Allosaurus fragilis AL-oh-SOR-us

"Different lizard"

Allosaurus silhouette — silhouette · Will Toosey (CC-BY) via PhyloPic

Length: 9 m (29.5 ft) — About 9 m long, ~2 tonnes — the lion of the Late Jurassic.
Period: Late Jurassic (155–145 Mya)
Place: North America · Western United States · Portugal
Food: Carnivore — Large prey — Stegosaurus, Camptosaurus, possibly young sauropods.
Clade: Allosauroidea Dinosauria Saurischia Theropoda Big Jurassic-Cretaceous predators: Allosaurus, Carcharodontosaurus.

The Morrison Formation's most abundant large predator, named for its oddly lightweight vertebrae.

Allosaurus reconstruction by Ddinodan — Reconstruction · Ddinodan (CC-BY)

Allosaurus was the top predator of the Late Jurassic. More than 60 individuals have been found at the Cleveland-Lloyd Quarry in Utah, making it the best-known large theropod by sheer number of bones. Allosaurus skulls had brow horns above the eyes and could open very wide — paleontologists nicknamed its bite the ‘hatchet jaw’ because it may have swung its head downward like an axe.

More about Allosaurus

Discovery

Named by Othniel Charles Marsh · 1877 · Felch Quarry, in the Garden Park area of Colorado

Marsh named Allosaurus fragilis in 1877 based on YPM 1930, a small collection of fragmentary bones including parts of three vertebrae, a rib fragment, a tooth, a toe bone, and the shaft of the right upper arm bone. The bones were uncovered by two of his collectors, Benjamin Mudge and Samuel W. Williston, in the autumn of 1877.

"Allosaurus" means "different lizard", from the Greek words for "strange" or "different" and "lizard" or "reptile". Marsh chose it because he believed the vertebrae were different from those of other dinosaurs due to their lightweight construction. The species name fragilis is Latin for "fragile", again referring to those lightening features in the vertebrae.

Portrait of Othniel Charles Marsh — Othniel Charles Marsh An American paleontologist who named Allosaurus during the Bone Wars, his rivalry with Edward Drinker Cope that drove a surge of fossil discoveries in the Western US.

Did Allosaurus hunt in packs or alone?

Scientists have debated whether Allosaurus had cooperative social behavior and hunted in packs, or was a solitary predator that merely formed congregations. The text notes there is evidence supporting both arguments, so the question remains unsettled.

For the curious → Why was its taxonomy such a tangle?

Marsh and Cope coined many names during the Bone Wars and gave terse descriptions, which complicated later research. As early as 1901 Samuel W. Williston noted that Marsh had never adequately distinguished Allosaurus from Creosaurus, and the genus passed through the name Antrodemus before Allosaurus was restored.

What we know

Over 60 individuals known from the Cleveland-Lloyd Quarry alone.
Two short brow horns above the eyes — likely display structures.
Strong neck muscles allowed a downward axe-like bite, unlike T. rex's crushing bite.
Lived alongside Stegosaurus, Apatosaurus, Diplodocus, Brachiosaurus.

What we guess

Whether Cleveland-Lloyd is a predator trap, a die-off site, or a pack burial.
How big the bite force was — much lower than T. rex, but a different killing style.
Whether brow horns were for ramming, display, or shading the eyes.