Millions of years ago, long before there were any people, there were dinosaurs. Dinosaurs were one of several kinds of prehistoric reptiles that lived during the Mesozoic Era, the "Age of Reptiles." Dinosaurs were reptiles and most hatched from eggs. No dinosaurs could fly and none lived in the water.The largest dinosaurs were over 100 feet (30 m) long and up to 50 feet (15 m) tall (like Argentinosaurus, Seismosaurus, Ultrasauros, Brachiosaurus, and Supersaurus). The smallest dinosaurs, like Compsognathus, were about the size of a chicken. Most dinosaurs were in-between.It is very difficult to figure out how the dinosaurs sounded, how they behaved, how they mated, what color they were, or even how to tell whether a fossil was male or female.There were lots of different kinds of dinosaurs that lived at different times. Some walked on two legs (they were bipedal), some walked on four (they were quadrupedal). Some could do both. Some were speedy (like Velociraptor), and some were slow and lumbering (like Ankylosaurus). Some were armor-plated, some had horns, crests, spikes, or frills. Some had thick, bumpy skin, and some even had primitive feathers.Most dinosaurs were plant-eaters (also called herbivores). For example, Triceratops was a plant-eating dinosaurSome dinosaurs were meat-eaters (also called carnivores). For example, T. rex was a meat-eating dinosaur.The dinosaurs dominated the Earth for over 165 million years during the Mesozoic Era, but mysteriously went extinct 65 million years ago. Paleontologists study their fossil remains to learn about the amazing prehistoric world of dinosaurs.When the dinosaurs lived, the Earth's continents were jammed together into a supercontinent called Pangaea and the Earth was warmer than it is now.