This is a mortality plate of over sixty, Ordovician trilobites from Tafraoute, Morocco. The entire piece of limestone is 11.6x8.1" with most of the trilobites being about 1 1/4" in length.
Trilobites were a very diverse group of extinct marine arthropods. They first appeared in the fossil record in the Early Cambrian (521 million years ago) and went extinct during the Permian mass extinction (250 million years ago). They were one of the most successful of the early animals on our planet with over 25k described species, filling nearly every evolutionary niche. Due in large part to a hard exoskeleton (shell), they left a excellent fossil record.