Editing Tetrapod (section)

=== Apomorphy-based definitions ===
{{See also|Stegocephali}}
A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct [[Digit (anatomy)|digits]] (fingers and toes), as well as legless vertebrates with limbed ancestors.<ref name=":2" /><ref name="RCQ03" /> Limbs and digits are major [[apomorphies]] (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. The first vertebrates with limbs and digits evolved in the [[Devonian]], including the [[Late Devonian]]-age ''[[Ichthyostega]]'' and ''[[Acanthostega]]'', as well as the trackmakers of the [[Middle Devonian]]-age [[Zachelmie trackways]].<ref name="NarkiewiczNarkiewicz2015" />

Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through [[convergent evolution]]. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalent [[Cladistics|cladistic]] definition. Cladistics is a modern branch of [[Taxonomy (biology)|taxonomy]] which classifies organisms through evolutionary relationships, as reconstructed by [[phylogenetic analyses]]. A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a [[Monophyly|monophyletic]] [[clade]], a group with all of its component taxa sharing a single common ancestor.<ref name="RCQ03" /> In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates.<ref name=":3" />