vertebrate

Vertebrates are animals classified within the subphylum Vertebrata of the phylum Chordata. They are defined by having a vertebral column, or spine, and a skull, which together provide a protective framework and support for a complex nervous system. Vertebrates include the major living groups of fishes, amphibians, reptiles, birds, and mammals.

Anatomically, vertebrates possess an endoskeleton of bone or cartilage, a cranium enclosing the brain, and a dorsal hollow nerve cord. The vertebral column typically replaces the notochord during development and encloses the spinal cord. They also show a closed circulatory system with a ventral heart and a high degree of cephalization, with advanced sensory organs and neural crest-derived tissues in many lineages. Most have paired limbs or fins and a relatively high metabolic rate compared with invertebrates.

Vertebrates inhabit a wide range of environments, from deep oceans to high mountains and deserts. Reproduction is predominantly sexual, with fertilization occurring internally in many terrestrial species and externally in many fish and amphibians. Development ranges from direct to larval stages that morph into adults, and strategies include oviparity, ovoviviparity, and viviparity.

The vertebrate lineage arose during the Cambrian period, and the evolution of the vertebral column enabled greater flexibility and mobility, facilitating active predation and diversification. The five major classes—fishes, amphibians, reptiles, birds, and mammals—represent the breadth of vertebrate life, with birds arising from a lineage of reptiles and mammals distinguished by hair and mammary glands.