521 million BCE • Earth
"Trilobites emerged fully formed upon the Cambrian scene. By the time, some 521 million years ago, that the initial members of this ancient line of arthropods began filling oceanic environments around the globe, they were already creatures with highly developed eyes, complex digestive systems and admirably functional calcite carapaces." Trilobites faced extinction around the end of the Permian period, approximately 252 million years ago, marking the conclusion of their remarkable 270-million-year evolutionary journey spanning the Cambrian to the Permian periods.
"The First Trilobites," American Museum of Natural History.
Image: James St. John via Flickr, CC BY 2.0 DEED, Attribution 2.0 Generic