How the structure of the axial skeleton in vertebrates will change during evolution

How the structure of the axial skeleton in vertebrates will change during evolution. Make an assumption about what is associated with an increase in the number of parts of the spine

The axial skeleton has undergone great changes in the process of evolution.
In the lower chordates, the axial skeleton is the chord, and in the higher chords, it is gradually replaced by developing vertebrae. In the vertebrae, the body, upper and lower arches are distinguished.
In primitive fish, in addition to the upper arches, lower arcs appear, and in higher fish and vertebral bodies. The vertebral bodies of most fish and higher animals are formed from the tissue surrounding the chord, as well as from the base of the arches. The remains of the chord in fish are preserved between the vertebral bodies.
In amphibians, in the early stages of development, the chord is replaced by the spine. There are already four sections in the spine: cervical, thoracic, sacral and caudal.
Reptiles have five sections in the spine: cervical, thoracic, lumbosacral and caudal. In the cervical region, different types of reptiles have a different number of vertebrae, but the largest is eight.
In birds, the spinal column is similar to the spine of reptiles, but has a certain specialization due to the presence of a tail.
In mammals, the spine has five sections: cervical thoracic, lumbar, sacral and caudal. There are seven vertebrae in the cervical region, and a variable number of vertebrae in the thoracic region (from 9 to 24, but more often 12-13).

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