Allosaurus

About Allosaurus

The Apex Predator That Ruled the Jurassic Ecosystem

Allosaurus was a carnivorous dinosaur that lived mainly in North America and Europe during the Late Jurassic period of the Mesozoic era, about 155 to 143.1 million years ago. It reigned as an overwhelming apex predator, especially in the ecosystem of the "Morrison Formation" in the United States, where many fossils have been found.

The Morrison Formation at the time was a harsh environment resembling a dry savanna with distinct wet and dry seasons. However, herds of massive herbivorous dinosaurs (sauropods) such as Apatosaurus and Diplodocus lived there in large numbers. It is thought that Allosaurus preyed on these giant dinosaurs while migrating through this environment.
Also, other large carnivorous dinosaurs such as Torvosaurus and Ceratosaurus lived in the same place. However, they are known to have coexisted perfectly by avoiding direct conflict, such as by targeting different types of prey or dividing their preferred hunting styles, like ambush hunting.

Species and Diversity of Allosaurus

The genus Allosaurus is currently divided into several valid species based on differences in habitat era, geographic distribution, and detailed shapes of the skull. In particular, the discovery of the new species "Allosaurus jimmadseni" described in 2020 was extremely important for understanding the evolutionary history of Allosaurus.

Hunting Method - "Strike and Tear"

Previously, it was said that Allosaurus hunted using a "hatchet bite," opening its mouth wide and swinging its upper jaw down on its prey like a lumberjack's axe to deliver a fatal wound. However, recent computer simulations have shown that this movement would place too much strain on its own bones, so it is now understood that this was not actually done.

Instead, the currently leading theory is a hunting method called "strike and tear." Allosaurus's jaw strength (bite force) itself was weak for its body size, but its skull was built to be extremely resistant to impact. And it was equipped with a wide-opening mouth and serrated teeth like steak knives.
It is thought that after biting into prey with their open jaws, instead of just crushing with jaw strength, they used the strong muscles of their neck and the spring of their whole body to quickly pull their head back, efficiently tearing the flesh and aiming for massive blood loss. This hunting style is very similar to that of modern birds of prey like falcons.

Also, Allosaurus was very fast, with an estimated top speed reaching 30 to 55 km/h. Unlike Tyrannosaurus, it had long forelimbs equipped with three sharply curved claws, so it could leap at fleeing prey at this speed, pinning it down firmly with its forelimbs while biting.

A Life-Threatening Hunt - The Death Struggle with Stegosaurus

Decisive fossils have been found showing that Allosaurus was not just a scavenger, but actively hunted large, living dinosaurs. These are the marks of fierce battles with the heavily armored herbivorous dinosaur "Stegosaurus," which had plates on its back.

A bone in the pelvis (from the groin to the abdomen) of one Allosaurus fossil bears a deep puncture wound that perfectly matches the thick spike (called a thagomizer) at the end of a Stegosaurus's tail. Because the fossil showed signs of the bone trying to heal and severe suppuration (infection) around the wound, we know that the Allosaurus survived for a while, enduring intense pain, even after being stabbed in a vital area and suffering a major injury.

On the other hand, fossils of Stegosaurus neck plates (osteoderms) have been found with "U-shaped bite marks" that perfectly match the shape of Allosaurus teeth. These clearly prove how dangerous it was to hunt herbivorous dinosaurs armed with powerful weapons, and that both sides were engaged in a literal life-and-death struggle.

Proof of a Harsh Life: "Big Al"

Discovered in Wyoming, USA, in 1991, the fossil nicknamed "Big Al" is a miraculous specimen with 95% of its skeleton preserved intact. Subsequent detailed investigations revealed that this individual is "Allosaurus jimmadseni," described as a new species.

When all the bones of Big Al were examined closely, surprisingly, traces of 19 fractures and diseases were found. In particular, the toe on its right foot suffered from a severe infection, leading researchers to believe it could not walk properly, let alone run. During its short life of about 17 years after hatching, Big Al survived desperately in the harsh nature of the Jurassic while sustaining severe injuries multiple times.

Differences from Tyrannosaurus and Evolution

Although often compared as "giant carnivorous dinosaurs," Allosaurus and Tyrannosaurus lived about 80 million years apart, so they never actually met. Furthermore, the direction of their evolution and their hunting styles are completely different.

Tyrannosaurus, which lived at the end of the Cretaceous period, had to deal with "heavily armored" dinosaurs such as Ankylosaurus, covered entirely in hard armor, and Triceratops, with its magnificent horns. Therefore, even at the cost of smaller forelimbs, it evolved a massive, robust head and became a "bone crusher," biting through prey bones and all with thick teeth.

On the other hand, Allosaurus primarily preyed on agile dinosaurs and giant but unarmored sauropods. As a result, instead of a one-hit kill with bite force, it evolved to specialize in "mobility and tearing"—cornering prey with its running speed, grappling with its well-developed long forelimbs, and repeatedly tearing flesh with its sharp teeth. Rather than asking which was stronger, it is more accurate to say that both were magnificent hunters perfectly tuned to the environment and prey of their respective eras.

History of Exhibition in Japan

Behind the fact that we can now see real dinosaur fossils in Japanese museums is the deep passion of Mr. Yukichi Ogawa, a Japanese-American. Mr. Ogawa, who moved to the United States before the war, learned that there were no real complete dinosaur skeleton fossils in post-war Japan. Driven by a strong desire to "show real dinosaurs to the children of Japan, my homeland, and nurture their dreams for science," he collected fossils with his own funds. He then donated valuable authentic fossil specimens, including an Allosaurus, to the National Museum of Nature and Science and others.

The Allosaurus skeleton exhibited at the National Museum of Nature and Science has inspired a fascination with dinosaurs in many people over the years. When it first went on display, according to the academic theories of the time, dinosaurs were thought to walk upright dragging their thick tails on the ground like Godzilla or a kangaroo (tripodal posture), so it was exhibited in that pose. However, subsequent biomechanical research revealed that dinosaurs walked while balancing by lifting their tails straight horizontally. As a result, the skeleton has now been reassembled into a dynamic horizontal posture, tail stretched taut, looking ready to pounce at any moment.
The Allosaurus exhibition continues to be loved today, not only for the charm of the dinosaur itself but also as an important presence that visually teaches us how scientific thinking progresses and changes.