Vertebrates

What is a Vertebrate?

A Vertebrate is an animal that has a backbone.

Even though vertebrates are a diverse group of animals, only around 50,000 species have been identified which is a very small fraction of all animal species. Vertebrates include all animals that have a backbone.

Reptiles and amphibians are vertebrates as well as mammals, birds and fish. They all have a flexible support column to which other body structures are attached.

The first vertebrates were primitive fish that account for more than half of all living vertebrate species. Tetrapods are the main core of vertebrates and it was the earliest tetrapods that first developed limbs and left that water to begin life on land. Those who left the water began to populate the land, take to the skies and some even returned to the water.

One of the ancestors of all vertebrate species was a small creature called a Pikaia who looked like a small eel with tail-fins and measured around 1.5 inches (5 centimetres) in length. The Pikaia was the first known Chordate (an animal with a stiff supporting rod (notochord) along its back). In later animals, the notochord developed into the backbone.

Fossil remains of the Pikaia were found in the 530 millions years old mudstone deposits of the Burgess Shale in Canada.

Main Vertebrate Groups

Gnathosomata – these are vertebrates with jaws and include all land vertebrates and all fish except Lampreys and Hagfish.

Elasmobranchii – these are cartilaginous fish such as sharks and rays which are jawed fish with a skeleton made from cartilage instead of bone.

Osteichthyes – these are bony vertebrates and include all vertebrates with a mineralised skeleton such as mammals, birds, reptiles, amphibians and most fish.

Tetrapods – these are bony vertebrates with four limbs or appendages. The first land-living vertebrate tetrapods include the earliest limbed vertebrates and all of its descendants including those that have since lost their limbs such as snakes.

Amphibians – these are cold-blooded tetrapods with porous, glandular skin and includes frogs, toads, salamanders and newts who are descendants of early tetrapods that did not develop an amnion (waterproof egg membrane).

Amniotes – these are tetrapods whose embryo grows inside a waterproof membrane called an amnion, which enables life outside of the water.

Reptiles – these are amniotes with thick skin and bony epidermal scales whose eggs have a mineralised shell. This group includes warm-blooded birds and scaly cold-blooded reptiles.

Lepidosaurs – these are reptiles that shed their skin in either large pieces are as a whole. These include tuataras, snakes, lizards and worm lizards.

Archosaurs – these are reptiles whose teeth are sunken into sockets such as birds and crocodiles.

Birds – these are archosaurs with symmetrical flight feathers. Many have shared features including a bony, toothless beak/bill and a keeled breastbone.

Mammals – warm-blooded animals which give birth to their young. The young feed on their mothers milk.