Allosaurus

***Allosaurus* (al·lo·saur·us)***(al-OH-sore-US)'(*March, 1877) meaning, "different lizard", is arguably one of the most well known Jurassic predators known, growing to about 28 - 29 feet (8.5 to 9 meters) on average and capable of reaching up to 12–13 meters (up to 42 feet.) long.[1]

The predator had a large skull with eye ridges, and claws designed for gripping. Its diet may have included many different dinosaurs, mainly smaller ones such as the Dryosaurus and *Camptosaurus*, but in packs it may, possibly, have hunted the stegosaurid Stegosaurus, young sauropods, even lonely and/or injured adult Diplodocus.

There are currently three species of *Allosaurus*known: ***A. fragilis***,***A. europaeus,*** ***A. jimmadseni***. And ***A. Maximus**(the largest subspecies).* Allosaurus is also known to hunt in packs to kill bigger dino*saurs* such as Camarasaurus.

*Allosaurus* was certainly not the only apex predator in the Jurassic, as it would have to compete against other predators, such as *Saurophaganax* and not the bigger but the main rivals *Ceratosaurus* and *Torvosaurus*. However, its weak bite force is misleading. Its skull would have been used almost like a hatchet to smack down on prey, causing heavy bleeding. It couldn't take down sauropods, but it was the perfect predator for smaller prey items. Smaller prey such as *Dryosaurus* and *Camptosaurus* would be ideal because *Allosaurus* was speedy with a "hatchet" mouth that could cause bleeding, and the smaller animals would bleed out quicker.

One particularly well-preserved specimen, nicknamed "Big Al", was found in Wyoming. While *Allosaurus* was a hunter, it was also a scavenger (like all predators). It would find dead animals or animals dying to feast on. "Big Al" had many different injuries from throughout its short, seven year life, yet surviving all but the last one, an infected toe wound. This would have left the animal unable to hunt, and it would have starved and/or died of thirst.*Allosaurus* skull was double hinged. This allowed it to open its mouth extremely wide, allowing it to have a huge bite radius, uncommon in other theropods of its age. One hinge was near the front of the jaw like a snake's, and the other was near the back of the jawbone. However, recent studies seem to show that *Allosaurus* had a relatively low bite force of only 350 pounds per square inch,much less than the value of sharks and crocodiles and even inferior to that of most big cats, despite being much bigger.

## Skull

Despite its apparently weak bite, *Allosaurus* skull was very strong, capable of withstanding a force of 12, 364 pounds. It seems that *Allosaurus* attacked like an axe, using its double-hinged jaw to open really wide and dropping the top part of its skull into its prey with a large amount of pressure while holding it steady with its strong arms.

A constant repetition of this method would have led to a massive amount of blood loss and shock. It was a very efficient way of killing, and it's probably why *Allosaurus* was so successful as a hunter.

## Description

*Allosaurus* was a typically large theropod once thought to be less than 10 meters but not after when the much large theropod Epanterias was proved to be synonymous to it, which means it grew up to 13 meters. The largest specimen of *Allosaurus fragilis* was about 9.7 meters in length. With the best known species showing it averaged on 8.5 meters. As with all dinosaurs there is much debate on the weight of *Allosaurus*. Some say it's weight was up to 2000 pounds, but possible synonym *Saurophaganax* had a weight of 4000 pounds. It is the most successful Jurassic predator. It's body was agile and sleek making it able to run in a speed of 25 mph. It had large claws on it's hands to grip prey while using it's "hatchet bite". It's legs were somewhat long but weak and containing less muscle mass. These strong body parts made *Allosaurus* to be one of the most successful hunters.

### Front limbs morphology

The forelimbs of *Allosaurus* were short in comparison to the hindlimbs (only about 35% the length of the hindlimbs in adults) and had three fingers per hand, tipped with large, strongly curved and pointed claws. The arms were powerful, and the forearm was somewhat shorter than the upper arm (1:1.2 ulna/humerus ratio). The wrist had a version of the semilunate carpal also found in more derived theropods like maniraptorans. Of the three fingers, the innermost (or thumb) was the largest, and diverged from the others. The phalangeal formula is 2-3-4-0-0, meaning that the innermost finger (phalange) has two bones, the next has three, and the third finger has four. The legs were not as long or suited for speed as those of tyrannosaurids, and the claws of the toes were less developed and more hoof-like than those of earlier theropods. Each foot had three weight-bearing toes and an inner dewclaw, which Madsen suggested could have been used for grasping in juveniles.There was also what is interpreted as the splint-like remnant of a fifth (outermost) metatarsal, perhaps used as a lever between the Achilles tendon and foot.

