Evolution of tetrapods

The evolutionary history of vertebrates descended from four-limbed ancestors, beginning with lobe-finned fishes and spanning lineages adapted to land and water.

Tiktaalik: A Devonian lobe-finned fish with wrist-like bones and other features associated with early tetrapods. Its anatomy documents an intermediate body plan near the fish–tetrapod transition.
Tetrapod limb: A paired appendage built around a skeleton with one proximal bone, two distal bones, and more distal elements. The transformation of fins into limbs is central to the origin of tetrapods.
Temnospondyli: A diverse group of extinct early tetrapods that lived from the Carboniferous to the Cretaceous. Their fossils document extensive early diversification, including lineages adapted to aquatic and terrestrial settings.
Lobe-finned fish: A bony fish in the group Sarcopterygii, whose paired fins have fleshy bases supported by internal bones. Tetrapods arose within this broader fish lineage.
Tetrapod trackway: A fossilized trail of footprints attributed to a four-limbed vertebrate. Some trackways may predate body fossils of known tetrapods, complicating the transition's chronology.
Acanthostega: An early Devonian tetrapod with limbs and digits that remained strongly adapted to aquatic life. It shows that digits evolved before tetrapods became fully terrestrial.
Digits: Bony or cartilaginous elements at the ends of limbs, including fingers and toes in tetrapods. Digits distinguish the limbs of tetrapods from the fins of most fishes.
Seymouria: A Permian terrestrial vertebrate with a mix of features historically associated with amphibians and reptiles. It illustrates the mosaic anatomy found among early land vertebrates.
Ray-finned fish: A bony fish in the group Actinopterygii, whose fins are supported mainly by slender rays. They are the other major bony-fish lineage, not the ancestors of tetrapods.
Devonian tetrapod tracks: Fossil footprints from the Devonian that have been interpreted as evidence of early tetrapod activity. Their dating and interpretation affect estimates for when tetrapods first moved across substrates.

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