Subphylum Vertebrata: the Vertebral Column
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Vertebrates are chordates whose notochord is replaced during development by a segmented vertebral column, a rigid, jointed structure protecting the underlying nerve cord.
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A shark's cartilaginous vertebral column and a human's bony one both replace an embryonic notochord present earlier in development.
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A vertebral column's material, cartilage or bone, does not determine vertebrate status — both compositions occur among different vertebrate lineages.