No Precambrian fossils are known from Oklahoma, and the state's fossil record begins in the Paleozoic. From the Cambrian to the Devonian, Oklahoma was covered by a sea. Cambrian life of Oklahoma included graptolites and trilobites in the area now surrounding Turner Falls in the Arbuckle Mountains, although fossils are relatively scarce. Oklahoma's Ordovician life included several species of brachiopods, bryozoans, primitive echinoderms, and ostracods. Their remains fossilized at Rock Crossing in the Criner Hills of southern Oklahoma. One common Oklahoman graptolite was Climacograptus. High quality specimens of the trilobite Isotelus were preserved southwest of Ardmore. During the Silurian, Oklahoma was home to brachiopods, bryozoans, the trilobite Calymene, echinoderms, and sponges, all of which left their remains south of Lawrence Creek. However, the best sources of Silurian fossils in Oklahoma are the northern flanks of the Arbuckle Mountains. Oklahoma was home to an extremely diverse Devonian fauna in the Lawrence and White Mound areas.