Marine sediments record the existence through time of a
fossil species because individuals of the species die and their shelly
remains or bones are preserved (fossilized) in the sediments. As time
goes by, more sediment accumulates and more individual fossils accumulate.
When a species goes extinct, the remains of the last living individual
of the species is preserved in sediments deposited at the time of extinction.
In a sediment core, the oldest sediments are at the bottom,
or deepest part of the core, and the youngest sediments are at the top,
or shallowest part of the core. Let's take an example of a particular
organism we will call species A. Its time of extinction will be
referred to as extinction 1. If a paleontologist were to examine sediments
just slightly older than (i.e. just below) extinction 1,
fossils of species A would be preserved in the sediment. However,
if sediments only slightly younger than (i.e. just above) extinction 1
were examined for this species A, none would be found in the sediment.
Why? Because by this time, all organisms of species A have died and the
species is now extinct.
