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Teeth are essential for many animals, as they are used to break down food (and sometimes for other purposes as well). Numerous instances of convergence are related to teeth, and as these hard, calcified structures fossilize well, they can also provide insights into the evolutionary history of extinct groups. The sabre-toothed cats, carnivores with long, dagger-like canines, represent one of the most spectacular tooth-related convergences, as only some of them were actually true cats. The sabre-tooth morphology evolved repeatedly in several groups, even in marsupials! On a rather more peaceful note, ceratopsians and hadrosaurs, dominant plant-eating dinosaurs during the Late Cretaceous, both independently evolved continuous batteries of serrated teeth that occluded at the back of the mouth to shear and slice vegetation. Today, analogues of these dental batteries are found in elephants.
Many interesting (and convergent) tooth specialisations have arisen in fish. Teleosts possess pharyngeal jaws, a second set of jaws in the pharynx that are functionally separate from the oral teeth and can take on different functions, thus allowing a huge diversity of feeding modes. Best known are probably the moray eels, which use their pharyngeal jaws to feed in a way that is strikingly reminiscent of snakes. Pharyngeal jaws can furthermore be modified as crushing tooth plates to eat hard-bodied prey, including molluscs and crustaceans. Such durophagy has arisen several times, for example in the freshwater cichlids and the marine labrids (which show further interesting feeding convergences, for instance the repeated evolution of mechanically fast jaw systems). Several groups of fish employ their oral teeth as crushers, including bony (e.g. pufferfish) as well as cartilaginous fish (e.g. eagle rays and bullhead sharks). In the latter group, this is particularly remarkable, as a suite of morphological and functional adaptations are needed to crush the protective armour of prey with the rather soft cartilaginous jaws.
Reptile dentition is highly instructive, too, and an interesting case is found in those taxa that have adapted to life in aquatic environments. Distinctly specialised teeth and jaws are critical to capturing and processing prey underwater. Despite the taxonomic diversity of aquatic reptiles, however, only three tooth types have been described, presenting us with a clear example of convergent evolution. In contrast, teiids, a group of skink-like lizards, show a stunning diversity of tooth types, from simple, single-cusped teeth to complex, multi-cusped molariforms. These cases provide rich evidence of convergence, within the teiids themselves as well as with distantly related reptile groups and even certain mammals and fish. The dentition of two unusual Early Cretaceous crocodiles, which show complex tooth occlusion, parallels that observed in a group of advanced proto-mammals called tritylodonts. Turtles and lizards of the genus Uromastyx have convergently lost their dentition and adopted specialised beak structures for feeding, which is, of course, reminiscent of birds. More famously, however, certain snakes such as vipers and cobras possess enlarged, hollow venom fangs in the upper jaw. Interestingly, these two groups independently arrived at the same developmental mechanism for formation of their anterior fangs. Venom fangs evolved convergently also outside the snakes, for example in some lizards but even in some mammals, for example solenodons, where the venom-delivering teeth are located in the lower jaw. Curiously, several groups of frogs also possess fangs, but it remains unknown whether they are involved in prey capture or have resulted from sexual selection.
Teeth are, inevitably, subject to wear and various strategies have evolved to deal with this. Of particular interest is the convergent incorporation of zinc to harden the teeth or equivalent structures. This is observed in, for example insects and polychaete annelids.