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Brontosaurus Makes a Comeback in Revised Dinosaur Family Tree

Image: Diplodocus and Brontosaurus

An artist's conception from 1897 shows Brontosaurus as semi-aquatic animal, with Diplodocus in the background on land. Charles R. Knight

More than a century after Brontosaurus was crossed off the official list of dinosaurs, an international team of paleontologists has restored the iconic name to the evolutionary family tree. It's not yet clear whether the revived label will stick, however.

"It's the classic example of how science works," Octavio Mateus, a paleontologist at Portugal's Universidade Nova de Lisboa, said in a news release. "Especially when hypotheses are based on fragmentary fossils, it is possible for new finds to overthrow years of research."

Mateus and two other researchers, study leader Emanuel Tschopp and Roger Benson, lay out the case for Brontosaurus in a paper published Tuesday by the open-access journal PeerJ.

The Brontosaurus is Back 1:59

The case goes back to the beginnings of modern paleontology in the 1870s. Among the early finds in the Western United States were two partial skeletons of enormous, long-necked dinosaurs. Influential Yale paleontologist Othniel Charles Marsh classified one of them as Apatosaurus ajax ("deceptive lizard") and the other as Brontosaurus excelsus ("thunder lizard").

In the years that followed, additional fossil discoveries led paleontologists to conclude in 1903 that Brontosaurus and Apatosaurus actually belonged in the same genus. Because Apatosaurus was named first, that name took precedence under the rules of the International Commission on Zoological Nomenclature.

The Brontosaurus label stuck in the public consciousness, however, thanks to cultural references ranging from the Sinclair Oil mascot to "Flintstones" cartoons. It wasn't until the 1970s that paleontologists resolved, seemingly once and for all, that Brontosaurus and Apatosaurus were the same animal.

The newly published study revisits the entire classification question. Tschopp and his colleagues were seeking to develop a more precise method for classifying Apatosaurus fossils as well as fossils from another closely related genus called Diplodocus. They looked at hundreds of characteristics for 81 fossil specimens drawn from museums around the world, including the specimens formerly known as Brontosaurus.

Image: Measuring femur
Paleontologists Emanuel Tschopp and Octávio Mateus measure a sauropod dinosaur femur. M. Ladeira / FCT-University Nova of Lisbon

When all the characteristics were taken into account, the researchers concluded that Brontosaurus was different enough from Apatosaurus to merit its own genus. For example, the vertebrae in Apatosaurus' neck were broader than those of Brontosaurus.

"Our research would not have been possible at this level of detail 15 or more years ago," Tschopp, who worked on the study while at the Universidade Nova de Lisboa, explained in the news release. "In fact, until very recently, the claim that Brontosaurus was the same as Apatosaurus was completely reasonable, based on the knowledge we had."

Benson, a paleontologist at the University of Oxford, said the differences between Brontosaurus and Apatosaurus "were at least as numerous as the ones between two other closely related genera."

Image: Brontosaurus history
An infographic traces the ups and downs in the debate over Brontosaurus. PeerJ

Download the Full-Size 'History of Brontosaurus' Infographic

For now, this is just one study. Eventually, the International Commission on Zoological Nomenclature will have to determine whether Brontosaurus goes back on the official list of dinosaur genera. "There does not exist a 'genus-o-meter,'" Thomas Holtz, a paleontologist at the University of Maryland, told NBC News.

Holtz, who did not play a role in the research published by PeerJ, said the paper lays out the best case to date for Brontosaurus.

"It is definitely not a proven case," Holtz said. "But as long as you recognize these as separate species, at least now there is a valid justification to return the species 'excelsus' to the old genus Brontosaurus."

The paper by Tschopp, Mateus and Benson, "A Specimen-Level Phylogenetic Analysis and Taxonomic Revision of Diplodocidae (Dinosauria, Sauropoda)," is freely available on the PeerJ website.