A 30-foot-long, two-legged cross between a crocodile and a mountatin lion. Add a huge skull and the predatory behavior of a shark. That's Allosaurus
This was how a 1993 issue of National Geographic appropriately described this remarkable animal. Allosaurus was the most common & successful predator of the late Jurassic. These theropods are often referred to as "the wolves of the Jurassic" because they so were so widespread (A. fragilis is the only theropod species positively identified on more than one continent) and they are believed to have been pack hunters. Thousands of fossilized bones (in a few cases,nearly complete skeletons) found throughout North America has made Allosaurus one the most well understood of all predatory dinosaurs.
S K E L E T O N
When the bones of Allosaurus were officially described in 1877, they were quite different from any other dinosaur fossils known at the time; hence the name Allosaurus (="other lizard"). A vast majority of what we know for certain about this animal has been learned from its skeletal remains.
The skull of Allosaurus was, by carnosaur standards, quite small in relation to its body. As with all members of the allosauridae family, the skull was adorned with prominent lacrimal horns and crests in front of each eye. One of the most noticeable features of the skull is the large window-like anorbital fenestra. The purpose of these wide openings was to lighten skull without sacrificing much in the way of strength andmay gave increased the area for jaw muscle attachment. Actually, a recent study indicates the skull of Allosaurus seems to have been able to withstand incredible amounts of force, to the point that it was seemingly over-engineered. For further explanation of this see "feeding and hunting." Allosaurus' jaws were extremely flexible. They possessed extra hinges that would allow the jaw to expand laterally, in order to bite of much larger chunks of flesh than would have been possible otherwise. Along the rim of allosaur jaws is a slightly raised edged that is marked with small grooves and pits that, in life, held blood vessels and nerves. This is where the lips would have been.
The teeth of Allosaurus were similar to steak-knives in form and function. The teeth were laterally flattened and serrated, 2-4 inches in length. Much like many modern-day reptiles, an Allosaurus would constantly shed and replaced its teeth throughout its lifetime.
The neck of Allosaurus was composed of nine cervical vertabrae. It exhibited the classic "S" curve found in birds and most other theropods. Large ribs were attached to the cervical (neck) vertabrae. These ribs protected some of the more vulnerable parts of the neck and at the same time served as an attachment point for muscles. Allosaurus' powerfully muscled neck was been of great assistance in rending flesh from its victims. The cervical vertabrae, as well as the first five dorsal (back) vertabrae, possessed chambers that are very similar to hollows located in the vertabrae of birds. In birds these chambers house a complex system of air-sacs that siginificantly increase the efficiency of the respiratory system beyond that of any mammal's. It is highly probable that Allosaurus had the benefit of an air-sac system as well.
The arms of Allosaurus were quite short, but they were powerful. They terminated in murderous talons that, on the first digits(thumbs), could be up to 10 inches in length. Allosaurus almost certainly employed its arms in dismembering prey and and carrion alike.
A very birdlike feature on Allosaurus is the presence of a furcula (wish-bone). When this bone was first found, it was thought to be one of the gastralia (belly ribs). The presence of this bone in Allosaurus and numerous other theropods atests to their kinship to modern birds.
The legs of Allosaurus were long and powerfully muscled. The claws on the hindlimbs were surprisingly blunt and hooflike. From this it can be inferred that Allosaurus was capable of running, kicking, and probably even jumping, but it is highly unlikely that it tore open its prey with its feet.
No complete Allosaurus tail has yet been found, but it is estimated that tail comprised more than half of the animal's total length. The main purpose of the Allosaurus tail was to serve as a counter-balance for its forward leaning body.
S P E C I E S
currently there are three distinct species of Allosaurus known- A.fragilis, A.jimmadseni, and A.maximus. The other A. species are too incomplete to be distinguished from the above named species.
A. fragilis (Marsh, 1877)- The type species for the genus Allosaurus is A.fragilis. For some time it was thought that two different animals that shared the name A. fragilis. One first with a rather triangular skull and pointed lacrinal horns. The other with a longer skull equipped somewhat blunt lacrimal horns, and a shorter neck. However recent research indicates that these differences are due to age, and individual factors. A.fragilis is known from literally thousands of fossilized bones found in the western United States. Interestingly, remains of this species have been identified in Portugal as well.
