Allosaurus
Allosaurus (AL-oh-SAWR-us; “different lizard”) is a genus of large carnivorous theropod dinosaur that roamed the Earth during the Late Jurassic period, approximately 155 to 145 million years ago. Its fossils have been found primarily in the Morrison Formation of North America, with some possible remains discovered in Portugal. The name Allosaurus comes from the Greek words “allos” (meaning “different” or “other”) and “sauros” (meaning “lizard”), referring to its vertebrae (backbones), which were lighter and different in structure compared to other dinosaurs known at the time of its discovery. Paleontologist Othniel Charles Marsh first named and described Allosaurus in 1877.
Description and Classification
Allosaurus was a formidable bipedal predator, meaning it walked on two powerful hind legs. It typically measured between 8.5 to 9.7 meters (28 to 32 feet) in length, though some exceptionally large individuals might have exceeded 12 meters (39 feet). Scientists estimate its weight to have been around 2 to 2.5 metric tons (about 4,400 to 5,500 pounds), making it one of the largest meat-eaters of its time.
Its massive skull, which could be up to 1 meter (3 feet) long, was equipped with dozens of large, sharp, serrated teeth. These teeth were constantly replaced as they wore down or broke off. The skull had large openings, called fenestrae, which helped reduce its weight without sacrificing too much strength. Allosaurus had a relatively short neck, a bulky body, and a long, heavy tail that provided balance. Its forelimbs were short but strong, each ending in three fingers tipped with sharp, curved claws, with the first claw being particularly large and formidable. Its powerful hind limbs also had three main toes, each with a strong claw.
Allosaurus belongs to the family Allosauridae, a group of large theropod dinosaurs within the larger clade Carnosauria. It is the type genus of this family. Several species of Allosaurus have been proposed over the years, but the best-known and most widely accepted species is Allosaurus fragilis. Another species, Allosaurus jimmadseni, was recognized more recently, based on distinct skull features and earlier geological occurrence.
Distinguishing Features
Allosaurus possessed several features that help paleontologists distinguish it from other dinosaurs:
- A pair of prominent, bony horns or crests located just above and in front of its eyes, the exact shape and size of which could vary between individuals.
- Large openings (fenestrae) in its skull, which made the skull lighter while still maintaining strength for biting.
- Relatively short but robust forelimbs with three-fingered hands, each finger bearing a large, recurved claw, especially the innermost (thumb) claw.
- Vertebrae (backbones) that were lighter and had a different shape (concave on both ends, or amphicoelous, with hollow spaces) than those of many other dinosaurs known during the 19th century, which is the characteristic that gave it its name.
- An S-shaped neck, typical of many theropod dinosaurs.
- Teeth that were blade-like, serrated, and recurved, designed for slicing through flesh.
Paleoenvironment and Diet
Allosaurus lived in what is now western North America, in an area known as the Morrison Formation. During the Late Jurassic, this region was a vast, semi-arid plain with rivers, floodplains, and open woodlands, similar to a savanna. It experienced distinct wet and dry seasons. Allosaurus shared this environment with a diverse range of other dinosaurs, including giant long-necked sauropods like Apatosaurus, Diplodocus, and Brachiosaurus, as well as armored dinosaurs like Stegosaurus, and smaller plant-eating ornithopods like Camptosaurus.
As an apex predator, Allosaurus was at the top of the food chain. Its diet consisted mainly of these herbivorous dinosaurs. Fossil evidence, including Allosaurus tooth marks found on the bones of Stegosaurus and sauropods, confirms its predatory behavior. There is ongoing discussion among scientists about whether Allosaurus hunted alone or in cooperative packs to bring down very large prey like adult sauropods. While some evidence suggests possible group attacks, many paleontologists believe it was more likely a solitary hunter, or perhaps engaged in mobbing behavior without true pack coordination. It likely scavenged on carcasses as well.
Significance and Ongoing Research
Allosaurus is one of the most common and best-understood large theropod dinosaurs from the Jurassic period. Hundreds of fossils, ranging from isolated bones to nearly complete skeletons, have been unearthed, particularly from sites like the Cleveland-Lloyd Dinosaur Quarry in Utah. This wealth of fossil material has allowed scientists to study its anatomy, growth, and even injuries in great detail. Famous specimens like “Big Al” (MOR 693) and “Big Al Two,” which show evidence of numerous injuries and infections, have provided valuable insights into the tough lives these predators led.
The study of Allosaurus is significant for understanding the predator-prey dynamics and overall ecosystem structure of the Morrison Formation. It helps researchers paint a clearer picture of life during the Late Jurassic.
Ongoing research continues to explore various aspects of Allosaurus biology. Scientists are investigating the biomechanics of its bite force and feeding strategies, using CT scans and computer modeling. The debate over its social behavior—whether it was a solitary hunter or engaged in group hunting—remains an active area of study. Bone histology (the microscopic study of bone structure) is used to determine growth rates and life history patterns. Furthermore, paleontologists continue to examine the diversity within the Allosaurus genus, working to clarify the exact number of valid species and their evolutionary relationships to other theropods like Saurophaganax or Torvosaurus, which shared its habitat.