Tetrapods: More on Morphology

Tetrapods have four limbs.

Tetrapod means "four feet," and all tetrapods -- except for highly modified forms, such as snakes -- have four limbs with the same basic structure. These limbs are most commonly used for locomotion -- crawling, walking, running, flying, or swimming -- and are modified accordingly, but all tetrapod limbs share a common basic structure. Both the forelimb and the hindlimb have one long bone that attaches to the body at one end and to two long bones at the other end.

The two long bones join a group of smaller carpal bones (in the forelimb) or tarsal bones (in the hindlimb) which form the "hand" (or manus) and "foot" (or pes) along with the digits (fingers and toes). The oldest tetrapods had seven or eight digits on each limb; the ancestral pattern for living tetrapods is five digits on each limb (such as the five fingers and toes of humans). However, there are a huge number of ways in which this basic pattern has been modified through evolution. To give a very few examples: horses have lost all but one of their digits; birds have lost or fused together their tarsal bones, carpal bones, and forelimb digits; whales and dolphins have rudimentary internal hindlimbs; most snakes have lost all trace of limbs.

The tetrapod skull is a useful feature for classification.

One of the most useful features in classifying tetrapods is the skull, a collection of bones which surrounds the brain, and includes the jaw. The structure and arrangement of the bones in the skull is what gives the major groups of amniote tetrapods their names. The Anapsida, Diapsida, and Synapsida are defined by the number of openings on each side of the skull: zero, two, and one, respectively.
This turtle fossil is an example of an Anapsid skull. Notice how there are no holes on the back of the skull behind the large eye holes.

Harris' Hawk photo courtesy Gerald and Buff Corsi © California Academy of Sciences.
Merriam's Kangaroo Rat photo courtesy Dr. Lloyd Glenn Ingles © California Academy of Sciences.
Pico Blanco Robber Frog photo courtesy © Andrew J. Crawford.
Green Sea Turtle photo courtesy Caroline Kopp © California Academy of Sciences.
Pronghorn Antelope photo courtesy Glenn and Martha Vargas © California Academy of Sciences.
Turtle skull fossil photo © UCMP.