The Bio-Chronologic Ages
Devonian Period Invertebrates

Taxon Range Zones

Biochronology refers to the use of fossils to subdivide the geologic time scale. This can be done on a large or small scale. For example, the Paleozoic, Mesozoic, and Cenozoic eras are divided based on their prefixes meaning “old”, “middle” and “new” life, respectively. These subdivisions arise from different groups of organisms that make up characteristic fossils for each time period. On a smaller scale, individual species can be used to subdivide the rock record into what are known as “Taxon Range Zones”. These zones are based on the occurrence (evolution) and disappearance (extinction) of different taxa and correspond to absolute ages determined by radiometric dating techniques.