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How did the notochord function in the earliest vertebrates?

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Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

In the earliest vertebrates, the notochord functioned as the primary longitudinal structural element, providing essential support for the body. As a stiff, rodlike structure featuring a turgid core and a fibrous sheath, it prevented the animal’s body from shortening during the muscular contractions required for locomotion.123

Beyond its mechanical role in movement, the notochord served as the first supporting structure for the developing embryo and played a critical organizational role in the development of the nervous system. In primitive vertebrates, such as cyclostome fishes, the notochord persists throughout the animal's life. In more advanced vertebrates, it serves as a developmental scaffold; while it is largely replaced or incorporated into the vertebral column as the animal matures, it remains a fundamental feature of early vertebrate anatomy.124