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Vertebrate: Definition, Anatomy, Evolution and Importance

An overview of vertebrates: animals with a spinal column. Covers defining features, major anatomical parts, evolutionary origins, groups (fish, amphibians, reptiles, birds, mammals) and ecological significance.

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: June 22, 2026

simple.wikipedia.org · CC BY-SA 3.0

A vertebrate is an animal distinguished by an internal skeletal axis formed of vertebrae that surrounds and protects a spinal cord. In living vertebrates that axis may be composed of bone or cartilage and supports a head with a protective braincase. The term comes from the vertebrae that make up the backbone; animals lacking this arrangement are called invertebrates. Vertebrates include familiar groups such as mammals, birds, reptiles, amphibians and several kinds of fish, and are members of the larger grouping of chordates that possess a dorsal nerve cord and (in early stages) pharyngeal structures.

Key anatomical features

The vertebrate body plan centers on an internal skeleton and a central nervous system organized around a dorsal spinal cord. Important components include:

Braincase (cranium): a bony or cartilaginous skull that shields the brain and sensory organs; see the brain for functions related to coordination and perception.
Vertebrae: a series of articulated elements forming the backbone; joints between vertebrae permit flexibility and movement while guarding the spinal cord. The term "vertebrae" links to the anatomical concept of the vertebral units that make up the spine.
Endoskeleton: composed of bone or cartilage, the skeleton supports tissues, transmits muscle forces, and provides attachment points for limbs and other structures; collectively it is often called the skeleton.
Pharyngeal (gill) arches: in fish and some larval amphibians these support gills; in many vertebrates some arches have been modified into jaws and related structures, a transformation that shaped feeding and respiration (see pharynx and jaws).

Additional common features are paired appendages (fins or limbs), a post-anal tail at some life stage, and an advanced endocrine and circulatory system. The vertebrate spinal arrangement allows a wide range of locomotion and specialized adaptations among major lineages.

Evolutionary origins and development

Vertebrates evolved from early chordate ancestors that possessed a dorsal nerve cord and pharyngeal slits. Fossil and comparative anatomical evidence indicate an ancient diversification in aquatic environments, followed by repeated colonizations of land and air. One of the most important innovations was the evolution of jaws from gill arches, which opened new ecological roles. Over geological time vertebrate groups radiated into many forms adapted to swimming, walking, flying and other modes of life.

Diversity and classification

Living vertebrates are commonly divided into major groups: jawless and jawed fish, amphibians, reptiles, birds and mammals. The label "fish" is used for several distinct vertebrate lineages rather than a single evolutionary group. Scientists have described tens of thousands of vertebrate species, and ongoing research continues to refine their relationships and number.

Classification is based on anatomical and genetic evidence; for example, the presence or absence of certain skull bones, limb structures, and reproductive modes are key to distinguishing groups. Modern molecular studies supplement traditional anatomy to trace evolutionary branching.

Importance, uses and notable distinctions

Vertebrates play crucial ecological roles as predators, prey, pollinators and ecosystem engineers. Humans rely on many vertebrates for food, clothing, companionship and scientific research. Vertebrate anatomy also serves as a comparative model for understanding organ systems, development and disease. A central distinction for lay audiences is simply: vertebrates possess a vertebral column that protects a spinal cord, whereas invertebrates do not. For introductory or more detailed information see general resources on animals and specialized entries for groups such as fish, amphibians, reptiles, birds and mammals.

For anatomical terms and structures referenced above, readers may consult dedicated articles about the skeleton, the brain, the mechanics of vertebral joints, the pharynx and the evolution of jaws. Further resources and taxonomic lists can be found through curated species databases and introductory overviews on vertebrate biodiversity.

Floor Plan

Vertebrate monophyly is supported by a set of common derived (new) basic plan features (synapomorphies):

Multi-layered epidermis: The epidermis differentiates into several superimposed cell layers. Within vertebrates, the "skin" with several layers and associated structures such as scales, feathers, etc. is formed.
Neurocranium: The brain and the large sensory organs are protected by a capsule (cranium).
Neural crest: An embryonic structure composed of pluripotent cells that arise from the ectoderm at the boundary between the epidermal ectoderm and the neural ectoderm. They form, among other things, skeletal structures of the head, pigment cells, neurons such as Rohon-Beard cells, ganglia and odontoblasts.
Placodes: thickenings of the embryonic epidermis. Cells of the placodes are involved in the formation of neural organs.
Inner ear: seat of the organ of equilibrium
Brain: The anterior part of the neural tube is differentiated into a (multi-part) brain.
Cerebral nerves: In the basic plan, ten nerves of a completely different nature, which connect the brain with the periphery. They are present quite constantly throughout the vertebrates.
Vascular system: The cardiovascular system is a self-contained system (almost completely).
Kidneys: Central urinary organ (excretory organ) of vertebrates.
Vertebrate eyes: highly developed and complex sensory organ for the perception of optical stimuli
Spinal ganglia: Ganglia can be assigned to the spinal nerves.

