Stem tetrapoda

Tiktaalik, a tetrapodomorph with wrists, straddles the fish-tetrapod divide

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. Sarcopterygian fish are considered to be stem tetrapods.

Content of the group

Stem tetrapods are members of Tetrapodomorpha, which unlike the Stem Tetrapoda, are a total group and thus a true clade, including stem tetrapods and their descendants, crown tetrapods:^[1]

The stem Tetrapoda encompass three distinct grades successively closer to crown group Tetrapoda:^[2]

• Osteolepiformes, a group of lobe-finned fishes that includes the families Rhizodontidae, 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.^[1]

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.^[3][4] 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),^[5] making the actual content of the group uncertain.^[6][7][8]

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.
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.
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, D; Laurin, M (2013). "The origin(s) of extant amphibians: a review with emphasis on the "lepospondyl hypothesis"". Geodiversitas. 35: 207–272. doi:10.5252/g2013n1a8.
5. Laurin, M; Anderson, JS (2004). "Meaning of the name Tetrapoda in the scientific literature: an exchange". Systematic Biology. 53: 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.
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.