Allosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Allosauridae

Genus: Allosaurus

Species: Allosaurus fragilis

Introduction

Allosaurus was a genus of carnivorous dinosaur with species what is modern-day North America, Portugal, Siberia, and Tanzania. One of the first carnivorous dinosaurs found, Allosaurus fossils have been important in the ongoing debate about the predatory and social behavior of dinosaurs.

In the Triassic period, the continents of the world were united in a single landmass called Pangaea. During the Jurassic and Cretaceous periods, this supercontinent split into two parts: Gondwanaland and Laurasia. Gondwanaland contained most of the modern southern hemisphere, and Laurasia contained North America, Northern Europe, and Asia. As the continents were splitting, dinosaur groups evolved and spread across the globe and diversified into a variety of species.

Allosaurus and similar dinosaurs were the dominant predators throughout the Jurassic period, and similar species are found in every continent. Allosaurus and its relatives would remain one of the most important predatory lineages until the end of the Mesozoic period, which lasted from 251 to 66 million years ago (Ma).

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Under the Linnaean classification system, Allosaurus was part of the Saurischia, or lizard-hipped, order, as well as part of the Theropoda suborder, a group of bipedal, generally carnivorous dinosaurs united by developments of the skull, teeth, and a few other skeletal features. Specifically, Allosaurus is a member of the Carnosauria infraorder, which comprises bipedal predators with common spinal and cranial characteristics.

Using a cladistic analysis, the bipedal Allosaurus falls under the following clades (from broadest to more specific): the Saurischia clade; the Neotheropoda clade; the Theropoda (“beast-footed”) clade, as it is bipedal; the Tetanurae (“stiff tailed”) clade; the Avetheropa clade; and the Carnosauria clade, which comprises large, predatory dinosaurs. The shared physical characteristics of a hinged lower jaw and wish bone indicate that the Allosaurus shares a common ancestor with other bipedal carnivorous dinosaurs such as Albertosaurus, Deinonychus, and Tyrannosaurus, which also belong to the Avetheropoda clade.

The theropod lineage eventually led to the evolution of birds, but Allosaurus is part of a theropod lineage not directly related to bird origins. Allosaurus is among the most common Jurassic carnosaurs.

Anatomy

Allosaurus was one of the largest theropod predators in the Late Jurassic. Fossils indicate a length of up to 12 meters (about 39 feet) from head to tail and a height of about 5 meters (16 feet). An adult Allosaurus might have reached 1,000 to 1,800 kilograms (2,204 to 3,968 pounds).

Allosaurus had a large head, relative to its body, with rows of curved, serrated teeth. The skull was thick at its base and narrowed toward the muzzle. The animal had forward-facing eyes and pronounced cranial crests above the eyes.

Allosaurus's skin was probably thick and pebbled, similar to fossilized skin impressions found for other dinosaurs. Like modern predators, Allosaurus may have been cryptically colored, allowing the animal to ambush its prey. Allosaurus had much longer legs than forearms, and its three-toed feet were likely covered in thick, hoof-like pads. While some later carnosaurs had greatly reduced forelimbs, the arms of Allosaurus were still muscular enough for hunting.

Intelligence

Allosaurus had EQ ratings in the range of 1.0 to 2.0, which is average for the dinosaurs. The animal's EQ rating was significantly higher than many of the herbivorous dinosaurs that shared its environment. While some biologists believe that EQ ratings provide a poor measure of intelligence, many paleontologists believe that it is safe to assume that many dinosaurs were not as intelligent as modern mammals and birds occupying similar niches within the environment.

Reproduction and Population

The discovery of Allosaurus nests containing eggs with fossilized embryos has given some clues to the animal's reproductive behavior. Paleontologists believe that Allosaurus may have laid eggs in underground burrows, a strategy used by modern crocodilians to protect eggs from predators. Allosaurus eggs had large pores, common in eggs laid underground, that allowed the embryos to breathe in the limited oxygen environment.

Because the nests of Allosaurus contained many eggs, paleontologists theorize that the dinosaur may have remained near its nesting site to protect the eggs. Some paleontologists believe that, like crocodiles and birds, Allosaurus may have also remained nearby its young to protect them from predators. Examinations of hatchling Allosaurus reveal that the young were probably capable of hunting for their own food after hatching.

Studies of Allosaurus bones indicate that the animal grew quickly, achieving its adult size in seven to ten years. Calcium rich deposits in young Allosaurus bones indicate that the animals were sexually active, though they had not yet reached adult size. Uncommon in mammals and birds, sub-adult breeding does occur among many reptiles.

