Allosaurus

Allosaurus (/ˌæləˈsɔːrəs/ AL-o-SAWR-us)[1] is a genus of theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period (Kimmeridgian to late Tithonian ages). The first fossil remains that could definitively be ascribed to this genus were described in 1877 by Othniel C. Marsh. The name "Allosaurus" means "different lizard", alluding to its lightweight vertebrae, which Marsh believed were unique. The genus has a very complicated taxonomy and includes at least three valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains come from North America's Morrison Formation, with material also known from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. It was known for over half of the 20th century as Antrodemus, but a study of the abundant remains from the Cleveland-Lloyd Dinosaur Quarry returned the name "Allosaurus" to prominence. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.

Allosaurus was a large bipedal predator for its time. Its skull was light, robust, and equipped with dozens of sharp, serrated teeth. It averaged 8.5 meters (28 ft) in length for A. fragilis, with the largest specimens estimated as being 9.7 meters (32 ft) long. Relative to the large and powerful legs, its three-fingered hands were small and the body was balanced by a long, muscular tail. It is classified in the family Allosauridae. As the most abundant large predator of the Morrison Formation, Allosaurus was at the top of the food chain and probably preyed on large herbivorous dinosaurs such as ornithopods, stegosaurids, and sauropods. Scientists have debated whether Allosaurus had cooperative social behavior and hunted in packs or was a solitary predator that forms congregations, with evidence supporting both arguments.

Allosaurus was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail, and reduced forelimbs. The average length of A. fragilis, the best-known species, has been estimated at 8.5 m (28 ft) and its average mass at 1.7 t (1.9 short tons), with the largest definitive Allosaurus specimen (AMNH 680) estimated at 9.7 m (32 ft) long,[48] with an estimated weight of 2.3–2.7 t (2.5–3.0 short tons).[48][49] In his 1976 monograph on Allosaurus, James H. Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of 12 to 13 m (39 to 43 ft).[3] As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between 1 and 4 t (1.1 and 4.4 short tons) for modal adult weight (not maximum).[50] John Foster, a specialist on the Morrison Formation, suggests that 1 t (1.1 short tons) is reasonable for large adults of A. fragilis, but that 700 kg (1,500 lb) is a closer estimate for average individuals. Using the subadult specimen nicknamed "Big Al", since assigned to the species Allosaurus jimmadseni, researchers using computer modeling arrived at a best estimate of 1.5 t (1.7 short tons) for the individual, but by varying parameters they found a range from approximately 1.4–2 t (1.5–2.2 short tons). A separate computational project estimated the adaptive optimum body mass in Allosaurus to be 2.3 t (2.5 short tons). A. europaeus has been measured up to 6–8 m (20–26 ft) in length and 1 t (1.1 short tons) in body mass.

The skull of Allosaurus was light and equipped with dozens of sharp, serrated teeth, and both were modestly proportioned for a theropod of its size. Paleontologist Gregory S. Paul gives a length of 845 mm (33.3 in) for a skull belonging to an individual he estimates at 7.9 m (26 ft) long. Each premaxilla (the bones that formed the tip of the snout) held five teeth with D-shaped cross-sections, and each maxilla (the main tooth-bearing bones in the upper jaw) had between 14 and 17 teeth; the number of teeth does not exactly correspond to the size of the bone. Each dentary (the tooth-bearing bone of the lower jaw) had between 14 and 17 teeth, with an average count of 16. The teeth became shorter, narrower, and more curved toward the back of the skull. All of the teeth had saw-like edges. They were shed easily and continually replaced, making them common fossils.

The lacrimal bones of Allosaurus extend above and in front of the eyes to form a pair of horns which varied in shape and size. Leading into the horns, the skull also features a pair of ridges running along the top of the nasal bones. The horns were probably covered in a keratin sheath and may have had a variety of functions, including acting as sunshades for the eyes, being used for display, and being used in combat against other members of the same species (although they were fragile). There was a ridge along the back of the skull roof for muscle attachment, as is also seen in tyrannosaurids.

Inside the lacrimal bones were depressions that may have held glands, such as salt glands. Within the maxillae were sinuses that were better developed than those of more basal theropods such as Ceratosaurus and Marshosaurus; they may have been related to the sense of smell, perhaps holding something like Jacobson's organs. The roof of the braincase was thin, perhaps to improve thermoregulation for the brain. The skull and lower jaws had joints that permitted motion within these units. In the lower jaws, the bones of the front and back halves loosely articulated, permitting the jaws to bow outward and increasing the animal's gape. The braincase and frontals may also have had a joint.

Allosaurus had nine vertebrae in the neck, 14 in the back, and five in the sacrum supporting the hips.[b] The number of tail vertebrae is unknown and varied with individual size; James Madsen estimated about 50, while Gregory S. Paul considered that to be too many and suggested 45 or less. There were hollow spaces in the neck and anterior back vertebrae. Such spaces, which are also found in modern theropods (that is, the birds), are interpreted as having held air sacs used in respiration. The rib cage was broad, giving it a barrel chest, especially in comparison to less derived theropods like Ceratosaurus. Allosaurus had gastralia (belly ribs), but these are not common findings, and they may have ossified poorly. In one published case, the gastralia show evidence of injury during life. A furcula (wishbone) was also present, but has only been recognized since 1996; in some cases furculae were confused with gastralia. The ilium, the main hip bone, was massive, and the pubic bone had a prominent foot that may have been used for both muscle attachment and as a prop for resting the body on the ground. Madsen noted that in about half of the individuals from the Cleveland-Lloyd Dinosaur Quarry, independent of size, the pubes had not fused to each other at their foot ends. He suggested that this was a sexual characteristic, with females lacking fused bones to make egg-laying easier.

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. A 2020 study by Motani and colleagues suggests that Allosaurus was also sexually dimorphic in the width of the femur's head against its length.

Skin impressions from Allosaurus have been described. One impression, from a juvenile specimen, measures 30 square centimeters and is associated with the anterior dorsal ribs/pectoral region. The impression shows small scales measuring 1 to 3 millimeters in diameter. A skin impression from the "Big Al Two" specimen, associated with the base of the tail, measures 20 centimeters by 20 centimeters and shows large scales measuring up to 2 centimeters in diameter. However, it has been noted that these scales are more similar to those of sauropods, and due to the presence of non-theropod remains associated with the tail of "Big Al Two" there is a possibility that this skin impression is not from Allosaurus.

Another Allosaurus fossil features a skin impression from the mandible, showing scales measuring 1 to 2 millimeters in diameter. The same fossil also preserves skin measuring 20 by 20 centimeters from the ventral side of the neck, showing scutate scales measuring 0.5 centimeters wide and 11 centimeters long. A small skin impression from an Allosaurus skull has been reported but never described. Additional undescribed skin impressions are known from various parts of the body in one specimen.