When I think of North American dinosaurs, my mind immediately jumps to the impressive giants like Diplodocus and Tyrannosaurus scattered in rock formations around the West. But there were East Coast dinosaurs, too. One of them, an enigmatic creature discovered at the close of the 19th century, even serves as the controversial official dinosaur of our nation's capital.

According to David Weishampel and Luther Young's book Dinosaurs of the East Coast, in 1898 construction workers found part of a dinosaur vertebra and other bone fragments while excavating a sewer at First and F Streets SE. It was one of several fragmentary dinosaurs found in the approximately 100-million-year-old, Early Cretaceous rock under parts of the city, but there was so little of it that paleontologists are still uncertain as to what this animal actually looked like. The most distinctive bone of the lot—the vertebra, which came from near the base of the tail—indicates that that animal was a large theropod dinosaur, but over the past century it has been bumped from one assignment to another. In 1911 the paleontologist Richard Swann Lull named the dinosaur Creosaurus potens, but this was overturned a decade later when his colleague Charles Gilmore noticed that the name "Creosaurus" was synonymous with Allosaurus. Furthermore, Gilmore proposed that the partial tail bone more closely resembled its counterpart in the New Jersey dinosaur Dryptosaurus, leading him to rename it Dryptosaurus potens.

The tailbone sat at the Smithsonian for another seven decades, but in 1990 the paleontologist Peter Kranz gave it another look. The tail bone did not closely match that of Dryptosaurus after all, and instead appeared to represent a unique type of dinosaur (which would make sense given that Dryptosaurus lived later during the Cretaceous and Allosaurus had lived earlier during the Jurassic). Little could be said beyond this with certainty, but Kranz called the dinosaur "Capitalsaurus" in a 1990 Washingtonian article and introduced the name, albeit in quotes, into the formal scientific literature in a 1998 review of Washington D.C.'s fossils.

Here's where things get tricky. Kranz never officially described the fossils according to the standards of modern paleontology, meaning that "Capitalsaurus" is an informal name and not a scientific designation for the dinosaur. This did not stop it from becoming the official dinosaur of Capitol Hill. The same year that Kranz published his review of D.C.'s fossils, he also worked with local schools to make the case to the D.C. Council that "Capitalsaurus" should be the district dinosaur. This passed, even if the body of the bill muddled the science itself, calling "Capitalsaurus" a potential ancestor of Tyrannosaurus while including a skeletal restoration of the distantly related allosauroid Acrocanthosaurus.

Valid or not, "Capitalsaurus" became embedded in the city's culture. The site of its discovery was renamed Capitalsaurus Court, and January 28 is Capitalsaurus Day to commemorate the date in 1898 when the dinosaur was found. Nevertheless, the name "Capitalsaurus" remains only a popular designation, and without more fossil material it will be impossible to definitively identify this dinosaur. Who knows if further remains will ever be found? The relevant deposits have been built over, although there is the possibility that future construction projects may inadvertently find more fossils. Until then, the true identity of "Capitalsaurus" will have to remain a mystery.