They evolved in the Upper Triassic period and survive to this day. The earliest fossils were discovered in Germany and already had a fully developed shell. The Cretaceous marine turtle called Archelon reached a size of nearly four metres. Turtles' shells are formed by greatly expanded ribs. They first evolved shells for life on land in the Late Triassic period, but these remained a useful protection from predators when various species returned to the sea.
Marine forms eat fish, marine invertebrates and some plants.
The Fact Files, Family Tree and Glossary have been written in consultation with leading palaeontologist Michael Benton, Professor of Vertebrate Palaeontology at the University of Bristol, and represent the latest scientific knowledge about these creatures.