What was the first dinosaur fossil to be described scientifically?

The first dinosaur recognized by science - the Megalosaurus – was described in 1824 from a partially preserved jaw. Most rocks making up England's underground were formed by ancient rivers depositing sand and mud in a shallow sea. Water currents and erosion will break and scatter animal remains. In most cases, only single bones survive the long and arduous fossilization process. For example, the Iguanodon, the first herbivore dinosaur recognized as such, was known at the time only from some teeth and bone fragments.

Source: American Museum of Natural History