Fossils can be grouped and studied in many ways, by size, habitat type, or phylogenetic groups. Large fossils like dinosaurs, corals, echinoderms, and macro-mollusks (macrofossils) were the primary emphasis of paleontologic studies in the 1700's, 1800's and early 1900's, because they were easily recognizable in the field and did not require special equipment to study them. As new and better microscopes were invented, more and more paleontologists began to use microfossils to solve a variety of geologic problems. Microfossils currently are the preferred tools at the USGS. These small fossils are generally much more widespread and abundant in sedimentary deposits than larger fossils, and because of their size, much smaller samples can be collected. Drilling of coreholes, which obtains important rock samples from beneath the earth's surface, has made microfossils an indispensable tool because only occasionally will macrofossils be preserved in the two-inch diameter cores that are the product of most coring operations.

In this section, you will learn about each of the fossil groups that are being used to solve geologic problems at the USGS.