Washington State is an anomaly in North American paleontology, a region surrounded by areas with rich terrestrial dinosaur fossil records, such as Alberta and Montana. Despite its proximity to these famous fossil beds, Washington has yielded surprisingly few signs of the giant reptiles that dominated the Mesozoic Era. For much of the time dinosaurs roamed the Earth, the geological conditions necessary for preserving their remains simply did not exist across most of the state. This scarcity has led to a unique scientific focus on the rare evidence that does exist.
The State’s Single Confirmed Dinosaur Fossil
The question of which dinosaurs lived in Washington State has a very specific answer centered on a single, confirmed bone fragment. This unique specimen is a partial left thigh bone, or femur, belonging to a large theropod dinosaur, the group of two-legged carnivores that includes Tyrannosaurus rex. The fossil was unearthed in 2012 by researchers on the shores of Sucia Island State Park in the San Juan Islands, though the discovery was not formally published until 2015.
Scientific analysis determined the bone is approximately 80 million years old, dating it to the Late Cretaceous period. While the fragment is incomplete, its size and anatomical features point specifically to a theropod. Paleontologists estimated the complete femur would have measured over three feet long, suggesting the dinosaur was a large predator, likely a member of the Tyrannosauridae.
The fossil was found preserved within the marine rocks of the Cedar District Formation. This suggests the dinosaur died near the coast, and its body or a disarticulated limb was washed out to sea, where it eventually settled among ancient clams and other marine life. This single bone represents the only diagnostic Mesozoic dinosaur fossil ever recovered from Washington State.
Geological Reasons for Scarce Dinosaur Finds
The primary reason for the extreme rarity of dinosaur remains in Washington lies in the geological history of the region during the Mesozoic Era. For most of the time dinosaurs existed, the western portion of the state was submerged beneath the ocean. Terrestrial fossils are typically preserved in sedimentary rock laid down on floodplains or in ancient lakebeds, environments that were largely absent in Washington during that time.
The state’s landscape is primarily composed of accreted terranes—fragments of crust that originated elsewhere and were later welded onto the North American continent by plate tectonics. These active tectonic processes, which created the Cascade Range, are highly destructive to fossil preservation. The intense pressure and heat from volcanism and continental collision often metamorphosed any potential fossil-bearing rock, destroying organic material.
Furthermore, the few sedimentary layers that may have contained terrestrial dinosaur fossils have often been eroded away or are deeply buried. The entire Puget Sound region was later covered by massive ice sheets during the Pleistocene Epoch. This extensive glaciation scraped and reshaped the landscape, covering potential fossil sites with hundreds of feet of younger glacial deposits, making the discovery of ancient rock layers difficult.
Significant Mesozoic Marine and Plant Life
While terrestrial dinosaur fossils are almost nonexistent, the evidence for other life from the Mesozoic Era is abundant, especially marine life. The same rock formations that yielded the single theropod bone are rich with the fossilized remains of invertebrates. Researchers initially found the dinosaur femur while collecting the coiled shells of ammonites, which were squid-like creatures that lived in the ancient seas.
Other common finds in the region include various species of bivalves, such as the fossilized clams (Crassatellites conradiana) found alongside the theropod bone, which helped confirm the marine burial environment. Even plant life from the dinosaur age is represented in the state’s fossil record. For example, recent discoveries on Sucia Island include fossilized winged fruits from the genus Ceratopetalum, a plant group now restricted to the Southern Hemisphere.
These Cretaceous plant and marine fossils confirm that the area was teeming with life 80 million years ago, even if it was largely underwater. The presence of these other fossils indicates that landmasses were nearby to wash material into the sea, supporting the idea that dinosaurs were present on the coast. The challenge remains in finding the specific non-marine rock layers where their bones might have been preserved.