Stem tetrapoda

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. 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==