Which group of organisms is typically used as index fossils due to wide distribution and short geological range?

• A. Benthic organisms
• B. Terrestrial plants
• C. Microfossils
• D. Swimming or floating organisms

Answer: (D)

Index fossils are fossils that help date rocks and correlate layers by being widespread geographically but existing for only a short span of time. The group that fits this best are swimming or floating organisms because they include planktonic and other mobile forms that disperse widely through the oceans. Their widespread distribution means their fossils appear in many distant locales, and their rapid evolution gives a relatively brief appearance in the rock record. As a result, finding their remains in different places at the same stratigraphic level provides a reliable marker for contemporaneous deposition.

Benthic organisms are tied to specific habitats on the seafloor, so their distribution and time ranges can be more restricted. Terrestrial plants fossilize less consistently in many sedimentary records, and their ranges can be long or climate-driven rather than tightly time-bound. Microfossils can indeed serve as excellent index fossils, but among the options given, the broad, easily dispersed swimming or floating groups best embody the characteristic combination of wide distribution and short geological range.