The Brontosaurus: Trials, Truimphs, and Taxonomy

"Apatosaurus Yale Peabody" by Ad Meskens - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

"Apatosaurus Yale Peabody" by Ad Meskens - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

Big news for dinosaur fans everywhere. The Brontosaurus is back! For over 100 years we were told that, despite its popularity, no such animal ever existed. The common story was that Othniel Charles Marsh first discovered the Apatosaurus in 1877, and then a similar fossil in 1879. He named the second one Brontosaurus and the specimen was soon on display in a museum. In 1903 another paleontologist looked at the second specimen and concluded that it was in fact an Apatosaurus. Some accounts even go on to criticize Marsh for attempting to name two species when he really only had one (there was a lot of competition to name new dinosaurs at the time). Fast forward to 2015, a new paper has caused quite a sensation; the authors have brought back the Brontosaurus.

That leaves one important question:

Where did the Brontosaurus go?

The truth is the Brontosaurus never went anywhere. The confusion stems from two areas: the process of how scientists name a species and how we talk about them.

How to name your dragon dinosaur

When a new species is described, it’s assigned to a series of hierarchical categories. This process of assignment is known as taxonomy, where each category is defined by several criteria.

For example, any organism that is multi-cellular, motile (can move itself around) for at least part of its life, and is heterotrophic (it has to eat things to get energy) is classified as an animal. Meanwhile, any animal that has a dorsal hollow nerve chord, pharyngial gills, a notochord, and a post-anal tail for at least part of its life is a chordate. Applying this process to extinct organisms (like dinosaurs) is very difficult because we can’t directly examine many of the traits normally used to classify extant (not extinct) organisms.

Would you be able to identify the male (circled in yellow) and female as the same species using only the skeletons? Image source: mynatureplace.org

Would you be able to identify the male (circled in yellow) and female as the same species using only the skeletons? Image source: mynatureplace.org

That’s one of the major challenges of paleontology. Taxonomists that work with extant animals use morphology, anatomy, physiology, DNA, RNA, behavior, embryology, and reproductive ability as evidence for a new species classification. For the most part, paleontologists only have skeletons to study. They can look at the size and shape (morphology), where they were found, and (approximately) when they were formed. Let's think about comparing skeletons for a moment. If you were to compare the skeletons of a German shepherd and a Chihuahua, you would probably classify them as two different species, but they are the same species. That’s a bit of a contrived example, since humans have bred them both for their specific traits, but what about an anglerfish? Compare the large female to the small male. How would a paleontologist, looking only at the skeletons, know that they are the same species? As if that weren’t enough of a challenge, sometimes they only have a few bones or fragments to work with (in fact, Marsh’s original Brontosaurus skeleton didn’t even have a skull). Imagine taking a jigsaw puzzle, and removing half of the pieces. Now throw away the box. Where do the remaining pieces fit? Now imagine that some of those pieces are six feet long and require a forklift to move, and you’re starting to get an appreciation for what paleontologists do.

The seven classic levels of taxonomy for Apatosaurus ajax. Image Credit: Seth Goodnight

The seven classic levels of taxonomy for Apatosaurus ajax. Image Credit: Seth Goodnight

So when Marsh brought back his first fossil, he examined its features and said it was close enough to other known organisms to be classified an animal, a chordate, and a dinosaur (the kingdom, phylum, and class). However, the new fossil didn’t fit into any known order, so Marsh assigned a new one: the Sauropoda (lizard footed). It also received a new family (Diplodocidae), genus, and species. Thus Apatosaurus ajax had been officially described.

Two years later, Marsh brought back a new fossil. It was pretty close to Apatosaurus ajax and was easily assigned to the family diplodocidae, but he believed that there were enough differences to warrant classifying it as a new genus, thus Brontosaurus excelsius, or the thunder lizard on high, was born (dinosaurs have the most impressive names). Several years later, another researcher examined those fossils again. He concluded that they were not quite so different and should be considered the same genus. Because the Apatosaurus genus came first, Brontosaurus excelsius became Apatosaurus excelsius., but the two fossils were still believed to represent two distinct species. Additionally the Brontosaurus genus went away because there were no longer any members. The recent study, published on April 7th, 2015 in PeerJ by Emanuel TschoppOctávio Mateus and Roger B.J. Benson, concluded that the original evaluation was correct, and that they should belong to different genera.

There is also a lot of confusion regarding how scientists talk about organisms. We commonly refer to organisms only by their genus name as a matter of convenience (Brontosaurus instead of Brontosaurus excelsius). It’s a bit like referring to a person by their family name; it works fine when everyone knows which Roosevelt you are talking about, but it can’t distinguish between Teddy, Franklin, and Eleanor without further context. The habit is especially common with dinosaurs where many genera have only one species. So when Brontosaurus was reclassified as Apatosaurus it became common for people to refer to them as the same animal.

Alitta succinea, a worm with an identity crisis. Image Credit: Hans Hillewaert via Wikimedia Commons

Alitta succinea, a worm with an identity crisis. Image Credit: Hans Hillewaert via Wikimedia Commons

While this story has gotten a lot of attention recently (I was certainly excited to read about it), it’s not all that uncommon for genera to be reassigned. Species can get moved around quite a bit as new techniques are developed to classify them. Take poor Neanthes succinea for example. When it was originally described in 1847, it was assigned to the Neanthes genus. It was later reclassified as a Neries species around 1909. Neries became Alitta in 2005, then back to Neanthes, then Alitta again, and so on. At this point it’s pretty common to see both genus names show up in scientific journals, usually as Neanthes (Alitta) succinea or Alitta (Neanthes) succinea. Ganoderma applanatum is a fungus with an even more complicated story (Andrew Tomes tells the story here).

There you have it. The name may have changed, but the actual animal never went anywhere. Everyone agreed that there were still two distinct species, it was just a question of how distinct.


Here are some other taxonomic tales you might enjoy.