dentition, kind, number, and arrangement of the teeth of humans and other animals. During the course of evolution, teeth were derived from bony body scales similar to the placoid scales on the skin of modern sharks. Tooth structures such as those found in humans are restricted to certain vertebrates, i.e., most fish, mammals, and reptiles, and some amphibians. The teeth of sharks, which are primitive vertebrates, consist of simple conelike structures, sometimes with serrated edges and sometimes flattened for crushing shelled prey. In many lower vertebrates the individual teeth are replaced throughout the animal's life; old tooth loss and new tooth growth follow wavelike patterns down the length of jaw and affect alternate teeth at any one time, so that half the teeth in a region are always functional. Fish and reptiles that have teeth have homodont dentition; that is, all teeth are identical. The mammals have heterodont dentition, or teeth of different basic types, including incisors for nipping or cutting, canines for piercing, and premolars and molars for shearing and grinding. Carnivorous animals have relatively small incisors, used for grasping rather than for cutting; long and strong canines; and relatively thin, sharp premolars and molars, used for severing muscle and other tissues. Herbivorous animals have well-developed incisors, used to cut grass and other vegetation; canines that are either smaller than those of carnivores or absent altogether; and broad, flat premolars and molars for grinding food. In some herbivores, the upper canines are absent, so they cut vegetation by the combined action of the tongue and lower incisors. Omnivorous animals such as man have less specialized dentition. Only part of the dentition of mammals is usually replaced; however, the incisors of rodents grow out at the base as fast as they wear down at the tip. Teeth, the hardest structures in the body, have been well preserved as fossils and have played an important role for paleontologists and physical anthropologists in the study of human evolution.