Based on the information provided, the most reasonable conclusion is:
Corals came into existence later than single-celled eukaryotes.
This conclusion is drawn from the fact that single-celled eukaryotes are generally considered to be among the earliest forms of life on Earth, while corals, which are multicellular and part of the animal kingdom, evolved later. The elevation at which the fossils are found does not directly indicate their existence or extinction status but suggests a timeline of their emergence.