Crown group

Inphylogenetics,thecrown grouporcrown assemblageis a collection ofspeciescomposed of the living representatives of the collection, themost recent common ancestorof the collection, and all descendants of the most recent common ancestor. It is thus a way of defining aclade,a group consisting of a species and all itsextantorextinctdescendants. For example,Neornithes(birds) can be defined as a crown group, which includes the most recent common ancestor of all modern birds, and all of its extant or extinct descendants.

The concept was developed byWilli Hennig,the formulator ofphylogenetic 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 byR. P. S. Jefferiesin 1979.^[2]Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 byGraham BuddandSören 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 thefamily treethat are descended from the most recent common ancestor of living members will still be part of a crown group. For example, if we consider the crown-birds (i.e. allextantbirds and the rest of the family tree back to their most recent common ancestor), extinct side branches like thedodoorgreat aukare still descended from the most recent common ancestor of all livingbirds,so fall within the bird crown group.^[4]One very simplified cladogram for birds is shown below:^[5]

Aves

†Archaeopteryx

	other extinct groups

	Neornithes(modern birds, some extinct like the dodo)

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),Archaeopteryxand 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. Bothbirdsandmammalsare traditionally defined by their traits,^[7]^[8]and contain fossil members that lived before the last common ancestors of the living groups or, like the mammalHaldanodon,^[9]were not descended from that ancestor although they lived later. Crown-Aves and Crown-Mammalia therefore differ slightly in content from the common definition of Aves and Mammalia. This has caused some confusion in the literature.^[10]^[11]

Other groups under the crown group concept

The cladistic idea of strictly using the topology of thephylogenetic treeto define groups necessitates other definitions than crown groups to adequately define commonly discussed fossil groups. Thus, a host ofprefixeshave been defined to describe various branches of the phylogenetic tree relative to extant organisms.^[12]

Pan-group

Apan-grouportotal groupis the crown group and all organisms more closely related to it than to any otherextantorganisms. In a tree analogy, it is the crown group and all branches back 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 tocrocodilians(their closest living relatives). The phylogenetic lineage leading back fromNeornithesto the point where it merges with the crocodilian lineage, along with all side branches, constitutes pan-birds. In addition to non-crown group primitive birds likeArchaeopteryx,HesperornisandConfuciusornis,therefore, pan-group birds would include alldinosaursandpterosaursas well as an assortment of non-crocodilian animals likeMarasuchus.

Pan-Mammalia consists of all mammals and their fossil ancestors back to the phylogenetic split from the remainingamniotes(theSauropsida). Pan-Mammalia is thus an alternative name forSynapsida.

Stem groups

Astem groupis aparaphyleticassemblage composed of the members of a pan-group or total group, above,minusthe crown group itself (and therefore minus all living members of the pan-group). This leaves primitive relatives of thecrown groups,back along the phylogenetic line to (but not including) thelast common ancestorof the crown group and their closest living relatives. It follows from the definition that all members of a stem group are extinct. The "stem group" is the most used and most important of the concepts linked to crown groups, as it offers a means to reify and name paraphyletic assemblages of fossils that otherwise do not fit into systematics based on living organisms.

While often attributed to Jefferies (1979), Willmann (2003)^[13]traced the origin of the stem group concept to Austrian systematistOthenio Abel(1914),^[14]and it was discussed and diagrammed in English as early as 1933 byA. 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 likeDimetrodonorAnteosaurusare 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.Cladistically,the new groups should then be added to the group, as paraphyletic groupings are not natural. In any case, stem groupings with living descendants should not be viewed as a cohesive group, but their tree should be further resolved to reveal the full bifurcating phylogeny.

