What term describes the main source of support for an animal that characterizes vertebrates?

The term that best describes the main source of support for an animal that characterizes vertebrates is the endoskeleton. In vertebrates, the endoskeleton is an internal framework made primarily of bone and cartilage, providing structure, support, and protection for vital organs. This framework allows for more complex movements and growth compared to an exoskeleton, which is characteristic of some other animal groups.

While the vertebral column is a significant part of the endoskeleton, specifically providing support for the back and housing the spinal cord, it is just one component of the overall endoskeletal system. The axial skeleton and pectoral girdle refer to specific sections or structures associated with the endoskeleton but do not encompass the entirety of the support system in vertebrates. Thus, the endoskeleton accurately describes the main supportive structure that characterizes vertebrate animals.