Direct evidence for diet
There are different types of evidence for links between an organism and what it eats (trophic links). Direct evidence is the most useful, such as fossilised stomach contents, or distinctive bite marks on the fossil prey.
Indirect evidence for diet
There are also a number of indirect pieces of evidence that can help us work out what an organism ate. We can analyse how the shape of a structure (known as morphology), such as a tooth or the whole body shape, is designed for a particular function - this is called a functional analysis. Close association of fossil finds, when they are found in rocks of the same age and place, can be useful. It is also possible to compare extinct animals with living or ‘extant’ animals. We can either compare animals that are related (homologous) or animals that are similar in size and shape (analogous). Stones found inside the stomach may indicate diet.
Studying the elements, such as carbon, found in fossilized bone and teeth can also give us clues to what the animal ate. Elements occur in different forms known as isotopes, carbon for example has three isotopes. Different foods have distinctive ratios of isotopes, and they are passed on to animals when they eat the food. This ‘chemical fingerprint’ can be preserved in fossils and measured using a machine known as a mass spectrometer. This calculates the ratios of isotopes by measuring their relative mass.