Determination of the biology of extinct organisms is hampered by the fact that, except in very unusual situations, only their hard parts (teeth, skeleton, shells) fossilize. In other words, a vertebrate paleontologist studying a particular dinosaur is almost always limited to the skeleton, and often only part of that skeleton. Information about the remainder of the animal's anatomy, its behavior, and other attributes must be inferred. This information is required in order to visualize the fossil as a living organism and to consider its paleoecology.