Allosaurus fragilis

Overview

Allosaurus was the king of the Jurassic—the dominant predator of North America long before T. rex existed! Living approximately 155-145 million years ago during the Late Jurassic, this powerful hunter was the most common large theropod of its time and is one of the best-studied dinosaurs thanks to numerous fossil discoveries.

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Taxonomy & Classification

• Clade: Theropoda
• Family: Allosauridae
• Diet: Carnivorous
• Locomotion: Bipedal

Allosaurus was distantly related to later giants like Carcharodontosaurus and Giganotosaurus.

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Physical Characteristics

Size & Build

• Length: 8-10 meters (26-33 feet)
• Height: About 3 meters (10 feet) at the hip
• Weight: 1,500-2,000 kg (1.7-2.2 tons)
• Skull Length: About 90 cm (3 feet)

Built for Killing

• Large skull with powerful jaws
• Serrated, blade-like teeth for slicing flesh
• Strong arms with three-fingered hands
• Large claws for gripping prey
• Bony crests above the eyes (distinctive horns)

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The Hatchet Attack

Unique Hunting Style

Allosaurus had an unusual attack method:

• Jaws opened very wide (gape up to 79 degrees!)
• Struck downward like a hatchet
• Teeth sliced through flesh in powerful bites
• Didn't crush bones like T. rex—sliced instead
• Could wound prey and wait for blood loss

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Apex Predator

What It Hunted

Allosaurus attacked some of the biggest dinosaurs:

• Stegosaurus (the plated dinosaur)
• Diplodocus and Apatosaurus (long-necked giants)
• Camptosaurus (medium-sized plant-eater)
• Young sauropods
• Possibly hunted in groups for big prey!

Evidence of Battle

• Fossil Stegosaurus shows Allosaurus bite marks
• One Allosaurus has a puncture from a Stegosaurus tail spike!
• Both predator and prey fought fiercely

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The Morrison Formation

Fossil Bonanza

Allosaurus is incredibly well-known:

• Thousands of bones discovered
• Most from the Morrison Formation (western USA)
• Multiple complete skeletons
• Found in Colorado, Utah, Wyoming, Montana
• One of the best-documented theropods

Famous Sites

• Cleveland-Lloyd Quarry in Utah has 40+ Allosaurus!
• Scientists don't know why so many died there
• Possibly a predator trap—attracted by dying prey

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Pack Hunter?

The Debate

Did Allosaurus hunt in groups?

• For: Multiple specimens found together
• For: Could take down giant sauropods easier
• Against: May have just been attracted to the same carcass
• Verdict: Still debated, but possible!

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Allosaurus vs. T. rex

Feature	Allosaurus	T. rex
Length	8-10m	12m
Weight	2 tons	8 tons
Arms	Strong, 3 claws	Tiny, 2 claws
Bite	Slicing	Crushing
Era	Late Jurassic	Late Cretaceous
Gap	Lived 80 million years BEFORE T. rex!

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"Big Al"

Famous Individual

One famous Allosaurus is called "Big Al":

• Found in Wyoming in 1991
• 95% complete skeleton!
• Shows 19 injuries from its life
• Featured in BBC's "Ballad of Big Al"
• Died young—only about 8 years old

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Cool Facts

• Allosaurus is the state fossil of Utah
• Its name means "different lizard"
• Could open its mouth wider than almost any other theropod
• Lived alongside Stegosaurus, Diplodocus, and Brachiosaurus
• One Allosaurus was killed by a Stegosaurus tail spike through its pelvis!
• Allosaurus was the most common predator of its time
• Scientists have found specimens ranging from babies to adults
• It probably had good eyesight and smell

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Allosaurus was the T. rex of its day—the undisputed ruler of the Late Jurassic, a hatchet-jawed hunter that struck fear into even the largest dinosaurs!