She noticed something odd in the rocks.
Her husband, Doctor Gideon Mantell, was visiting a patient nearby, and they took the rocks home.
Gideon and Mary Mantell
The teeth Mary Mantell had found were like those of an iguana, a well-known lizard, but much larger.
Doctor Mantell chose the name Iguanodon for the animal which had once owned the teeth.
Over the next twenty years various other ancient bones and teeth were linked to giant land-living reptiles.
In 1842 Richard Owen, superintendent of the Natural History Museum in London, called these animals dinosaurs.
At first, not many full skeletons were found, so rebuilding dinosaurs was difficult.
Richard Owen thought Iguanodon looked like this model, made in the 1850s.
Lots of fossils have been found since, which makes it easier to see a better idea of this amazing animal.