The principle that fossil organisms succeed one another in a definite and determinable order is a cornerstone of biostratigraphy. This concept posits that sedimentary rock layers contain fossilized flora and fauna which appear in a specific vertical sequence. An observation of this principle would be, for example, that trilobites consistently appear in older rock layers than ammonites, reflecting their respective periods of prevalence on Earth.
This ordered appearance of fossils is critical for relative dating of geological strata and correlating rock units across vast distances. It allows geologists to establish a chronological framework even in the absence of absolute dating methods. Historically, it facilitated the construction of the geologic timescale and provided crucial evidence supporting the theory of evolution by demonstrating the changing nature of life through time.
