In 1991, paleontologist David Gillette announced that he had found the largest of the enormous sauropod dinosaurs. He called it Seismosaurus halli, and based on the parts of the skeleton that had been prepared at the time, Gillette believed Seismosaurus to be between 127 and 170 feet long! Even giants such as Diplodocus would have looked puny next to it, but if Seismosaurus was such a gargantuan dinosaur, why doesn't anyone talk about it anymore?
Figuring out which dinosaur was the biggest of them all has been a question fraught with controversy, especially since the 1980s. Over the past three decades numerous sauropod dinosaurs have been proposed to push the boundaries of body size based upon fragmentary remains, but these dinosaurs have not always turned out to be what they seemed. Seismosaurus is one such animal, known today by a different name, but to understand why we need to go back to the beginning.
Although it was not formally described until 1991, the bones of what Gillette would call Seismosaurus were initially discovered in northwestern New Mexico by hikers Arthur Loy, Jan Cummings, Frank Walker and Bill Norlander in 1979. They reported their discovery to the Bureau of Land Management, but the government body lacked the proper tools to remove the bones. Then, in 1985, Gillette stepped in to collect the bones for the New Mexico Museum of Natural History. It was not easy work. The sheer size of large sauropod dinosaurs make them extremely difficult to collect, with the added challenge that it takes many years to fully prepare the enormous bones.
Based on the quarry map Gillette published, his field crew had collected portions of the rear half of the dinosaur, the ribs and vertebra from parts of the torso, the hips, and parts of the tail. By 1991, only a few tail vertebrae and portions of the hips had been fully prepared, but based on his observations of these parts Gillette believed that he had found a new type of dinosaur. Since all of these parts appeared to be longer than their corresponding bones in the skeleton of Diplodocus—a dinosaur Seismosaurus was closely related to—it seemed clear that the new dinosaur was 150 to 200 percent larger than its better-known cousin.
As paleontologists continued to prepare the skeleton of Seismosaurus, however, they noticed that it was beginning to shrink. While Gillette had thought that the large tail bones were from a more distal part of the tail, it turns out that they came from closer to the hips. This placed the dinosaur much closer to the lower end of Gillette's size estimate; a more modest, but still huge, 110 feet long. Nor was Seismosaurus as distinct as Gillette had believed. As the skeleton was prepped, each of the characteristics used to designate the skeleton as a new genus were tossed out. As announced at the annual GSA conference in 2004 (and detailed in print since that time), Seismosaurus was really an especially large Diplodocus, although paleontologists have (so far) retained its amended species name to designate the dinosaur as Diplodocus hallorum.
As with the recent public controversy over the proposed reclassification of the dinosaur Torosaurus as mature Triceratops, the changing status of "Seismosaurus" reminds us that dinosaur names are useful labels that are subject to change. Misidentifications are sometimes made—"pygmy" species have turned out to be juveniles of known species and partial skeletons of giants have been discovered to be difficult-to-interpret parts of more modestly-sized animals—but science self-corrects as it goes along. In fact, I am glad that mistakes and misidentifications are regularly ferreted out and corrected. Such revisions are a sign that paleontologists are constantly reexamining the evidence and finding new ways to investigate the evolution and paleobiology of dinosaurs.
David D. Gillette (1991). Seismosaurus halli, gen. et sp. nov., A New Sauropod Dinosaur from the Morrison Formation (Upper Jurassic/Lower Cretaceuos) of New Mexico, USA Journal of Verterbrate Paleontology, 11 (4), 417-433