Trilobites are extinct arthropods that are the distant relatives of modern lobsters, horseshoe crabs and spiders. Their fossilized remains have been found on every continent on earth with over 20,000 scientifically recognized trilobite species. Trilobites were probably the first life forms with complex eyes, with some species having hundreds of individual lenses per eye. Many trilobite fossils represent remnants of their shed exoskeletons, rather than of the trilobites themselves. Collecting trilobites has a long history. Some have been found in human burial mounds dating back more than 50,000 years. Elrathia is a famous genus of trilobite that had an amazing ability to thrive where other ancient animals could not. They patrolled the deep, muddy waters offshore of the ancient continent Laurasia. While other organisms enjoyed the good life of sunshine and warmth along equatorial shorelines, Elrathia filled the dark benthic zone on the border of very little oxygen and none at all.