Supersaurus

A reconstructed skeleton, Museum of Ancient Life, Utah, USA

Supersaurus is present in stratigraphic zone 5 of the Morrison, dating from the Tithonian.[3] The original fossil remains of Supersaurus were discovered in the Dry Mesa Quarry in 1972. This find yielded only a few bones: mainly the shoulder girdle, an ischium, and tail vertebrae. Paleontologist James A. Jensen described Supersaurus; he designated a scapulocoracoid BYU 9025 (originally labeled as BYU 5500) as the type specimen. This shoulder girdle stood some 2.4 meters (8 ft) tall, if placed on end. The specimen was given the name "Supersaurus" informally as early as 1973,[4][5] but was not officially described and named until more than a decade later, in 1985.[6] Sauropod researcher Jack McIntosh at one time thought that the BYU Supersaurus material might represent a large species of Barosaurus but later felt that there was evidence for Supersaurus being a valid genus.[7]

A reconstruction of WDC DMJ-021, nicknamed "Jimbo", Wyoming Dinosaur Center

A much more complete specimen WDC DMJ-021, was found in Converse County, Wyoming in 1986 by Brandon Flyr and Bart Lesco while out hiking and was reported to the people who owned the land at the time. The discovery was later named "Jimbo" in 1996 by the family that purchased the land, it was described and assigned to Supersaurus in 2007. The specimen represented approximately 30% of the skeleton. Its bones are being held at the Wyoming Dinosaur Center. A comparison of WDC DMJ-021 and other specimens previously assigned to Supersaurus was done in order to help decide what material from the Dry Mesa Quarry belonged to the genus. It indicated that a series of tail vertebrae and an ulna may have belonged to some other diplodocid.[8]

Ultrasauros

James A. Jensen with the reconstructed front leg of Ultrasauros

Jensen, who described the original Supersaurus specimen, simultaneously reported the discovery of another gigantic sauropod, which would later be named "Ultrasaurus" macintoshi[6] (later renamed Ultrasauros macintoshi). The type specimen (the specimen used to define a new species) of Ultrasauros, being a backbone (dorsal vertebra, labeled 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 synonym of Supersaurus, which had been named first and thus retains priority, and the name Ultrasauros was abandoned.[9]

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.[9] The Brachiosaurus bones indicate a large, but not record-breaking individual, a little larger than the "Brachiosaurus" brancai (Giraffatitan brancai) mount in the Berlin's Natural History Museum.[10]

Dorsal vertebra BYU 9044, the holotype of Ultrasauros, now assigned to Supersaurus Museum of Ancient Life

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,[11] which placed it in the same category as the blue whale and the equally problematic Bruhathkayosaurus.

The naming of the chimeric Ultrasauros has a similarly complicated history. Ultrasaurus (with the final "u") was the original choice, and was widely used by the media after the discovery in 1979. However, the name of a new species must be published with a description to become official.[12]

Pelvis BYU 130185, currently assigned to Supersaurus, Museum of Ancient Life - Thanksgiving Point

Before Jim Jensen published his discovery in 1985, another paleontologist, Kim Haang Mook, 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 humerus for an ulna. 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.[10][12] 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, with the final "o".[12]

When it was later discovered that the new name referred to bones from two separate, and already known species, the name Ultrasauros was considered invalid and 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. The name Supersaurus was kept instead of Ultrasaurus as the animal is a diplodocid and Ultrasaurus had always referred to a brachiosaurid.[9]

Diagram showing the size of Supersaurus (orange) compared with selected giant sauropods

Life restoration of Supersaurus based primarily on Wyoming Dinosaur Center's more complete "Jimbo"

Supersaurus is among the largest dinosaurs known from good remains, with the WDC and BYU specimens reaching 33–35 meters (108–115 ft) in length and approximately 35–40 metric tons (39–44 short tons) in body mass.[8][14] A larger specimen indicating a body length over 39 metres (128 ft) has been described in a 2021 abstract.[15]

The first described specimens of Supersaurus were individual bones that suggested a large diplodocid. A large cervical vertebra BYU 9024 from the same quarry was later assigned to Supersaurus.[16]. This vertebra measures 1.38 m (4.5 ft) in length and is the longest cervical known.[17] This enormous vertebra was reclassified as a Barosaurus vertebra, by Mike Taylor and Matt Wedel.[18] However Brian Curtice has reassigned it to Supersaurus on the basis of additional specimens.[15]

The assignment of the more complete specimen, WDC DMJ-021, to Supersaurus suggests that in most respects it was very similar in anatomy to Apatosaurus but less robustly built with especially elongated cervical vertebrae, resulting in one of the longest known sauropod necks.[8]

Most studies of diplodocid relationships have found it to contain two primary subgroups: Diplodocinae (containing those diplodocids more closely related to Diplodocus than to Apatosaurus) and Apatosaurinae (diplodocids more closely related to Apatosaurus than to Diplodocus). Originally, it was thought that Supersaurus was related to the long-necked diplodocid Barosaurus, and therefore a member of the subfamily Diplodocinae, however, with the assignment of the more complete WDC DMJ-021 most later studies found Supersaurus to be a close relative of the familiar Apatosaurus in the group Apatosaurinae.[8] However, some later studies cast doubt on this paradigm. One comprehensive study of diplodocoid relationships published by Whitlock in 2011 found Apatosaurus itself to lie at the base of the diplodocid family tree, and other "apatosaurines", including Supersaurus, to be progressively more closely related to Diplodocus (making them diplodocines).[19]

Restoration

In 2015, a specimen level phylogenetic study of diplodocids found that Dinheirosaurus lourinhanensis grouped with Supersaurus. The study considered that it should be a new species of Supersaurus, in a new combination S. lourinhanensis.[2]

• "Why do mass estimates vary so much?", by Mike Taylor, August 27, 2002. (see footnote)