Trillobite Fossil in Sandstone
Trillobite are are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago) (Wikipedia has lots of information on these creatures).
Our example is 3" wide and 3" long formed in sandstone with intricate details of the beastie.