In mammals teeth have reached their highest peak of evolution, mammalian teeth are both more complicated and more efficient than in other vertebrates. Teeth are heavy and require considerable muscle to operate efficiently, this has made an important contribution to the evolution of the mammal skull. Mammals are heterodonts, this means some of our teeth are different. In fishes and reptiles the teeth are all basically the same, some bigger than others but the same basic shape. Mammals needed their teeth to do several different jobs and so mammal teeth evolved into different forms.
Teeth are the hardest part of any mammal and therefore they are the part most often fossilised.
The number, size, organisation and shape of the teeth are different in every species of mammal and can be used in taxonomy, especially of fossils. In fact without teeth the fossil record would be much harder to understand. Mammals have only two sets of teeth, the first set they get soon after birth, often called the 'milk teeth' and a larger set they acquire as an adult. In all other toothed vertebrates teeth just keep coming, no matter how many you lose there is always another one ready to take its place. In other words fish amphibians, reptiles and birds either have no teeth or numerous sets.