Allosaurus was a prominent predatory dinosaur during the Late Jurassic period (roughly 155 to 145 million years ago). This formidable theropod, whose remains are abundant across North America and Europe, was an apex predator characterized by a massive skull, a short neck, and a long, counterbalancing tail. Its significance as the most common large carnivore in the Morrison Formation has led to extensive study. However, determining its actual size is complex due to the variability found in the fossil record. Understanding the dimensions of Allosaurus requires looking beyond a single number to appreciate the range of sizes that existed within the genus.
Standard Measurements of the Allosaurus
The most well-known and best-represented species, Allosaurus fragilis, serves as the benchmark for the genus’s dimensions. An average adult specimen of A. fragilis typically reached a length of about 8.5 meters (28 feet), from the tip of its snout to the end of its tail. This length is comparable to that of a modern articulated city bus. The height of the animal at the hip, which represents the highest point of its back when standing naturally, was approximately 2.4 meters (eight feet).
The weight of an average Allosaurus fragilis is estimated to be around 1.7 metric tons, although estimates can vary widely between 1.0 and 2.0 metric tons for a modal adult. This mass is equivalent to a large modern rhinoceros. These figures represent the size of a fully grown, healthy individual, excluding the rare, exceptionally large specimens. The sheer volume of fossil material for A. fragilis allows paleontologists to establish this relatively tight range for the typical adult size.
Variation in Size and Maximum Known Specimens
The size of Allosaurus was not uniform, fluctuating based on age, individual variation, and species differences. The most complete, large specimen, AMNH 680, indicates that some individuals of A. fragilis grew to be about 9.7 meters (32 feet) long and could weigh up to 2.7 metric tons. This represents a substantial increase over the average, illustrating the growth potential within the species.
Even larger size estimates have been proposed, though they are based on more fragmentary material. The genus Allosaurus has historically included gigantic specimens, some of which may belong to other closely related genera. For instance, the large remains once classified as A. maximus are now often referred to the separate genus Saurophaganax, which is estimated to have reached lengths of 10.9 meters (36 feet).
The size of an individual also depended heavily on its growth stage, as demonstrated by the famous specimen nicknamed “Big Al.” This fossil, now assigned to the species Allosaurus jimmadseni, measured about 8 meters (26 feet) long and weighed an estimated 1.5 metric tons. Researchers determined that “Big Al” was still a sub-adult, estimated to be only about 87% grown at the time of its death. This indicates that a fully mature adult of this species would have exceeded 8 meters, emphasizing the large gap between sub-adults and the largest mature specimens.
Furthermore, recent studies have identified a new, robust species, Allosaurus anax, based on fragmentary but massive bones. The estimated body mass for this new species is significantly higher than A. fragilis, ranging from 3.8 to 4.6 metric tons. This suggests that the Allosaurus lineage included multiple body plans with different maximum sizes.
Paleontological Methods for Size Estimation
Determining the dimensions and weight of an extinct animal like Allosaurus relies on two primary scientific approaches. The first, known as extant scaling, uses the relationship between limb bone circumference and body mass found in living bipedal animals. Paleontologists measure the circumference of the femur (thigh bone) and scale that measurement against modern animals to predict the dinosaur’s body mass. This technique is valuable because it requires only a single, well-preserved bone to generate an estimate.
The second, more complex method is the volumetric density approach, which involves creating a three-dimensional model of the dinosaur’s entire skeleton. Researchers digitally “flesh out” this model by adding soft tissue and calculating the total volume. This volume is then multiplied by an assumed body density to arrive at a body mass estimate. Both methods are complementary, but a major challenge in both techniques is accurately estimating soft tissue mass, as factors like muscle density and fat deposits significantly influence the final weight figure.