One of the main factors that restricts the evolutionary change of the vertebral column in mammals is speed. Mammals are relatively fast-moving animals, and the vertebral column must be able to withstand the forces generated by running, jumping, and other activities. Any major changes to the vertebral column could potentially compromise an animal's ability to move quickly and efficiently.
For example, if the vertebrae were to become longer, the spine would become more flexible, but it would also be less stable and more prone to injury. Similarly, if the vertebrae were to become shorter, the spine would be more rigid, but it would also be less flexible and more likely to restrict movement.
In addition to the need for speed, the vertebral column is also constrained by the need to protect the spinal cord and other vital organs. The vertebrae must be strong enough to protect these structures from damage, but they must also be flexible enough to allow for movement. Any changes to the vertebral column must take into account these conflicting requirements.
As a result of these constraints, the vertebral column of mammals has evolved relatively slowly over time. While there have been some changes, such as the reduction in the number of vertebrae in the neck and tail, the overall structure of the vertebral column has remained largely the same.
This is not to say that the vertebral column is not capable of change. In fact, the vertebral column of mammals has undergone significant changes over the course of evolution. However, these changes have been gradual and incremental, and they have been constrained by the need to maintain the essential functions of the vertebral column.
