A geologic period is a subdivision of geologic time that divides an era into smaller timeframes. The equivalent term used to demarcate rock layers and the fossil record is the system; thus the rocks of the Devonian System were laid down during the Devonian Period. While paleontologists often refer to faunal stages rather than geologic periods, they are often used in popular presentations of paleontology. Such references include the book and movie Jurassic Park.