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The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a node-based clade , a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).
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.
Articles relating to the Stegocephalia, a name used for four-limbed stem-tetrapods, and their amphibian-grade descendants, and in phylogenetic nomenclature for all tetrapods. The term was coined in 1868 by American paleontologist Edward Drinker Cope.
Tetrapodomorpha (also known as Choanata [3]) is a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish.
Stem tetrapods are the animals belonging to the lineage leading to tetrapods from their divergence from the lungfish, our nearest relatives among the fishes. In addition to a series of lobe-finned fishes , they also include some of the early labyrinthodonts .
Example: the tetrapods consist of the first ancestor of humans (A) from which humans inherited limbs with fingers or toes (M) and all descendants of that ancestor. These descendants include snakes (B), which do not have limbs. Several other alternatives are provided in the PhyloCode, [4] (see below) though there is no attempt to be exhaustive.
Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as Pan-Amniota [2] [3]) is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians).