vertebrates

Vertebrates are animals in the subphylum Vertebrata within Chordata. They are characterized by a segmented, internal skeleton of bone or cartilage and a protective skull around the brain, along with a dorsal, hollow nerve cord and a notochord that is typically replaced by a vertebral column during development.

The vertebrate body plan includes a backbone of vertebrae, a closed circulatory system with a heart, and complex organ systems. Vertebrates are deuterostomes with bilateral symmetry and relatively high metabolic rates compared with many invertebrates.

Major groups include jawless fishes, jawed fishes, amphibians, reptiles, birds, and mammals. Jawed vertebrates are divided into cartilaginous fishes (such as sharks and rays) and bony fishes. Tetrapods encompass amphibians, reptiles, birds, and mammals, reflecting the transition to land-dwelling lifestyles in many lineages.

Development typically features a skull and brain, paired limbs in most lineages, and a variety of reproductive strategies ranging from eggs laid externally to internal gestation. Many vertebrates undergo metamorphosis or exhibit parental care, and these traits vary widely among groups.

Vertebrates first appeared in the fossil record during the early Cambrian period, with early armored jawless forms such as ostracoderms. The evolution of jaws, skeletons, and later flight and endothermy in some lineages contributed to extensive diversification through the Paleozoic, Mesozoic, and Cenozoic eras.

Today vertebrates inhabit nearly all environments, from oceans to deserts. They face threats such as habitat loss, pollution, and overexploitation, making conservation efforts important for many species.