Vertebrates (Vertebrata) are a group of chordates that includes lampreys, mammals, birds, amphibians, reptiles, and fishes. There are about 57,000 species of vertebrates alive today, which accounts for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates. Vertebrates have a vertebral column in which the notochord is replaced by multiple vertebrae that form a backbone. The vertebrae surround and protect a nerve cord and provide the animal with structural support.
Vertebrates have a well-developed head, a distinct brain that is protected by a skull, and paired sense organs. They also have a highly efficient respiratory system, a muscular pharynx with slits and gills (in terrestrial vertebrates the slits and gills are greatly modified), a muscularized gut, and a chambered heart. Another notable character of vertebrates is their endoskeleton. An endoskeleton is an internal assemblage of notochord, bone or cartilage that provides the animal with structural support. The endoskeleton grows as the animal grows and provides a sturdy framework to which the animal's muscles are attached.
The vertebral column in verebrates is one of the group's defining characteristics. In most vertebrates, a notochord is present early in their development. The notochord is a flexible yet supportive rod that runs along the length of the body. As the animal develops, the notochord is replaced by a series of vertebrae that form the vertebral column.
Basal vertebrates such as cartilaginous fishes and ray-finned fishes breath using gills. Amphibians have external gills in the larval stage of their development and (in most species) lungs as adults. Higher vertebrates—such as reptiles, birds and mammals—have lungs instead of gills.
For many years, the earliest vertebrates were thought to be the ostracoderms, a group of jawless, bottom-dwelling, filter-feeding marine animals. But during the past decade, researchers have discovered several fossil vertebrates that are older than the ostracoderms. These newly discovered specimens, which are about 530 million years old, include Myllokunmingia and Haikouichthys. These fossils exhibit numerous vertebrate traits such as a heart, paired eyes, and primitive vertebrae.
The origin of jaws marked an important point in vertebrate evolution. Jaws enabled vertebrates to capture and consume larger prey than their jawless ancestors. Scientists believe that jaws arose via the modification of the first or second gill arches. This adaptation is thought to have at first been a way of increasing gill ventilation. Later, as musculature developed and the gill arches bent forward, the structure functioned as jaws. Of all living vertebrates, only the lampreys lack jaws.
The key characteristics of vertebrates include:
- vertebral column
- well-developed head
- distinct brain
- paired sense organs
- efficient respiratory system
- muscular pharynx with slits and gills
- muscularized gut
- chambered heart
Vertebrates are classified within the following taxonomic hierarchy:
Vertebrates are divided into the following taxonomic groups:
- Bony fishes (Osteichthyes) - There are about 29,000 species of bony fishes alive today. Members of this group include ray-finned fishes and lobe-finned fishes. Bony fishes are so named because they have a skeleton made of true bone.
- Cartilaginous fishes (Chondricthyes) - There are about 970 species of cartilaginous fishes alive today. Members of this group include sharks, rays, skates, and chimaeras. Cartilaginous fishes have a skeleton that is made of cartilage instead of bone.
- Lampreys and Hagfishes (Agnatha) - There are about 40 species of lamprey alive today. Members of this group include pouched lampreys, Chilean lampreys, Australian lampreys, northern lampreys, and others. Lampreys are jawless vertebrates that have a long narrow body. They lack scales and have a sucker-like mouth.
- Tetrapods (Tetrapoda) - There are about 23,000 species of tetrapods alive today. Members of this group include birds, mammals, amphibians, and reptiles. Tetrapods are vertebrates with four limbs (or whose ancestors had four limbs).
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