Allosaurus

Type Species: Allosaurus fragilis

Classification: Dinosauria – Saurischia – Theropoda - Carnosauria – Allosauroidea – Allosauria – Allosauridae 

Time Period: Late Jurassic

Location: North America and Europe

Diet: Carnivore   

Allosaurus’ name means ‘different lizard,’ and it refers to its ‘unique’ concave vertebrae (only ‘unique’ in quotes because, since its discovery, many theropods have been discovered with such vertebrae). Allosaurus was described in 1877 by paleontologist Othniel Charles Marsh, and as it was one of the first well-known theropod dinosaurs, it’s long been popular in paleontological and cultural circles (though we must emphasize that Allosaurus was not the ancestor of the later, and bigger, Tyrannosaurus). Interestingly, for over half of the 1900s, Allosaurus was known as Antrodemus; only later did the name Allosaurus come back to prominence to be established as one of the best-known dinosaurs!

Allosaurus lived from the Kimmeridgian to the Tithonian stages of the Late Jurassic. Though its primary stomping ground was the Morrison Formation of western North America, its remains have also been discovered in Portugal. Radiometric dating puts the age of the Morrison Formation and its associated fossil-beds to between 156 and 146 million years ago, safely nestled in the Late Jurassic and including the late Oxfordian stage, the Kimmeridgian stage, and the early Tithonian stage of the period. The Morrison Formation was a semiarid environment with distinct wet and dry seasons. The Morrison Basin stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains. These environments were dominated by herbivorous sauropods such as Camarasaurus, Brachiosaurus, Apatosaurus, and Diplodocus; other herbivorous dinosaurs included the plated Stegosaurus and ornithopods such as Camptosaurus and Dryosaurus. Other large theropods, in addition to Allosaurus, were the equally-sized Torvosaurus and the smaller Ceratosaurus. The top predator of the area was Allosaurus, which accounts for up to seventy-five percent of theropod specimens. Non-dinosaurian organisms included ray-finned fishes, frogs, salamanders, turtles, lizards, terrestrial and aquatic crocodylomorphs, and several species of pterosaurs. Fossilized flora of the Morrison Formation includes green algae, fungi, mosses, horsetails, cycads, ginkgoes, and conifers. The Morrison Formation was a wide tract of territory with sub-environments that included river-lined forests of tree ferns and ferns (called ‘gallery forests’); swampy ‘backwaters’; lakeside ‘resorts’; and fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.

The bulky, well-muscled Allosaurus was the top predator in its environment: it grew between thirty and forty feet in length and weighed between 1200 to 1500 pounds. It had a barrel-chested ribcage, giving it a bulkier appearance in contrast to other theropods such as its smaller contemporary Ceratosaurus. It had a wishbone. Its forearm was shorter than its upper arm, but its arms were powerful and capped with three fingers per hand, each finger equipped with ten-inch long strongly curved, pointed claws. Its legs were built for running, though not to the same extent of the later tyrannosaurs; Allosaurus’ top speed has been estimated between twenty and thirty-four miles per hour.  Its feet claws were less developed, too, and more hoof-like than those of earlier theropods. Each foot had three weight-bearing toes and an inner dewclaw, which some scientists have speculated was used for grasping in juveniles. It also had a splint-like remnant of a fifth toe that was perhaps used as a lever between the Achilles tendon and foot. 

Its massive skull attached to a short, S-shaped neck. Allosaurus had a pair of horns above and in front of the eyes; these horns were composed of extensions of the lacrimal bones, varied in shape and size among specimens, and would’ve been covered in a keratinous sheath in life. There were also lower paired ridges running along the top edges of the nasal bones that led to the horns. The function of the horns has been debated: they may have been used for display, for interspecies combat between rival males fighting for territory or females, for species recognition, or even as sunshades for the eyes! They would’ve been too weak and fragile to be used in any serious combat, however. There’s evidence for salt glands in the lacrimal bones. The eyes ‘shaded’ by these horns were placed on the skull in such a way that Allosaurus had binocular vision, though it was limited to twenty degrees. This meant that Allosaurus needed to keep it prey directly in front of it, because if the prey turned sharply, the prey could break from the hunter’s cone of vision. When this happened, Allosaurus could see the prey but only with one eye, and it would lack depth perception which could lead to a failed strike. Particularly agile prey may have been able to dodge Allosaurus’ bites by keeping itself out of its narrow binocular vision. Studies of Allosaurus’ braincase indicate its brain was similar to those found in crocodiles. Allosaurus had large olfactory bulbs, but the area associated with assessing smells was undeveloped. Scientists believe this meant that Allosaurus had a great sense of smell but the ability only to recognize a few, perhaps those of prey animals, carrion, or other members of its species. The design of its inner ear implies it was best suited to hearing low frequency sounds; the design of its vestibular system – which controls balance – shows that its head was usually held horizontally level to the ground. The braincase roof was thin, perhaps to improve thermoregulation for the brain. 

Allosaurus’ skull was massive and armed with dozens of sharp, serrated teeth up to four inches long. Each premaxilla of the tip of the snout held five teeth; each maxilla in the upper jaw had between fourteen and seventeen teeth. Each tooth-bearing cone of the lower jaw had between fourteen and seventeen teeth, as well. The teeth became shorter, narrower, and more backwards-curved toward the back of the skull. All of Allosaurus’ teeth had saw-like edges. Its teeth were shed easily and were replaced continually, making them common fossils in the Morrison Formation. The skull and lower jaws had points that permitted motion; in the lower jaws, the bones of the front and back halves loosely articulated, enabling the jaws to bow outward and increasing Allosaurus’ gape. Allosaurus’ hunting methods and preferred prey may have changed with age. Juvenile specimens had proportionally longer legs than adults; they would be ‘quick on their feet,’ perhaps the speediest dinosaurs of the Morrison Formation. They would’ve likely preyed on smaller animals, such as early mammals and ornithopods. As Allosaurus matured, it needed more sustenance – and thus it turned to the larger dinosaurs. As Allosaurus grew, its legs became shorter and more robust, which made them better able to handle the stresses in tackling large prey. For such large prey, high speed wouldn’t be needed, and longer legs would’ve made Allosaurus more susceptible to injury. An adult Allosaurus, at the top of the ‘feeding pyramid,’ would’ve been the terror of ornithopods, stegosaurs, and even sauropods.

There’s plenty of evidence that Allosaurus tackled stegosaurs; we have evidence of an Allosaurus tail vertebra with partially healed puncture wounds that fit the spikes of a Stegosaurus thagomizer, and the neck plate of one Stegosaurus specimen has a U-shaped wound that correlates well with an Allosaurus snout. Many sauropod bones have scrapings that fit Allosaurus teeth, and we even have shed Allosaur teeth present with many sauropod bones. Does this mean, then, that Allosaurus could take down fully-crown sauropods? Many scientists are doubtful. In comparison to an adult Brachiosaurus, for instance, Allosaurus wouldn’t be much of a threat. Perhaps the evidence of Allosaurus feeding on sauropods is due to scavenging; a sauropod carcass could be a windfall for a predator. Another theory is that Allosaurus did indeed hunt full-grown sauropods, and it was able to do this because it was a pack hunter (we will examine that concept in a moment). If Allosaurus hunted alone, it may have preyed upon sick or young sauropods, but attacking a healthy, full-grown adult would’ve been tantamount to foolishness. Because ornithopods were common in the Morrison Formation, they were likely a staple in the Allosaurus diet. 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. Allosaurus’ forelimbs were strong and capable of restraining prey with the three-fingered claws, so this hunting method seems likely.

