Some of the largest animals to ever walk on Earth were the long-necked, long-tailed dinosaurs known as the sauropods—and the most famous of these giants is probably Brontosaurus, the "thunder lizard." Deeply rooted as this titan is in the popular imagination, however, for more than a century scientists thought it never existed.



The first of the Brontosaurus genus was named in 1879 by famed paleontologist Othniel Charles Marsh. The specimen still stands on display in the Great Hall of Yale's Peabody Museum of Natural History. In 1903, however, paleontologist Elmer Riggs found that Brontosaurus was apparently the same as the genus Apatosaurus, which Marsh had first described in 1877. In such cases the rules of scientific nomenclature state that the oldest name has priority, dooming Brontosaurus to another extinction.



Now a new study suggests resurrecting Brontosaurus. It turns out the original Apatosaurus and Brontosaurus fossils appear different enough to belong to separate groups after all. "Generally, Brontosaurus can be distinguished from Apatosaurus most easily by its neck, which is higher and less wide," says lead study author Emanuel Tschopp, a vertebrate paleontologist at the New University of Lisbon in Portugal. "So although both are very massive and robust animals, Apatosaurus is even more extreme than Brontosaurus."



The nearly 300-page study analyzed 477 different physical features of 81 sauropod specimens, involving five years of research and numerous visits to museum collections in Europe and the U.S. The initial goal of the research was to clarify the relationships among the species making up the family of sauropods known as the diplodocids, which includes Diplodocus, Apatosaurus and now Brontosaurus.



The scientists conclude that three known species of Brontosaurus exist: Brontosaurus excelsus, the first discovered, as well as B. parvus and B. yahnahpin. Tschopp and his colleagues Octávio Mateus and Roger Benson detailed their findings online April 7 in PeerJ. "We're delighted that Brontosaurus is back," says Jacques Gauthier, curator of vertebrate paleontology and vertebrate zoology at Peabody, who did not participate in this study. "I grew up knowing about Brontosaurus—what a great name, 'thunder lizard'—and never did like that it sank into Apatosaurus."



For vertebrate paleontologist Mike Taylor at the University of Bristol in England, who did not take part in this research, the most exciting thing about this study is "the magnificent comprehensiveness of the work this group has done, the beautifully detailed and informative illustrations and the degree of care taken to make all their work reproducible and verifiable. It really sets a new standard. I am in awe of the authors," he says. Vertebrate paleontologist Mathew Wedel at Western University of Health Sciences in Pomona, Calif., who also did not collaborate on this paper, agrees, saying "the incredible amount of work here is what other research is going to be building on for decades."



Tschopp notes their research would have been impossible at this level of detail 15 or more years ago. It was only with many recent findings of dinosaurs similar to Apatosaurus and Brontosaurus that it became possible to reexamine how different they actually were and breathe new life into Brontosaurus, he says.



Although while Kenneth Carpenter, director and curator of paleontology at Utah State University Eastern's Prehistoric Museum, finds this study impressive, he notes the fossil on which Apatosaurus is based has never been described in detail, and suggests the researchers should have done so if they wanted to compare it with Brontosaurus. "So is Brontosaurus valid after all?" he asks. "Maybe. But I think the verdict is still out."



All in all, these findings emphasize "that sauropods were much more diverse and fascinating than we've realized," Taylor says. Indeed, the recognition of Brontosaurus as separate from Apatosaurus is "only the tip of the iceberg," he adds. "The big mounted apatosaur at the American Museum of Natural History is probably something different again, yet to be named. Yet another nice complete apatosaur, which is in a museum in Tokyo, is probably yet another new and distinct dinosaur."



This sauropod diversity emphasizes "that the Late Jurassic [period] of North America in which they lived may have been a weird time," Wedel says. "You basically had an explosion of these things in what could be harsh environments, which raises the question of how they could have found enough food to have supported them all." In other words, research that helped resurrect Brontosaurus may have birthed new mysteries as well.