### "Big Al" and "Big Al Two"

In 1991 "Big Al" (MOR 693), a 95% complete, partially articulated specimen of *Allosaurus* was discovered. It measured about 8 meters (about 26 ft) in length. MOR 693 was excavated near Shell, Wyoming, by a joint Museum of the Rockies and University of Wyoming Geological Museum team. This skeleton was discovered by a Swiss team, led by Kirby Siber. In 1996 the same team discovered a second *Allosaurus*, "Big Al Two", which is the best preserved skeleton of its kind to date.

The completeness, preservation, and scientific importance of this skeleton gave "Big Al" its name; the individual itself was below the average size for *Allosaurus fragilis*, and was a subadult estimated at only 87% grown. The specimen was described by Breithaupt in 1996. Nineteen of its bones were broken or showed signs of infection, which may have contributed to "Big Al's" death. Pathologic bones included five ribs, five vertebrae, and four bones of the feet; several damaged bones showed osteomyelitis, a bone infection. A particular problem for the living animal was infection and trauma to the right foot that probably affected movement and may have also predisposed the other foot to injury because of a change in gait. Al had an infection on the first phalanx on the third toe that was afflicted by an involucrum. The infection was long lived, perhaps up to 6 months. Big Al Two is also known to have multiple injuries.

### Feeding

Paleontologists accept *Allosaurus* as an active predator of large animals. There is dramatic evidence for allosaur attacks on *Stegosaurus*, including an *Allosaurus* tail vertebra with a partially healed puncture wound that fits a *Stegosaurus* tail spike, and a *Stegosaurus* neck plate with a U-shaped wound that correlates well with an *Allosaurus* snout. Sauropods seem to be likely candidates as both live prey and as objects of scavenging, based on the presence of scrapings on sauropod bones fitting allosaur teeth well and the presence of shed allosaur teeth with sauropod bones. However, as Gregory Paul noted in 1988, *Allosaurus* was probably not a predator of fully grown sauropods, unless it hunted in packs, as it had a modestly sized skull and relatively small teeth, and was greatly outweighed by contemporaneous sauropods.Another possibility is that it preferred to hunt juveniles instead of fully grown adults. Research in the 1990s and first decade of the 21st century may have found other solutions to this question. Robert T. Bakker, comparing *Allosaurus* to Cenozoic sabre-toothed carnivorous mammals, found similar adaptations, such as a reduction of jaw muscles and increase in neck muscles, and the ability to open the jaws extremely wide. Although *Allosaurus* did not have sabre teeth, Bakker suggested another mode of attack that would have used such neck and jaw adaptations: the short teeth in effect became small serrations on a saw-like cutting edge running the length of the upper jaw, which would have been driven into prey. This type of jaw would permit slashing attacks against much larger prey, with the goal of weakening the victim.

Similar conclusions were drawn by another study using finite element analysis on an *Allosaurus* skull. According to their biomechanical analysis, the skull was very strong but had a relatively small bite force. By using jaw muscles only, it could produce a bite force of 805 to 2,148 N, less than the values for alligators (13,000 N), lions (4,167 N), and leopards (2,268 N), but the skull could withstand nearly 55,500 N of vertical force against the tooth row. The authors suggested that *Allosaurus* used its skull like a hatchet against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away without splintering bones, unlike *Tyrannosaurus*, which is thought to have been capable of damaging bones. They also suggested that the architecture of the skull could have permitted the use of different strategies against different prey; the skull was light enough to allow attacks on smaller and more agile ornithopods, but strong enough for high-impact ambush attacks against larger prey like stegosaurids and sauropods. Their interpretations were challenged by other researchers, who found no modern analogues to a hatchet attack and considered it more likely that the skull was strong to compensate for its open construction when absorbing the stresses from struggling prey. The original authors noted that *Allosaurus* itself has no modern equivalent, that the tooth row is well-suited to such an attack, and that articulations in the skull cited by their detractors as problematic actually helped protect the palate and lessen stress. Another possibility for handling large prey is that theropods like *Allosaurus* were "flesh grazers" which could take bites of flesh out of living sauropods that were sufficient to sustain the predator so it would not have needed to expend the effort to kill the prey outright. This strategy would also potentially have allowed the prey to recover and be fed upon in a similar way later. An additional suggestion notes that ornithopods were the most common available dinosaurian prey, and that allosaurs may have subdued them by using an attack similar to that of modern big cats: grasping the prey with their forelimbs, and then making multiple bites on the throat to crush the trachea. This is compatible with other evidence that the forelimbs were strong and capable of restraining prey. Studies done by Stephen Lautenschager *et al.* from the University of Bristol also indicate *Allosaurus* could open its jaws quite wide and sustain considerable muscle force. When compared with *Tyrannosaurus* and the therizinosaurid *Erlikosaurus* in the same study, it was found that *Allosaurus* had a wider gape than either; the animal was capable of opening its jaws to a 79 degree angle. The findings also indicate that large carnivorous dinosaurs, like modern carnivores, had wider jaw gapes than herbivores.