A polyphyodont is any animal whose

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

are continually replaced. In contrast,

diphyodont A diphyodont is any animal with two ss of tooth (animal), teeth, initially the ''deciduous teeth, deciduous'' set and consecutively the ''permanent teeth, permanent'' set. Most mammals are diphyodonts—as to chew their food they need a strong, dura ...

s are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes, many reptiles such as

crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...

s and

gecko Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from . Geckos ar ...

s, and most other vertebrates, mammals being the main exception.

Growth

New, permanent teeth grow in the jaws, usually under or just behind the old tooth, from

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

s in the

dental lamina The dental lamina is a band of epithelial tissue seen in histologic sections of a developing tooth. The dental lamina is first evidence of tooth development and begins (in humans) at the sixth week in utero or three weeks after the rupture of th ...

. Young animals typically have a full set of teeth when they hatch; there is no tooth change in the egg. Within days, tooth replacement begins, usually in the back of the jaw continuing forward like a wave. On average a tooth is replaced every few months.

Crocodilia

Crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...

are the only non-mammalian vertebrates with

tooth socket Dental alveoli (singular ''alveolus'') are sockets in the jaws in which the roots of teeth are held in the alveolar process with the periodontal ligament. The lay term for dental alveoli is tooth sockets. A joint that connects the roots of the ...

Alligators An alligator is a large reptile in the Crocodilia order in the genus ''Alligator'' of the family Alligatoridae. The two extant species are the American alligator (''A. mississippiensis'') and the Chinese alligator (''A. sinensis''). Additionall ...

grow a successional tooth (a small replacement tooth) under each mature functional tooth for replacement once a year, each tooth being replaced up to 50 times in the alligator's life. Crocodilia are researched for

tooth regeneration Tooth regeneration is a stem cell based regenerative medicine procedure in the field of tissue engineering and stem cell biology to replace damaged or lost teeth by regrowing them from autologous stem cells. As a source of the new bioengineered tee ...

in humans.

Evolution in mammals

Manatees Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...

elephants Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae and ...

and

kangaroos Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...

are unusual among mammals because they are polyphyodonts, in contrast to most other mammals which replace their teeth only once in their lives (diphyodont). Although most other extant mammals are not polyphyodont, mammalian ancestors were. During the evolution of Therapsida, there was a period during which mammals were so small and short-lived that wear on the teeth yielded no significant selection pressure to constantly replace them. Instead, mammals evolved different types of teeth which formed a unit able to crack the exoskeleton of

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

. Molars came later in their evolution (as earlier in

cerapods Cerapoda ("ceratopsians" and "ornithopods") is a clade of the dinosaur order Ornithischia, that includes pachycephalosaurs, ceratopsians and ornithopods Classification Cerapoda is divided into two groups: Ornithopoda ("bird-foot") and Marginoc ...

and ''

Diplodocus ''Diplodocus'' (, , or ) was a genus of diplodocid sauropod dinosaurs, whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek δι� ...

''). Mammals chew (

masticate Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, t ...

) their food which requires a set of firmly attached, strong teeth and a "full" tooth row without gaps. The manatees have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and

premolar The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...

s. These teeth are continuously replaced throughout their life with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, a process known as "hind molar progression" or “marching molars”.

Growth

Crocodilia

Evolution in mammals

See also

References

Further reading