Stem Tetrapoda
Encyclopedia
Stem Tetrapoda is a cladistically
defined group, consisting of all animals more closely related to extant four legged vertebrates than to their closest extant relatives (the lungfish
), but excluding the crown group
Tetrapoda. It is thus paraphyletic, though acceptable in phylogenetic nomenclature as the group is defined by strict reference to phylogeny rather than to traits as in traditional systematics.
, which unlike the Stem Tetrapoda is a total group and thus a true clade, including stem tetrapods and their descendants, crown tetrapods:
The stem Tetrapoda encompasses three distinct grades
successively closer to crown group
Tetrapoda:
Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grade
s rather than clades. The stem tetrapods may also include one or both of Temnospondyli
and Lepospondyli
, depending on author. This is due to the uncertain origin of the modern amphibians
, whose position in the phylogenetic tree dictates what lineages goes in the crown group
Tetrapoda. Neither is there for the moment a consensus of the phylogeny of stem tetrapods, nor how Tetrapoda itself should be defined, making the actual content of the group uncertain.
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
defined group, consisting of all animals more closely related to extant four legged vertebrates than to their closest extant relatives (the lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
), but excluding the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
Tetrapoda. It is thus paraphyletic, though acceptable in phylogenetic nomenclature as the group is defined by strict reference to phylogeny rather than to traits as in traditional systematics.
Content of the group
Stem tetrapods are members of TetrapodomorphaTetrapodomorpha
Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish...
, which unlike the Stem Tetrapoda is a total group and thus a true clade, including stem tetrapods and their descendants, crown tetrapods:
The stem Tetrapoda encompasses three distinct grades
Evolutionary grade
In alpha taxonomy, a grade refers to 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.-Definition:...
successively closer to crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
Tetrapoda:
- OsteolepiformesOsteolepiformesOsteolepiformes are a group of prehistoric lobe-finned fishes which appears first time during the Devonian period. The order contains five families: Canowindridae, Elpistostegidae, Megalichthyidae, Osteolepidae and Tristichopteridae...
, a group of lobe-finned fishes that includes the families Rhizodontidae, CanowindridaeCanowindridaeCanowindridae is a family of prehistoric lobe-finned fishes which lived during the Devonian period . Fossils of fishes that belonged to this family have been found in Australia, Antarctica and Europe....
and MegalichthyidaeMegalichthyidaeMegalichthyidae is a family of prehistoric lobe-finned fishes which lived during the Carboniferous period ....
, OsteolepidaeOsteolepidaeOsteolepidae is a family of prehistoric lobe-finned fishes which lived during the Devonian period.... - ElpistostegaliaElpistostegaliaElpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period . They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes...
, the more advanced lobe-finned fishes (TristichopteridaeTristichopteridaeTristichopterids , were a diverse and successful group of tetrapodomorph fishes throughout the Late Devonian stage. They first appeared in the Givetian epoch of the Middle Devonian stage...
) and the "fishapods" (genera like PanderichthysPanderichthysPanderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...
and TiktaalikTiktaalikTiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
) - Ichthyostegalia, the primarily aquatic primitive labyrinthodonts such as AcanthostegaAcanthostegaAcanthostega is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto...
, IchthyostegaIchthyostegaIchthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
, TulerpetonTulerpetonTulerpeton is a fossil of an extinct genus of Devonian labyrinthodont that was found in the Tula Region of Russia at a site named Andreyevka...
, and probably loxommatidsLoxommatidaeBaphetidae is an extinct family of early tetrapods. Baphetids were large labyrinthodont predators of the Late Carboniferous period of Europe. Fragmentary remains from the Early Carboniferous of Canada have been tentatively assigned to the group...
.
Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grade
Evolutionary grade
In alpha taxonomy, a grade refers to 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.-Definition:...
s rather than clades. The stem tetrapods may also include one or both of Temnospondyli
Temnospondyli
Temnospondyli is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent...
and Lepospondyli
Lepospondyli
Lepospondyli are a group of small but diverse Carboniferous to early Permian tetrapods. Six different groups are known, the Acherontiscidae, Adelospondyli, Aïstopoda, Lysorophia, Microsauria and Nectridea, and between them they include newt-like, eel- or snake-like, and lizard-like forms, along...
, depending on author. This is due to the uncertain origin of the modern amphibians
Lissamphibia
The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...
, whose position in the phylogenetic tree dictates what lineages goes in the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
Tetrapoda. Neither is there for the moment a consensus of the phylogeny of stem tetrapods, nor how Tetrapoda itself should be defined, making the actual content of the group uncertain.