By Stacey Venzel
First appearing on the planet 245 million years ago, the turtles of today have not changed much from the turtles of the past. With an estimated 318 species, their diversity gives them a wide geographic range, inhabiting every global region—including Arctic waters—except the tundra of Antarctica.
Members of the order Testudines, sometimes classified as Chelonia, “turtles” is a broad term that includes three different types of these reptiles: tortoises, turtles and terrapins.
All chelonians share five traits in common that distinguish them from other reptiles. First, every species has a shell made of a carapace and a plastron. Second, all of them have four appendages. Additionally, they all have a tail, hatch from eggs, and have a specialized skull which lacks teeth.
The group of shelled reptiles can be further divided into three categories based on anatomy and habitat. Tortoises live strictly on land. Because of this, they have adapted flat feet for traversing terrain. Acting as protection from predators, the shell of a tortoise is also highly domed, like that of the rare radiated tortoise of Madagascar.
Aquatic-dwelling individuals living in or nearby water are called turtles. Their anatomy is modified for their habitat. Typically the shell of a turtle is much flatter than a tortoise for more streamlined swimming. Turtles also developed long-clawed appendages specialized for life in the water. Freshwater turtles like the river cooter have webbed feet. The box turtle is sometimes referred to as a tortoise because it has a relatively domed shell and spends a great deal of time on land, but the webbed feet classify it as a turtle. Sea turtles and the freshwater pig-nosed turtle spend almost their entire lives in water and so they evolved flippers, which act as rudders and paddles.
Terrapins are also members of the Testudines, distinguished by their habitat in swampy or brackish waters. Like turtles, terrapins have webbed feet and usually have a less prominent dome shape to their shell. The diamondback terrapin has a long habitat range, stretching from Cape Cod down to southern Florida.
Based on their adaptation to retract extremities, all turtles can be further divided into one of two groups: the hidden-necked or side-necked turtles. The majority of Testudines are hidden-necks (Cryptodira), characterized by the ability to completely pull their head inside the shell. The gigantic Galapagos tortoise is one well-known example. Side-necked species (Pleurodira), such as the northern Australian snake-necked turtle, rely on long necks that fold backward to rest the head, exposed but somewhat protected, against the shell. Though sea turtles have lost the ability to retract their head and limbs into their shell, they fall into the hidden-neck category.
Karen Eckert, David Gulko, Sea Turtles: An Ecological Guide (Honolulu, Mutual Publishing, 2004), 14, 26.
Carl J Franklin, Turtles: An Extraordinary Natural History 245 Million Years in the Making (St. Paul, Voyageur Press, 2007), 9, 17-18, 53, 57, 132.
Whit Gibbons, Judy Greene, Turtle: The Animal Answer Guide (Baltimore, The John Hopkins University Press, 2009), 1-3, 5, 9, 21.