Trilobite

Trilobites were marine arthropods from the Paleozoic era. They ranged in sizes: some were small, some were large.

Facts
They were prey for a lot of animals including Anomalocaris and various sea scorpions. Trilobites themselves ate both algae on the sea floor and small organisms. Some of them ate plankton, others - primitive plants, or even other animals. Trilobites were armored sea creatures that looked a bit like giant woodlice. These sea creatures were arthropods and evolved sturdy skeletons on the outside of their bodies. There’re no relatives of these creatures alive in the modern times but up to 15,000 species of trilobites existed at one time (Palaeozoic).

Size
They ranged in size from a microscopic size, a millimeter in length, to the largest species, Isotelus, which grew to an enormous 72 cm long (which was as big as they got).

Their eyes were made of calcite. They became extinct at the end of the Permian due to climate change. The name trilobite means "three parts".

Water Dwellers
A group of trilobites were seen feeding on the sea floor. One was then eaten by an Anomalocaris.

Trivia

 * The species featured in Walking with Monsters was possibly Paradoxides.