Answer:
In undisrupted layers of rock, fossils occurred in a definite sequential order, with more modern-appearing ones closer to the top. Because bottom layers of rock logically were laid down earlier and thus are older than top layers, the sequence of fossils also could be given a chronology from oldest to youngest. The fossil record provides consistent evidence of systematic change through time—of descent with modification. That is, amphibians will not appear before fishes, nor mammals before reptiles, and no complex life will occur in the geological record before the oldest eucaryotic cells.
To answer your second question, relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.
Scientists infer relationships among fossils if two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous. E.g. Tiktaalik, having many features akin to those of tetrapods. It was technically a fish, complete with scales and gills - but it had the flattened head of a crocodile and unusual fins. So the scientists classified it as an intermediate form between fish and amphibians.
Step-by-step explanation: