Chelonian is the preferred term to describe turtles, terrapins and tortoises. Although technically all tortoises and terrapins are turtles, the vernacular usage of these three terms often makes these designations ambiguous. For the most part, “turtle” usually refers to both fresh and salt water species, “terrapin” refers to brackish water species and “tortoise” refers to terrestrial species.

Chelonia as an order is believed to be the oldest group of reptiles; the earliest fossil evidence of them date back about 260 million years ago. Interestingly, they have changed little in all of that time. Although the very earliest ancestors of chelonians lacked a full shell (they lacked the carapace), a species called Odontochelys semitestacea from around 220 million years ago had all of the other key features of present day turtles.

There are currently nearly 300 described species of chelonians, found on all continents except Antarctica, and in all of the oceans. They range in size from the giant Galapagos tortoise, Geochelone elephantopus capable of weighing well over 800 pounds, and the huge leatherback sea turtle, Dermochelys coriacea, weighing nearly a ton, to the tiny cape speckled padloper tortoise, Homopus signatus signatus maxing out at four inches long, and the bog turtle, Glyptemys muhlenbergii with a maximum length of 4 ½ inches.