Final answer:
The trait distinguishing vertebrates from more primitive chordates is the presence of a backbone or spine, which is a series of bones that form the vertebral column and provides shape support and spinal cord protection.
Step-by-step explanation:
The trait that sets vertebrates apart from simpler chordates is the presence of a backbone or spine. The backbone or vertebral column is a distinctive feature of vertebrates, which encompasses a series of bones aligned to form the spine, replacing the embryonic notochord in adult forms. This structure not only allows vertebrates to maintain their shape and provides support but also protects the spinal cord.
The vertebrae that make up the backbone consist of separate, irregularly shaped bones that are joined together and often interspersed with shock-absorbing discs in many species. Members of phylum Chordata, such as fish, amphibians, reptiles, birds, and mammals, exhibit this hallmark vertebral column, distinguishing them from invertebrate chordates like tunicates and lancelets, which do not have this structure.