Final answer:
A scientist inferring that fossil A is older than fossil B from their positions in rock layers is using relative dating principles. The determination is based on understanding that lower rock layers, which are older, are assumed to contain older fossils according to the Law of Superposition.
Step-by-step explanation:
When a scientist observes that fossil A is older than fossil B, and infers this information based on their positions in sedimentary rock, they are using the principles of relative dating. This is because in sedimentary rock layers, lower layers are generally older than the layers above them due to the way sediment accumulates over time. Thus, if fossil A was found in a layer of rock below fossil B, then it is inferred to be older according to the principle known as the Law of Superposition.
Additionally, fossils are an important part of the fossil record, providing evidence for when organisms lived on Earth and how they evolved. The presence of specific fossils in these layers can also assist in biostratigraphic correlation, which allows geologists to relate different layers of rock and their assemblages to specific periods of geological time, supporting the inference of the age of fossils without the need for radiometric dating.