Imagine you discovered a fossil from an adult organism that you suspect might be an ancient chordate. What structure would you look for to confirm your suspicion?

To confirm the suspicion that a fossil comes from an ancient chordate, you would look for evidence of structures that are characteristic of the chordate phylum. Chordates are a diverse group of animals that include vertebrates (animals with backbones), as well as some invertebrates. The defining features of chordates that you would look for in a fossil include:

1. **Notochord**: A flexible, rod-shaped body that runs the length of the organism's body, providing skeletal support. In many vertebrates, the notochord is replaced by the vertebral column (spine) during development, but evidence of its existence might be found in the fossilized remains.

2. **Dorsal Hollow Nerve Cord**: Unlike other animal phyla that have solid nerve cords, chordates have a nerve cord that is dorsal (on the back) and hollow. In vertebrates, this structure develops into the spinal cord and brain.

3. **Pharyngeal Slits or Pouches**: Openings in the throat area (pharynx) that are used for filter-feeding in invertebrate chordates and are present during the embryonic development of vertebrates. In fish and some amphibians, these slits develop into gills, while in terrestrial vertebrates, they give rise to various structures in the ear and neck.

4. **Post-anal Tail**: An extension of the body past the anal opening. In many chordates, this tail contains skeletal elements and muscles, and it can be quite prominent during some stage of development.

5. **Endostyle or Thyroid Gland**: The endostyle is a glandular groove in the floor of the pharynx that is present in invertebrate chordates, which in vertebrates develops into the thyroid gland.

When examining a fossil for signs of these structures, it's important to note that not all of them may be preserved. Fossilization can preferentially preserve certain types of tissues or structures, depending on the conditions under which the organism was fossilized. Hard structures like bones or shells are more likely to be preserved than soft tissues. Therefore, in the case of a suspected chordate fossil, the most likely features to be identifiable would be the notochord (or vertebral column if it's a vertebrate), and possibly the pharyngeal slits if the conditions allowed for their preservation.

If the fossil shows evidence of any of these structures, it would strongly support the hypothesis that the organism was indeed a chordate.