In phylogenetics, a crown group is a group of species 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, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to extinct ones. Though formulated in the 1970s, the term was not commonly used until its reintroduction in the 2000s.[1]
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The usual definition of a crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all extant members, and all of that ancestor's descendants. Extinct side branches on the family tree 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 or great auk are still descended from the last common ancestor of all living birds, so fall within the bird crown group.[2] One very simplified cladogram for birds is shown below:[3]
| Aves |
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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".[4] 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.
The cladistic idea of strictly using the topography of the phylogenetic tree to define groups, necessitate other definitions than crown groups to adequately define commonly discussed fossil groups like dinosaurs and various Burgess Shale fauna. Thus, a host of prefixes has been defined to describe various branches of the phylogenetic tree relative to extant organisms.[5]
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,[6]
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:
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stem group birds
crown group birds
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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 and Confuciusornis, 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.[7] The stem mammals is more or less identical with the traditional use of the term Synapsida as a subclass of Reptilia.[7]
Stem arthropods is a group that has seen attention in connection with the Burgess Shale fauna. Several of the finds, including the enigmatic Opabinia and Anomalocaris have some, though not all features associated with arthropods, and are thus considered stem arthropods.[8][9] 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.[9]
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). 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 Thecodonts 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.
The name plesion has a long history in biological systematics, and plesion group has acquired several meanings over the years. One use is as "nearby group" (plesion means close to in Greek),[10] i.e. sister group to a given taxon, whether that group is a crown group or not.[11] the term may also mean a group, possibly paraphyletic, defined by primitive traits (i.e symplesiomorphies).[12]
In phylogenetic nomenclature, the term has been interpreted as derived from plesiomorphy, meaning a primitive character.[13] Though the exact meaning varies, it is generally taken to mean a side branch splitting off lower on the phylogenetic tree than group in question.
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,[14] 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 and Hadrocodium.[15]
Placing fossils 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 animals.
The application of the stem group concept also radically reformed the interpretation of the organisms of the Burgess shale. Their classification in stem groups to extant phyla, rather than in phyla of their own, made the Cambrian explosion much easier to understand without invoking unusual evolutionary mechanisms.[16]
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.[17] This approach has not been universally accepted for known groups. A number of paleontologists have opted to apply this approach anyway.[18] This has led to a confusion over the exact extension of well known taxa like birds and mammals.[19][20]
Budd, G. (2001), "Climbing Life's Tree", Nature 412 (6846): 487, doi:10.1038/35087679, PMID 11484029
Budd, G. E. (2001), "Tardigrades as 'stem-group' Arthropods: the Evidence from the Cambrian Fauna", Zoologischer Anzeiger 240 (3–4): 265–279, doi:10.1078/0044-5231-00034
Craske, A. J.; Jefferies, R. P. S. (1989), "A New Mitrate from the Upper Ordovician of Norway, and a New Approach to Subdividing a Plesion", Palaeontology 32: 69–99
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