Trilobites create one of the most primitive recognized groups of arthropods. These are a group of fossilized extinct marine arachnomorph arthropods. Trilobites seem to have been wholly marine organisms, since the petrified remains of trilobites are always found in rocks containing fossils of other salt-water animals. So therefore, the answer is the hill you are hiking on is at one time, were under water. Since Trilobites are found under water.