6. Individual fossil species have much shorter geologic ranges than the group as a whole. If you were to find an ostracode preserved in a sedimentary rock, it might be of any age from Cambrian to Recent. However, if you could identify the particular species of ostracode, it might give you the age of the rock within a few million years. Commonly, assemblages of several types of fossils are found together in a rock. This can also help you determine the age of the rock more closely. For example, if you had an ostracode fossil and a planktonic foram fossil together in a rock, what would this tell you about the age of the rock