The notocord evolution is unique to vertebrates, while the fusion of the pelvic girdle to the backbone, alongside the emergence of the humerus-radius-ulna and expansion of the atlas/axis, are shared developments in tetrapods and vertebrates.
Evolution of the notocord is an example of a development seen in vertebrates but not in tetrapods. The notocord is a defining feature of chordates and present in early vertebrates, providing support and structure before vertebrae develop.
The fusion of the pelvic girdle to the backbone is another element that happens in both tetrapods and vertebrates, contributing to the stability and locomotion of animals along with the emergence of the humerus-radius-ulna and expansion of the atlas/axis.
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