26-523: See also below . Tetrapodomorpha (also known as Choanata ) is a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish . Advanced forms transitional between fish and the early labyrinthodonts , such as Tiktaalik , have been referred to as " fishapods " by their discoverers, being half-fish, half-tetrapods, in appearance and limb morphology. The Tetrapodomorpha contains
52-766: A common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
78-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
104-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
130-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
156-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
182-596: A synapomorphy with more crownward tetrapodomorphs. Eusthenopteron differs significantly from some later Carboniferous tetrapods in the apparent absence of a recognized larval stage and a definitive metamorphosis . In even the smallest known specimen of Eusthenopteron foordi , with a length of 29 millimetres (1.1 in), the lepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera like Polydon (the American paddlefish ). This might indicate that Eusthenopteron developed directly, with
208-498: Is a genus of prehistoric sarcopterygian (often called "lobe-finned") fish known from several species that lived during the Late Devonian period , about 385 million years ago. It has attained an iconic status from its close relationship to tetrapods . Early depictions of animals of this genus show them emerging onto land, but paleontologists now think that eusthenopteron species were strictly aquatic animals , though this
234-570: Is a medium- to large-sized tristichopterid . The species E. foordi is estimated to have exceeded 1.5 m (4 ft 11 in) in length, while the species E. jenkinsi probably reached 2.1 m (6 ft 11 in). Eusthenopteron may have weighed around 50 kilograms. The earliest known fossilized evidence of bone marrow has been found in Eusthenopteron , which may be the origin of bone marrow in tetrapods. Eusthenopteron shares many unique features among fishes but in common with
260-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
286-471: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
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#1732851943415312-403: Is not completely known. The genus was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha , Quebec , Canada . Some 2,000 Eusthenopteron specimens have been collected from Miguasha, one of which was the object of intensely detailed study and several papers by paleoichthyologist Erik Jarvik between the 1940s and the 1990s. Eusthenopteron
338-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
364-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
390-1740: The Middle Devonian period . After Benton, 2004; and Swartz, 2012. Other clades include the Eotetrapodiformes ( Tinirau , Platycephalichthys , the Tristichopteridae and Elpistostegalia ). Older taxa which include late stem tetrapods and early tetrapods are the Labyrinthodontia and Ichthyostegalia . The cladogram is based on a phylogenetic analysis of 46 taxa using 204 characters by B. Swartz in 2012. † Kenichthys † Rhizodontidae [REDACTED] † Marsdenichthys † Canowindra † Koharalepis † Beelarongia † Gogonasus [REDACTED] † Gyroptychius † Osteolepis [REDACTED] † Medoevia † Megalichthyidae [REDACTED] † Spodichthys † Tristichopterus † Eusthenopteron [REDACTED] † Jarvikina † Cabonnichthys † Mandageria † Eusthenodon [REDACTED] † Tinirau † Platycephalichthys † Panderichthys [REDACTED] † Tiktaalik [REDACTED] † Elpistostege † Elginerpeton [REDACTED] † Ventastega [REDACTED] † Acanthostega [REDACTED] † Ichthyostega [REDACTED] † Whatcheeriidae [REDACTED] † Colosteidae [REDACTED] † Crassigyrinus [REDACTED] † Baphetidae [REDACTED] Tetrapoda ( crown group ) [REDACTED] The following cladogram follows
416-408: The crown group tetrapods (the last common ancestor of living tetrapods and all of its descendants) and several groups of early stem tetrapods , which includes several groups of related lobe-finned fishes, collectively known as the osteolepiforms . The Tetrapodomorpha minus the crown group Tetrapoda are the stem Tetrapoda , a paraphyletic unit encompassing the fish to tetrapod transition. Among
442-454: The characteristics defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Another key trait is the internal nostril or choana . Most fish have two pairs of nostrils, one on either side of the head for incoming water (incurrent nostrils) and another pair for outgoing water (excurrent nostrils). In early tetrapodomorphs like Kenichthys ,
468-406: The earliest known tetrapods as well. Unlike the early tetrapods, Eusthenopteron did not have larval gills . Like other fish-like sarcopterygians, Eusthenopteron possessed a two-part cranium , which hinged at mid-length along an intracranial joint . Eusthenopteron ' s notoriety comes from the pattern of its fin endoskeleton , which bears a distinct humerus , ulna , and radius in
494-421: The earliest-known tetrapods . It shares a similar pattern of skull roofing bones with stem tetrapoda forms such as Ichthyostega and Acanthostega . Eusthenopteron , like other tetrapodomorph fishes, had internal nostrils (or a choana ), one of the defining traits of tetrapodomorphs, including tetrapods. It also had labyrinthodont teeth, characterized by infolded enamel , which characterizes all of
520-558: The excurrent nostrils had shifted towards the mouth's perimeter. In later tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana . The nearly-equivalent clade Choanata often refers to these later forms specifically. Tetrapodomorph fossils are known from the early Devonian onwards, and include Osteolepis , Panderichthys , Kenichthys and Tungsenia . Tetrapodomorpha evolved from ancient lobe-finned fish ( sarcopterygians ) around 390 million years ago in
546-565: The fore-fin and femur , tibia , and fibula in the pelvic fin . These appendicular long bones had epiphyseal growth plates that allowed substantial longitudinal growth through endochondral ossification , as in tetrapod long bones. These six appendicular bones also occur in tetrapods and are a synapomorphy of a large clade of sarcopterygians, possibly Tetrapodomorpha (the humerus and femur are present in all sarcopterygians). Similarly, its elasmoid scales lack superficial odontodes composed of dentine and enamel; this loss appears to be
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#1732851943415572-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
598-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
624-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
650-470: The results found by Clement et al . (2021). Dipnomorpha Kenichthys Tungsenia Hongyu Gooloogongia [REDACTED] Sauripterus Barameda Screbinodus Rhizodus Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of
676-535: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Eusthenopteron Eusthenopteron (from Greek : εὖ eû , 'good', Greek : σθένος sthénos , 'strength', and Greek : πτερόν pteron 'wing' or 'fin')
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