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<p>Polyphyodonts include most toothed fishes, many reptiles such as <a href="https://en.wikipedia.org/wiki/Crocodile" title="Crocodile">crocodiles</a> and <a href="https://en.wikipedia.org/wiki/Gecko" title="Gecko">geckos</a>,<sup><a href="https://en.wikipedia.org/wiki/Polyphyodont#cite_note-2">[2]</a></sup><sup><a href="https://en.wikipedia.org/wiki/Polyphyodont#cite_note-FUENZALIDA2000-3">[3]</a></sup><sup><a href="https://en.wikipedia.org/wiki/Polyphyodont#cite_note-Gaete2013-4">[4]</a></sup> and most other vertebrates, mammals being the main exception.</p>
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<h2>Evolution in mammals</h2>
<p><a href="https://en.wikipedia.org/wiki/Manatees" title="Manatees">Manatees</a> 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 extant mammals are not polyphyodont, mammalian ancestors were. During the evolution of <a href="https://en.wikipedia.org/wiki/Therapsida" title="Therapsida">Therapsida</a>, 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 <a href="https://en.wikipedia.org/wiki/Arthropods" title="Arthropods">arthropods</a>. <a href="https://en.wikipedia.org/wiki/Molar_(tooth)" title="Molar (tooth)">Molars</a> came later in their evolution (as earlier in <a href="https://en.wikipedia.org/wiki/Cerapods" title="Cerapods">cerapods</a> and <em><a href="https://en.wikipedia.org/wiki/Diplodocus" title="Diplodocus">Diplodocus</a></em><sup><a href="https://en.wikipedia.org/wiki/Polyphyodont#cite_note-D%E2%80%99Emic2013-9">[9]</a></sup>). Mammals chew (<a href="https://en.wikipedia.org/wiki/Masticate" title="Masticate">masticate</a>) their food which requires a set of firmly attached, strong teeth and a "full" tooth row without gaps.</p>
<p>The manatees have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and <a href="https://en.wikipedia.org/wiki/Premolar" title="Premolar">premolars</a>. These teeth are continuously replaced throughout 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”.<sup><a href="https://en.wikipedia.org/wiki/Polyphyodont#cite_note-10">[10]</a></sup></p>
<p><a href="https://en.wikipedia.org/wiki/Elephant" title="Elephant">Elephants</a> and <a href="https://en.wikipedia.org/wiki/Kangaroo" title="Kangaroo">kangaroos</a> are the only two other mammalian polyphyodonts.</p>
