Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig
Willi Hennig
Emil Hans Willi Hennig was a German biologist who is considered the founder of phylogenetic systematics, also known as cladistics. With his works on evolution and systematics he revolutionised the view of the natural order of beings...

, the formulator of phylogenetic systematics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

, as a way of classifying living organisms relative to extinct ones. Though formulated in the 1970s, it was not commonly used until its reintroduction in the 2000s.

Crown group extensions

The usual definition of a crown group is the smallest monophyletic group, or "clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

", to contain the last common ancestor of all extant members, and all of that ancestor's descendants. Extinct side branches on the family tree
Family tree
A family tree, or pedigree chart, is a chart representing family relationships in a conventional tree structure. The more detailed family trees used in medicine, genealogy, and social work are known as genograms.-Family tree representations:...

 which are within this clade will still be part of a crown group. For example, if we consider the crown-birds (i.e all extant birds and the rest of the family tree down to their last common ancestor), extinct side branches like the dodo
The dodo was a flightless bird endemic to the Indian Ocean island of Mauritius. Related to pigeons and doves, it stood about a meter tall, weighing about , living on fruit, and nesting on the ground....

 or great auk
Great Auk
The Great Auk, Pinguinus impennis, formerly of the genus Alca, was a large, flightless alcid that became extinct in the mid-19th century. It was the only modern species in the genus Pinguinus, a group of birds that formerly included one other species of flightless giant auk from the Atlantic Ocean...

 are still descended from the last common ancestor of all living bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, so fall within the bird crown group. One very simplified cladogram for birds is shown below:
In this diagram, the clade labelled "Neoaves" is the crown group of birds: it includes the ancestor of all living birds and its descendants, living or not. Although considered to be birds (i.e. members of the clade Aves), Archaeopteryx and other other extinct groups are not included in the crown group, as they fall outside the Neoaves clade, being descended from an earlier ancestor.

An alternative definition does not require all members of a crown group to be extant, only to have resulted from a "major cladogenesis event". The first definition forms the basis of this article.

Often, the crown group is given the designation "crown-", to separate it from the group as commonly defined. Both birds and mammals are traditionally defined by their traits, and contain fossil members that lived before the last common ancestors of the living groups. Crown-Aves and Crown-Mammalia therefore differ slightly in content from the common definition of Aves and Mammalia.

Other groups under the crown group concept

The cladistic idea of strictly using the topography of the phylogenetic tree
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...

 to define groups, necessitate other definitions than crown groups to adequately define commonly discussed fossil groups like dinosaurs and various Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

 fauna. Thus, a host of prefix
A prefix is an affix which is placed before the root of a word. Particularly in the study of languages,a prefix is also called a preformative, because it alters the form of the words to which it is affixed.Examples of prefixes:...

es has been defined to describe various branches of the phylogenetic tree relative to extant organisms.

Stem groups

A stem group is a group composed of all organisms more closely related to the crown group than to any other extant organisms, but minus the crown group itself. This leaves primitive relatives of the crown groups, back along the phylogenetic line to (but not including) the last common ancestor of the crown group and their nearest living relatives. As all living species are by definition in a crown group, it follows that all members of the stem-group of a clade are extinct. Any known member of a stem group is necessarily fossil. Despite being a paraphyletic assemblage, the "stem group" is the most used and most important of the concepts linked to crown groups, as it offer a purely phylogenetic route to classify fossils that otherwise do not obey systematics based on living organisms.

Stem group organisms always lack one or more features that are present at the base of the crown group to which they are attached. As a group evolves away from the last common ancestor of it and its nearest living relative, it accumulates the distinctive features seen in the crown group.

The crown-and-stem group concept was first mooted in 1979,


Stem birds is perhaps the most cited example of a stem group, as the phylogeny of this group is fairly well known. Following the above logic, the stem group of birds can be shown on the same cladogram used above to illustrate a crown group:
The crown group here is Neoaves, all modern bird lineages down to their last common ancestor. The closest living relatives of birds are the crocodilians. If we follow the phylogenetic lineage leading to Neoaves downwards, the line itself and all side branches belong to stem-birds, down until the lineage merges with that of the crocodilians. In addition to non-crown group primitive birds like Archaeopteryx, Hesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...

and Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...

