Tetrapods may be defined as vertebrates with four limbs and digits (that is, arms and legs bearing fingers and toes). The term includes not only the extinct forms described here, but all their relatives and descendants, including dinosaurs, snakes, frogs and salamanders, and even mammals like ourselves. It has, however, become increasingly difficult to draw a line between some of the earliest fossil tetrapods and their closest relatives with fins, in part as a result of the increase in the number of transitional fossils that are being discovered. Thus, the definition of the word “tetrapod” has become almost as murky as the swamps from which the first four-legged animals emerged.