Dinosaurs were members of the wider "archosaur" group of reptiles that first appeared around 250m years ago. Some other archosaurs often get wrongly labelled dinosaurs - notably everyone's favourite flying reptile, Pterodactylus (often referred to as a pterodactyl). It was one of the pterosaur reptiles that evolved powered flight before birds or bats, and lived alongside dinosaurs until their parallel extinction at the end of the Cretaceous period.

Today's crocodiles are also archosaurs, and their ancestors are believed to date back 230m years to the time of dinosaurs. They, however, survived the great extinction event, probably because by the Cretaceous they lived predominantly in water.

As well as "non-dinosaur" archosaurs, there are many other ancient reptiles that are wrongly termed dinosaurs - many of them marine creatures (true dinosaurs lived only on land).

The large sea creatures commonly depicted from the start of the Jurassic period were "plesiosaurs" - carnivorous marine reptiles more closely related to modern lizards, often with a broad body and long neck. Many of these reptiles' fossils have been found in Britain, including some virtually complete skeletons, and there is an important collection in the Natural History Museum. Plesiosaurs also went extinct at the end of the Cretaceous epoch.

Another group of giant marine reptiles were the "ichthyosaurs" ("fish lizards"). Resembling dolphins and fish, the oldest examples of their fossils date back 245m years - and they thrived until 90m years ago, 25m years before dinosaurs were wiped out. Their extinction may simply have been caused by excessive competition with other marine reptiles.

Another creature commonly mistaken for a dinosaur is Dimetrodon, with its distinctive "sail" back and sprawling reptilian stance. In fact, it pre-dated the dinosaurs and lived during the Permian period approximately 275m years ago. Dimetrodon is classified as a "pelycosaur" or "basal synapsid", and is actually more closely related to mammals than reptiles.