Allosaurus

One of the Largest Predators During the Jurassic Period

The Allosaurus roamed the Earth some 155 – 145 million years ago in the Late Jurassic period. This large bipedal Theropod ruled the Earth as one of the largest predators in North America and terrorized smaller dinosaurs with its huge carnivorous jaws. While much about the Allosaurus remains a mystery enough is known about this two and a half short ton beast to know its impact on surrounding life during the Jurassic period.

Allosaurus Size

The Allosaurus stood around sixteen and a half feet tall and measured in at around thirty eight feet long. This large carnivore weighed in somewhere around two and a half short tons and is recognized for the short brow horns on its skull and the bony knobs over its eyes and on the top of its head. The Allosaurus had large serrated teeth within its moderately sized skull; the large teeth were particularly long and sharply serrated to allow adequate hunting and feeding. The Allosaurus teeth became smaller as they moved towards the back of the mouth with the last teeth in the mouth being particularly narrow and short and curving back towards the back of the head. Allosaurus teeth are commonly discovered due to the fact that they were repeatedly shed and re-grown. This is a particularly important feature in such large carnivorous Theropods since if shed teeth were not re-grown the dinosaur would have been unable to hunt and feed and would have starved to death.

Horns

The bony protuberances above the Allosaurus’ eyes were also accompanied by a pair of horns which grew in as extensions of the lacrimal bones of the skull and differed in size based on the individual dinosaur as well as that dinosaur’s place along the line of maturity. It is thought that the horns of the Allosaurus could possibly have acted as sunshades to keep the sun from these vicious hunters’ eyes; however it is also believed that these horns led a part in sexual prowess and in fighting ability. The Allosaurus is known to have been one species of dinosaur with a bad temper and as such they commonly fought within their own species as well as out of their own species. The Allosaurus’ skull also holds a key to this large hunters hunting ability. The skull of the Allosaurus contains large sinus cavities that were particularly more developed than other dinosaurs of the time which lead paleontologists to believe that the Allosaurus had a rather keen sense of smell. Some paleontologists believe that the cavities held something akin to Jacobson’s organ (also referred to as a veomeronasal organ, the Jacobson’s organ is an additional olfactory organ found in some mammals) and provided the Allosaurus with a particularly good sense of smell, therefore leading it to be a particularly good hunter.

Gastralia

Aside from the presence of potential Jacobson’s organs the Allosaurus had quite a few other peculiarities which made it stand out above other dinosaur species. The Allosaurus not only was a barrel chested dinosaur which enabled larger air sacs but it also had gastralia – ribs in its belly area. Gastralia are not commonly found in dinosaurs and acted to protect soft organs within the belly of the dinosaur when it found itself in a combat situation. Another interesting fact in regards to the Allosaurus skeleton is that it featured a pubic bone; this large bone had a “foot” which is thought by paleontologists to have allowed the dinosaur to rest on the ground using this foot as a prop. One discovery by Madsen, however, noted that in females the pubic foot was not fused together and he suggested that perhaps this was a function of reproduction. That were the pubic bone fused in females egg laying would have been increasingly difficult. While this finding seems to hold some truth it has not yet been proven or disproven.

Regardless of the fusing or lack of fusing in the pubic bone all Allosaurus were good hunters. Aside from large jaws packed with large serrated teeth the Allosaurus also had particularly powerful hind limbs. The hind limbs of the Allosaurus were not as long or as suited for running as the hind limbs of the similar Tyrannosaurus Rex, however, they were powerful and each possessed three toes which bore the brunt of the Allosaurus weight. A fourth toe served as a dew claw. Much like the Tyrannosaurus Rex, however, the Allosaurus had short but powerful arms, each of which had three fingers with large claws which were well suited to grasping prey.

Eating Habits Debated

As such a large carnivorous Theropod the Allosaurus are believed to have fed on larger dinosaurs rather than smaller herbivores. Much like the Tyrannosaurus Rex, however, paleontologists believe that the Allosaurus could also have been a scavenger as well as a hunter based on the finding of Allosaurus teeth with larger Sauropod bones. It is debated by some paleontologists that perhaps the Allosaurus hunted in packs and as such would have been able to take done much larger Sauropods as prey, although this finding is looked upon with skepticism by other paleontologists who know that these large Theropods commonly fought amongst themselves. These skeptics assert that instead of hunting healthy larger Sauropods the Allosaurus hunted sick or dying large Sauropods or else picked of juveniles. Analysis of the teeth and jaws of the Allosaurus have led paleontologists to believe that the Allosaurus used its jaws like a hatchet, attacking prey with an open mouth and driving the teeth in to the flesh where it would slash and tear at its prey while sometimes splintering bone as well. Such high impact attacks are thought to be the only feasible way for the Allosaurus to have attacked and fatally wounded much larger Sauropods. Some paleontologists argue that this hatchet type of attack was unlikely as the particular movements needed from the skull to attack and feed in this manner are impractical, instead these paleontologists suggest that the Allosaurus could also have been a carnivorous grazer which too chunks of flesh from prey and left them to heal or die as they moved on.