Systematics

External systematics

The vertebrates have the rank of a subphylum in conventional biological systematics. Together with the tunicates and the species-poor skulls, they form the phylum Chordata.

According to the Notochordata-Urochordata hypothesis, the vertebrates are considered to be the sister group of the skullless animals (Acranians/Cephalochordata), hence they are often referred to as "skull animals" (Craniota or Craniata). In contrast, the alternative Olfactores-Cephalochordata hypothesis, which appeared later, states that the mantle animals (Urochordata/Tunicata) are the sister group of vertebrates. Which hypothesis is correct is still not clear today.

Inner systematics

In the past, vertebrates were subdivided according to the criterion of whether or not they had jaws. This approach is outdated: the jawed (jawed animals) are no longer contrasted with the jawless (Agnatha), but with the round jawed (Cyclostomata).

However, the internal systematics of vertebrates remains controversial, particularly the question of whether there is a sister group relationship between jawed mouths and lampreys, or between hagfishes and lampreys:

Hagfishes + (jaws + lampreys)

(hagfishes + lampreys) + pine mouths

The following representation also takes extinct groups into account. The classic large groups are highlighted in bold.

Vertebrates (Vertebrata): over 70,300 species

cyclostomes (Cyclostomata) about 131 species

Hagfishes (Myxini): about 83 species
lampreys (Petromyzontida): about 47 species

"Ostracodermi" † (paraphyletic).
pine mouths (Gnathostomata): over 70,200 species

Placodermi † (paraphyletic)
Acanthodii † (paraphyletic)
cartilaginous fish (Chondrichthyes): about 1227 species
Bony fish (Osteichthyes) (paraphyletic)

Ray fins (Actinopterygii): over 33,400 species
Meat fins (Sarcopterygii): 8 species

terrestrial vertebrates (Tetrapoda): over 35,600 species

Amphibians (Amphibia): over 7,800 species
Amniotes: over 27,800 species

Sauropsida

Reptiles (Reptilia) (paraphyletic): about 10,700 species
Birds (Aves): about 10,700 species, in addition 158 species have become extinct in historical times

Mammals (Mammalia): about 6,400 species, in addition 96 species have become extinct in historical times

Extinct groups

The extinct, often heavily armored, jawless taxa are grouped together as Ostracodermi, the armored, jawed ones as Placodermi. However, both groups are not monophyletic taxa, nor are the Acanthodii, which are partly basal to cartilaginous fishes or to bony fishes.

The affiliation of the extinct conodonts to the Vertebrata is controversial.

Diversity of recent vertebrates: proportions of groups in the total number of species. "Fishes" = bony and cartilaginous fishes.

Vertebrate evolution since the Cambrian according to Benton (1998). Extinct are the Placodermi and the "spiny sharks". The width of the branches illustrates the number of vertebrate families, which, however, only gives an approximate indication of the respective species diversity. In particular, the species richness of the Mesozoic reptiles is not sufficiently evident. Also, the large number of newly discovered amphibian and reptile species to date is not yet reflected here

Questions and Answers

Q: What is a vertebrate?

A: A vertebrate is an animal with a spinal cord surrounded by cartilage or bone.

Q: What are animals that are not vertebrates called?

A: Animals that are not vertebrates are called invertebrates.

Q: What parts make up the vertebrate skeleton?

A: The parts of the vertebrate skeleton include the braincase, which protects the brain, a series of short, stiff vertebrae separated by joints to protect the spinal cord, bones to support and protect soft tissues and provide points for muscle attachment, and gill arches in some species to support their gills.

Q: What types of animals are examples of vertebrates?

A: Examples of vertebrates include mammals, birds, and fish.

Q: How many species of vertebrates have been identified?

A: A few tens of thousands of species have been identified.

Q: How many different kinds of fish exist as part of the Vertebrata class?

A: The term "fish" is used to describe at least four different kinds of vertebrates.