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The Stem Tetrapoda are 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. They are 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. Thus, some finned sarcopterygians are considered to be stem tetrapods.

Content of the group

Stem tetrapods are members of Tetrapodomorpha, 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, Canowindridae, Megalichthyidae, and Osteolepidae
• Elpistostegalia, the more advanced lobe-finned fishes (Tristichopteridae) and the "fishapods" (genera such as Panderichthys and Tiktaalik)
• Ichthyostegalia, the primarily aquatic primitive labyrinthodonts such as Acanthostega, Ichthyostega, Tulerpeton, and probably loxommatids.

Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grades rather than clades, with amniotes and modern amphibians branching off at some point from the latter. 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 go 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 (i.e. as a crown group, or as an apomorphy-based group, using the limb with digits), making the actual content of the group uncertain.

References

1. ^ Coates, M.I.; Ruta, M.; Friedman, M. (2008). "Ever Since Owen: Changing Perspectives on the Early Evolution of Tetrapods" (PDF). Annual Review of Ecology, Evolution, and Systematics. 39: 571–92. doi:10.1146/annurev.ecolsys.38.091206.095546. Archived from the original (PDF) on 29 October 2013.
2. Ahlberg, P. E.; Johanson, Z. (1998). "Osteolepiforms and the ancestry of tetrapods" (PDF). Nature. 395 (6704): 792–794. Bibcode:1998Natur.395..792A. doi:10.1038/27421. S2CID 4430783. Archived from the original (PDF) on 29 March 2017.
3. Sigurdsen T, Green D (2011). "The origin of modern amphibians: a re-evaluation". Zoological Journal of the Linnean Society. 162 (2): 457–469. doi:10.1111/j.1096-3642.2010.00683.x.
4. Marjanović, David; Laurin, Michel (2013). "The origin(s) of extant amphibians: a review with emphasis on the "lepospondyl hypothesis"". Geodiversitas. 35: 207–272. doi:10.5252/g2013n1a8. S2CID 67823991.
5. Laurin, M; Anderson, JS (2004). "Meaning of the name Tetrapoda in the scientific literature: an exchange". Systematic Biology. 53 (1): 68–80. doi:10.1080/10635150490264716. PMID 14965901.
6. Carroll, R.L. (2001). "The origin and early radiation of terrestrial vertebrates" (PDF). Journal of Paleontology. 75 (6): 1202–1213. doi:10.1666/0022-3360(2001)075<1202:toaero>2.0.co;2. S2CID 59359868. Archived from the original (PDF) on 2012-09-30.
7. Laurin, M. (1996): Phylogeny of Stegocephalians, from the Tree of Life Web Project
8. Laurin, M (1998). "The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics, middle ear evolution, and jaw suspension". Annales des Sciences Naturelles, Zoologie, Paris. 13e Série. 19: 1–42. doi:10.1016/S0003-4339(98)80132-9.

v t e Tetrapodomorpha (Pan-Tetrapoda)
Kingdom: Animalia Phylum: Chordata Clade: Osteichthyes Clade: Sarcopterygii Clade: Rhipidistia Gnathostomata see Gnathostomata Tetrapodomorpha see below↓
Tetrapodomorpha Tetrapodomorpha †Bogdanovia †Chrysolepis †Geptolepis †Glyptopomus †Gogonasus †Greiserolepis †Gyroptychius †Harajicadectes †Kenichthys †Litoptychus †Lamprotolepis †Latvius †Medoevia †Megadonichthys †Muranjilepis †Osteolepis †Peregrina †Platyethmoidia †Shirolepis †Thaumatolepis †Thursius †Tungsenia †Viluichthys †Rhizodontida Archichthys Aztecia Barameda Gooloogongia Hongyu Letognathus Propycnoctenion Pycnoctenion Rhizodus Rossichthys Sauripterus Screbinodus Strepsodus Thysanolepis? Vorobjevaia? †Canowindridae Beelarongia Canowindra Koharalepis Marsdenichthys? Owensia? †Megalichthyidae Askerichthys Cladarosymblema Cryptolepis? Ectosteorhachis Lohsania? Mahalalepis Megalichthys Megapomus? Megistolepis? Palatinichthys Rhizodopsis Sengoerichthys Sterropterygion? Eotetrapodiformes †Palaeospondylus? †Tristichopteridae Bruehnopteron Cabonnichthys Callistipterus Canningius Devonosteus Edenopteron Eusthenodon Eusthenopteron Heddleichthys Hyneria Jarvikina Langlieria Litoptychus? Mandageria Marsdenichthys? Notorhizodon Platycephalichthys Spodichthys Tinirau Tristichopterus Elpistostegalia †Howittichthys? †Livoniana †Panderichthys †Parapanderichthys †Qikiqtania †Rubrognathus †Tiktaalik Stegocephali (Tetrapoda sensu lato) see below↓
Stegocephali (Tetrapoda sensu lato) Devonian taxa †Acanthostega †Brittagnathus †Densignathus †Elpistostege? †Hynerpeton †Ichthyostega †Jakubsonia †Metaxygnathus †Parmastega †Sinostega †Tiktaalik? †Tulerpeton †Tutusius †Umzantsia †Ventastega †Ymeria †Elginerpetontidae Elginerpeton Obruchevichthys Webererpeton Post-Devonian taxa †Antlerpeton †Aytonerpeton †Crassigyrinus †Diploradus †Doragnathus †Gaiasia †Ichthyerpeton †Koilops †Mesanerpeton †Occidens †Ossirarus †Perittodus †Sigournea †Tantallognathus †Aistopoda Andersonerpeton Lethiscus Ophiderpeton Oestocephalidae Coloraderpeton Oestocephalus Phlegethontioidea Pseudophlegethontia Phlegethontiidae Phlegethontia Sillerpeton †Whatcheeriidae Ossinodus? Pederpes Whatcheeria †Colosteidae Colosteus Deltaherpeton Greererpeton Pholidogaster †Adelospondyli Acherontiscus Adelogyrinidae Adelogyrinus Adelospondylus Dolichopareias Palaeomolgophis †"Nectridea" see Nectridea †Baphetoidea Eucritta? Spathicephalus Baphetidae Baphetes Kyrinion Loxomma Megalocephalus †Embolomeri Anthracosaurus Archeria Aversor Calligenethlon Carbonoherpeton "Cricotus" Diplovertebron Eobaphetes Eoherpeton? Leptophractus Neopteroplax Palaeoherpeton Papposaurus Pholiderpeton Proterogyrinus Pteroplax Seroherpeton Spondylerpeton †Gephyrostegidae Bruktererpeton Eusauropleura Gephyrostegus †Other "anthracosaurs" Caerorhachis Casineria? Eldeceeon Eoherpeton Silvanerpeton Termonerpeton Crown group Tetrapoda Temnospondyli (Batrachomorpha see Temnospondyli includes Lissamphibia Reptiliomorpha (Pan-Amniota) see Reptiliomorpha includes Amniota
Related topics Paraphyletic / Polyphyletic groups Stem tetrapoda Osteolepiformes Osteolepididae Megalichthyiformes? Tristichopteridae? Labyrinthodontia Rachitomi Ichthyostegalia Lepospondyli Holospondyli Phyllospondyli Anthracosauria Other topics Evolution of tetrapods Polydactyly in stem-tetrapods Romer's gap Timeline Zachelmie trackways
Category

• Tetrapodomorphs
• Tetrapod taxonomy
• Paraphyletic groups