Utilisateur:DDCantor/Groupe Tronc

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Modèle:For

Two distinct crown groups (in red) are illustrated, connected by an ancestor (black circle). The two groups form a larger crown group (lilac).

Modèle:Evolutionary biology

Sur un arbre phylogénétique, un groupe-couronne (encadré en rouge). Le plus proche ancêtre commun est en jaune. Un groupe-tronc encadré en bleu.

En biologie systématique, un groupe-couronne est un groupe monophylétique contenant tous les organismes vivants du groupe, leur plus proche ancêtre commun, et tous les groupes aujourd'hui disparus relevant de cet ancêtre.

Le nom a été inventé par le père de l'école cladistique de la biologie systématique, Willi Hennig, en raison de la nécessité d'inclure les organismes éteints dans les arbres phylogénétiques. Bien que le nom ait été créé dans les années 1970, il n'a été utilisé régulièrement que dans les années 2000[1].


Notion de groupe couronne[modifier | modifier le code]

La définition traditionnelle d'un groupe couronne est le plus petit groupe monophylétique, ou clade, qui contient le dernier ancêtre commun d'un ensemble d'organismes vivants et tous les descendant de cet ancêtre. Les branches éteintes de l'arbre phylogénétique de cette clade font partie de ce groupe couronne. Par exemple, si l'on considère le groupe couronne des oiseaux, les espèces éteintes telles que le dodo ou le grand pingouin font bien partie de ce groupe en tant que descendant du dernier ancêtre commun de tous les oiseaux actuellement vivants[2].

Ci-dessous se trouve un cladogramme très simplifiés des oiseaux tiré de l'article sur l'histoire évolutive des oiseaux.

Avien (oiseaux) 

†Archaeopteryx




d'autres groupes éteints



Neornithes (Oiseaux actuelles et certaines espèces éteintes comme le dodo)




Modèle:Arbre début

Modèle:Arbre fin


Dans ce diagramme, la clade Neornithes est le groupe couronne des oiseaux : il inclut l'ancêtre commun des oiseaux actuels et ses descendants qu'ils soient vivants ou disparus. Cependant, l'archeopterix bien qu'étant considéré comme étant un oiseaux (il est effectivement membre du clade avien) ne fait pas partie du groupe couronne des oiseaux, n'étant pas descendant de l'ancêtre commun des oiseaux actuellement vivant.


Les oiseaux et les mammifères sont traditionnellement définies par un ensemble de caractéristiques (l'oreille interne chez les mammifères par exemple) et contiennent alors certains membres qui vivaient avant le dernier ancêtre commun des groupes actuels. Ainsi, les groupes couronnes des mammifères et celui des oiseaux diffèrent des groupes des mammifères et des oiseaux telles qu'il sont traditionnellement définis.

Other groups under the crown group concept[modifier | modifier le code]

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.[3]

Pan-group[modifier | modifier le code]

A pan-group or total group is the crown group and all organisms more closely related to it than to any other extant organisms. In a tree-analogy, it is the crown group and branch down to (but not including) the split with the closest branch to have living members. The Pan-Aves thus contain the living birds and all (fossil) organisms more closely related to birds than to crocodilians (their closest living relatives). If we follow the phylogenetic lineage leading from Neoaves downwards, the line itself and all side branches belong to pan-birds, down until the lineage merges with that of the crocodilians. In addition to non-crown group primitive birds like Archaeopteryx, Hesperornis and Confuciusornis, pan-group birds would include all dinosaurs, an assortment of non-crocodilian "thecodonts" and quite likely the pterosaurs.

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.

Stem groups[modifier | modifier le code]

A stem group is a group composed of a pan-group or total-group, above, but minus the crown group itself (and therefore minus all living members of the pan-group). 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. It follows from the definition that all members of a stem group are extinct. 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,[4]

Examples[modifier | modifier le code]

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:

Aves 

 †Archaeopteryx 




 other extinct groups 



 Neoaves (modern birds, some extinct like the dodo) 




stem group birds
crown group birds

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.[5] The stem mammals is more or less identical with the traditional use of the term Synapsida as a subclass of Reptilia.[5]

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.[6][7] 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.[7]

Zygon-group[modifier | modifier le code]

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.

