What chordate trait evolved first?

The first chordate trait that evolved is believed to be the presence of a notochord, a flexible rod-like structure that provides support. This feature appears in the earliest chordates, such as cephalochordates and early vertebrates, serving as a precursor to the vertebral column in more advanced chordates. Other key traits, such as the dorsal nerve cord, pharyngeal slits, and post-anal tail, evolved subsequently.