Allosaurus, meaning “different lizard,” is one of the most recognized and studied predatory dinosaurs. Understanding its time and environment reveals much about Earth’s past ecosystems.
The Late Jurassic Period
Allosaurus lived during the Late Jurassic Period, approximately from 155 to 145 million years ago. This epoch was a time of significant global change, with the supercontinent Pangaea continuing its breakup into smaller landmasses like Laurasia and Gondwana. The climate during this period was warm and humid, lacking polar ice caps, which contributed to lush vegetation across many regions.
Vast coniferous forests, fern prairies, and cycad-dominated landscapes characterized the plant life. This abundant flora supported a diverse array of herbivorous dinosaurs. Large sauropods, such as Brontosaurus, Diplodocus, Apatosaurus, and Supersaurus, roamed North America, alongside armored stegosaurids like Stegosaurus and ornithopods such as Camptosaurus and Dryosaurus. This rich prey base provided ample resources for large predators like Allosaurus.
Allosaurus in its Ecosystem
As the most abundant large predator of its era, Allosaurus occupied the top of the food chain in its ecosystem. An average Allosaurus fragilis could reach lengths of 8.5 meters (28 feet) and weigh around 1.7 metric tons, with some larger individuals possibly extending up to 12 meters (39 feet) in length. It possessed a massive skull equipped with dozens of sharp, serrated teeth, some measuring 5 to 10 centimeters long. The skull also featured small, bony horns above its eyes and ridges along its nasal bones.
Allosaurus was a bipedal dinosaur with powerful hind limbs and a long, muscular tail that provided balance. Its forelimbs were relatively small but ended in three-fingered hands with curved claws, useful for grasping prey. While its bite force was not exceptionally strong, it likely used its skull in a “hatchet-like” motion to deliver powerful downward strikes, tearing flesh from its prey. Evidence suggests Allosaurus preyed on large herbivorous dinosaurs, including sauropods and stegosaurids, and may have hunted cooperatively or scavenged carcasses.
Unearthing Allosaurus’s Story
Our understanding of Allosaurus largely stems from extensive fossil discoveries, particularly within the Morrison Formation in western North America. This geological formation, spanning parts of Wyoming, Colorado, and Utah, has yielded numerous Allosaurus specimens, including nearly complete skeletons. These fossil findings, first described in 1877, have allowed paleontologists to reconstruct its anatomy and infer its behavior.
Scientists determine the age of these fossils through radiometric dating of volcanic ash layers found above or below the fossil-bearing sedimentary rocks. This method relies on the decay of radioactive isotopes within the rock to establish precise age ranges. Skeletal remains provide insights into Allosaurus’s size, physical characteristics, and potential hunting strategies.