The Alsea Formation in Oregon preserves fossils from the Rupelian stage of the Oligocene and records marine life from the early Oligocene. Among the fossils found there are mammals including Aetiocetus cotylalveus, Maiabalaena nesbittae, and Simocetus rayi, along with the fish Orthechinorhinus davidae. These discoveries make the formation an important source for understanding ancient marine ecosystems and the evolution of early whales in the North Pacific.