From bottom to top:
1. Common Snapping Turtle (Chelydra serpentina)
2. Geometric Tortoise (Psammobates geometricus)
3. Galapagos Tortoise (Dipsochelys dussumieri)
4. Argentine Snake-Necked Turtle (Hydromedusa tectifera)
5. Mata Mata (Chelus fimbriata)
6. Leatherback Turtle (Dermochelys coriacea)
7. Hawksbill Turtle (Eretmochelys imbricata)
The Testidunes (or Chelonii, often called Chelonians) are the reptiles belonging to the order Testudines, and include all of the turtles and tortoises. This order is characterized by their protective bony carapaces, which developed from their ribs millions of years ago. Ribs still line the inside of the shell.
As they’re reptiles, they’re ectothermic, and their body temperature adjusts to the surroundings. They’re all amniotes, as well - that is, they lay eggs outside of the water.
The carapace of most chelonians is covered by bony, overlapping plates, called scutes. However, some species, such as the leatherback sea turtle, have a thick, oily skin covering their carapace instead. Chelonians also have a protective chest plate, called a plastron.
The difference between “turtle”, “tortoise”, and “terrapin” is defined differently depending upon your field and what part of the world you live in. In general, turtles live in either freshwater or the ocean. Tortoises live on dry land and cannot swim. Terrapin is a more specific term for some turtles, referring to the small, edible, hard-shelled turtles.
While all chelonians can be long-lived (as their organs do not suffer age-related decay), large torotoises are the best-known for living over a hundred years. Jonathan, a Seychelles Giant Tortoise, is 183 years old next week, and is the oldest living vertebrate!
Kuntsformen der Natur. Ernst Haeckel, 1904.