The first dinosaur ever discovered by paleontologist Gideon Mantell (1790–1852) was an Iguanodon. The iguanodons evolved in the Early Cretaceous period, were larger than the heterodontosaurs and hypsilophodontids, and were fast and strong. They had long, heavy forelegs, and probably walked on all fours. They measured about 33 feet (10 meters) long, had up to 29 teeth per tooth row (on the sides of their jaws for chewing)—and a unique, conical thumb “spike” on the first digit of their hands that may have been used for defense.