The law of faunal succession holds that different strata contain particular types of fossilised flora and fauna, and that these fossil forms succeed each other in a specific and predictable order that can be identified over wide distances. A fossilised Neanderthal bone will never be found in the same stratum as a fossilised Tyrannosaurus rex, for example.
This principle, first identified during the early 19th century by the geologist William Smith, is of great importance in determining the relative age of rocks and strata. The fossil content of rocks can be correlated with the law of superposition to determine the sequence in which the rocks were laid down and over what period this took place.
The law of faunal succession is also of great importance to the theory of evolution, which predicts that archaic biological features and organisms will be succeeded in the fossil record by more complex versions. For instance, paleontologists investigating the evolution of birds predicted that feathers would first be seen in primitive forms on flightless predecessor organisms such as feathered dinosaurs. This is precisely what has been discovered in the fossil record: simple feathers, incapable of supporting flight, are succeeded by increasingly large and complex feathers.