Looking at an animal's teeth can provide a lot of information on how the animal lived. This is particularly useful when the animal is extinct, or no longer exists in the world we live in. Dinosaur teeth can tell you many things about it, such as the type of food that it ate, how it got its food, and how it digested its food (did it chew food, crush or grind food, or just eat it whole?).
Teeth are much harder than bone, and so they fossilize more readily. Countless fossilized dinosaur teeth have been uncovered. Some dinosaur species such as Cardiodon, Trachodon and Deinodon are only known to have existed because of their fossilized teeth.
By looking at the shape of the dinosaurs teeth we can figure out what they ate when they were alive. Allosaurus had long teeth with sharp edges that were pointed and curved back towards its throat. Even today, every reptile that has this type of teeth is a meat-eater. Reptiles that browse for food, such as the iguana, have low and tiny leaf-shaped teeth used for shredding plants, much like dinosaurs before them did (Stegosaurs, Ankylosaurs and Hypsilophodontids). Animals that graze today, like sheep and horses, have grinding teeth with flat tops to mash tough and fibrous, low-growing plants like grass. The Hadrosaurs family of duck-billed dinosaurs had teeth similar to this.
The number of teeth dinosaurs had varied dramatically, depending on the type of dinosaur. Some dinosaurs, like Ornithomimus and Gallimimus, did not have teeth. On the other end of the spectrum, Tyrannosaurus rex had 50 to 60 solid cone-shaped teeth as big as bananas. Hadrosaurs, or duck-billed dinosaurs, had the most teeth: up to 960 cheek teeth!
Dinosaur teeth were replaceable. If a dinosaur broke or lost a tooth, another grew in to take the place, behaving much like sharks teeth do today.
Plant-eating sauropod dinosaurs (Apatosaurus, Diplodocus, Brachiosaurus, Supersaurus and many more) were equipped with peg-like or spoon-shaped teeth they used for stripping leaves off of plants. These teeth were not used for chewing, however, because of their shape. The plant material that these dinosaurs ate was swallowed and digested in their guts, maybe in fermentation chambers where the materials would break down, often with the help of gastroliths, or stones that the dinosaur swallowed to help break up the leaves and twigs in its gut.
Meat-eating theropods (Tyrannosaurus rex, Carcharodontosaurus, Allosaurus, Gigantosaurus, Spinosaurus and many more) had sharp, pointed teeth they used to tear flesh and sometimes even crush bones. Recently, a Tyrannosaurus rex coprolite (fossilized feces) was discovered containing bits of crushed bone, which tells scientists that the dinosaur did in fact crush its food with his powerful teeth and strong jaws.
Plant-eating Ornithischians, as well as some prosauropods had varying teeth but many had horny beaks and many leaf-like cheek teeth for nipping and chewing through tough foliage.
Stegosaurids (Kentrosaurus and Stegosaurus as well as others) had leaf-shaped teeth that were built for slicing at weeds that grew close to the ground.
Hadrosaurs (Edmontosaurus, Maiasaura, Lambeosaurus, Parasaurolophus and many more) were duck-billed dinosaurs and had around 960 self-sharpening cheek teeth; the most teeth of all of the dinosaurs.
Iguanodontids (Iguanodon, Probactrosaurus, and Ouranosaurus among others) had teeth that look similiar to today's iguanas. They were rounded outward, notched on top and curved, indicating that perhaps today's iguanas originated as iguanodontids.
Heterodontosaurus was a small dinosaur that had three different types of teeth in addition to a beak. It had sharp upper teeth which it used with its beak to bite and cheek teeth for grinding its food and two pairs of long canine-type teeth that fit into sockets when Heterodontosaurus closed its mouth.
Ceratopsians (Triceratops, Monoclonius and Styracosaurus belonged to this group) had toothless beaks they used to gather food and lots of flat cheek teeth they used to grind and chew tough plant material.
Ankylosaurs (such as Euoplocephalus, Sauropelta and Ankylosaurus) were unable to chew their food so they may have had large fermentation chambers where they were able to digest the tough plant fibers. Ankylosaurs had teeth shaped like a hand with the fingers together. Ornithomimids (like Ansermimus, Gallimimus, Ornithomimus and Struthiomimus) did not have teeth, but they had beaks with which they ate plants and insects and small animals.
For more information on dinosaurs and their teeth, explore the following resources: