Ultrasauros (formerly Ultrasaurus) is at present invalid sauropod genus, with a complicated history. When a compilation of bones from it seems that hitherto undiscovered dinosaur was set up in 1979 at the Dry Mesa Dinosaur Quarry in Utah by James A. Jensen, it was hailed as the main dinosaur known to date. Unfortunately, the specimen was in fact composed of bones from a Supersaurus and a large Brachiosaurus.
The backbone (a dorsal vertebra, labeled BYU 9044) that was second-hand to define the new species in fact belongs to a Supersaurus. In fact, it almost certainly belongs to the original Supersaurus, which was exposed in the same quarry in 1972. Other bones, like the shoulder girdle (scapulocoracoid, BYU 9462) belong to a Brachiosaurus, perhaps a large specimen of Brachiosaurus altithorax. (Curtice, 1996)
The Brachiosaurus bones point to a large, but not record-breaking individual, a little larger than the Giraffatitan brancai in the Humboldt Museum of Berlin, which was the main known dinosaur for decades. Larger specimens of Brachiosaurus are known from the Tendaguru Beds of Tanzania, in east Africa.
Originally, the bones were supposed to represent an single dinosaur that was about 25 to 30 meters (80 to 100 feet) long, 8 meters (25 feet) high at the shoulder, 15 meters (50 feet) in total height, and weighing maybe 70 metric tonnes (75 tons). At the time, mass estimates ranged up to 180 tons, which located it in the same group as the blue whale and the equally problematic Bruhathkayosaurus. More present estimates are far more conservative.