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Brontosaurus thunders back!

Pretty much every person who ever read a dinosaur book or went to a natural history museum learned that Brontosaurus is just an outdated name for a big long-necked dinosaur that should be called Apatosaurus. Two different names were applied to the same type of animal, but Apatosaurus wins out because it was named first.

But…it’s just not that simple. It’s never that simple! In a move that is sure to generate considerable discussion by scientists and non-scientists alike, Brontosaurus is back. This topic is being covered in immense detail by many other writers, so I’m going to give just a quick tutorial here. In the interests of full disclosure, I should also note that I was the volunteer editor who handled this paper for the open access journal PeerJ. The opinions expressed here are solely my own.

An old school Brontosaurus, as envisioned in the late 1800s. After Marsh 1896.
An old-school Brontosaurus, as envisioned in the late 1800s. After Marsh 1896.

So, how do we resurrect Brontosaurus? It’s not just a matter of arbitrary scientific opinion–the authors of the new study, Emanuel Tschopp, Octávio Mateus, and Roger Benson–didn’t wake up one morning and say, “Hmm…let’s mess with people.” The work entailed a ridiculously awesome amount of hands-on study of specimens around the world, as well as detailed analysis of the features of the fossils to determine their evolutionary relationships.

Evolutionary relationships are the key–erecting, demolishing, or maintaining species and genera requires an understanding of how these organisms are related. Basically, if species of Brontosaurus are all mixed in with species of Apatosaurus, then the two animals should be recognized under one name (Apatosaurus). If, on the other hand, what used to be called Brontosaurus is evolutionarily well separated from species of Apatosaurus, then Brontosaurus stands up.

In order to see how the species are related, Tschopp and colleagues built a massive database documenting the anatomical features for numerous relatives of Apatosaurus–at the individual specimen level. This was the largest-scale study of its type ever done; most other studies (with few exceptions) have lumped specimens together under species, or have lumped species together into genera. A fine-scale approach, looking at individual specimens, turns out to be key. Once all of the specimens were coded, they were subjected to a phylogenetic analysis to determine which possible evolutionary trees best fit the data. In other words, a computer algorithm took the data and found which possible arrangement of evolutionary relationships made the most sense in light of the anatomical evidence. Animals that share more features changed from the ancestral condition (“derived features” in paleontological parlance) are assumed to be more closely related.

In the diagram below, which is roughly similar to a previously published analysis by Paul Upchurch and colleagues, note that the original Brontosaurus species (Brontosaurus excelsus) is in the middle of a whole bunch of Apatosaurus. Species more closely connected are more closely related–and here, the original Brontosaurus species is more closely related to the original Apatosaurus species than anything else. Unless one wanted to name each of those other species its own genus (a practice  employed for many other dinosaur groups), it made the most sense to keep Brontosaurus the same as Apatosaurus.

Tentative evolutionary tree of Brontosaurus and Apatosaurus
A hypothetical  view of the evolutionary relationships of Apatosaurus and Brontosaurus, based in part on a previously published analysis by Upchurch and colleagues. The dates at right indicate when each species was named. If this arrangement is correct, then Brontosaurus should be called Apatosaurus.

In their own analysis, Tschopp and colleagues found that Brontosaurus excelsus is well separated from the two species most traditionally considered Apatosaurus. Not only does the shape of the evolutionary tree show this, but the sheer quantity of differences also supported Brontosaurus as something different from Apatosaurus. By the paper’s standard of a “genus”, Brontosaurus fits the bill.

A new evolutionary hypothesis showing Apatosaurus and Brontosaurus as different genera.
In this arrangement, based on that found by Tschopp and colleagues, Apatosaurus and Brontosaurus are two different genera. As an interesting side-note, Brontosaurus parvus originally got its own name, Elosaurus. But, because it is “nested” within Brontosaurus, it keeps that name. As another side-note, Brontosaurus yahnaphin has also been called Eobrontosaurus. So, one could make an argument that BrontosaurusElosaurus, and Eobrontosaurus are all valid! This is where things can get a little subjective, but Tschopp et al. posit that the differences are few enough so that they should also be Brontosaurus.

So, Brontosaurus lives again! In terms of pop cultural significance, this is a pretty big deal–but in terms of scientific nomenclature, these kinds of things happen all of the time. In fact, the real treasure of the paper by Tschopp and colleagues is its in-depth approach to unraveling the evolutionary relationships of ApatosaurusBrontosaurus, and their relatives. This level of detail is becoming increasingly common for dinosaur research, and as demonstrated here can be a pretty handy tool for unraveling long-standing problems in identifying and naming species.

Brontosaurus has an unparalleled hold on the public imagination–its name aptly means “thunder lizard,” after all. As such, it’s a prime example for showing how scientists classify and reclassify organisms in light of new information.  Papers such as this, beyond taking care of naming species and genera, provide an important backbone for other analyses. If we want to understand the migrations and interchanges of dinosaurs between continents, we have to know how the various species are related. If we want to correlate the origination or extinction of species to climate changes or other global events, we have to have a solid grasp on these same relationships. Scientific progress thunders onward!

Tschopp E, Mateus O, Benson RBJ. (2015) A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda) PeerJ 3:e857

[post edited to add clarification on shared derived features, add links to other blog posts, fixed a typo or two]

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