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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”. “which revolutionized the way biologists and paleontologists view life.
The terms are defined as follows. A crown group includes all living species in the group plus all extinct descendants back to the common ancestor of all living species. The stem group includes all species that are not part of the crown group. By definition, all members of the radical group must become extinct. If they were not extinct, they would be defined as part of the crown group.
Stem-group animals, such as those represented by the many early fossils of tetrapods, mammals, and reptiles that have been unearthed, give us important insights into the course of evolution and how animals experimented with different strategies to adapt to their environments. Stem groups are necessarily paraphyletic, meaning they can be more complex than just containing a particular species and all of its descendants. A stem group may contain several early branches from a group, only one of which has evolved into the crown group.
Stem clusters are quite common in paleontology. An example would be the mammals of the trunk group, or synapsids – although mammals are also technically synapsids since they are descended from them – which were called “mammal-like reptiles” until it was understood that they were not reptiles but mammals of the group of the trunk. The earliest synapsids were called “naked lizards” because they would have looked like lizards, but without scales. As synapsids continued to evolve, they became more mammalian in appearance. Synapsids are sometimes called “basic mammals” or “stem group mammals”.
Fish have various groups of stalks, including the acanthodians, or spiny sharks, and the placoderms, or armor fish. Acanthodians, despite their name, are considered close relatives of the ancestors of bony fishes, while placoderms, more closely related to the ancestors of modern sharks, included the first vertebrate apex predator, Dunkleosteus telleri, which was between 8 and 11 m (26 and 36 ft). Both groups lived during the Paleozoic era, about 300-400 million years ago.