Fossilized remains were discovered in November 1994 by geology student Richard Thompson in the
Turney Ranch Formation in the
Whetstone Mountains, located in the
Chihuahua Desert region of the
Sonoran Desert in southern Arizona. Thompson had investigated a previously almost unexplored region, where fossils proved to be plentiful and directly accessible on the surface. A relatively complete sauropod skeleton was weathering out on a rock wall. He informed paleontologist
Ronald Paul Ratkevich of the
Arizona-Sonora Desert Museum,
Tucson of the find. Ratkevich assembled a team of volunteers and began securing the bones in the spring of 1995; excavations would only end in 1999. He assumed the fossils represented a sauropod but was no expert on that taxon. The curator of geology of the museum, David W. Thayer, thought it might be a
therizinosaur, mistaking a tail chevron bone for the long hand claw typical of that group. In 1995, Ratkevich and Tayer first reported the find, already using the name "Sonorasaurus" but informally, so that it remained a
nomen nudum. Both men now asked dinosaur expert
Edwin Harris Colbert to identify the animal. Colbert, having seen only pictures, suggested it might be a member of the
Hadrosauridae. Ratkevich and Thayer then visited the displays in the
American Museum of Natural History, concluding their find was rather dissimilar to the hadrosaurid skeletons shown there, so that it must represent a species new to science. Ratkevich considered naming it "Chihuahuasaurus" but ultimately shied away from the comical contrast between the gigantic sauropod and the minute dog breed. In 1996, a subsequent article tried to fit the bones found, in a diagram of the hadrosaurid
Kritosaurus. This attempt largely failed, with an ilium being mistaken for a shoulder blade. Again, the name "Sonorasaurus" was used but still invalidly.
Sonorasaurus was finally formally described in 1998 by Ratkevich, who identified it as a brachiosaurid sauropod. Dating of the specimen found it to be the earliest known brachiosaurid to have lived in the 'middle'
Cretaceous Period of
North America. ==Description==