Although the recognition of fossils goes back hundreds of datings of fossils lab ca, the systematic cataloguing and assignment of relative ages to different organisms from the distant past — paleontology — only dates back to the earliest part of the 19th century.
The oldest undisputed fossils are from rocks dated around 3. The oldest well-understood fossils are from rocks dating back to around Ma, and the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history of life.
However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils or that they are easy to find. Fossils alone cannot provide us with numerical ages of rocks, but over the past century geologists have acquired dating of fossils lab ca isotopic dates from rocks associated with fossil-bearing rocks such as igneous dykes cutting through sedimentary layers to be able to put specific time limits on most fossils.
If the rock we are studying has several types of fossils in it, and we can assign time ranges to those fossils, we might be able to narrow the time range for the age of the rock considerably. Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area and so can be used to compare rocks from different regions.