Turtles are reptiles of the order Testudines, characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic. The earliest known turtles date from 215 million years ago, making turtles one of the oldest reptile groups and a more ancient group than lizards, snakes and crocodiles.