Allosaurus, a large predatory dinosaur, roamed the Earth during the Late Jurassic period (155 to 145 million years ago). Its fossil remains provide insights into prehistoric ecosystems and the evolution of large theropods. The distribution of these fossils across various geological formations helps paleontologists understand its ancient habitat and geographic spread.
The Morrison Formation Discoveries
The most prolific source of Allosaurus fossils is the Morrison Formation, Upper Jurassic sedimentary rock spanning the western United States. This vast formation extends across states such as Colorado, Utah, Wyoming, and New Mexico. The abundance of Allosaurus remains, including many nearly complete skeletons, makes it a primary focus for paleontological study.
The Cleveland-Lloyd Dinosaur Quarry in Utah is significant within the Morrison Formation. This quarry contains one of the densest concentrations of Jurassic dinosaur bones ever discovered. Allosaurus fossils make up 67% to 75% of the carnivorous dinosaur remains, representing at least 46 individual animals. Other notable sites include Como Bluff in Wyoming, Dry Mesa Quarry and Garden Park in Colorado, and Dinosaur National Monument in Utah, all contributing to the extensive record of Allosaurus. The richness of the Morrison Formation is attributed to ancient river systems and floodplains that created ideal conditions for the rapid burial and preservation of remains.
Allosaurus Finds Beyond Morrison
While the Morrison Formation in North America dominates Allosaurus discoveries, fossil evidence indicates a wider, though less common, geographical distribution. Discoveries outside the core Morrison region in North America include specimens of Allosaurus jimmadseni, found in equivalent geological units in parts of Utah, Colorado, and Wyoming, representing an earlier species within the genus. These finds add to the understanding of Allosaurus evolution within the continent.
Beyond North America, the presence of Allosaurus has been confirmed in Europe, notably in Portugal. Fossils assigned to Allosaurus europaeus have been unearthed from the Lourinhã, Bombarral, and Alcobaça formations. These European discoveries suggest faunal exchange between the continents during the Late Jurassic, highlighting the broad ancient range of this predatory dinosaur.
The Ancient World of Allosaurus
The Late Jurassic period featured a vastly different global landscape that shaped where Allosaurus fossils are now found. During this time, the supercontinent Pangaea was continuing its process of breaking apart, leading to the formation of distinct landmasses like Laurasia to the north and Gondwana to the south. This continental drift influenced the climate and environments available for large dinosaurs.
In the regions where Allosaurus thrived, particularly across what is now western North America, the landscape featured extensive floodplains and intricate river systems. The climate was generally warmer than today, lacking polar ice caps, and characterized by a less severe temperature gradient between the equator and the poles. These conditions supported lush conifer forests and diverse ecosystems, providing ample prey for Allosaurus.