Placodermi - extinct
Chondrichthyes (cartilaginous fish)
Acanthodii - extinct
Actinopterygii (ray-finned fish)
Vertebrata is a subgroup of chordates, specifically, those with backbones or spinal columns. Vertebrates are definied as chordates whose notocord is protected by bones called vertebrae, sometimes referred to as a vertebral column. Vertebrata is the largest and most advanced subphylum, and contains most of the animals with which we are familiar. Fish (including lampreys but excluding hagfishes), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Additional characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone.
The internal skeleton which defines vertebrates consists of cartilage or bone, or in some cases both. The skeleton provides support to the organism during the period of growth. For this reason vertebrates can achieve larger sizes than invertebrates, and on average vertebrates are in fact larger. The skeleton of most vertebrates, that is excluding the most primitive ones, consists of a skull, the vertebral column and two pairs of limbs. In some higher forms of vertebrates, one or both of these pairs of limbs may be absent, such as in snakes or whales. These limbs have been lost in the course of evolution.
The skull is thought to have facilitated the development of intelligence as it protects vital organs such as the brain, the eyes and the ears. The protection of these organs is also thought to have positively influenced the development of high responsiveness to the environment often found in vertebrates.
Both the vertebral column and the limbs support the body of the vertebrate overall. This support facilitates movement. Movement is normally achieved with muscles that are attached directly to the bones or cartilages. The contour of the body of a vertebrate is formed by the muscles. A skin covers the inner parts of a vertebrate's body. The skin sometimes acts as a structure for protective features, such as horny scales or fur. Feathers are also attached to the skin.
The trunc of a vertebrate is hollow and houses the internal organs. The heart and the repiratory organs are protected in the trunc. The heart is located behind the gills, or where there are lungs, in between the lungs.
The central nervous system of a vertebrate consists of the brain and the spinal cord. Both of these are characterizes by being hollow. In lower vertebrates the brain mostly controls the functioning of the sense organs. In higher vertebrates the size of the brain relative to the size of the body is larger. This larger brain enables more intensive exchange of information between the different parts of the brain. The nerves from the spinal cord, which lies behind the brain, extend to the skin, the inner organs and the muscles. Some nerves are directly connected to the brain, linking the brain with the hear and lungs.
Vertebrates have been traced back to the Silurian period (444 million to 409 million years ago).