The genus Allosaurus, meaning “different lizard,” was a massive, bipedal theropod dinosaur that dominated the landscape of the Late Jurassic Period. This apex predator typically reached lengths of 8.5 to 10 meters. Its success was rooted in its dozens of sharp, serrated teeth and robust, agile structure, allowing it to hunt the largest herbivores of its time. Allosaurus flourished during the Late Jurassic, disappearing around the transition into the Cretaceous Period.
Allosaurus in the Late Jurassic Period
Allosaurus reached its ecological peak during the Late Jurassic, spanning the Kimmeridgian and Tithonian stages (approximately 155 to 145 million years ago). The abundance of its fossil remains makes it the most commonly found large theropod in North America. The primary source of these fossils is the Morrison Formation, a vast stretch of sedimentary rock deposits covering much of the western United States.
This environment consisted of semi-arid floodplains and lush river valleys, supporting immense herds of sauropods and armored dinosaurs, which served as the Allosaurus’s main prey. Fossil evidence, such as bite marks on Stegosaurus plates and Apatosaurus bones, confirms its predatory role at the top of this complex ecosystem. Fossils found in locations like Portugal also indicate a broad geographic distribution.
Defining the Extinction Boundary
The disappearance of Allosaurus from the fossil record occurs around the boundary between the Jurassic and Cretaceous periods, approximately 145 million years ago. This geological demarcation represents the transition from the Tithonian stage of the Late Jurassic to the Berriasian stage of the Early Cretaceous. The decline of Allosaurus was a gradual process of faunal turnover, unlike the sudden, catastrophic impact event that ended the Cretaceous Period.
Paleontologists struggle to pinpoint the exact moment of its extinction because the boundary itself is poorly defined in the rock record, lacking a single, universally agreed-upon global indicator. The decline is marked by a noticeable absence of the large, specialized Late Jurassic fauna—including Allosaurus—in the rock layers immediately following this boundary. This gradual fading makes it difficult to distinguish whether the final species truly went extinct or if the fossil record simply became too sparse to capture the last remaining individuals.
Environmental Shifts and Replacement
The scientific consensus suggests that the decline of Allosaurus was driven by broad environmental shifts and competitive replacement by new predatory lineages. The Late Jurassic-Early Cretaceous transition was a time of major global change, including the continued breakup of the supercontinent Pangea. This continental drift led to the development of new landmasses and isolated habitats.
This period saw a significant drop in global sea levels, known as a eustatic lowstand, which altered coastal environments and changed local climates. These environmental pressures caused a substantial turnover in the dominant Late Jurassic plant and animal communities, on which Allosaurus relied. As the ecosystem changed, the specialized Allosaurus lineage may have been less adaptable than emerging theropods. Newer groups of large theropods, such as early Carcharodontosaurids, began to appear in the Early Cretaceous, likely outcompeting the remaining Allosaurus populations in the shifting habitats.