Question

what are all the ways a scientist can date fossils

Answer 1

1. Radiometric dating: This is based on dating the rocks found in the same layers of soil. Because we know how many kinds of rocks form volcanically, and we also know that they contain ratios of elements that change at a known rate (because some elements undergo radioactive decay at a known rate), we can compute the approximate age of the rocks, and thus the fossils.

2. Index fossils: There are certain species of organisms that we find unique to certain layers of soil of a certain age (as determined by #1). For example, trilobites are found only in Cambrian layers (about 540 to 490 million years ago), and ammonites are found only in late Silurian and early Devonian layers (about 400 million years ago). Once we have documented this pattern, those fossils are "index fossils" that allow us to date other fossils in the same layer.