Abstract
Four-legged land animals first appeared in the fossil record about 365 million years ago. This was the fish-tetrapod transition, which was characterized by innovation in limb structure and adaptation to habitual air-breathing. Acanthostega gunnari is the most primitive tetrapod known, and the most recent work on its fossil remains shows, among other things, that the formation of the tetrapod limb went well ahead of changes in the respiratory system.
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