Until the 1800s, it was generally believed that animals did not become extinct. This changed with the discovery of prehistoric animal fossils, most notably dinosaurs. One of the most influential fossil hunters, Mary Anning, is also one of the least well known. Anning did not have a formal education as a scientist, but she was a self-taught fossil hunter from an early age. She excavated and recovered a skeleton of a Plesiosaurus dinosaur—the first intact Plesiosaurus skeleton ever discovered—along with many other significant fossils. She also collaborated with some of the most respected scientists of her time. The tongue twister "she sells seashells by the seashore" is derived from a poem that was written to honor Anning's memory.