Approximately 80 million years ago, dinosaurs roamed much of North America.  Although it would still be another 12 million years before Tyrannosaurus rex (T. rex) emerged as one of the largest apex predators of the dinosaur kingdom, other tyrannosaurs were just starting to evolve.  Tyrannosaurs were just one of many types of theropods, i.e., two-legged carnivores such as T. rex and Velociraptor that paved the way for birds.

When an 80-million-year-old theropod fossil was found in the San Juan Islands in April 2012 by a group of researchers from the Burke Museum of Natural History and Culture in Seattle, it was the first dinosaur fossil discovered in Washington.  The fossil was classified as a piece of a theropod's left femur—about 17 inches of what scientists estimate would have been a 4-foot-long leg bone.  The fossil was deposited in the Cedar District Formation (Formation).  Scientists disagree on where this Formation was located when the theropod died; estimates range from Baja California, Mexico, to Northern California.

The theropod fossil likely immigrated to present-day Washington as the Formation traveled north tens of millions of years ago.  The rock eventually became part of Sucia Island, an island in the San Juan Islands that today houses a state park accessible only by watercraft.

In the first academic paper describing the fossil, University of Washington paleontologist Christian Sidor and then-PhD student Brandon Peecook hypothesized that the theropod was likely a tyrannosaur, and they included images comparing the femur to that of a Daspletosaurus.  As scientists lack enough of the theropod's bones to classify the species definitively, the theropod to which the femur fossil belonged has been nicknamed the Suciasaurus rex in recognition of the island where it was discovered.

Dinosaur fossils are rarely found in the Pacific Northwest because active tectonic plate boundaries nearby have resulted in significant geological turmoil.  In addition, the dense development of cities in Western Washington has curtailed scientists' ability to dig for fossils.  Washington has previously designated the Columbian mammoth as the state fossil.  According to many scientists, the state bird—the Willow (or American) Goldfinch—is also considered a type of theropod.  Twelve states and Washington D.C. have official state dinosaurs. 

The Suciasaurus rex is the state dinosaur of Washington.