Oregon stands apart from dinosaur-rich neighbors like Montana and Utah, which boast expansive fossil beds yielding complete skeletons. Oregon’s geologic history makes direct evidence of terrestrial dinosaurs exceedingly rare, leading to a long-held view that the state was largely dinosaur-free. However, the few confirmed finds reveal that these animals were present, even though their remains are difficult to find.
Why Terrestrial Dinosaur Fossils Are Scarce
The scarcity of dinosaur bones in Oregon is primarily due to the state’s geographic location during the Mesozoic Era. For most of that time, the landmass that would become modern Oregon was largely submerged beneath the Pacific Ocean. This environment favored the formation of marine sedimentary rock, which preserves oceanic creatures but not land-dwelling dinosaurs.
The small portions of land that did exist were mostly volcanic islands or coastal fringes, where conditions for fossilization were poor. Subsequent geological activity, including massive volcanism and faulting, has either buried or destroyed the few terrestrial rock layers that might have contained dinosaur remains. Much of the exposed rock in the state is marine in origin or from time periods before or after the dinosaurs.
Confirmed Dinosaur Evidence: Trace Fossils and Fragments
Despite the geological challenges, a few isolated fragments provide definitive proof that dinosaurs inhabited the region. The most significant find is a fossilized toe bone, or pedal phalanx, belonging to a plant-eating ornithopod dinosaur. This bone was discovered in the Early Cretaceous-aged Hudspeth Shale near Mitchell in Eastern Oregon.
The ornithopod was estimated to be a large herbivore, possibly over 20 feet long, related to genera like Tenontosaurus or Eolambia. Found embedded in marine rock alongside mollusk fossils, the toe bone is believed to have reached the ocean via the “bloat and float” phenomenon. This occurs when a dinosaur dies on land, its remains wash out to sea, and the buoyant carcass floats until decomposition causes the dense bone to sink and fossilize.
A second bone, a vertebra likely from the same type of dinosaur, was later found in the same area. Another confirmed fragment is a sacrum piece from a hadrosaur, or duck-billed dinosaur, found in marine sandstone at Cape Sebastian on the southern coast.
Oregon’s Mesozoic Giants: Marine Reptiles
Since Oregon was mostly covered by a shallow sea during the Age of Dinosaurs, its most abundant and complete Mesozoic fossil discoveries are marine reptiles. These animals, which include Ichthyosaurs, Plesiosaurs, and Mosasaurs, were ocean-dwelling reptiles that evolved separately from terrestrial dinosaurs.
The fossil record in Oregon includes several major groups of these ancient swimmers. Ichthyosaurs, often described as resembling dolphins, are known from Triassic-aged limestones in the Wallowa Mountains. Plesiosaurs, with their long necks and four large flippers, are represented by finds such as a partial skull discovered near Mitchell, belonging to a 25-foot-long predator. Mosasaurs, which were giant, serpentine marine lizards, also patrolled Oregon’s Cretaceous waters, with some species reaching lengths of over 50 feet.