On Thanksgiving weekend in 1995, Richard Thompson, a geology student looking for petrified wood in the Sonoran Desert about 40 miles southeast of Tucson, came upon bone fragments exposed along a sandstone ridge. His chance find turned out to be the first glimpse of a 51-foot- long, 35-ton dinosaur--probably a brachiosaur--that lived about 100 million years ago. The Sonoran fossil is the only dinosaur skeleton ever found in southern Arizona. More surprisingly, brachiosaurs--long-necked, long-tailed herbivores--were thought to have died out some 125 million years ago. Ronald Ratkevich, a paleontologist at the Arizona Sonora Desert Museum, says that about half the skeleton has been unearthed, including a complete hind foot (shown here), numerous vertebrae, a humerus, the pelvis, and the entire skull. Although the skull had been crushed, it features notable brachiosaur traits, such as a nasal opening on top of the head, as well as sockets for wide, spoonlike teeth. On the off chance that the dinosaur turns out to be a new genus of brachiosaurid, the museum plans to name it Sonorasaurus thompsoni. Since the dinosaur was found in a part of the Sonoran Desert called the Chihuahua Desert, Ratkevich at first wanted to name the dinosaur Chihuahuasaurus but felt it wasn’t appropriate for a 51- foot-long animal.