Crown group
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The concept was developed by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to their extinct relatives in his "Die Stammesgeschichte der Insekten",[1] and the "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979.[2] Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 by Graham Budd and Sören Jensen.[3]
Contents of the crown group
It is not necessary for a species to have living descendants in order for it to be included in the crown group. Extinct side branches on the
Aves
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In this diagram, the clade labelled "Neornithes" is the crown group of birds: it includes the most recent common 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 extinct groups are not included in the crown group, as they fall outside the Neornithes clade, being descended from an earlier ancestor.
An alternative definition does not require any members of a crown group to be extant, only to have resulted from a "major cladogenesis event".[6] 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
Other groups under the crown group concept
The cladistic idea of strictly using the topology of the phylogenetic tree to define groups necessitates other definitions than crown groups to adequately define commonly discussed fossil groups. Thus, a host of prefixes have been defined to describe various branches of the phylogenetic tree relative to extant organisms.[12]
Pan-group
A pan-group or total group is the crown group and all organisms more closely related to it than to any other
Pan-Mammalia consists of all mammals and their fossil ancestors back to the phylogenetic split from the remaining
Stem groups
A stem group is a
While often attributed to Jefferies (1979), Willmann (2003)[13] traced the origin of the stem group concept to Austrian systematist Othenio Abel (1914),[14] and it was discussed and diagrammed in English as early as 1933 by A. S. Romer.[15]
Alternatively, the term "stem group" is sometimes used in a wider sense to cover any members of the traditional taxon falling outside the crown group. Permian synapsids like Dimetrodon or Anteosaurus are stem mammals in the wider sense but not in the narrower one.[16]
Often, an (extinct) grouping is identified as belonging together. Later, it may be realized other (extant) groupings actually emerged within such grouping, rendering them a stem grouping.
Examples of stem groups (in the wider sense)
Stem birds perhaps constitute the most cited example of a stem group, as the phylogeny of this group is fairly well known. The following cladogram, based on Benton (2005),[8] illustrates the concept:
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birds' closest living relatives
stem group birds
crown group birds |
The crown group here is
Under the widely used total-group perspective,[17] the Crocodylomorpha would become synonymous with the Crocodilia, and the Avemetatarsalia would become synonymous with the birds, and the above tree could be summarized as
Archosauria
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An advantage of this approach is that declaring Theropoda to be birds (or
Stem mammals are those in the lineage leading to living mammals, together with side branches, from the divergence of the lineage from the Sauropsida to the last common ancestor of the living mammals. This group includes the
Stem arthropods constitute a group that has seen attention in connection with the
Stem priapulids are other early Cambrian to middle Cambrian faunas, appearing in Chengjiang to Burgess Shale. The genus Ottoia has more or less the same build as modern priapulids, but phylogenetic analysis indicates that it falls outside the crown group, making it a stem priapulid.[3]
Plesion-group
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
Palaeontological significance of stem and crown groups
Placing
The application of the stem group concept also influenced the interpretation of the organisms of the
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), however the crown group should have no prefix.[27] The latter has not been universally accepted for known groups. A number of paleontologists have opted to apply this approach anyway.[28]
See also
References
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- ^ S2CID 39772232.
- ^ "DNA yields dodo family secrets". BBC News. London. 2002-02-28. Retrieved 2006-09-07.
- ISBN 978-0-86840-413-4
- ^ "UCMP Glossary: Phylogenetics". www.ucmp.berkeley.edu. University of California Museum of Paleontology. 2009-11-12. Archived from the original on 2017-09-29.
- ISBN 0787654329.
- ^ ISBN 0-632-05637-1. Archived from the originalon 2008-10-19.
- S2CID 4317817. Archived from the original(PDF) on 2012-11-24.
- PMID 12396594. Retrieved 28 December 2011.
- PMID 14965901.
- ^ 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" (PDF). Palaeontology. 32: 69–99.
- .
- ^ Abel, O. (1914), Die vorzeitlichen Saugetiere, G. Fischer Verlag, Jena
- ^ Romer, A.S. (1933), Vertebrate Paleontology, University of Chicago Press, Chicago
- )
- S2CID 198156530.
- ISBN 0-7167-1822-7
- S2CID 67823991.
- .
- ^ PMID 18761282.
- hdl:2246/1224.
- ^ Kluge, N. (2000). Modern Systematics of Insects. Part I. Principles of Systematics of Living Organisms and General System of Insects, with Classification of Primary Wingless and Paleopterous Insects (in Russian). St. Petersburg, Russland: Lan'. p. 336. Archived from the original on 2012-09-14. Retrieved 2011-12-31.
- S2CID 2723134.
- .
- ISBN 978-1-893882-20-1.
- ^ Lauterbach, K-E. (1989): Das Pan-Monophylum – ein Hilfsmittel für die Praxis der Phylogenetischen Systematik. Zoologischer Anzeiger, no 223, pp 139–156.
- ^ Gauthier, J., and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves." Pp. 7-41 in New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom (J. A. Gauthier and L. F. Gall, eds.). Peabody Museum of Natural History, Yale University, New Haven, Connecticut, U.S.A.
Further reading
- S2CID 27322225
- Budd, G. E. (2001), "Tardigrades as 'stem-group' Arthropods: the Evidence from the Cambrian Fauna", Zoologischer Anzeiger, 240 (3–4): 265–279,