The stem group/crown group classification was created by Willi Hennig to classify relationships between living and extinct organisms. The crown group includes all living species and their extinct descendants up to the common ancestor, while the stem group includes all species not in the crown group. Stem groups provide insights into evolution and can contain several early offshoots. Examples include stem-group mammals and stem-group fishes like acanthodes and placoderms.
The stem group/crown group terminology was invented to classify the relationship between living and extinct organisms by Willi Hennig, a German taxonomist and 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 in the group, plus all extinct descendants up to the common ancestor of all living species. The stem group includes all species that are not part of the crown group. By definition, every member of the stem group must be extinct. If they weren’t extinct, they would be defined as part of the crown group.
Stem-group animals, such as those represented by the many fossils of early tetrapods, mammals, and reptiles that have been unearthed, provide us with important insights into the course of evolution and how animals tried different strategies to adapt to their environments. Stem groups are necessarily paraphyletic in the sense that they can be more complex than containing only a certain species and all of its descendants. A stem group may contain several early offshoots of a group, only one of which has evolved into the crown group.
Stem groups are quite common in paleontology. An example would be stem-group mammals, or synapsids – although mammals are also technically synapsids, since they were descended from them – which were called “mammal-like reptiles” until it was realized that they weren’t reptiles at all, but mammals of the stem group . Early synapsids were called “naked lizards” because they would have resembled lizards in appearance, but without scales. As synapsids continued to evolve, they acquired a more mammalian appearance. Synapsids are sometimes called “basal mammals” or “stem group mammals”.
Fishes have several groups of stems, including acanthodes, or spiny sharks, and placoderms, or armored fishes. Acanthodians, despite their name, are considered to be closely related to bony fish ancestors, while placoderms, more closely related to modern shark ancestors, included the first vertebrate apex predator, Dunkleosteus telleri, which measured 8–11 m (26 -36 feet). Both of these groups lived during the Paleozoic, about 300-400 million years ago.
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