Apatosaurus louisae

Artwork depicting an apatosaurus dinosaur

NPS / Bob Walters and Tess Kissinger


Apatosaurus louisae is a species of sauropod dinosaur from the Late Jurassic of North America found in the Morrison Formation. Its remains are present on the Wall of Bones at Dinosaur National Monument. Apatosaurus grew up to 69 ft (21 m) long and ate plants. This sauropod (long necked dinosaur) was discovered and named Apatosaurus, or "false lizard," because of its unbelievably large size. Discovered by paleontologist, Earl Douglass, in 1909, Apatosaurus was the first dinosaur found in what would soon become the Carnegie Quarry of Dinosaur National Monument.

A black and white photograph of the eight tail bones that prompted Earl Douglass to begin digging. The bones are naturally eroding out of the rock as Douglass would have seen them in 1909.
The original 8 tail bones of an Apatosaurus that Earl Douglass saw naturally eroding out of the rock face in 1909.

NPS Douglass Collection, edited by Evan Hall

Finding Apatosaurus

In 1909, paleontologist Earl Douglass worked for the Carnegie Museum in Pittsburgh, Pennsylvania. He was tasked by the museum's director to find and excavate dinosaur fossils that the museum could put on display. Douglass chose to come to northeastern Utah, an area he had been to before and whose geology he was already familiar with. He knew that the Morrison Formation, a sequence of sedimentary rock from the Late Jurassic Period known to harbor dinosaur fossils, was exposed here. Upon arrival, Douglass sought the help of a local man named George Goodrich to help with the search. The men looked for several weeks. While they found plenty of dinosaur bones, most were just fragmentary pieces unworthy of museum display. In his journal, Douglass lamented that the only promising specimen they found had already been located before. He could tell because someone had apparently attempted to excavate, but was inexperienced and ruined the specimen. Though he was on the verge of giving up, on August 17, 1909, Douglass climbed atop a ridge of Morrison rock in Jensen, Utah. There, he made a spectacular discovery...

"I saw eight tailbones of a Brontosaurus in exact position. It was a beautiful sight." — Earl Douglass

Further investigation revealed that the vertebrae actually belonged to Apatosaurus louisae, a huge sauropod (long-necked, long-tailed) dinosaur. This was the first dinosaur discovered by a scientist at what would soon become Dinosaur National Monument in 1915. It was thanks to this find that the Carnegie Quarry was eventually uncovered.

The Trouble with Skulls

The First Apatosaurus Skull Discovered
The first bones of Apatosaurus louisae found by Earl Douglass in 1909 were vertebrae (back bones) from the animal's tail. It was the discovery of those tail bones that led to the excavation of the Carnegie Quarry and the eventual extraction of more than 350 tons of fossils. However, the most surprising thing about this nearly-complete specimen of Apatosaurus louisae was that it also had an intact skull — the first Apatosaurus skull ever found.
Why Skulls Are Rare
Of all the fossils uncovered, skulls are easily one of the rarest body parts to find. Unlike leg and arm bones, skulls are hollow and have many holes in them. In life, these holes are filled with soft tissues, like brains, eyeballs, tongues, and sinuses. Dinosaur skulls had additional additonal holes in the skulls (called fenestrae) that likely housed air sacs, which were part of the respiratory system. Air sac systems are common in birds, the only descendendents of dinosaurs alive today. They probably helped dinosaurs breathe more efficiently and reduced the weight of otherwise heavy structures like the head and neck. Because the bone structure of dinosaur skulls is so fragile, skulls are easily crushed during the fossilization process. Even in rare cases where the skulls survive, they're usually flattened like soda cans so that most of the 3-dimensional structure is lost.
Skulls at the Carnegie Quarry
Fortunately, the sand sized grains of the Carnegie Quarry were extremely fine. Their small size allowed them to permeate empty spaces in the delicate bone structure of some skulls. This enabled the skulls to fossilize without getting crushed. Excavations of the Carnegie Quarry between 1909 and 1924 revealed an astounding 10 skulls from different dinosaurs! Although excavations of the Carnegie Quarry did uncover an Apatosaurus skull, it was unattached to the rest of the body. Because the body was found first, it was extracted and mounted in the Carnegie Museum without a head in 1915. The skeleton stayed headless for the next 17 years. Most paleontologists reasoned that Apatosaurus would probably have had a head similar to that of another dinosaur, Camarasaurus. After much debate, a Camarasaurus skull was placed on the Apatosaurus body at the museum in 1932. It stayed there for 47 years until scientists at the Carnegie Museum reasoned that the narrow skull found near the body back in 1910 was the right one. A skull cast of the accurate Apatosaurus skull was finally added to the Apatosaurus body in 1979. While casts of the Apatosaurus skull are occasionally on display in museums, the real skull cannot be displayed because of its delicate structure.

Apatosaurus or Brontosaurus?

While we can't say who would win in a fight, we can tell you that the heaviest sauropod of the Carnegie Quarry is definitely Apatosaurus louisae. Many people often confuse Apatosaurus for another famous sauropod dinosaur, Brontosaurus. However, as of 2024, no fossils attributed to the genus Brontosaurus have ever been found at Dinosaur National Monument. So where does the confusion come from?
Late 1800s: Discovery and Naming
Although Apatosaurus louisae was the first dinosaur found in the Carnegie Quarry in 1909, it was not the first Apatosaurus ever discovered. The first Apatosaurus fossils were uncovered by Arthur Lakes and Henry C. Beckwith in the Morrison Rock Formation in 1877. The best-preserved of those specimens was scientifically described in 1877 by Yale University's professor of Paleontology, Othniel Charles Marsh. Marsh named that specimen Apatosaurus ajax. Eventually, Marsh described another specimen of sauropod dinosaur from the same rock formation. He named this one Brontosaurus excelsius. Although the fossils of the new dinosaur were uncovered in 1874, even before the first bones of Apatosaurus, Marsh didn't produce a scientific description for Brontosaurus until 1879.
Early 1900s: Restructuring the Family
Apatosaurus and Brontosaurus were considered distinct genera until 1903, when a paleontologist named, Elmer Riggs, published his perspective. After carefully reviewing the type specimens described by Marsh, he argued that Apatosaurus and Brontosurus weren't different enough to be classified under seperate genera (the plural of genus). In the paper, he argued that the two specimens ought to be reclassified under one genus. Evidently, his argument was strong enough to convince most paleontologists at the time, because the sauropod family was restructured. According to scientific naming conventions, whenever two organisms are reclassified under one genus, the genus that was named and described first has priority. In this case, that was Apatosaurus, so the genus Brontosaurus was discontinued. After 1903, any specimens that had previously belonged to Brontosaurus were reclassified under the genus, Apatosaurus. However, many museums chose to keep the name Brontosaurus on exhibits. This may explain why the name Brontosaurus is more well-known.
Today: New Research & Controversy
In 2015, a group of scientists published an extensive study looking at the Diplodocid family tree, the group that Apatosaurus (and Brontosaurus) belong to. In their paper, the scientists conclude that Apatosaurus is much larger and more robust than Brontosaurus. They claim that the degree of difference is great enough that the two dinosaurs could be reclassified as separate genera. While many paleontologists agree with their claims, there are also many who disagree. In order for this controversy to be resolved, more specimens will likely need to be found and examined for comparison.

Last updated: March 30, 2024

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