tetrapod

Tetrapod is a vertebrate that belongs to the clade Tetrapoda, defined by having four limbs or their evolutionary descendants. The group includes all living amphibians, reptiles, birds, and mammals, together with numerous extinct lineages. Some modern tetrapods, such as snakes and caecilians, have secondarily lost their limbs but are still descended from limbed ancestors.

Tetrapods originated from lobe-finned fishes (sarcopterygians) during the late Devonian period, about 395–360 million years ago. The transition involved structural changes to paired fins, shoulder and hip girdles, a longer neck, and a more robust skeleton adapted for life on land. Early fossils such as Acanthostega and Ichthyostega show transitional features, and Tiktaalik is often cited as one of the bridging forms between fish and tetrapods.

Anatomically, tetrapods typically have limbs with digits and reinforced pectoral and pelvic girdles, enabling weight-bearing locomotion on land. The skulls and vertebral columns were modified to allow head movement independent of the body, and most possess lungs and a rib cage, though respiratory modes can vary among lineages and life stages.

Diversity and reproduction: Today tetrapods inhabit a wide range of environments, from aquatic to terrestrial. Reproduction varies: many amphibians lay eggs in water, while amniotes (reptiles, birds, and mammals) produce eggs or young with membranes that permit development away from water.

Classification: Tetrapoda is a major vertebrate clade descended from sarcopterygian ancestors. Living tetrapods are divided into Amphibia and Amniota (reptiles, birds, and mammals).