The Allosaurus, a formidable predator that roamed the Earth during the Late Jurassic period (approximately 155 to 145 million years ago), is one of the most thoroughly studied theropods in the fossil record. This large, bipedal carnivore’s abundance, particularly in North America, has provided scientists with a deep understanding of its anatomy, growth, and behavior. The genus name, meaning “different lizard,” was inspired by the unusual, lightweight structure of its vertebrae. Paleontologists recognized this structure as distinct from other known dinosaurs at the time.
Initial Discovery and Naming
The scientific recognition of Allosaurus began in the Western United States during intense fossil exploration in the 1870s. The type specimen, a collection of fragmentary bones, was first discovered in the Colorado region, likely within the rock layers of the Morrison Formation. Paleontologist Othniel Charles Marsh formally described and named the species Allosaurus fragilis in 1877, though the initial remains were scant. Marsh’s work occurred during the “Bone Wars,” a fierce rivalry that drove rapid documentation of North American dinosaurs. This initial discovery established the presence of a massive, previously unknown predator, and subsequent expeditions soon unearthed more complete specimens, including a significant skeleton found at Como Bluff, Wyoming.
The Primary Fossil Hotspot
The vast majority of Allosaurus fossils have been recovered from the Morrison Formation, a massive sequence of sedimentary rock layers stretching across multiple U.S. states, including Colorado, Utah, Wyoming, and New Mexico. Dating to the Late Jurassic, this formation represents an ancient semi-arid landscape of floodplains and river systems. The formation’s extensive exposure and favorable preservation conditions have made it the richest source of dinosaur fossils in North America.
Within the Morrison Formation, the Cleveland-Lloyd Dinosaur Quarry in Utah stands out as the most significant location for this species. This quarry contains the densest concentration of Jurassic dinosaur bones ever found, with Allosaurus remains making up a disproportionately large percentage of the total. Over 46 individual specimens have been identified there, accounting for up to 75% of the carnivore fossils recovered. This unusual abundance of predators suggests the site was a “predator trap,” possibly a muddy waterhole where animals became stuck.
The sheer volume of specimens from this quarry has allowed scientists to study the species in unprecedented detail, from juveniles to fully grown adults. This wealth of material has provided deep insights into the growth stages, population variability, and overall biology of Allosaurus fragilis.
Global Finds and Unique Contexts
While North America is the primary home of Allosaurus remains, the genus has also been discovered across the Atlantic in Europe, specifically in Portugal. Fossils have been found in formations dating to the same Late Jurassic period as the Morrison Formation. These European finds include a partial skull and vertebrae that were distinct enough to be named a separate species, Allosaurus europaeus, in 2006. The presence of this distinct species suggests that a land bridge allowed for faunal exchange between the continents before the North Atlantic Ocean fully separated them. This discovery confirms a broader, intercontinental distribution for the large theropod lineage, showing it was a dominant predator across interconnected landmasses.