It's the most famous dinosaur that never was — or at least that's what paleontologists have believed for 112 years. After an exhaustive comparative analysis of over 500 physical characteristics, researchers have concluded that Brontosaurus is sufficiently distinct from Apatosaurus to warrant its own genus. Bronto, it appears, is back.
Sauropods, the suborder to which both Brontosaurus and Apatosaurus belong, are among the most recognizable dinosaurs. They had long necks and tails, a tiny head, and of course, those absolutely massive bodies; they're considered the largest creatures to have ever walked on land.
First (above, by Othniel C Marsh, 1883) and second (below, by Othniel C Marsh, 1891) skeletal reconstruction of Brontosaurus excelsus. Rearranged by Michael Taylor (2010).
But as virtually everyone has been taught, Brontosaurus — the aptly named "thunder lizard" — isn't one of them. The genera was retired in 1903, after scientists decided it too closely resembled that of another giant dinosaur, Apatosaurus. Because the differences between Brontosaurus excelsus and Apatosaurus were so small, the scientists at the time thought it better to put them both in the same genus. And because Apatosaurus was discovered first, its name was retained owing to scientific naming conventions. Brontosaurus excelsus became Apatosaurus excelsus.
Now, following a thorough and careful phylogenetic analysis, a paper published Tuesday in PeerJ makes a compelling case that Brontosaurus is deserving of its own genus.
To reach this conclusion, a group of European experts led by Emanuel Tschopp of Universidade Nova de Lisboa in Portugal analyzed the anatomy of all species belonging to the Diplodocidae group of dinos, including Brontosaurus, Apatosaurus, and Diplodocus. Using laser scanning technologies and computer rendering to visualize and contrast the bones, the researchers were able to analyze nearly 500 anatomical traits.
Such a detailed analysis has only recently been made possible. It also helped that a number of new, very well-preserved specimens recently became available for study at the Sauriermuseum Aathal in Switzerland, close to where Tschopp grew up. Owing to this collection of anatomical information, the researchers were able to reassess the diversity and taxonomy of the entire group in extraordinary detail.
"Based on this comparison, and with the help of two different statistical approaches calculating the number of differences between individual skeletons, or groups of skeletons, we were able to establish certain guidelines for distinguishing species and genera — which finally led to the resurrection of Brontosaurus," Tschopp told io9. "So, our results are supported by a detailed analysis of the actual bones, and by two different statistical approaches that led to the same conclusion."
All told, Tschopp and his colleagues compared 477 morphological characteristics across 81 different dinosaurs. Among the most notable differences between Apatosaurus and Brontosaurus was the former's neck, which was wider – and presumably stronger – than that of the latter.
Tschopp says his group, which included palaeontologists Octávio Mateus and Roger Benson, sought to be as objective and consistent as possible when applying their statistical methods. According to the researchers, the differences found between Brontosaurus and Apatosaurus specimens were at least as numerous as the ones between other closely related genera, and much more than what would be expected between species. And so, the researchers say, the Brontosaurus genus should be revived.
But what's in a name, really?
According to Mark Norell, Chair and Curator-in-Charge of the American Museum of Natural History's Paleontology Division, the debate over Brontosaurus' status is inconsequential, from a scientific standpoint.
"Most of the press is really concentrating on whether or not Brontosaurus is back," says Norell, who was not involved in the the study, "but that's not really a scientific question — it's more semantics. And as a scientific question, it's not even really that interesting. What is interesting, however, is the detailed and complex phylogenetic treatment of a group of dinosaurs that's hard to study."
Indeed, it's the first time this group of dinosaurs has been subjected to such a complete phylogenetic analysis. Norell is satisfied that the researchers have found enough evidence to warrant a separate genus. But what's more important, he says, is that we now have a new-and-improved family tree that will help paleontologists better understand the various biological characteristics of these animals, including where they lived, how fast they grew, and how their shapes changed over time.
"It's the first step — but the hardest step — to solving any biological problem and in understanding how animals are related to each other," says Norell, "If you want to understand the comparative features of a species, you have to have a family tree."
According to John Whitlock, an anatomist and physiologist at Mount Aloysius College who was unaffiliated with the study, the real heart of the debate is the vagueness involved when distinguishing genera from species.
"The thing is, genera and species aren’t real things in the most technical sense," Whitlock told io9. "They are what we might call subjective units, in that they are constructs we apply to the patterns we perceive in nature, so that we can talk about them. So right off the bat, no matter how much data we have, we’re talking about something that we as humans have to define rather than having them be discrete entities — for example: you, me, your pet cat."
Consequently, Whitlock says we have an abundance of criteria and guidelines that we use to define what a species is — but these methods don't always agree with one another.
"So, really, a species or a genus can mean anything we want it to, so long as we define it," he says. "And that, for me, is a very interesting thing."
He says the challenge is in determining whether a physical change in a creature represents a small genetic difference or a large one, and if such genetic changes warrant a distinction between genus and a different species.
"These are big questions, and ones that we’re still arguing about with modern animals, where we have molecules and behavior and ecological data on a scale we cannot even dream about in paleontology," he says.
Because we lack the answers, he says these are more terms of convenience when talking about animals.
"So in cases like this, where all the specimens still make up a monophyletic group [a group of organisms that includes its ancestral species and descendants], I wonder if it's more useful to distinguish specimens as Brontosaurus and Apatosaurus than it is to distinguish A. excelsus from A. louisae?," he says. "Unfortunately, I don’t have an answer to that — there will be people who say 'of course!' and some who will say 'no way!'"
Whitlock says it's an old and interesting question, one he expects the new paper will revive.
"I do hope it’s the start of a conversation about these questions, since we all bump up against them from time to time," he told me.
I asked Tschopp what kind of reaction he expects to get from the scientific community.
"I think it will mostly be positive, because we provide a huge amount of data, and are very clear in what we did and how we did it," he responded. "This is always appreciated, because like this everybody can test our results. The name changes we propose might spark some discussion, and I'm happy it does, because this is how science works, and how we get to better understand the world we live in."
Looking ahead, Tschopp expects some paleontologists to start using Brontosaurus in their publications, while others will wait to see if other research groups put forward their own evidence against a separation.
Read the entire study at PeerJ: “A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda)”.