Plesion-group[modifier | modifier le code]

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),[8] i.e. sister group to a given taxon, whether that group is a crown group or not.[9] the term may also mean a group, possibly paraphyletic, defined by primitive traits (i.e symplesiomorphies).[10]

In phylogenetic nomenclature, the term has been interpreted as derived from plesiomorphy, meaning a primitive character.[11] 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.

Scion-group[modifier | modifier le code]

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,[12] 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.[13]

Palaeontological significance of stem- and crown groups[modifier | modifier le code]

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.[14]

Stem-groups in systematics[modifier | modifier le code]

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.[15] This approach has not been universally accepted for known groups. A number of paleontologists have opted to apply this approach anyway.[16] This has led to a confusion over the exact extension of well known taxa like birds and mammals.[17][18]

References[modifier | modifier le code]

  1. ↑ G.E. Budd et S. Jensen, « A critical reappraisal of the fossil record of the bilaterian phyla », Biological Reviews, vol. 75, no 02,‎ , p. 253–295 (DOI 10.1017/S000632310000548X, lire en ligne)
  2. ↑ « DNA yields dodo family secrets », BBC News, London, (consulté le )
  3. ↑ 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 no 32, pages 69–99
  4. ↑ « {{{1}}} »
  5. ↑ a et b Romer, A.S. (1966): Vertebrate Paleontology. University of Chicago Press, Chicago; 3rd edition (ISBN 0-7167-1822-7)
  6. ↑ « {{{1}}} »
  7. ↑ a et b « {{{1}}} »
  8. ↑ (en) C. Lyell, Principles of geology, being an attempt to explain the former changes of the earth's surface, by reference to causes now in operation, vol 3, London, 1st, , 109 p., p. 77
  9. ↑ C. Patterson, « Review of ichthyodectiform and other Mesozoic teleost fishes and the theory and practice of classifying fossils », Bulletine of American Natural History, vol. 158, no 2,‎ , p. 85–172
  10. ↑ (ru) N. Kluge, 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, St. Petersburg, Russland, Lan', , 336 p. (lire en ligne)
    An English translation has been published by Entomological Review
  11. ↑ « Volume 1: Phylogenetics », UCMP Glossary, University of California Museum of Paleontology (consulté le )
  12. ↑ Padian, K. & Chiappe, L.M. (1997): Bird Origins. In Encyclopedia of Dinosaurs (red. Currie, P.J & Padian, K., Academic Press, San Diego, pages 41–96, (ISBN 0-12-226810-5)
  13. ↑ Luo, Z.-X., Kielan-Jaworowska, Z. & Cifelli, R.L. (2002): In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica no 47: pages 1-78.
  14. ↑ « {{{1}}} »
  15. ↑ Lauterbach, K-E. (1989): Das Pan-Monophylum – ein Hilfsmittel für die Praxis der Phylogenetischen Systematik. Zoologischer Anzeiger, no 223, pp 139–156.
  16. ↑ 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.
  17. ↑ Jason S. Anderson, « Use of Well-Known Names in Phylogenetic Nomenclature: A Reply to Laurin », Systematic Biology, vol. 51, no 5,‎ , p. 822–827 (PMID 12396594, DOI 10.1080/10635150290102447, lire en ligne, consulté le )
  18. ↑ Laurin, M. & Anderson, J.S. (2004): Meaning of the Name Tetrapoda in the Scientific Literature: An Exchange. Systematic Biology no 53 (vol 1): pp 68-80. doi: 10.1080/10635150490264716

Further reading[modifier | modifier le code]

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Modèle:Phylogenetics

Category:Phylogenetics Category:Evolutionary biology Category:Fossils Category:Transitional fossils Category:Paleontology Category:Zoology Category:Phylogenetics fr:Groupe-couronne

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