While the actual feeding habits of the Allosaurus are debated among paleontologists it is known that these large carnivores were made for tearing in to flesh and with a top speed of between 20 to 35 miles an hour it is certain that this hunter was quite capable of chasing prey if it needed to. The Allosaurus vision, however, gives paleontologists a clue that perhaps this large Theropod was an ambush hunter rather than an active predator. The vision of the Allosaurus was binocular vision and its limited vision is compared by paleontologists to that of modern day crocodiles that depend on distance and timing for a successful hunt rather than chasing and bringing down prey.

Abundance of Fossils

Due to its ability to hunt (regardless of how the hunting was done) and its unique physiologies the Allosaurus became one of the most abundant Theropods in the Western United States. Around 70% of the fossils located in the Morrison Formation in the Western United States are fossils of Allosaurus. The first of the Allosaurus’ fossils is believed to have been formally identified in 1877 and consisted of parts of three vertebrae, a portion of rib, a tooth, a toe bone and the shaft of the right upper arm. In 1877 these bones were labeled as those of Allosaurus fragilis by Othniel Charles Marsh. The name “Allosaurus” means “different” “lizard”, the Allosaurus was given its name because in comparison to other dinosaur specimens of the time the Allosaurus had much different vertebrae. This small sample of Allosaurus bones was located in the Morrison Formation in Garden Park, north of Canon City. As time went on very many Allosaurus remains were to be located in the Morrison Formation giving it the label as the most discovered fossil in this area. In 1983 in this same area of the Morrison Formation M.P. Felch, would discover a nearly complete Allosaurus specimen as well as several other partially complete Allosaurus specimens.

As time progressed more and more specimens of the Allosaurus were to be recovered; however, it was in 1991 that the biggest Allosaurus discovery to date was made. In 1991 near Shell, Wyoming a 95% complete Allosaurus specimen was uncovered; this specimen would be referred to as “Big Al” and was partially articulated at the time of discovery. Big Al measures around twenty six feet long and was discovered by Kirby Siber and his Swiss team of paleontologists, they would also go on to discover the second most complete Allosaurus specimen which they would name “Big Al Two”. Big Al is not known to be the largest specimen of Allosaurus but due to his status as the most complete Allosaurus fossil his importance is paramount to the Allosaurus as a whole. It is unsure what brought Big Al to his end but analysis of his bones show signs of infection and broken bones which could well have brought this large creature to its end. Specifically some of the pathologic bones showed signs of osteomyelitis, a bone infection that would have led to a particularly painful end for Big Al.

Injury Prone Dinosaur

As such a large carnivore the Allosaurus was prone to injury during attacking prey as well as during squabbles with other Allosaurus’. In addition due to their slightly clumsy frame it is thought that the Allosaurus could frequently have fallen and Dr. Bruce Rothschild of Northeast Ohio has even suggesting that these falls due to tripping while running are the culprit of fourteen fractured ribs in a single Allosaurus specimen. While the injuries quite obviously had begun to heal they were no doubt painful for the Allosaurus and could have contributed to the death of other Allosaurus specimens had the bones become infected like those of Big Al. Ironic as it may be that such small bacteria could bring down one of the most feared carnivores of the late Jurassic period it is often the case that the larger beasts are brought down by the smallest of organisms.

The Allosaurus is seen as one of the most known about dinosaur species of the late Jurassic period. Due to their success on the whole the Allosaurus has been located throughout Western North America, Australia, Africa, Portugal and Europe. While the scattering of specimens of the Allosaurus do seem to be widespread it is evident that they thrived the most in the Western United States where the semiarid environment that brought specific wet and dry seasons allowed various species to thrive. Where there was a bounty of life there could survive one of the most feared carnivores of the period. Discovery of fossil beds in this particular area show rich dinosaur life which lent itself to a ripe feeding ground for the Allosaurus. While the Allosaurus did have to contest with the Ceratosaurus and the Torvosaurus for food there seems to have been plenty to go around, particularly in Colorado where the rainy seasons brought plant life which in turn brought herbivores.

While the actual number and types of Allosaurus species are still unknown, paleontologists do know that the Allosaurus was a thriving species that had very little trouble locating acceptable prey. Fossils show the teeth of Allosaurus piercing the neck of such immense Sauropods as the Apatosaurus, proving that these huge Theropods never went without a meal unless something brought them to their knees. Very little is known about what, if anything chose to hunt the Allosaurus but there were certainly a couple of factors which were able to bring this powerful carnivore to its knees. Whether it was clumsiness and the onset of bacterial infection and disease that brought the large Allosaurus to its knees or whether it was the teeth of a fellow Allosaurus the Allosaurus certainly did have its own fair share of worries in the dinosaur world. One would think that as such a powerful predator with so few predators of its own that the Allosaurus would have been successful in complete domination; however, between design faults in the physiology of the dinosaur and self sabotage of the species, a portion of these giant beasts came to their end.

Video Recreation of the Allosaurus Dinosaur