Final answer:
Fossils in the deepest sedimentary layer of an undisturbed canyon wall are the oldest and can reveal information about a past environment, including events like mass extinctions. Scientists also utilize the abundance of fern spores and ichnofacies to infer ecological conditions and disturbances, such as those found at the K-Pg boundary.
Step-by-step explanation:
The fossils found in the bottom layer of sedimentary rock in an undisturbed canyon wall are likely to be the oldest in the geological record, given that sedimentary layers are formed sequentially with the oldest layers at the bottom. These fossils can tell us about the ancient environment and the organisms that existed at the time the sediment was deposited.
Paleontologists can conclude that a mass extinction event occurred if there is a sharp decrease in the diversity and abundance of fossils from one layer to the next. A layer that correlates to a mass extinction might also show a shift in the types of fossils found, representing the species that survived the event.
When analyzing rock samples across the Cretaceous-Paleogene (K-Pg) boundary, scientists often find a sudden increase in fern spores just above the boundary, which indicates a disruption in the ecosystem, as ferns are typically the first plants to colonize an area after a disturbance.
Ichnofacies, which are associations of trace fossils, can provide additional insight into the environment at the time of deposition, including water depth, salinity, energy, and turbidity. The mode of preservation of fossils in carbonate layers can involve fluids, which may contain minerals that replace the original biological material, preserving the structure of the organisms.