The Stem Tetrapoda are a
cladistically defined group, consisting of all animals more closely related to
extantfour-legged vertebrates than to their closest extant relatives (the
lungfish), but excluding the
crown groupTetrapoda. 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.
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]