Between about 157 and 145 million years ago, Allosaurus — a large predatory dinosaur — stalked North America and Europe.
The fossil record suggests the beast was rather common. And did we mention it was big? A full grown Allosaurus could grow to be 34 feet (10.4 meters) long, 9 feet (2.8 meters) tall at the hip and around 3.2 tons (2.9 metric tons) in weight.
Yet even giant carnivores take their lumps from time to time. Ladies and gents, meet "Big Al."
A subadult Allosaurus with a killer nickname, "Big Al" lived in what's now north-central Wyoming. Scientists would eventually recover 95 percent of this flesh-eating dinosaur's skeleton.
Look closely at his (or maybe her?) remains and you'll find no fewer than 19 separate bone fractures. Somehow, Al sustained injuries to multiple backbones, toe bones and ribs. There's also evidence of a serious infection on the right foot.
If you liked the original, you'll love the sequel. "Big Al" was unearthed at a Wyoming quarry in 1991. Five years later, fossil hunters working in the same state found another Allosaurus skeleton that's come to be known as "Big Al 2."
Just like its predecessor, this specimen was pretty banged up. Many of its bones had been fractured or otherwise damaged — only to be re-healed during the dinosaur's lifetime. (Although one hip injury apparently never healed over and may have been implicated in the demise of Big Al 2.)
Other Allosaurus fossils also bear the tell-tale signs of serious wounds. To fully appreciate these, we need to take a step back and consider the animal's role in its environment.
Snapshot of the Jurassic
Allosaurus lived during the Late Jurassic Period. Back then, giant herbivorous dinosaurs called "sauropods" thundered across the planet. We often find their bones in close association with Allosaurus material.
At Dinosaur National Monument, which straddles the Utah-Colorado border, there's a protected quarry where visitors can look at a jumbled collection of fossils that've been lying together for the past 149 million years.
Besides Allosaurus remains, this quarry includes the bones of such long-necked sauropods as Diplodocus, Camarasaurus and Apatosaurus. Fossils belonging to the unrelated spiky-tailed plant-eater Stegosaurus are also present.
These were just some of the vegetarian dinos Allosaurus interacted with. And there was competition at the buffet line. Two of its rival predators in Late Jurassic North America were the 39-foot (11.9-meter)- long Torvosaurus and Ceratosaurus, a horn-nosed carnivore that could grow over 19 feet (6 meters) long. The latter had a short cameo in "Jurassic Park III" (2001).
Compared to some other dinosaurs, Allosaurus had a weak bite force. The bite of Tyrannosaurus rex — a famous carnivore that evolved tens of millions of years after the last Allosaurus died out — may have been four times stronger.
Even so, Allosaurus performed well under pressure; mathematical models show the beast's head could withstand lots of physical strain.
In 2013, paleontologist Eric Snively and his colleagues used computer simulations to learn more about how this dinosaur dismembered its prey. According to their research, Allosaurus may have sometimes behaved like an overgrown falcon at dinner.
The animal's skull was light and its neck muscles were peculiar. With their simulations, Snively and company showed Allosaurus would've had an easy time plucking meat off corpses by grabbing a hunk of flesh in its jaws and then yanking its head backwards. Falcons do the same thing today.
But while those hunting birds have flight-ready wings, Allosaurus had clawed hands. Noting their size and range of motion, dino expert Kenneth Carpenter wrote in 2002 that Allosaurus could use its arms to "grasp moderately large prey and pull it towards the body."
When Hunger Strikes
What counted as "moderately large prey" for a ravenous Allosaurus? Maybe beaked herbivores like the 23-foot (7-meter) Camptosaurus fit the bill, or perhaps juvenile sauropods.
If Allosaurus hunted adult sauropods, scientists aren't sure how. Some species alive in the Late Jurassic were around 24 to 34 times heavier than even the biggest Allosaurus, so the predators might not have bothered. A few experts think Allosaurus only targeted young, sick or dead sauropods, leaving healthy grownups alone.
On the other hand, it's possible the dinosaur used its serrated teeth to rip the flesh off of giant, still-living sauropods in nonfatal attacks. There's a pretty awesome name for this hypothetical feeding technique: "flesh grazing." Wicked.
Bite marks tell us Stegosaurus was on the menu. Only sometimes, attacking an armored dinosaur might not have been the brightest idea. One Allosaurus pubic bone shows a gaping wound which matches the size and shape of a Stegosaurus tail spike. That's a roundabout way of saying the carnivore was likely stabbed in the crotch.
Eating Your Own
Here's another grim development: Allosaurus has been suspected of cannibalism.
Stephanie Drumheller, an earth scientist at the University of Tennessee, led a survey, published in the May 2020 issue of the journal Plos One, of 2,368 fossil bones all recovered at the same Jurassic quarry in Colorado. Almost 29 percent bore the bitemarks of meat-eating dinosaurs with serrated teeth.
Many of these gnawed-on bones belonged to sauropods and other herbivores. However, some of them came from Allosaurus.
Since Allosaurus is also the most abundant predatory dinosaur found at this dig site, it's entirely possible that we're looking at evidence of cannibalistic behavior.
Graveyard of Killers
No discussion about Allosaurus would be complete without mentioning Utah's mysterious Cleveland-Lloyd Dinosaur Quarry (CLDQ).
Fossils of 10 different dino species have been excavated here, including plant-eaters like Stegosaurus and Apatosaurus. Yet a remarkable 66 percent of all the dinosaur bones found in the CLDQ were left behind by Allosaurus.
Altogether, the quarry has yielded parts of at least 46 Allosaurus skeletons. Some were juveniles, others were full-grown adults. But they all died in the same spot.
The question is, why? Why are the numbers so skewed? Why is Allosaurus so overrepresented at the CLDQ? Several explanations have been put forth over the years.
Maybe the CLDQ was once a Jurassic predator trap.
Basically, that's a place where herbivores get caught in thick mud (or another substance) and then attract carnivores who die the same way. All those bodies attract even more carnivores, with dead and dying predators vastly outnumbering prey species.
Critics say the rarity of bite marks on the quarry's dinosaur bones is a strike against this hypothesis, though.
It's also possible the CLDQ was the home of a poisoned watering hole — or a normal one that dried up, leaving thirsty dinosaurs to die on its banks. Another proposed scenario blames flooding for the accumulation of skeletons.
Originally Published: Mar 20, 2008