USA Biology Olympiad (USABO) Practice Exam

Which characteristic is essential for all chordates at some developmental stage?

1. Post-anal tail

2. Pharyngeal slits

3. Vertebrae

4. Notochord

The correct answer is: Notochord

The notochord is a flexible rod-like structure that provides support and is a defining characteristic of all chordates at some stage in their development. This structure plays a critical role during early development, serving as a primary support structure and as a point of attachment for muscles. It is present in the embryonic stage of all chordates and is significant in defining the subphylum. In vertebrates, the notochord is eventually replaced by the vertebral column (the backbone) but remains an essential feature in the early development stages of all chordates, including fish, mammals, and reptiles. The presence of a notochord is indicative of the evolutionary lineage of chordates and is fundamental to their biological classification. Other options, such as the post-anal tail and pharyngeal slits, are characteristics that are present at different life stages or may be more transient; they may not be found in all members of the phylum at every developmental stage. The vertebrae are also specific to a subset of chordates (vertebrates) and do not apply to all chordate members. Thus, the notochord is the most consistent and defining characteristic across all chordates during their development.