In phylogenetics , the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. It is thus a way of defining a clade , a group consisting of a species and all its extant or extinct descendants. 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.
39-567: Stem group genera: The ”Fallotaspidoidea” are a superfamily of trilobites , a group of extinct marine arthropods . It lived during the Lower Cambrian ( Atdabanian ) and species occurred on all paleocontinents except for the Gondwana heartland (currently Latin America, most of Africa, Australia, Antarctica, India and China). A member of this group, Profallotaspis jakutensis , has long been
78-428: A host of prefixes have been defined to describe various branches of the phylogenetic tree relative to extant organisms. 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 all branches back to (but not including) the split with the closest branch to have living members. The Pan-Aves thus contain
117-432: 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
156-506: Is an early Cambrian trilobite genus found in Morocco . Like the closely related Choubertella and Wolynaspis , but unlike any other Fallotaspidoidea, it lacks genal spines. Cambrian of the Anti-Atlas, Morocco, 30.4° N, 8.8° W (Daguinaspis trilobite zone, Tazemmourt Section and Tiout Section. Amouslek Formation, Middle Atdabanian (520.0 - 516.0 Ma) ) Daguinaspis occurs together with
195-488: Is closest to the trilobites with dorsal cephalic sutures, such as Bigotina and Lemdadella . Bradyfallotaspis fusa , Geraldinella corneiliana and Selindella gigantea , all have a long frontal glabellar lobe (L4), and eye ridges that contact only the posterior part of L4, and so these genera belong to the “Nevadioidea“. As with most early trilobites, the Fallotaspidoidea have an almost flat exoskeleton, that
234-618: Is no consensus phylogeny. Stem arthropods constitute 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. 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
273-454: Is only thinly calcified, and has crescent-shaped eye ridges. As part of the Olenellina suborder, the Fallotaspidoidea lack dorsal sutures. The superfamily can be distinguished from all other Olenellina by features of the cephalon and in particular the glabella . The glabella tapers forward. The frontal lobe of the glabella (because it is counted from the back, it is numbered L4) is as long as
312-425: Is shown below: † 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), Archaeopteryx and other extinct groups are not included in
351-453: Is thought by some to make the Cambrian explosion easier to understand without invoking unusual evolutionary mechanisms; however, application of the stem group concept does nothing to ameliorate the difficulties that phylogenetic telescoping poses to evolutionary theorists attempting to understand both macroevolutionary change and the abrupt character of the Cambrian explosion . Overemphasis on
390-405: The dinosaurs and 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. 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
429-466: The last common ancestor of 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) traced
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#1732858582428468-426: The lungfish , our nearest relatives among the fishes. In addition to a series of lobe-finned fishes , they also include some of the early labyrinthodonts . 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. This example shows that crown and stem group definitions are of limited value when there
507-557: The paleocontinent Siberia . The group quickly spread to southern Europe and northwestern Africa, which were part of the margin of Gondwana (or Peri-Gondwana), and northwestern Laurentia . The sutured Redlichiina developed from the fallotaspidoids somewhere in Siberia and the Gondwana-margin, and from there spread to all of Gondwana, including current China and Australia, where fallotaspidoids and other Olenellina were absent. In Laurentia
546-400: The "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979. Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 by Graham Budd and Sören Jensen . 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 family tree that are descended from
585-711: The Archaeaspididae, next to the Nevadiidae, the Neltneriidae and the Judomiidae. This opinion was reflected in the Revised Treatise of 1997. More recent cladistic analysis suggests that the ”Fallotaspidoidea” are the sister group of all other Olenellina (Judomiidoidea, Nevadiidoidea and Olenelloidea). The ”Fallotaspidoidea” remain paraphyletic (hence the quotation marks), even after reorganizing it, because it gave rise to
624-521: 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 mammals are those in the lineage leading to living mammals, together with side branches, from
663-795: The Daguinaspidinae and the Fallotaspidinae (including Andalusiana , and Bradyfallotaspis ), together comprising the current ”Fallotaspidoidea”, in addition to the Nevadiinae, the Neltneriinae and the Callaviniinae. This was followed by the view that the Archaeaspididae (including Bradyfallotaspis ), the Fallotaspididae and the Daguinaspididae were all families of their own and sistergroups of
702-529: The Fallotaspidoids were succeeded by Nevadioids , Judomioids and Olenelloids , the latter remaining the dominant group of trilobites until the extinction of all Olenellina at the very end of the Lower Cambrian, after which Redlichiina, Ptychopariida and Corynexochida took over. The views of scholars on the relationships of the taxa now assigned to the ”Fallotaspidoidea” have changed regularly over
741-797: The Olenellidae, Homiidae, and Nevadiidae. Ahlberg et al., moved the Archaeaspis -group as a subfamily to the Callaviidae, leaving the Fallotaspis -group and the Daguinaspis -group as subfamilies in the Daguinaspididae. Palmer and Repina erected two superfamilies, the Olenelloidea and the Fallotaspidoidea, the latter consisting of the Fallotaspididae (comprising the Fallotaspidinae and the Daguinaspidinae), and
