Therizinosaur: Mystery of the Sickle-Claw Dinosaur


A once-in-a-lifetime find in 2000 by Museum of Northern Arizona paleontologists led to the discovery of the most complete therizinosaur skeleton ever found. The 93-million-year-old bones were excavated near Big Water, in the desert landscape of southern Utah. The dinosaur was found in a location that was once the bottom of the Cretaceous Western Interior Seaway, an ancient sea that covered the middle of North America. The initial discovery, a single toe bone, led to the recovery of the nearly complete skeleton. But a mystery remains - how did the whole animal get buried in a seafloor, 60 miles from shore?

The dinosaur’s identity was a mystery well into the excavation. “We weren’t thinking ‘therizinosaur’ at first, because at that time they were known only from Asia,” said Dr. David D. Gillette, exhibit curator and MNA’s Colbert Chair of Vertebrate Paleontology. “From that first toe bone, we thought maybe we had a big ‘raptor’ (an agile, hunting dinosaur). But when we found peculiar bones of the massive hips, we knew we had a sickle-claw dinosaur. They were like nothing we’d ever seen.”

Most dinosaurs in this mysterious family are known only from partial skeletons. And the lifestyle of these lumbering, pot-bellied, sickle-clawed forms has been debated for decades; MNA’s skeleton fills in some major gaps in what is known about therizinosaur anatomy and habits.

“In the past two decades, new studies have regrouped therizinosaurs with carnivorous (meat-eating) dinosaurs,” noted Gillette, “but there are many questions. Was this animal truly carnivorous as indicated by its shared ancestry with forms like Tyrannosaurus rex? How did it use its three slashing sickles on each hand? Did this small-headed predator actually prey on plants?”

The therizinosaur was featured in an exhibit at MNA titled Therizinosaur: Mystery of the Sickle-Claw Dinosaur from 2007-2009. A cast of the exhibit is currently housed in MNA Collections. Its technical name is Nothronychus graffami, for MNA volunteer Merle Graffam of Big Water, Utah.