Washington state has very few dinosaur fossils compared to places like Montana or Utah, but at least three types of dinosaurs left evidence of their presence there during the Late Cretaceous period, roughly 65 to 90 million years ago. The state’s volcanic history, tectonic activity, and repeated glaciation have destroyed or buried most of the rock layers where dinosaur remains would normally be preserved, making every discovery remarkably significant.
The Suciasaurus: Washington’s Only Dinosaur Bone
The first and so far only diagnostic dinosaur bone found in Washington came from Sucia Island, a small state park island in the San Juan archipelago. The fossil is the upper portion of a left thigh bone from a large, meat-eating theropod. It measures 42.5 cm long and 22.4 cm across at its widest point, near the hip joint. Researchers identified it as a theropod based on the hollow internal cavity of the bone, a hallmark of the group that includes tyrannosaurs and raptors. The animal it came from was big, possibly in the size range of a Tyrannosaurus, though the fragment isn’t complete enough to pin down the exact species.
The bone was collected from a dark gray silty sandstone in the Cedar District Formation, part of the Nanaimo Group, a set of shallow marine rocks from the Campanian stage of the Late Cretaceous. That marine setting is important: this dinosaur didn’t live in the ocean. Its carcass most likely washed out to sea from a nearby coastline before settling on the seafloor and becoming fossilized. A 2015 study published in PLOS One formally described the specimen, making it the first scientifically confirmed dinosaur from the state.
In 2023, the Washington state legislature officially designated this dinosaur “Suciasaurus rex” as the state dinosaur, giving it a nickname even though it doesn’t have a formal scientific species name. The fossil is housed at the University of Washington’s Burke Museum in Seattle.
Dinosaur Footprints in the Cascades
While the Suciasaurus is the only body fossil, a separate discovery along the eastern Cascade Mountains revealed the first dinosaur tracks ever recorded in Washington. These footprints, preserved as casts in Upper Cretaceous rock, came from at least three different types of dinosaurs: theropods (two-legged meat-eaters), ornithopods (two-legged plant-eaters, the group that includes duckbills), and an ankylosaur (a heavily armored, tank-like herbivore).
This trackway evidence is especially valuable because it dramatically expands the known diversity of dinosaurs in the state. A single bone tells you one animal was there. Tracks from three separate groups tell you Washington supported a functioning dinosaur ecosystem during the Late Cretaceous, with both predators and prey walking the landscape. The age of the track-bearing rock layers was confirmed in part by the discovery of a mosasaur tooth crown in the same strata. Mosasaurs were large marine reptiles, not dinosaurs, but their presence helps geologists date the surrounding rock with confidence.
Why Dinosaur Fossils Are So Rare Here
Washington sits in one of the most geologically active corners of North America, and that activity has been terrible for fossil preservation. The Cascade Range is a volcanic arc that has been erupting on and off for millions of years, burying older rock layers under thick flows of lava and ash. The Columbia Plateau in eastern Washington is covered by some of the largest basalt floods in Earth’s history, sheets of volcanic rock that obliterated whatever fossils may have existed beneath them.
On top of that, the Pleistocene ice ages sent massive glaciers across the northern half of the state, scraping away surface rock and rearranging sediments. Much of western Washington’s surface geology dates to the last 2 million years or so, far too young for dinosaurs. The rocks that do survive from the Cretaceous period are mostly marine sediments (like those on Sucia Island) or isolated pockets in the Cascades. Terrestrial sedimentary rock from the Mesozoic era, the kind most likely to contain dinosaur bones, is exceptionally scarce.
This contrasts sharply with neighboring Montana, where vast exposures of Cretaceous terrestrial rock have yielded thousands of dinosaur specimens, including the famous duckbill Maiasaura. Washington almost certainly had a comparable diversity of dinosaurs roaming its landscape. The fossils just didn’t survive.
What the Cretaceous Landscape Looked Like
The plant fossil record fills in some of the picture that dinosaur bones cannot. The Winthrop Formation in north-central Washington preserves a rich flora from the Albian stage of the Early Cretaceous, roughly 100 to 113 million years ago. This fossil plant assemblage, found in the Methow basin, includes a variety of conifers and other gymnosperms that would have formed the forests and understory of the dinosaur-age landscape. By the Late Cretaceous, flowering plants had also diversified across the region.
The combination of marine sediments on the coast, terrestrial tracks in the mountains, and plant fossils in the basins paints a picture of a Washington that was warmer, lower in elevation, and partially submerged along its western edge. Large theropods hunted near the coastline. Armored ankylosaurs and plant-eating ornithopods moved through forested inland areas. The ecosystem was rich, even if the fossil record capturing it is frustratingly thin.
Dinosaurs Confirmed in Washington
Based on all available evidence, the dinosaurs documented in Washington include:
- Large theropod (Suciasaurus rex): Known from a single thigh bone found on Sucia Island. A big meat-eater, possibly tyrannosaur-sized, from the Late Cretaceous.
- Theropods (tracks): Footprints from smaller meat-eating dinosaurs found in the eastern Cascades.
- Ornithopods (tracks): Footprints from plant-eating dinosaurs in the duckbill lineage, also from the eastern Cascades.
- Ankylosaur (tracks): At least one set of prints from an armored herbivore, found alongside the other trackways.
That list will almost certainly grow. Washington’s Cretaceous rocks are underexplored, and each new discovery has expanded the known diversity significantly. The jump from zero dinosaurs to one bone to three track-making groups happened over just a few years of focused research. More accessible exposures of Mesozoic rock, particularly in the eastern Cascades, remain promising targets for future fieldwork.