780-430: The closely knit group of Choubertella , Daguinaspis and Wolynaspis , with Eofallotaspis slightly further removed, and near the basis a subgroup consisting of Profallotaspis and Pelmanaspis . The monotypical family Fallotaspididae is next to branch-off. This is followed by the branch of Parafallotaspis all on its own, followed by the monotypical family Archaeaspididae . Finally Fallotaspidella
819-514: The closest living relatives of arthropods. 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. The name plesion has a long history in biological systematics, and plesion group has acquired several meanings over
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#1732858582428858-582: 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". 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
897-483: 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 influenced the interpretation of the organisms of the Burgess shale . Their classification in stem groups to extant phyla, rather than in phyla of their own,
936-509: The divergence of the lineage from the Sauropsida to the last common ancestor of the living mammals. This group includes the synapsids as well as mammaliaforms like the morganucodonts and the docodonts ; the latter groups have traditionally and anatomically been considered mammals even though they fall outside the crown group mammals. Stem tetrapods are the animals belonging to the lineage leading to tetrapods from their divergence from
975-439: The earliest known trilobite, but recently the redlichiid Lemdadella has been claimed as occurring even earlier. ”Fallotaspidoidea” occur in the Lower Cambrian of North America (Cordilleran region and northern Greenland), Europe (United Kingdom, Comley area; Ukraine), northwestern Africa and northern Asia (Siberia). Lieberman (2002) suggests that fallotaspidoids, the first hard-shelled trilobites (or Eutrilobites), originate in
1014-505: The extinct moa ) The crown group here is Neornithes , all modern bird lineages back to their last common ancestor. The closest living relatives of birds are crocodilians . 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 like Archaeopteryx , Hesperornis and Confuciusornis , stem group birds include
1053-432: The full bifurcating phylogeny. 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), illustrates the concept: Crocodilia Pterosauria Hadrosauridae Stegosauria Sauropoda Tyrannosauridae Archaeopteryx Neognathae (including the extinct dodo ) Paleognathae (including
1092-522: The last common ancestors of the living groups or, like the mammal Haldanodon , 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. 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,
1131-628: The living birds and all (fossil) organisms more closely related to birds than to crocodilians (their closest living relatives). The phylogenetic lineage leading back from Neornithes to 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 like Archaeopteryx , Hesperornis and Confuciusornis , therefore, pan-group birds would include all dinosaurs and pterosaurs as well as an assortment of non-crocodilian animals like Marasuchus . Pan-Mammalia consists of all mammals and their fossil ancestors back to
1170-539: The most backward lobe (L0), less than in the other Olenellina. The eye ridges (or ocular lobes ) contact, but do not merge with, the entire frontal margin of the glabella. This key complements the key in the Olenellina article. Crown group#Stem groups 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", and
1209-442: 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. all extant birds and the rest of the family tree back to their most recent common ancestor), extinct side branches like the dodo or great auk are still descended from the most recent common ancestor of all living birds , so fall within the bird crown group. One very simplified cladogram for birds
Fallotaspidoidea - Misplaced Pages Continue
1248-464: The narrower one. 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
1287-474: The origin of the stem group concept to Austrian systematist Othenio Abel (1914), and it was discussed and diagrammed in English as early as 1933 by A. S. Romer . 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
1326-696: The past half century or so. Initially, the Olenellidae , which also encompassed the subfamily Fallotaspidinae, was regarded the sistergroup of the Daguinaspididae, a view that made it into the Treatise of 1959. Later, the genera now in the ”Fallotaspidoidea” were considered part of the Holmiinae , the sistergroup of the Olenellinae and the Nevadiinae . Bergström recognized within the family Daguinaspididae five subfamilies:
1365-493: The phylogenetic split from the remaining amniotes (the Sauropsida ). Pan-Mammalia is thus an alternative name for Synapsida . A stem group is a paraphyletic assemblage composed of the members of a pan-group or total group, above, 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)
1404-454: The stem group concept threatens to delay or obscure proper recognition of new higher taxa. 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. The latter has not been universally accepted for known groups. A number of paleontologists have opted to apply this approach anyway. Daguinaspis Daguinaspis
1443-513: The trilobites with dorsal sutures. Applying a strictly cladistic approach would call for including the ”Fallotaspidoidea” in the Redlichiina , and possibly raising the status of the remaining Olenellina to become an order (and be renamed to Olenellida). Repinaella is closest to the common ancestor with the other Olenellina. Second to split away is the Daguinaspididae -family, which comprises
1482-597: The widely used total-group perspective, 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 Crocodilia Birds An advantage of this approach is that declaring Theropoda to be birds (or Pan-aves ) is more specific than declaring it to be a member of the Archosauria, which would not exclude it from
1521-487: The years. One use is as "nearby group" (plesion means close to in Greek ), i.e. sister group to a given taxon , whether that group is a crown group or not. The term may also mean a group, possibly paraphyletic , defined by primitive traits (i.e. symplesiomorphies ). It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question. Placing fossils in their right order in
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