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.
Stem tetrapods are members of Tetrapodomorpha, which unlike the Stem Tetrapoda is a total group and thus a true clade, including stem tetrapods and their descendants, crown tetrapods:[1]
The stem Tetrapoda encompasses three distinct grades successively closer to crown group Tetrapoda:[2]
Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grades 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.[3] 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.[4][5]