it precisely in Earth's timeline, like marking a specific chapter in the longest story ever told.