There were thousands of different species of dinosaur, and we have discovered only a small proportion of these. It is important to realize how rare fossils are. A few dinosaur species must have been very common, for example Iguanodon, which left many fossils. Others were probably scarce when they were alive or lived in regions where fossilization was unlikely, and so there are few remains of these dinosaurs.
Herbivores often lived in groups or herds, but carnivores were usually more solitary and their fossils are therefore more rare. Many of the most interesting dinosaurs are known from a single fossil, or sometimes from just one or two bones. Palaeontologists must deduce the shape and size of the animal from these few remains.
Picture credit: google