WHAT IS AN APATOSAURUS? Apatosaurus was a gigantic, long-necked dinosaur that lived during the Late Jurassic period, around 150 million years ago. It was one of the large sauropods — the group of plant-eating dinosaurs known for their long necks, long tails, and massive bodies. Here are the key facts: Name meaning: “Deceptive lizard” (from Greek apatao, meaning “to deceive”) Length: Up to about 75 feet (23 meters) Weight: Around 20–25 tons — as heavy as several elephants Diet: Herbivore — it ate