A. amplexus (Cope, 1878) [nomen dubium]- Known from massive bones found in the Uppermost parts of the Morrison Formation. The bones of this species are so large, upon their intial discovery they were thought to be those of a Sauropod. This species is estimated to have have been more than 12 meters in length and to have weighed about 5 tons, making it one of the largest of all predatory dinosaurs. Too incomlpete to distinguish from any of the recognized species.
A. atrox (Marsh, 1878) [nomen dubium]- Probably A. fragilis.
A. ferox (Marsh, 1896)- Based on a pathologic dentary with teeth
A. jimmadseni (Chure, 2000)- A newly described species based on a skeleton that is over 90% complete.
A. maximus (Chure,1995)- A massive predator from Oklahoma. It is estimated to have been 11 meters in length and weighed 2.5 tons.A.maximus is known from elements of the limbs and vertebral column. Sometimes placed in a separate genus, Saurophaganax.
A. tendagurensis (Janensch, 1925) [nomen dubium]- This species is based on a tibia from the Tendaguru Formation of Africa.
A. meriani (Greppin, 1870) [nomen dubium]- Known only from a tooth
A. trihedrodon (Cope, 1877) [nomen dubium]
F E E D I N G & H U N T I N G
Allosaurus was almost definitely an active hunter, but, like any other carnivore, it would not have been above scavenging. We do know that Allosaurus fed upon the numerous sauropods that shared its environment - Allosaurus teeth have been found embedded in Apatosaurus bones. They probably targeted young or ill sauropods most of the time, because a healthy 20+ ton adult would have been quite dangerous. When dealing with large animals, Allosaurus probably ambushed its prey and used "hit & run" tactics. In other words, it would run up to the brontosaur from behind, bite a massive chunk of flesh from its flank or a limb, and then retreat in order to avoid being crushed, or struck down by the sauropod's whip-like tail. It would then wait for the brontosaur to succumb to blood loss, and maybe deadly bacteria saturated saliva, much like a Komodo dragon's. Allosaurus may have used its powerful arms to grapple with smaller animals, such as Camptosaurus, holding the victim in place with its powerful talons while dispatching it with its jaws. Many researchers think Allosaurus may have been a pack hunter. This is examined further on the "Intraspecific Interactions".
The Bite of Allosaurus
A recent study found that the true power behind Allosaurus' bite did not lie in the muscles of its jaws, but in its neck & reinforced skull. It would gape and cleave flesh from its prey by using its powerfully-muscled neck to wield its impact-resistant skull like an axe. This would have done far more damage than simply opening and closing its jaws.
Pursuit of Prey
It has been said that Allosaurus could not have been an effective hunter because it was too slow and too heavy. First of all, the weight factor is not really an issue here. Modern day rhinos weigh as much as 3 1/2 tonnes (only an exceptionally large Allosaurus would have been heavier) and they are capable of running at speeds in excess of 35 mph. No matter how slow Allosaurus was, most of its potential prey (Sauropods and Stegosaurs) were slower, less agile animals that often weighed many times more than Allosaurus. Keeping pace with them certainly would not have been a problem.
Another argument against Allosaurus pursuing prey was that, with its weight and short arms, it would not survive a fall if it tripped while running. Therefore Allosaurus would not risk injury to pursue prey. The results of a study conducted by Bruce Rothschild of the Arthritis Center of Northeast Ohio seem to be contrary to this idea. X-ray analysis of pathologic Allosaurus ribs showed healed injuries consistent with what one would expect if the beast had tumbled while running. So it seems Allosaurus was more resilient than proponents of the pure scavenger theory once believed.
It is not at all unusual to find the remains of more than one Allosaurus skeleton in close proximity to one another, and in some cases, such as the Clevland-Lloyd quarry, the bones of hundreds of individuals are found jumbled together. From this we can infer Allosaurus was, if not a social animal, not certainly not afraid to gather in groups.
