A new and much more complete specimen of Supersaurus, nicknamed 'Jimbo', has been found in Converse County, Wyoming. It is currently being excavated and its bones are being held at the Wyoming Dinosaur Center. Originally, it was thought that Supersaurus was related to the long-necked diplodocid Barosaurus (a member of the subfamily Diplodocinae), but the new specimen makes it clear that Supersaurus is actually more closely related to Apatosaurus (a member of the subfamily Apatosaurinae).
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 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 tonnes (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.
Before Jim Jensen published his discovery in 1985, another paleontologist, Haang Mook Kim, used the name Ultrasaurus in a 1983 publication to describe what he believed was a giant dinosaur in South Korea. This was a different, much smaller dinosaur than Jensen's find, but Kim thought it represented a similarly gigantic animal because he confused a femur (leg bone) for a humerus (arm bone). While the logic of naming was incorrect, the Ultrasaurus from Kim's find fulfilled the requirements for naming and became regarded as a legitimate, if dubious genus. Thus, because Jensen did not publish his own "Ultrasaurus" find until 1985, Kim's use retained its official priority of name, and Jensen was forced to choose a new name (in technical terms, his original choice was "preoccupied" by Kim's sauropod). In 1991, at his suggestion, George Olshevsky changed one letter, and renamed Jensen's sauropod Ultrasauros.
When it was later discovered that the new name referred to bones from two separate, and already known species, the name Ultrasauros became a junior synonym for Supersaurus. Since the bones from the Brachiosaurus were only used as a secondary reference for the new species, Ultrasauros is not a junior synonym for Brachiosaurus. Since Supersaurus was named slightly earlier, the name Ultrasauros has been discarded in favor of Supersaurus.