In sedimentary rocks, fossils are the only widely applicable tool for time correlation.:229 Evolution leaves a record of progressive change, sequential and nonrepeating.:230 A rock unit has a characteristic assemblage of fossils, independent of its lithology.:229 Thus, the fossils can be used to compare the ages of different rock units.
The basic unit of biochronology is the biostratigraphic zone, or biozone, a collection of fossils found together in a rock unit. This is used as the basis of a biochron, "a unit of time in which an association of taxa is interpreted to have lived.":229 However, a biozone may vary in age from one location or another. For example, a given taxon may migrate, so its first appearance varies from place to place. In particular, facies-controlled organisms (organisms that lived in a particular sedimentary environment) are not well suited for biochronology because they move with their environment and may change little over long periods of time.:230–231 Thus, biostratigraphers search for species that are particularly widespread, abundant, and not tied to particular sedimentary environments. This is particularly true of free-swimming animals such as benthic foraminifera, which readily spread throughout the world's oceans.:230
Another challenge for stratigraphy is that there are often large gaps in the fossil record at a given location. To counter this, biostratigraphers search for a particularly well-preserved section that can be used as the type section for a particular biostratographic unit. As an example, the boundary between the Silurian and Devonian periods is marked by the first appearance of the graptolite Mongraptus uniformus uniformus in a section in Klonk, Czech Republic.:237
In terrestrial deposits, fossils of land mammals and other vertebrates are used as stratigraphic tools, but they have some disadvantages relative to marine fossils. They are seldom evenly distributed through a section, and they tend to occur in isolated pockets with few overlaps between biozones. Thus, correlations between biozones is often indirect, inferred using a knowledge of their sequence of evolution.:240 This practice was first proposed by H. S. Williams in 1941.
In the United States, biochronology is widely used as a synonym for biostratigraphy, but in Canada and Europe the term is reserved for biochronology that is not tied to a particular stratigraphic section. This form of biochronology is not recognized by the International Stratigraphic Guide, but it is "really what a great many paleontologists and stratigraphers are after ... an optimum network of fossil correlations, thought to embody a reliable and high-resolution isochronous time (lines) framework."