Examples of stem groups (in the wider sense)

Stem birdsperhaps 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:

Archosauria

Crocodylomorpha

	Crocodilia

Ornithopoda	Hadrosauridae

Thyreophora	Stegosauria

	Neognathae (including the extinctdodo)

	Paleognathae (including the extinctmoa)

birds' closest living relatives

stem group birds

crown group birds

The crown group here isNeornithes,all modern bird lineages back to their last common ancestor. The closest living relatives of birds arecrocodilians.If we follow the phylogenetic lineage leading to Neornithes to the left, the line itself and all side branches belong to the stem birds until the lineage merges with that of the crocodilians. In addition to non-crown group primitive birds likeArchaeopteryx,HesperornisandConfuciusornis,stem group birds include thedinosaursand thepterosaurs.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. As the bird stem group evolved, distinctive bird features such as feathers and hollow bones appeared. Finally, at the base of the crown group, all traits common to extant birds were present.

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

	Crocodilia

	Birds

An advantage of this approach is that declaring Theropoda to be birds (orPan-aves) is more specific than declaring it to be a member of the Archosauria, which would not exclude it from the Crocodilia branch. Basal branch names such as Avemetatarsalia are usually more obscure. However, not so advantageous are the facts that "Pan-Aves" and "Aves" are not the same group, the circumscription of the concept of "Pan-Aves" (synonymous with Avemetatarsalia) is only evident by examination of the above tree, and calling both groups "birds" is ambiguous.

Stem mammalsare 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 thesynapsidsas well asmammaliaformslike themorganucodontsand thedocodonts;the latter groups have traditionally and anatomically been considered mammals even though they fall outside the crown group mammals.^[18]

Stem tetrapodsare the animals belonging to the lineage leading totetrapodsfrom their divergence from thelungfish,our nearest relatives among the fishes. In addition to a series oflobe-finned fishes,they also include some of the earlylabyrinthodonts.Exactly what labyrinthodonts are in the stem group tetrapods rather than the corresponding crown group is uncertain, as the phylogeny of early tetrapods is not well understood.^[19]This example shows that crown and stem group definitions are of limited value when there is no consensus phylogeny.

Stem arthropodsconstitute a group that has seen attention in connection with theBurgess Shalefauna. Several ofthe finds,including the Enigma ticOpabiniaandAnomalocarishave some, though not all, features associated witharthropods,and are thus considered stem arthropods.^[20]^[21]The sorting of the Burgess Shale fauna into various stem groups finally enabled phylogenetic sorting of this Enigma tic assemblage and also allowed for identifyingvelvet wormsas the closest living relatives of arthropods.^[21]

Stem priapulidsare other early Cambrian to middle Cambrian faunas, appearing in Cheng gian g to Burgess Shale. The genusOttoiahas more or less the same build as modernpriapulids,but phylogenetic analysis indicates that it falls outside the crown group, making it a stem priapulid.^[3]

Plesion-group

The nameplesionhas a long history in biological systematics, andplesion grouphas acquired several meanings over the years. One use is as "nearby group" (plesion meansclose toinGreek), i.e.sister groupto a giventaxon,whether that group is a crown group or not.^[22]The term may also mean a group, possiblyparaphyletic,defined by primitive traits (i.e.symplesiomorphies).^[23]It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question.

Palaeontological significance of stem and crown groups

Placingfossilsin 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 thetetrapods,mammals,andanimals.

The application of the stem group concept also influenced the interpretation of the organisms of theBurgess shale.Their classification in stem groups to extant phyla, rather than in phyla of their own, is thought by some to make theCambrian explosioneasier to understand without invoking unusual evolutionary mechanisms;^[21]however, application of the stem group concept does nothing to ameliorate the difficulties that phylogenetic telescoping^[24]^[25]poses to evolutionary theorists attempting to understand both macroevolutionary change and the abrupt character of theCambrian explosion.Overemphasis on the stem group concept threatens to delay or obscure proper recognition of new higher taxa.^[26]

Stem groups in systematics

As originally proposed byKarl-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 ofpaleontologistshave opted to apply this approach anyway.^[28]

References

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