The Stem Tetrapoda are a cladistically defined group, consisting of all animals more closely related to

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

four-legged vertebrates than to their closest extant relatives (the

lungfish Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

), but excluding the

crown group 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 ...

Tetrapoda. They are thus

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, though acceptable in phylogenetic nomenclature as the group is defined by strict reference to

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

rather than to traits as in traditional systematics. Thus, some finned

sarcopterygians Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

are considered to be stem tetrapods.

Content of the group

Stem tetrapods are members of

Tetrapodomorpha 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 f ...

, the total group and clade that also includes their descendants, the crown tetrapods: The stem Tetrapoda encompass three distinct grades successively closer to crown group Tetrapoda: * Osteolepiformes, a group of lobe-finned fishes that includes the families

Tristichopteridae Tristichopterids (Tristichopteridae) were a diverse and successful group of fish-like tetrapodomorphs living throughout the Middle and Late Devonian. They first appeared in the Eifelian stage of the Middle Devonian. Within the group sizes ranged ...

, Canowindridae, Megalichthyidae, and Osteolepidae *

Elpistostegalia Elpistostegalia is a clade containing ''Panderichthys'' and all more derived Tetrapodomorpha, tetrapodomorph taxa. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, such as ''Tiktaalik'', are sometimes called fisha ...

, the more advanced lobe-finned fishes (

) and the " fishapods" (genera such as ''

Panderichthys ''Panderichthys'' is a genus of extinction, extinct Sarcopterygii, sarcopterygian (lobe-finned fish) from the late Devonian period, about 380 Myr, Mya. ''Panderichthys'', which was recovered from Frasnian (early Late Devonian) deposits in Latvia, ...

'' and ''

Tiktaalik ''Tiktaalik'' (; ) is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian Period, about 375 Mya (million years ago), having many features akin to those of tetrapods (four-legged animals). ''Tiktaalik'' is est ...

'') *

Ichthyostegalia Ichthyostegalia is an obsolete order of early amphibians, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade. While the group are recognized as having feet rather than fins, most, if ...

, the primarily aquatic primitive

labyrinthodont "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally conside ...

s such as ''

Acanthostega ''Acanthostega'', from Ancient Greek ἄκανθα (''ákantha''), meaning "spine", and στέγη (''stégē''), meaning "roof", is an extinct genus of stem tetrapoda, stem-tetrapod, among the first vertebrates, vertebrate animals to have recogn ...

'', ''

Ichthyostega ''Ichthyostega'', from Ancient Greek ἰχθύς (''ikthús''), meaning "fish", and στέγη (''stégē''), meaning "roof", is an Extinction, extinct genus of limbed tetrapodomorpha, tetrapodomorphs from the Devonian, Late Devonian of what is ...

'', '' Tulerpeton'', and probably loxommatids. Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...

s rather than clades, with

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

s and modern amphibians branching off at some point from the latter. The stem tetrapods may also include one or both of

Temnospondyli Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order of small to giant tetrapods—often considered primitive amphibians—that flourished ...

and

Lepospondyli Lepospondyli is a diverse clade of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco ('' Diplocaulus minimus''), lepospondyls lived from the Visean stage of the Early Carboniferous to the Ea ...

, depending on author. This is due to the uncertain origin of the modern amphibians, whose position in the phylogenetic tree dictates what lineages go in the

Tetrapoda. Neither is there for the moment a consensus of the phylogeny of stem tetrapods, nor how Tetrapoda itself should be defined (i.e. as a

, or as an apomorphy-based group, using the limb with digits), making the actual content of the group uncertain. Laurin, M. (1996)
Phylogeny of Stegocephalians
from the

Tree of Life Web Project The Tree of Life Web Project (ToL) is an Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world. The s ...

References

{{Tetrapodomorpha, R. Tetrapodomorpha Tetrapod taxonomy Paraphyletic groups