Final answer:
Biostratigraphy is a relative dating method that uses fossil associations in strata to determine the relative ages of rock layers. It relies on the law of superposition, where lower layers are presumed to be older, and it can be used to calculate the rate of speciation of organisms.
Step-by-step explanation:
A relative dating method that uses the associations of fossils in strata to determine each layer's approximate age is called biostratigraphy. This method is based on the principle of stratigraphy, which observes that soil and sediment are deposited in successive layers, or strata. The deepest layers are considered older than the layers above them, a concept known as the law of superposition. Biostratigraphy specifically uses fossils to correlate and establish the relative ages of sedimentary rocks, helping to determine which fossils are older or younger, although not providing their exact age in years.
Indicator artifacts such as certain species of elephant, pig, and horse fossils have been used in Africa to establish relative dates of geological strata, like those observed at Olduvai Gorge. This relative dating technique is crucial for determining the geological ages of fossils so researchers can calculate the rate of speciation of early organisms without needing to know their specific age in years.