One of the most well-known—and earliest discovered dinosaurs—was the Iguanodon (“iguana tooth”). Ornithopods also included the hadrosaurs (the “duck-billed” dinosaurs with the famous crests), the iguanodontids, the heterodontosaurs, the hypsilophodontids, and numerous other dinosaurs. But many of these dinosaurs were not prolific until the Cretaceous period. In general, the ornithopods were bipedal, ranging in size from less than 3 feet (1 meter) tall and six feet (two meters) long, to as big as about 23 feet (7 meters) tall and 66 feet (20 meters) long. The ornithopods were plant eaters, evolving early in the Jurassic period and continuing to the end of the Cretaceous. They lived on every continent, including Antarctica. They were the first herbivorous dinosaurs to have multiple tooth rows, cheek pouches, and the ability to truly chew.