Ultrasauros was a chimeric species of dinosaur. While it existed it was fairly well known as one of the tallest dinosaurs ever but it was often misspelled Ultrasaurus, with a U replacing the O (Ultrasaurus was a smaller (but still large) dinosaur from Korea) and (Argentinosaurus is bigger).
Ultrasauros was actually a mix of bones from Supersaurus and Brachiosaurus, giving the impression that it had the size of Supersaurus and the height of Brachiosaurus. Paleontologist James A. Jensen, who described the original Supersaurus specimen, simultaneously reported the discovery of another gigantic sauropod, which would later be named "Ultrasaurus" macintoshi (later renamed Ultrasauros macintoshi). The type specimen (the specimen used to define a new species) of Ultrasauros, being a backbone (dorsal vertebra, labelled BYU 9044), was later found to have come from Supersaurus. In fact, it probably belonged to the original Supersaurus specimen, which was discovered in the same quarry in 1972. Therefore, Ultrasauros became a junior objective synonym of Supersaurus, which had been named first and thus retains priority, and the name Ultrasauros was abandoned.
Diagram showing the size of different sauropods compared to a human, Supersaurus in orange Other bones that were found at the same location and originally thought to belong to Ultrasauros, like a shoulder girdle (scapulocoracoid, BYU 9462), actually belonged to Brachiosaurus, possibly a large specimen of Brachiosaurus altithorax. The Brachiosaurus bones indicate a large, but not record-breaking individual, a little larger than the "Brachiosaurus" brancai (Giraffatitan brancai) mount in the Humboldt Museum of Berlin. Larger specimens of Brachiosaurus are known from the Tendaguru beds of Tanzania, in east Africa. Originally, these Supersaurus and Brachiosaurus bones were believed to represent a single dinosaur that was estimated to reach about 25 to 30 meters (80 to 100 ft) long, 8 meters (25 ft) high at the shoulder, 15 meters (50 ft) in total height, and weighing maybe 70 metric tons (75 short tons). At the time, mass estimates ranged up to 180 tons, which placed it in the same category as the blue whale and the equally problematic Bruhathkayosaurus.