Ichthyosaurus

The Ichthyosaurus was a species of marine reptile from the Age of the Dinosaurs. It was similar to a modern shark, complete with a triangular fin on its back and rows of sharp teeth. It was, however, significantly larger and more dangerous than the sharks of the 20th century.

The Fourth Doctor showed his companion an Ichthyosaurus and a Plesiosaurus during a trip to 75 million years before the 20th century. (PROSE: The Doctor Who Dinosaur Book)

The first fossilised remains of an Ichthyosaurus known to humanity were discovered by Mary Anning in Dorset in 1810. The discovery was witnessed by the Fifth Doctor, Tegan Jovanka and Nyssa. She described her discovery as a dinosaur; (PROSE: Amazing Fossil Fish) according to the Fourth Doctor, Ichthyosaurus and Plesiosaurus were not technically Dinosaurs, although they were related and similar enough that according to the Doctor, one "could count them as a kind of honorary Dinosaur". (PROSE: The Doctor Who Dinosaur Book)