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A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. [1] Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, [2] [3] [4] and most other vertebrates, mammals being the main exception, though not absolute.
A diphyodont is any animal with two sets of teeth, initially the deciduous set and consecutively the permanent set. [1] [2] [3] Most mammals are diphyodonts—as to chew their food they need a strong, durable and complete set of teeth. Diphyodonts contrast with polyphyodonts, whose teeth are constantly replaced.
Horse teeth can be used to estimate the animal's age. Between birth and five years, age can be closely estimated by observing the eruption pattern on milk teeth and then permanent teeth. By age five, all permanent teeth have usually erupted. The horse is then said to have a "full" mouth.
These are called deciduous teeth, primary teeth, baby teeth or milk teeth. [7] [8] Animals that have two sets of teeth, one followed by the other, are said to be diphyodont. Normally the dental formula for milk teeth is the same as for adult teeth except that the molars are missing.
These two components are what remains following fusion between a larger set of bones. An additional inset tooth row with up to 20 teeth lies parallel to the main marginal tooth row of the jaw. [20] All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in other amphibians.
Comparison of carnassial teeth of wolf and typical hyaenodontid and oxyaenid. Creodonts had two or three pairs of carnassial teeth, but only one pair performed the cutting function: either M1/m2 or M2/m3, depending on the family. [8] In Oxyaenidae, it is M1 and m2 that form the carnassials. Among the hyaenodontids it is M2 and m3.
An African elephant in Tanzania, with visible tusks. Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammal species. They are most commonly canine teeth, as with narwhals, chevrotains, musk deer, water deer, muntjac, pigs, peccaries, hippopotamuses and walruses, or, in the case of elephants, elongated incisors.
A 380-million-year-old fossil of one species represents the oldest known example of live birth. [18] The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions, and the class was entirely extinct by the end of the Devonian. Chondrichthyes: Cartilaginous fishes