Ichnology is the study of the behaviour of extinct organisms by examination of their traces, tracks or burrows. These are left in sediments and subsequently preserved as trace fossils. They were produced by the activity of animals (usually invertebrates, sometimes vertebrates). Many animals such as worms, containing soft tissues and no hard skeletal tissue, are known only from trace fossils, as after death the animal decays and the body is not preserved. Trace fossils are useful in determining the environmental conditions that prevailed when the traces were made.