Phacops rana are best known for their self-defense mechanism and their elaborate eyes.  In order to protect themselves from predators, Phacops rana would roll into a ball with their hard exoskeleton on the outside as protection, much like the pill bug of today does.  Many other trilobites have adopted the same feature, but the Phacops rana nearly perfected it.  The slightest amount of sediment would trigger their senses and the Phacops rana would be hidden in a tiny shelter.  Although this safety feature often helped them to evade predators, occasionally it backfired and the Phacops rana would be buried under heavy sediment.  Scientists find them in their balled up position 400 million years later. 
 
 

Facts about Phacops rana:

Pennsylvania State Fossil Phacops rana is found in Pennsylvania's Devonian-age rocks  One animal probably produced ten to twelve potentially preservable skeletons in its lifetime. Best known for their self-defense mechanism and elaborate eyes.