In Biology, what is the Difference between a Stem Group and a Crown Group?
The stem group/crown group terminology was invented to classify the relationship of living and extinct organisms by Willi Hennig, a German taxonomist, the father of cladistics, in the late 1940s. It was part of his “theory of phylogenetic systematics” that revolutionized the way biologists and paleontologists look at life. The terms are defined as follows. A crown group includes all living species of the group, plus all extinct descendants back to the common ancestor of all living species. The stem group includes all species not part of the crown group. By definition, every member of the stem group must be extinct. If they weren’t extinct, they’d be defined as part of the crown group. Stem group animals, like those represented by the numerous early tetrapod, mammal, and reptile fossils that have been dug up, give us important information about the course of evolution and how animals tried out different strategies to adapt to their environments. Stem groups are necessarily paraphyletic,
