Editing Clade (section)

=== Age ===
The age of a clade can be described based on two different reference points, [[crown group|crown]] age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its [[sister group|sister]] clade. A clade's stem age is either the same as or older than its crown age.{{sfn|Harmon|2021}} Ages of clades cannot be directly observed. They are inferred, either from [[stratigraphy]] of [[fossil]]s, or from [[molecular clock]] estimates.<ref>Brower, A. V. Z., Schuh, R. T. 2021. Biological Systematics:  Principles and Applications (3rd edn.). Cornell University Press, Ithaca, NY.</ref>