Iguanodon [Gr., = iguana tooth], herbivorous ornithiscian dinosaur , characterized by teeth similar to those of the iguana , a horny beak, spikelike thumbs, and a powerful tail. It may have been capable of walking on either two or four feet. Fossils remains of the 30-ft-long (9-m) reptile have been found in Europe, Asia, Africa, and the W United States in strata of the early Cretaceous period, approximately 135-110 million years ago. It possessed characteristic spikelike thumbs. Iguanodon, the second dinosaur to be formally named, was first described by Gideon Mantell, an English physician and geologist. He credited his wife, Mary Ann Woodhouse Mantell, with the discovery. An impressive find by miners in a Belgian coal mine in 1878 turned up more than 20 nearly complete fossil skeletons.