Despite some question about the event, it is generally believed that in 1822 Mary Ann Mantell stumbled upon a tooth unlike anything she had ever seen before. She presented the tooth to her husband, Gideon, a physician who had an interest in fossils. Based on the location of discovery, Gideon was able to trace the tooth back to its source and discovered several other teeth as well as some other bones. Although Dr. Mantell was unable to link the other bones with the teeth, he still had these highly unusual teeth to interpret.
Dr. Mantell ended up sending the teeth to renowned scientist Baron Georges Cuvier. Although Cuvier was initially skeptical of the teeth and said they were part of a rhinoceros, he later recanted. In a letter that Gideon reprinted in his announcement of Iguanodon, Cuvier expresses his opinion, that based on the structure of the teeth, they were from a very large, unknown, herbivorous reptile. Cuvier recommended finding more material though, of which Mantell was unable to do.
The teeth were found in the sandstone of the Tilgate forest, of which crocodile, plesiosaur, and Megalosaurus, material was already identified within the rock unit. These teeth, and some other bones, were the only pieces they weren't able to identify. Even though he didn't have much material to go on, Dr. Mantell would not be dissuaded and upon visiting the Royal College of Surgeons in London he was shown a specimen of an iguana, brought back from the West Indies by Darwin. Looking at the teeth, Dr. Mantell noticed the extreme similarity between the two, except his was much, much larger. He eventually published on the specimen in 1825 in a journal article entitled "Notice on the Iguanodon, a newly discovered fossil reptile, from the sandstone of Tilgate Forest, in Sussex".
"If however any inference may be drawn from the nature of the fossils with which its remains associated, we may conclude, that if amphibious, it was not of marine origin, but inhabited rivers or fresh-water lakes; in either case the term Iguanodon, derived from the form of the teeth, ... will not, it is presumed, be deemed objectionable."
"That the latter equalled, if not exceeded the former in magnitude [Megalosaurus], seems highly probable; for if the recent and fossil animal bore the same relative proportions, the tooth, fig. 1. must have belonged an individual upwards of sixty feet long; a conclusion in perfect accordance with thatdeduced by Professor Buckland from a femur, and other bones in my possession."The initial reconstruction for Iguanodon must also wait, like with Megalosaurus, another decade for a more complete specimen to be found. It is at that time that Mantell will return to the Iguanodon in order to create a what will become one of the first dinosaur reconstructions.