Illaenus sarsi, Ordovician trilobite fossil. Trilobites were arthropods that fed as they crawled on the seabed. They lived between 500 and 300 million years ago and are now extinct. This specimen dates from the Early to Middle Ordovician, and was found in the Arenig Series, in the Billingen-Vaana Substage of Vaginatum Limestone, in Ljungsbro, Ostergotland, Sweden. This specimen is rolled up into a ball.