A vertebrate is an animal that has a backbone, which protects the spinal cord inside and forms part of an internal skeleton. In contrast, invertebrates are animals that lack a backbone and, instead, may have soft bodies, fluid-filled internal structures or hard outer coverings such as shells and exoskeletons.
Vertebrates also tend to have more complex organ systems, including a well-developed brain enclosed in a skull, a closed circulatory system and specialized sensory organs. In contrast, invertebrates display a much wider range of body plans and physiological strategies.
Evolutionarily, vertebrates belong to a broader animal phylum called chordates. Long before true backbones evolved, early chordates developed a flexible, rod-like support structure called a notochord. Today, a few unusual marine animals, such as sea squirts and tiny, fish-like lancelets, still possess this primitive structure.
Vertebrates constitute only a small fraction of animal diversity. Currently the total number of scientifically described vertebrate species is around 65,000 to 73,000. In contrast, invertebrates, which span dozens of different animal phyla, represent roughly 95 to 97 percent of all known animal species, with well over 1.3 million described species and many more likely still undiscovered.
Vertebrates are traditionally classified into five major branches, with fishes accounting for the largest share at more than 31,000 described species. This domination is primarily because they have had far more evolutionary time to diversify, occupy an exceptionally wide array of habitats and possess biological traits that promote evolution into new species.
The remaining branches include reptiles (roughly 11,000 to 12,000 species), birds (about 10,000 to 11,000 species), amphibians (around 8,000 species) and mammals (roughly 6,000 to 6,500 species).
Among vertebrates, amphibians are consistently identified as the most threatened group, with about 41 percent of assessed species at risk of extinction according to recent IUCN evaluations. Sharks, rays and chimeras follow closely, with roughly 37 percent of species threatened, while mammals face a risk at around 27 percent, reptiles at about 21 to 28 percent and birds at roughly 12 to 13 percent. Freshwater fishes also show high levels of threat, with some assessments indicating more than a third of species are endangered or vulnerable.