Iguanodon is a rather famous dinosaur. It was the second dinosaur species to be described (the first was Megalosaurus, named by William Buckland in 1824.). Iguanodon, "iguana tooth lizard", was described by Dr. Gideon Mantell in 1825. Dr. Mantell was a medical doctor, and also an ardent amateur geologist and fossil collector.
Although there is no actual proof to the story, supposedly Mantell's wife Mary Anne, who was also interested in paleontology, had accompanied him on a medical visit in Sussex, England. While Dr. Mantell saw to the patient, Mary passed the time by walking about and looking at some rocks. She discovered a peculiar-looking tooth, which looked like it belonged to a gigantic iguana, which she showed to her husband. Dr. Mantell realized that Mary Anne had found something new to science, and he later wrote a scientific description of the animal using other teeth and bones that had been discovered.
Because Iguanodon's remains were sparse, and because very little was known about dinosaurs at the time (remember, Richard Owen did not name the Order Dinosauria until 1842!), Mantell's reconstruction of what he thought Iguanodon looked like is rather different from our current knowledge of the animal. Mantell had discovered a peculiar bone that he thought must be a short horn that he placed on Iguanodon's snout, this was later discovered to be its "thumb".
Iguanodon was an herbivore (a plant eater) belonging to the Family Iguanodontidae, which now contains Camptosaurus, Callovosaurus, Vectisaurus, Ouranosaurs, Muttaburrasaurs, and Probactrosaurus, all have prominent thumb spikes. The Iguanodontidae belong to a suborder of dinosaurs called the Ornithopods ("bird feet"), a group of bipedal/quadrupedal (they could walk on either two or four legs) dinosaurs.
The Iguanodontids developed in the middle Jurassic Period (195 to 140 million years ago), and spread throughout the world. (Their fossils have even been found in the Arctic Circle--this area would have been ice-free at the time. Be sure to look at the Paleomap Project.) The Iguanodontids reached the peak of their diversity towards the end of the early Cretaceous Period (140 to 65 million years ago), they then began to decline, and became extinct at the end of the Cretaceous Period during the Cretaceous-Tertiary Extinction Event.
So what was an herbivore doing with those nasty-looking thumb spikes? We do not know at this time. Some paleontologists have theorized that they may have used the spikes to tear down the foliage they fed on; or maybe they could have been used to defend against predators, such as the contemporary Megalosaurus. Some paleontologists theorize that they might have been used in courtship and mating.
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