, stem-group birds would include all dinosaurs, an assortment of non-crocodilian "thecodonts" and possibly the pterosaurs. The last common ancestor of birds and crocodilians – the first crown group archosaur - was neither bird nor crocodilian, and possessed none of the features unique to either. Evolution up the bird stem group allowed the accumulation of distinctive bird features such as feathers and hollow bones, until all were finally present at the base of the crown group.

Stem mammals are the lineage leading to mammals, from it split off from the early reptiles until the last common ancestor of all living mammals, including side branches. This group is composed of the mammal-like reptiles and the Mammaliformes, the latter a group traditionally and anatomically considered mammals, but falling outside the crown group mammals. The stem mammals is more or less identical with the traditional use of the term Synapsida as a subclass of Reptilia.

Stem arthropods is a group that has seen attention in connection with the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

 fauna. Several of the finds
Fossils of the Burgess Shale
The fossils of the Burgess Shale, like the Burgess Shale itself, formed around in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 60,000 specimens in a series of field trips up from 1909 to 1924...

, including the enigmatic Opabinia
Opabinia is an animal genus found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale of British Columbia, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed,...

and Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...

have some, though not all features associated with arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, and are thus considered stem arthropods. The sorting of the Burgess Shale fauna into various stem groups finally enabled phylogenetic sorting of this enigmatic assemblage, and also allowed for identifying velvet worms as the closest living relatives of arthropods.


A crown group and its stem group considered together are known as the pan-group or total group. The Pan-Aves thus contain the living birds and all (fossil) organisms more closely related to birds than to crocodilians (their closest living relatives). Pan-Mammalia are all mammals and their fossil ancestors down to the phylogenetic split from the remaining amniotes (the Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...

). Pan-Mammalia is thus an alternative name for Synapsida.


A zygon-group is a crown group containing the group in question and their most closely related crown-group and ancestors down to their last common ancestor. Thus, Zygon-Aves contain birds and crocodilians - their closest living relatives - and a host of extinct groups like dinosaurs and various Thecodont
Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period...

s that hails from the last common ancestor of birds and crocodilians. Zygon-Aves and Zyogn-Crocodilia are thus just two ways of naming the same crown group, a group normally known as Archosauria.


Any side branch on the phylogenetic tree splitting off before the most recent common ancestor of the crown group itself is termed a plesion-group. By the very definition, all members of a plesion-group are extinct. As it is very unlikely that any fossil we find will be a member of an actual ancestor
An ancestor is a parent or the parent of an ancestor ....

 species, all fossils found not to be in a crown group will be considered members of a plesion-group.


A Scion-group is a group consisting of a crown group and one or more plesion-groups, down to the last common ancestor between the crown-group and the most removed plesion-group. Groups that contain one or more extinct "early offshoots" of the family tree are plesion-groups. Under the common definition of birds as including Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

, Aves is a scion-group, as Archaeopteryx represent an early side-branch without extant representatives. The same goes for the common understanding of Mammalia, as it includes extinct side-branches like Morganucodon
Morganucodon is an early mammalian genus which lived during the Late Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast...

and Hadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...


Palaeontological significance of stem- and crown groups

Placing fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s in their right order in a stem group allows the order of these acquisitions to be established, and thus the ecological and functional setting of the evolution of the major features of the group in question. Stem groups thus offer a route to integrate unique palaeontological data into questions of the evolution of living organisms. Furthermore, they show that fossils that were considered to lie in their own separate group because they did not show all the diagnostic features of a living clade, can nevertheless be related to it by lying in its stem group. Such fossils have been of particular importance in considering the origins of the tetrapods, mammals, and animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...


The application of the stem group concept also radically reformed the interpretation of the organisms of the Burgess shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

. Their classification in stem groups to extant phyla, rather than in phyla of their own, made the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

 much easier to understand without invoking unusual evolutionary mechanisms.

Stem-groups in systematics

As originally proposed by Karl-Ernst Lauterbach, stem-groups should be given the prefix "stem" (i.e. Stem-Aves, Stem-Arthropoda), and the crown group no prefix. This approach has not been universally accepted for known groups. A number of paleontologists
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 have opted to apply this approach anyway. This has led to a confusion over the exact extension of well known taxa like birds and mammals.

Further reading

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