tetrapods

Tetrapods are a clade of vertebrates that possess, or evolved from ancestors with, four limbs. They arose from lobe-finned fishes during the late Devonian period, roughly 390 million years ago. The earliest clearly terrestrial vertebrates in this group include fossils such as Acanthostega and Ichthyostega, which show limbs with digits and adaptations for life on land, though they retained aquatic features. The term Tetrapoda encompasses all descendants of the last common ancestor of living tetrapods, including amphibians, reptiles, birds, and mammals.

Morphology among tetrapods is defined by limbs that evolved from paired fins, with segments becoming the upper arm (stylopod), forearm (zeugopod), and hand or foot (autopod), supported by shoulder and pelvic girdles. Most have a more robust rib cage and a neck region enabling head movement independent of the torso. Respiratory systems commonly include lungs, though some amphibians retain skin-based respiration. Reproduction varies, with many amphibians laying eggs in water and undergoing metamorphosis, while amniotes possess watertight eggs and membranes that support terrestrial life.

Evolutionary radiation of tetrapods led to diversification across terrestrial and freshwater habitats. The major split is between amphibians (salamanders, frogs, caecilians) and amniotes, the latter giving rise to reptiles, birds, and mammals. Birds are descendants of theropod dinosaurs within the reptile lineage, while mammals include monotremes, marsupials, and placentals. Today tetrapods display a wide range of body plans and life histories, from fully aquatic to highly terrestrial forms.