The precise methodology of an ‘Allosaurus attack’ has been the subject of fierce debate. The renowned paleontologist Robert Bakker compared Allosaurus to Cenozoic saber-toothed carnivores and noted similar adaptations, such as a reduction in jaw muscles and increase in neck muscles, and the ability to open the jaws wide. Bakker suggested that in the absence of ‘saber teeth,’ Allosaurus’ short teeth became small serrations on a saw-like cutting edge that ran the length of the upper jaw. Allosaurus would use the length of its jaws to execute slashing attacks against much larger prey in the hope of weakening the prey by pain, hewn muscle, and blood loss. Other paleontologists have suggested that Allosaurus used its skull like a machete against prey: it would attack open-mouthed, slashing flesh with its teeth, and tearing the flesh away without splintering bone. The strength of the skull would’ve helped Allosaurus withstand the impact forces associated with repeated attacks against prey. Building upon this theory, some scientists have suggested that Allosaurus would attack large prey such as full-grown sauropods but without the intent of actually killing them; in this scenario, the fleet-footed Allosaurus would lunge up, take a bite of flesh out of the Brachiosaurus haunch, and then retreat to feed in peace (leaving the wounded Brachiosaurus moaning with a chunk of flesh missing). Such a strategy would enable Allosaurus to fill its stomach while also preserving the prey to be fed upon another day. Once Allosaurus brought down prey – whether a juvenile sauropod, a Stegosaurus, or a hapless ornithopod – how did it eat? A 2013 biomechanical study showed that Allosaurus could make rapid, forceful vertical movements with its skull. This suggested that Allosaurus ate by vertical movements in a manner akin to that of modern falcons: Allosaurus could have gripped the prey with its skull and feet, then jerked back to tear flesh from bones. This method of eating was different than that envisioned for tyrannosaurs, which probably tore flesh with side-to-side shakes of the skull, similar to the prey-handling of crocodiles.  

were allosaurs family-friendly pack hunters?

Many scientists believe that Allosaurus worked in family groups or packs to take down large prey. This concept was promoted during the 1970s and has become a staple of theropod studies. Robert Bakker extended social behavior in hunting to parental care; he interprets shed allosaur teeth and chewed bones of large prey animals as evidence that adult allosaurs brought food to lairs for their young to eat until they were grown. Evidence for ‘pack hunting’ – or at least gregarious behavior among Allosaurus – comes from multiple bone-beds that include multiple Allosaurus specimens in varying stages of growth. However, another explanation for these ‘family’ bone-beds is that they are fossilized scenes of territorial infighting or even cannibalism run amuck. That allosaurs didn’t always get along is evident, as we have interspecies injuries recorded in allosaur pathologies, such as allosaur bite wounds to Allosaurus skulls. Head-biting may have been a quick way to establish dominance in a pack or to settle territorial disputes among rogue allosaurs. There’s significant evidence of cannibalism among Allosaurus: these include allosaur teeth among allosaur rib fragments, allosaur tooth markings on allosaur bones, and even fully-cannibalized allosaur skeletons among the bones at Bakker’s ‘lair sites.’ The large bone-beds of ‘family groupings’ may not represent family groups but feeding sites. Most carnivores are territorial and will kill or cannibalize intruders of the same species; young organisms of the same species are often killed by older organisms when they become too bold. The accumulation of multiple Allosaurus remains may be due to individual allosaurs gathering at the same site not due to pack hunting but to feed on other disabled or dead allosaurs; in the ‘feeding frenzy’ other allosaurs were killed, as well – and in a domino effect, the situation gets worse and worse as more bodies start piling up. This could explain the high proportion of juvenile and sub-adult allosaurs present, since they are disproportionally killed at modern group feeding sites of modern predators like crocodiles and Komodo dragons. This may be what was taking place at Bakker’s ‘lair sites’ (in which case, Bakker’s ‘paternal’ allosaurs become frenzied baby-killers). Whether or not Allosaurus was a family-friendly pack hunter (as Bakker thinks) or a bloodthirsty killer with no qualms about killing members of its own kind (as others think) is still a matter of debate. 

or were they loners who had a tendency to fight and kill one another? jury's out!