Which fossil is older?
The simple answer is that the fossils at the bottom are older than the ones at the top. This is because rocks with fossils i.e. sedimentary rocks, are, simplistically-speaking, laid down one on top of another in layers (geologist call these “strata”). It’s just a sequence of events: The oldest ones therefore have to be at the bottom, which in geology is known as the “Law of Superposition”. The law applies where the rocks are nicely preserved as horizontal layers, such as in the Grand Canyon, but this is not always the case. Sedimentary rocks are not always laid down in horizontal layers! In many instances, the rocks have been folded or faulted, sometimes even overturned, due to tectonic activity, and whole sections may be mising due to erosion (unconformities). These can make the picture considerably more complex!
