Temporal range: Albian–Cenomanian
Sonorasaurus is a genus of brachiosaurid dinosaur from the middle Cretaceous ( Albian to Cenomanian stages, around 112 to 93 million years ago). It was a herbivorous sauropod whose fossils have been found in southern Arizona in the United States. Its name, which means "Sonora lizard", comes from the Sonoran Desert where its fossils were first found. It is estimated to have been about 15 meters long (49 ft) and 8.2 meters (27 feet) tall, about one third of the size of Brachiosaurus.
Fossilized remains were discovered by geology student Richard Thompson, in November 1994, 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 the paleontologist of the Arizona-Sonora Desert Museum at Tucson, Ronald Paul Ratkevich. 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. 
Dating of the find makes this the first known brachiosaur to have lived in the middle Cretaceous Period of North America. The type species is S. thompsoni, described by Ratkevich in 1998. Since August 3, 2018, it is the state dinosaur of Arizona.
- Ratkevich, R (1998). "New Cretaceous brachiosaurid dinosaur, Sonorasaurus thompsoni gen et sp. nov, from Arizona." Arizona-Nevada Academy of Science 31; 71-82.
- Thayer D.W. and Ratkevich R., 1995, "In progress dinosaur excavation in the mid-cretaceous Turney Ranch Formation, southeastern Arizona", Proceedings of the Fossils of Arizona Symposium. Bulletin No.3., Mesa Southwest Museum, Southwest Paleontological Society
- David W. Thayer, Ronald P. Ratkevich & Stan E. Krzyzanowski, 1996, "A new Dinosaur for the Arizona-Sonora Desert Museum Tucson, Arizona", Rocks & Minerals, 71(1): 34-38
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