Apatosaurus

About Apatosaurus

History of Discovery and the Mystery of the Skull

Apatosaurus is a massive herbivorous dinosaur of the Diplodocidae family that lived in North America during the Late Jurassic period of the Mesozoic era. The first fossils were discovered in Colorado in 1877 by Arthur Lakes and others, and were described as a new genus and species by paleontologist Othniel Charles Marsh. At that time, a fierce fossil hunting competition known as the "Bone Wars" was unfolding in the United States.

The name "Apatosaurus" means "deceptive lizard" in Greek. This is because the shape of the chevron bones under its tail resembled those of the marine reptile Mosasaurus, which confused (deceived) researchers. In 1909, extremely well-preserved fossils of Apatosaurus louisae were discovered in Utah, greatly advancing dinosaur research.

One of the most famous episodes in Apatosaurus research is the misidentification of its skull. Because the skull of Apatosaurus was thin and fragile, it was rarely preserved as a fossil, leading to a long-held misconception that it had a round, short skull like that of Camarasaurus. However, it wasn't until 1979 that an elongated skull, similar to that of Diplodocus, was finally proven to be the authentic one, putting an end to a debate that lasted over half a century.

Skeletal Anatomy and Morphological Features

The most prominent feature of Apatosaurus is its overall thicker, heavier, and extremely robust skeleton compared to its close relative Diplodocus. It had a very small head relative to its massive body, and its snout was square-shaped. Its jaws were lined with peg-like teeth for stripping plants, but since there are no scratches on the teeth surface indicating they crushed hard plants, it is thought they swallowed the stripped leaves whole without chewing.

The neck bones (cervical vertebrae) were extremely thick and sturdy, and the inside of the bones was hollowed out (pleurocoels) like in birds. This was an adaptation to lighten its weight while supporting its massive body. In addition, its limbs were thick and straight like pillars to support its immense weight, and the thumb of its front foot had a giant claw. This claw may have been used for defense against carnivorous dinosaurs or for gripping trees when rearing up. Based on fossilized footprints, it is estimated that they traveled 25 to 40 km a day and reached a maximum speed of 20 to 30 km/h.

Function and Role of the Tail and Neck Combat

Apatosaurus's tail made up more than half of its body length, and the tip was shaped like a thin whip. It was once thought that by swinging this tail, the tip would exceed the speed of sound, creating a loud boom like a cannon called a "sonic boom" used for communication with peers or intimidation.

However, a biomechanical study using computers in 2022 proved that the maximum speed of swinging the tail was only about 30 meters per second (about 110 km/h), not reaching the speed of sound. Mechanically, it was found that if it tried to accelerate to the speed of sound, the tail itself would tear and break apart due to the strong force. Currently, the prevailing theory is that this long tail was used as a "tactile organ" to keep in touch with peers and avoid getting lost in a herd with poor visibility.

Additionally, the neck of Apatosaurus was unusually thick even compared to other sauropods and was sturdily built to withstand strong impacts from below. Recent studies suggest that this thick neck was not simply for eating plants from high places but may have been used as a weapon in "neck combat," where males would violently crash into each other seeking mates. Like modern elephant seals, they might have fought by directly slamming their thick necks and chests against their opponents.

Ontogeny and Growth Model of Apatosaurus

How Apatosaurus grew into a behemoth weighing tens of tons has been understood in detail by examining the "lines of arrested growth" in the cross-sections of fossil bones. Unlike modern reptiles that grow slowly over their lifetime, it was characterized by having a "spurt period" of rapid growth during its youth. According to research, during its peak growth, it gained up to 5 tons of weight in a single year, reaching a massive 25 tons in just about 15 years.

As can be seen from the data above, it has been confirmed that Apatosaurus reached sexual maturity and began breeding before its body had fully grown. This is thought to have been a strategy to survive and reliably leave offspring in a harsh environment with many natural enemies like carnivorous dinosaurs. Also, juvenile Apatosaurus had shorter necks and tails compared to adults, and the process by which the pelvic bones (sacral vertebrae) fused and became sturdier as they grew has also been observed.

The Problem with the Name Brontosaurus

Apatosaurus was confused with the famous "Brontosaurus (Thunder lizard)" for many years. Apatosaurus was named in 1877, and Brontosaurus two years later in 1879, both by Othniel Marsh. However, in 1903, it was concluded that "Brontosaurus is merely a juvenile Apatosaurus."

However, in 2015, a study that re-analyzed a vast amount of fossil data using computers caused a major paradigm shift. It was proven that while the skeleton of Apatosaurus is extremely thick and robust, Brontosaurus is more gracile and fundamentally different in structure. As a result, Brontosaurus made a comeback as an independent, valid dinosaur genus for the first time in 112 years. However, there are objections to this theory, and debate continues among experts today.

Recent Taxonomic Status of Species Once Assigned to Apatosaurus

Over the course of history, many fossils were classified into the genus Apatosaurus, but large-scale phylogenetic analyses in 2015 and later revealed that some belong to entirely different lineages (like Camarasauridae) or to the independent genus Brontosaurus. Below is the current classification of major species previously considered Apatosaurus.

Habitat Environment

Late Jurassic North America (Morrison Formation), where Apatosaurus lived, was an environment with vast floodplains and savannas, experiencing cycles of severe droughts and rainy seasons. Camarasaurus and others also lived in the same environment, but Apatosaurus avoided competition for food by stripping ferns and soft leaves at lower levels, rather than raising its neck high like Brachiosaurus.

It was once thought that they swallowed stones (gastroliths) into their stomachs to digest plants. However, in recent years, the role of gastroliths in sauropods generally is being dismissed, and it is believed they used stomach acid and microbes to ferment and digest food in their massive gastrointestinal tracts.

"Medium-scale" Survival Strategy Adapted to the Environment

It was generally believed that dinosaurs grew increasingly larger as time progressed from the Jurassic to the Cretaceous periods. However, research on new fossils discovered in 2026 from Arches National Park in Utah revealed an interesting fact. These fossils belong to a rather recent (geologically later) period among Apatosaurines, but their bodies remained a "medium size," contrary to the trend of gigantism.

This is a significant discovery proving that when the climate and environment changed at the end of the Jurassic, not all dinosaurs simply took the path to gigantism. Instead, some populations deliberately kept their bodies from becoming too large, remaining flexible to adapt to the amount of water and plants available, and thus survived.