Which of the following is NOT a shared characteristic of all chordates?

The characteristic that is not shared by all chordates is the presence of a four-chambered heart. While a four-chambered heart is a fascinating adaptation found in certain advanced vertebrates, such as birds and mammals, it is not a universal feature among all chordates.

In contrast, pharyngeal slits, notochords, and dorsal hollow nerve cords are fundamental characteristics that define the chordate group. Pharyngeal slits, for instance, are present at some stage of development in all chordates and may serve various functions, including filter feeding in aquatic larval forms. The notochord, a flexible rod that provides support, is also characteristic of all chordates during at least some stage of their life cycle. Lastly, the dorsal hollow nerve cord is another key characteristic, forming the basis of the central nervous system in chordates.

Thus, the four-chambered heart does not apply to all chordates, making it the correct answer to the question.