Description and Classification

The story of Ultrasaurus is primarily one of mistaken identity, particularly concerning the fossils found in the United States. In 1979, paleontologist James A. Jensen discovered very large dinosaur bones in the Morrison Formation of Colorado. He informally announced the name “Ultrasaurus” for these remains in 1985, suggesting it was an enormous sauropod, possibly a type of brachiosaurid, even larger than Brachiosaurus. These remains included a massive shoulder blade (scapulocoracoid) and some vertebrae.

However, further research revealed that the material Jensen called “Ultrasaurus” (later formally named Ultrasauros macintoshi) was actually a chimera – a collection of bones from at least two different types of dinosaurs. The very large shoulder blade was found to belong to an exceptionally large individual of Supersaurus vivianae, a diplodocid sauropod. Other bones mistakenly attributed to Ultrasauros were identified as belonging to Brachiosaurus. Therefore, Ultrasauros macintoshi is no longer considered a valid dinosaur genus. The name Ultrasaurus itself had also been previously used in 1983 by paleontologist Haang Mook Kim for fossils found in South Korea (Ultrasaurus tabriensis). The Korean material consists of a partial upper arm bone (humerus) and some other fragments, but its identity is also uncertain, and it is often considered a nomen dubium (a doubtful name) by paleontologists. Due to the prior use by Kim, Jensen’s name was invalid anyway.

So, while the popular image of Ultrasaurus was that of a gigantic brachiosaurid, the fossils that inspired this image mostly belong to Supersaurus, which is a very long diplodocid, not a super-tall brachiosaurid.

Distinguishing Features

Since Ultrasaurus (referring to Jensen’s North American finds) is not a valid genus, it has no distinguishing features of its own. The features once attributed to it were actually those of other dinosaurs:

• An exceptionally large shoulder blade: This prominent bone, one of the first to draw attention, is now recognized as belonging to Supersaurus. It indicated an animal of immense size.
• Tall vertebrae: Some vertebrae found at the site were similar to those of Brachiosaurus, suggesting a very tall animal with a giraffe-like posture.

The misunderstanding arose from combining these characteristics from different animals into a single, hypothetical super-giant.

Paleoenvironment and Diet

The fossils initially named Ultrasauros macintoshi were discovered in the Morrison Formation of Colorado, USA. This geological formation dates back to the Late Jurassic period, about 156 to 147 million years ago. During this time, the area was a vast, semi-arid plain with rivers, floodplains, and sparse forests of conifers, cycads, and ferns. This environment supported a rich diversity of dinosaur life.

The dinosaurs whose bones were mixed up (Supersaurus and Brachiosaurus) were herbivores. Like other sauropods, Supersaurus would have used its long neck to browse on vegetation, possibly at medium to high levels. Brachiosaurus, with its taller build, would have fed on leaves from the highest treetops. The Morrison Formation was home to many other famous dinosaurs, including the predatory Allosaurus, the plate-backed Stegosaurus, and other large sauropods like Apatosaurus and Diplodocus.

Significance and Ongoing Research

The story of Ultrasaurus is significant in paleontology as a prime example of how scientific understanding evolves with new evidence and re-evaluation of existing fossils. It highlights the challenges of working with incomplete skeletons and the possibility of creating “chimeras” by mistakenly combining bones from different individuals or species. For a time, the idea of “Ultrasaurus” captured public and scientific imagination as potentially the largest dinosaur ever, even though this was based on a misinterpretation.

Ongoing research is not focused on Ultrasaurus as a valid genus. Instead, research continues on the dinosaurs whose fossils were involved in the original confusion, particularly Supersaurus. Paleontologists are still working to understand the full size and anatomy of Supersaurus, which is indeed one of the longest known dinosaurs. Studies of the Korean Ultrasaurus tabriensis material are less common, and its true identity remains unclear. The tale of Ultrasaurus serves as a valuable lesson in the scientific process of discovery, hypothesis, and correction.