A 1998 discovery in Wyoming revealed what seems to be an allosaur nest. Remains of adults and very young individuals were found together, as well numerous herbivore bones. These herbivore bones bore gnaw marks from adults and infants alike. This indicates that allosaurs may have brought food to their young.
Presently it certainly can not be determined if Allosaurus was capable any vocal communication beyond a hiss. Since their closest living relatives, birds an crocodiles, are capable of vocalization, it is generally accepted that Allosaurus probably was too. It is almost certain that Allosaurus use visual communication to some degree; The ornamentation on its skull is evidence of this. In life, its lacrimal crests were probably vividly colored. Head-bobbing rituals may have been a key element in courtship and warding off rivals. Also, with those huge jaws bristling with teeth, it is hard not to imagine an open-mouthed gape being used as a threat display.
Some fossil trackways seem to show groups of large theropods (though not specifically Allosaurus) stalking and perhaps persuing large sauropods. This evidence, along with the fact Allosaurus gathered in groups, indicates Allosaurus may have been a pack hunter. Since sauropods were often ten times the size of a lone allosaur, a group using some form of cooperative hunting bring down such massive prey much more quickly more safely than an individual.
R E L A T I V E S
The Superfamily Allosauroidea
All members of this group are typically characterized by light skulls decorated with hornlets and/or crests, hands equipped with three talons, and a powerful S-curved neck. Their bodies are laterally flattened to a large extent. It seems most basal forms possessed larger crests. Allosauroid remains have been found on every continent, including antarctica. Some members of this group were among the largest land predators in the Earth's history. This page gives a brief description of several Allosauroids.
H A B I T A TThe World of Allosaurus
The habitat in which Allosaurus lived was not a lush rainforest or swamp as it has often been portrayed. Allosaurus' environment was like the Serengeti in that it had seasons of rainfall punctuated by periods of severe drought. The landscape was rather open with patches of conifers, ferns, and cycads. Conspicuously absent is grass - it had not yet appeared. There would have been shallow lakes and streams, most of which would have dissapeared during the long dry season. This dry season certainly took a seemingly heavy toll on life in this ecosystem. The position in which many late jurassic dinosaur skeletons are found indicate that they were victims of drought; the neck and tail of these dinosaurs are pulled back toward each other in an arc as a result of the animals' bodies dessicating under the sun in extremely dry conditions.
Dinosaurs that co-existed with Allosaurus
At least two genera of large predators shared Allosaurus' environment, Torvosaurus and Ceratosaurus. It is not known if they directly competed with Allosaurus for food, or if, to some extent, each specialized in feeding on particualr prey items. Whatever the case neither of these predators were nearly as numerous as Allosaurus. Among the small predatory dinosaurs in this ecosystem, Ornitholestes is the most well known. It had a proportionately small head so it can reasonably be speculated that it specialized in killing small prey such as lizard-like animals, insects, fish, and hatchling dinosaurs, including Allosaurus chicks. In turn, Ornitholestes may have been a prey item for older Allosaurus.
The gigantic sauropods were seemingly by far the most numerous herbivores in this environment. Some of these creatures were over 100 ft. in length and weighed in excess of 50 tons. These animals certainly had a profound effect on the vegetation their ecosystem, both in the amount of devastation they caused and in the benefit of the "fertilizer" they left behind.
With its tiny head, huge body and plated-back, Stegosaurus is the most easily distinguishable dinosaur in this habitat. Though it had a short neck, it was not restricted to feeding on low growing plants. It could have easily reared onto its hindlegs,using its tail as a support, in order to feed on higher vegetation. For protection against predators such as Allosaurus, Stegosaurus possessed a powerfully muscled tail equipped with massive spikes. A few Allosaur bones have been found with puncture marks that may have been caused Stegosaurus tail-spikes.
The ornithopods, small to mid-size bipedal herbivores, were probably a favorite prey item of Allosaurus. With the exception of the speed of the smaller individuals, these animals seem to have possessed no defense mechanisms against large predators