The Big Dinosaur Debate: Is It an Apatosaurus or Brontosaurus?

You've got to admit that thunder lizard sounds awfully cool. Fewer people get psyched about the name Apatosaurus, which means "deceptive lizard."

What was so deceptive about it? Well, the first Apatosaurus fossils Marsh received were a collection of pelvic and back bones. The latter reminded him of the vertebrae found on extinct marine reptiles called mosasaurs. Yet Marsh still recognized his fossils for the sauropod bones they truly were.

Apatosaurus lived in Colorado, Wyoming, Utah, South Dakota and Oklahoma around 155 to 150 million years ago. It was therefore a resident of the Jurassic.

A geologic time period made famous by that Stephen Spielberg movie you might've seen, the Jurassic lasted from 199.6 to 145.5 million years before the present. Tyrannosaurus rex hadn't evolved yet, but Apatosaurus had to look out for other predators like the 28-foot (8.5-meter) Allosaurus and the horned Ceratosaurus.

Killing a full-grown Apatosaurus would've been quite a challenge, though.

Most had hip heights of around 13.4 to 14.7 feet (4.1 to 4.5 meters). From snout to tail, a typical Apatosaurus probably measured somewhere between 72 and 77 feet (22 and 23.5 meters) long, making this animal an American colossus.

One unusually big — but very incomplete — specimen from Oklahoma suggests Apatosaurus could grow even longer, maybe pushing 98 feet (30 meters) in total length.

Just remember that this individual was a lot larger than average. According to the 2020 reference book "Dinosaur Facts and Figures: Sauropods and Other Sauropodomorphs" by Rubén Molina-Pérez and Asier Larramendi, more typically sized Apatosaurus likely weighed 15 to 22 tons (14 to 20 metric tons). Kind of makes you wonder how the Oklahoma giant compared.

Western North America was full of other sauropods who brushed shoulders with Apatosaurus. These included its close cousins Diplodocus and Barosaurus (one guards the rotunda at the American Museum of Natural History in New York City).

Like Apatosaurus, they were "diplodocids," meaning they had front legs that were shorter than the back pair and long, flexible necks.

Apatosaurus was more robust than other diplodocids. To scientists, its weirdest feature is the neck. Thickened by large, downward-facing cervical ribs, Apatosaurus had an extraordinarily wide neck for a sauropod.

Maybe it served a combat function. Some experts wonder if rival Apatosaurus used to bludgeon each other with their broad, durable necks. Sideways or downward blows probably would've packed the most punch. Laugh all you like, but both male elephant seals and giraffes use similar fighting techniques today.

Back when Apatosaurus drew breath, the greater Rocky Mountain region was dominated by lakes, swamps and floodplains. Paleontologist John A. Whitlock has suggested Apatosaurus mainly ate plant matter that grew at or near ground level. As evidence, he cites its wide snout, pencil-shaped teeth and the dental microwear patterns seen on some fossils.

Meanwhile, it seems two contemporary sauropods — the towering Brachiosaurus and blunt-faced Camarasaurus — preferred browsing on tree limbs. More distantly related to Apatosaurus, they both lived right alongside Marsh's "deceptive lizard" in the Late Jurassic.

Like we said earlier, Marsh named Brontosaurus excelsus on the basis of the skeleton he reviewed in 1879. This happened to be the most complete sauropod specimen ever found at the time. Too bad it was missing a head.

When Marsh published illustrations of his Brontosaurus, he used his imagination to fill in missing details. His drawings depict the animal with a blunt skull that (somewhat) resembles those of Camarasaurus and Brachiosaurus.

So for many years, the Brontosaurus skeletons mounted at museums were given boxy, artificial heads. We now know the real animal had a long, horse-like skull — much like the one Diplodocus possessed.

But that's not what put the validity of Brontosaurus in doubt.

In 1903, paleontologist Elmer Riggs took a fresh look at Marsh's Apatosaurus ajax and Brontosaurus excelsus. Now for those who might need a refresher on science jargon, Apatosaurus and Brontosaurus are two genus names. On the other hand, Apatosaurus ajax and Brontosaurus excelsus are species names.

Under the system we use to classify living things, every genus contains one or more species.

Now Marsh felt there were fundamental differences between the sauropod fragments he described in 1877 and the more complete skeleton he examined in 1879. He not only believed they represented two distinct species, but also two separate genera (i.e., "genuses").

Riggs didn't see it that way. He argued that Brontosaurus excelsus and Apatosaurus ajax actually belonged to the same genus. And since the genus name Apatosaurus was older (by two years), it had seniority.

Dinosaur specialists overwhelmingly agreed with Riggs. Brontosaurus was discarded as an invalid genus name by just about every working paleontologist (with some notable exceptions).

For the next 112 years, the animal Marsh had called Brontosaurus excelsus was referred to as Apatosaurus excelsus instead.

Right when it seemed like the debate was effectively settled, a 2015 paper published in the journal Paleontology and Evolutionary Science shook things up — again.

Written by Emmanuel Tschopp, Octavio Mateus and Roger B.J. Benson, this was a deep dive into the world of diplodocids. In their review of the group, the authors contradicted Riggs. According to their analysis, Brontosaurus excelsus really was distinct enough from Apatosaurus ajax — and other Apatosaurus species — to deserve its very own genus.

If they're right, then by the rules of scientific nomenclature, the genus name Brontosaurus should be reinstated.

What's more, the paper reassigned a grand total of three Apatosaurus species — A. excelsus, A. parvus and A. yahnapin — to the genus Brontosaurus.

Before any dino enthusiasts start dancing in the streets, you should know the 2015 study received some pushback. Tschopp, Mateus and Benson may consider Brontosaurus a legitimate genus, but a few of their colleagues disagree.

No matter how this debate unfolds, there's one thing we can never take away from Brontosaurus. It's the only dinosaur name mentioned in MGM's "The Wizard of Oz." That's got to count for something, right?