Diet

Allosaurus was built for eating meat, and many paleontologists believe it preyed on the sauropods that shared its range. These large herbivores could reach more than 50 meters (164 ft) in length, leading some to speculate that Allosaurus may have specialized in hunting young sauropods, or they may have tackled weak adults in groups.

Evidence suggests that Allosaurus occasionally hunted Stegosaurus. Such evidence includes the discovery of several Allosaurus fossils with wounds that appear to have been caused by blows from a Stegosaurus tail.

The structure of the Allosaurus skull indicates that the animal would have had a weak bite force, relative to many modern carnivores. Some paleontologists believe that Allosaurus may have used its large teeth to wound its prey and then wait for the animal to weaken before attempting to kill it. Also, examination of the skull has revealed large olfactory bulbs; this common trait of predators provides them with a superior sense of smell to help track prey and carrion. Analyses of skull scars and simulations of Allosaurus neck musculature suggest it may have pulled meat off much as modern birds of prey do.

Judging from the bones in Allosaurus’ legs, the animal was slower than later relatives in the carnosaur group. Estimates place the maximum speed between 30 to 55 kilometers per hour (19–34 miles per hour). Given that, and its large maximum jaw gape of 79 to 92 degrees, Allosaurus may have been an ambush predator, approaching to within striking distance before attacking its prey.

Behavior

Paleontologists have found the remains of several adult Allosaurus that appear to have been attacking the same prey animal. This discovery led some to speculate that the dinosaur hunted cooperatively to bring down large prey. Alternatively, Allosaurus may have gathered around wounded or dead animals, attacking together but also competing for meat. However, evidence does not suggest that groups of Allosaurus displayed any social behavior besides gathering around a food source.

Habitat and Other Life Forms

During the Jurassic period, the continent of Western Laurasia was covered in a variety of habitats, ranging from lush forests to open scrub plains. The dominant plants were gymnosperms, such as conifers, cycads, and ferns. (Flowering plants would not develop until the Cretaceous.) Allosaurus occupied semi-forested scrub and the plains of Western Laurasia.

There were a variety of small mammals, reptiles, and amphibians living alongside Allosaurus. The skies were filled with a variety of insects, the earliest birds, and flying reptiles known as Pterosaurs.

Allosaurus lived in an environment dominated by large dinosaurs. The herbivorous sauropods, like Apatosaurus and Diplodocus, were one of the largest herbivore groups, joined by members of the family Stegosaurida, like the large Stegosaurus. There were several varieties of smaller herbivorous dinosaurs also occupying the territory.

In addition to Allosaurus, a variety of other theropods were living in Western Laurasia, from small insectivores to large predators that rivaled Allosaurus in size. The large carnivores Ceratosaurus and Torvosaurus have also been found in Jurassic deposits from North America.

Research

Allosaurus was named and first described by pioneering paleontologist Othniel C. Marsh in 1877 based on skeletal fragments. In 1883, rancher M. P. Feltch found one of the first nearly complete Allosaurus specimens in a fossil bed in Colorado. More than sixty specimens of Allosaurus have been uncovered from Colorado's Morrison Formation since the late 1870s, leading paleontologists to suspect that the species was fairly common.

Paleontologists have uncovered similar species in Africa, Australia, and parts of Europe. In addition to the type species, Allosaurus fragilis, three other species have been identified as belonging to the genus Allosaurus: A. europaeus (dating back 155.7 to 150.8 Ma) in 2012, A. lucasi (150.8 to 145.5 Ma) in 2014, and A. jimmadseni (150.8 to 145.5 Ma) in 2020. Further cladistic analysis continues to revise and refine the genus Allosaurus. Cladistic analysis of Allosaurus has helped to revise the carnosaur group, in terms of how species like Allosaurus fit in the lineage with later carnosaurs and offshoots like the dromeosaurs.

A juvenile skeleton, estimated to be 70 percent complete, was found in the Morrison Formation in Wyoming in 2013, put up for auction in 2015, and purchased privately in 2018. At that time of the sale, the skeleton was billed as a new Allosaurus species, as it reportedly had longer shoulder blades and more teeth; however, its sale to a private individual generated controversy as paleontologists would likely not be allowed to study the specimen or verify claims about it.

A 2017 analysis by Matthew Baron and his colleagues found twenty-one shared traits suggesting theropods, such as Allosaurus, were more closely related to ornithischians, such as Stegasaurus and Triceratops, than to the sauropods in the Saurischia clade. Thus, the researchers proposed therapods should be reclassified into a new clade named Ornithoscelida.

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