Allosaurus fragilis, the best-known species, was the apex predator in the Late Jurassic period, dominating the ecosystems of the Morrison Formation around 150 million years ago. Its remains are the most abundant of any large meat-eating dinosaur of its time, providing scientists with an unparalleled view into the life of a prehistoric carnivore. The sheer quantity of its fossil material makes it one of the best-understood large theropods in the world. However, arriving at a single, precise number is complicated by the nature of fossil discovery and counting methods.
Defining the Count: Specimens Versus Individuals
Providing a definitive number for Allosaurus finds requires distinguishing between a “specimen” and an “individual.” A specimen, formally the Number of Identified Specimens (NISP), is a count of every fossilized bone fragment or collection of associated fragments identified in a quarry. This raw count can easily reach into the thousands, but it does not represent the number of animals that died.
A more meaningful metric is the Minimum Number of Individuals (MNI), which estimates the fewest number of unique animals necessary to account for all the bones found in a specific location. Paleontologists calculate MNI by identifying the most frequently occurring single bone element from one side of the body, such as the highest count of left femurs. This method helps prevent overcounting when dealing with jumbled bonebeds, but it is always a conservative estimate of the actual population. For Allosaurus, most recovered fossils are disarticulated and scattered, meaning the MNI count is far lower than the true number of animals that contributed bones to the fossil record.
The Primary Source of Mass Allosaurus Finds
The abundance of Allosaurus material is overwhelmingly attributed to the Cleveland-Lloyd Dinosaur Quarry (CLDQ) in Utah. This quarry, part of the Jurassic National Monument, represents one of the densest concentrations of Jurassic dinosaur bones ever discovered, yielding over 12,000 fossils. The fossil assemblage is heavily skewed toward carnivores, with Allosaurus remains making up over 75% of the bones found.
The MNI estimate for Allosaurus at the CLDQ is cited as representing at least 46 to 50 distinct individuals. This extraordinary figure makes the location the most concentrated deposit of large theropods known anywhere. The bones range from small juveniles to massive adults, providing information on the species’ growth and variation.
Allosaurus remains dominate the theropod record across the entire Morrison Formation, which stretches across several western U.S. states. In many localities, Allosaurus fossils comprise 70% to 75% of all identified theropod bones. This widespread abundance suggests that Allosaurus was the most common meat-eater in its ecosystem, vastly outnumbering competitors like Ceratosaurus and Torvosaurus. Overall, the number of confirmed Allosaurus individuals recovered globally is well over 100.
Significant Individual Discoveries and Scientific Insights
Beyond the mass finds in quarries, several individual, high-quality Allosaurus specimens have provided unique scientific insights that quantity alone cannot offer. These rare, more complete skeletons allow researchers to study specific anatomical details and life histories. The subadult specimen nicknamed “Big Al,” for instance, is one of the most complete Allosaurus skeletons ever found, now assigned to the species Allosaurus jimmadseni.
The fossil of “Big Al” gained fame not for its completeness but for the extensive evidence of injuries and pathologies it carried. Researchers have identified numerous healed and unhealed fractures, infections, and signs of disease throughout its skeleton, which detail a challenging and injury-prone life. Such specific evidence allows paleontologists to reconstruct the behavior and physical resilience of individual dinosaurs.
Other significant finds include high-fidelity skulls that have allowed for precise study of bite mechanics and sensory capabilities, showing a robust yet flexible skull structure. These singular discoveries move the science beyond simple population estimates, providing tangible proof of growth rates, individual variation, and the harsh realities of life for a Late Jurassic predator. These individual fossils, even if they contribute only one to the MNI count, are invaluable for their qualitative information.