The teeth of most vertebrates are replaced continuously throughout the animal's life. Teeth erupt from the jaw, they wear and perhaps break, and they drop out of the jaw as new teeth grow in at their roots. This eminently practical process doesn't work well, however, for animals that depend on the precise meeting ("occlusion") of upper and lower teeth for chewing, as it creates gaps where teeth are absent or still growing in. For an analogy, consider what it is like to try to cut with scissors that have a deep notch in one blade. Such a row of teeth might work well for grabbing and holding prey, but it would be much less efficient than an uninterrupted row when it comes to slicing and grinding.
The pattern found instead in most mammals (and believed to be primitive for the group) is diphyodonty, a term derived from Greek words meaning "twofold production of teeth." Most mammals are born with a special set of usually smaller, weaker teeth called milk teeth or deciduous teeth. In many species, milk teeth erupt soon after birth; in a few, they erupt and are replaced by adult teeth in utero; and in some, they appear to be absent altogether. The milk teeth consist of incisors, canines, and premolars; molars come in later as part of the adult dentition and are not replaced.
In most species, the main period of growth of the jaws takes place during the period when milk dentition is present. Continual growth of the skull and jaws makes precise occlusion difficult or impossible; teeth that barely fit in a newborn's jaws may be separated by considerable gaps by the time they are replaced. But young mammals feed on milk, a source of nourishment that requires no chewing, so for them, accurate occlusion is probably less important. Milk teeth generally are replaced by the time of weaning, and molars are added as space becomes available in the growing jaws.
Replacement of milk teeth and eruption of molars takes place in a relatively fixed, species-specific sequence. This means that it is often possible to estimate the age of a young animal by determining if milk teeth are still present, and if so, which they are. One can also determine if all molars are present.
Teeth -- adult as well as milk -- wear down as they are used. The rate of wear can be quite rapid, as in the case of rodent incisors, or slow, as in the case of human molars. The rate depends partly on the conformation of the tooth's occlusal surface, partly on the abrasiveness of the animal's diet, partly on how the tooth is used, and partly on whether it is rooted or unrooted. It is a fairly common practise of mammalogists to estimate an animal's age by examining the wear on its molars. This gives an idea of the age relative to that of others with more or less worn teeth, and if proper controls are present, may also provide evidence of absolute (chronological) age.
There are a few exceptions to the pattern of diphydont replacement of teeth. Toothed whales have a single set of (unreplaced) teeth. In pinnipeds and many rodents, both sets of teeth are present but replacement takes place in utero, that is, before the young is born. In elephants, the cheek teeth (molars and premolars) erupt from the rear of the jaw and gradually move forward, dropping out as they reach the anterior end of the jaw. Elephants eat a very abrasive diet, and the teeth wear strongly as they move forward. By the time the tooth is lost, it is largely worn away. Generally only two teeth are active at any time. Elephants have a maximum of six cheek teeth on each side of the upper and lower jaws; captives often live beyond the time when they lose their last tooth and must be fed a diet of soft materials. Manatees, which also feed on abrasive vegetation, have a similar pattern, but their dentition includes up to 20 teeth per quadrant, six to eight functional at one time. Kangaroos also have a similar pattern of tooth replacement. In young kangaroos, two blade-like premolars erupt first on each side. These are soon shed and replaced by a third premolar. The molars erupt in succession, with the anterior-most appearing first, then move forward as the animal ages. A young kangaroo may have the first two molars exposed; a middle-aged kangaroo will have four, but the first will be in the position originally occupied by the third premolar. An old individual may have just one or two teeth remaining, and these will be in the anterior part of the jaw. One species of kangaroo, the Nabarlek (Peradorcas concinna), has a seemingly unlimited supply of supernumerary molars, which continue to erupt throughout its life.
Phil Myers (author).