Allosaurus fragilis has been interpreted to be the top predator in the food web represented in the Morrison Formation (1). The Morrison Formation environment was arid, with alternating wet and dry seasons on flat floodplains (2). Plants found in this environment includes bryophytes, tree ferns, conifers, and ginko trees (3). This period is characterized by higher CO2 levels than the present time (4).
Numerous other species shared the environment with Allosaurus. Invertebrate species include a variety of mollusks, while trace fossils show signs of ants, termites, and other insects. Ray-finned fish were also found, as well as several different frog and salamander species. Of dinosaurs, sauropod dinosaurs appear to dominate the Morrison ecosystem, such as Diplodocus and Camarasaurus. Other dinosaurs are found in lower abundance, including both small and large theropods and more herbivorous species. Small mammals were also present, with the largest being no bigger than a modern squirrel (5).
The diversity in aquatic species has led paleontologists to believe that, though arid, the Morrison environment supported wetlands, lakes, and rivers as well. Allosaurus lived in and was an important participant in a unique environment, without any satisfactory modern analog (5).
Allosaurus fragilis walked upright on two legs with its body and tail held parallel to the ground (1). Allosaurus fragilis was an apex predator of the late Jurassic period, approximately 155 to 150 million years ago, eating an entirely carnivorous diet (2). The predatory instincts of Allosaurus may have occasionally led this extinct animal into danger. At the Cleveland-Lloyd quarry, where 40-60 Allosaurus skeletons have been found in a single deposit, it has been suggested that these animals became trapped in the mud during an attempt to scavenge or prey upon other trapped dinosaurs (3). There are multiple alternative hypotheses for this mass gravesite, such as pack hunting and cannibalism, but there is little hard evidence to support any of these claims (4).
Allosaurus fragilis bones have rings that can be used to determine age and yearly rates of growth for the animal, much like counting the rings of a tree. Using this, paleontologists can reconstruct growth curves. Paleontologists have estimated that Allosaurus had a maximum lifespan between 22-28 years with fastest growth (148 kg/year) occurring at approximately 15 years of age. The femur, or thigh bone, of Allosaurus grew at an increased pace relative to the humerus, ulna, and tibia. That is, the forelimbs and the shins grew more slowly than the thighs. This process leading to similar limb proportions as other theropod dinosaurs. This gave Allosaurus very short forelimbs relative to the rest of the body(1). The adult weight of Allosaurus has been highly debated, with estimates ranging from 1,500 kg to 4,000 kg (2).
Allosaurus fragilis, a member of the Tetanurae group (1), was first discovered in 1877 by Othniel Charles Marsh (2). It averages 8.5 m in length with the largest specimen at 9.7 m long. This large theropod dinosaur lived during the late Jurassic period (155 to 150 million years ago) and is known primarily from abundant deposits in the Morrison Formation in western America (3). At the Cleveland-Lloyd quarry, an estimated 40-60 Allosaurus skeletons have been discovered at a single site (4). Their habitat was relatively arid, with alternating wet and dry seasons (5).
During its time, Allosaurus was the top predator in its ecosystem, preying on large herbivorous dinosaurs and possibly even smaller predators. Like many theropod dinosaurs, Allosaurus had a large skull, sharp teeth, and small forelimbs (4). Allosaurus is part of the same evolutionary pathway that gave rise to many other well-known dinosaur species, such as Tyrannosaurus rex, Velociraptor mongoliensis, and all birds (6).
Researchers have been able to look at the brain of Allosaurusin greatdetail usingspiral computed tomography (CT) scanning. This is the same CT scanning used in hospitals to diagnose a variety of conditions in humans. In many cases, paleontologists have partnered with hospitals and medical schools to use their facilities.The brain of Allosaurus is more similar to crocodiles than to birds. Like crocodiles, Allosaurus would have been able to discern lower frequencies best. Allosaurus had very large olfactory bulbs, implying heavy reliance on a sense of smell. Olfactory bulbs are structures in the front of the brain of a vertebrate. Evidence for them is preserved in the structure of the interior of the skull and can be mapped using CT scanning (1).
Using this method, inner ear anatomy can be determined as well. The vestibular system of the ear, the part of the inner ear that contributes to balance. It contains three semicircular canals oriented perpendicular to each other. The angle of these semicircular canals can be used to determine head position in extinct animals (2). One canal is always in the horizontal position. How that canal is placed within the skull allows paleontologists to determine the natural head position of the animal. In Allosaurus, this canal is oriented on the same angle as the length of the skull, meaing that the skull is held parallel to the ground. Thus, reconstructions of the skeleton are similar to the modern interpretation of Tyrannosaurus rex with the head and tail both extended at approximately the same height.The rest of their brain was not very complex, leading researchers to infer that Allosaurus followed a set pattern of behaviors with less social complexity than seen in modern mammals or birds (1).
The skull of Allosaurus fragilis has typical proportions for a theropod dinosaur of its size (1). Theropoda is the group whose evolutionary pathway that gave rise to many other well-known dinosaur species, such asTyrannosaurus rex,Velociraptor mongoliensis, and all birds. Research on the skull of Allosaurus shows surprisingly low bite strength for this dinosaur, similar to that of living animals such as leopards and wolves (2, 3) but much lower than other predatory dinosaurs, such as Tyrannosaurus rex (4). Rayfield et al. suggest that the low bite force implies a feeding strategy consisting of swift slashing bites to soft tissues, leading to massive blood loss in prey animals (2). Contrary to what a low bite force might suggest, the skull of Allosaurus was strongly built. The structure of the skull is such that the force of a bite would be spread throughout the skull, protecting the skull against mechanical stress (2). The skull ofAllosaurus has prominent ridges over the eyes. This ornamentation seems to have a less functional use, perhaps supporting soft tissue or functioning in sexual display (5).
