Allosaurus Died From Stegosaur Spike to the Crotch, Wyoming Fossil Shows

Analysis of an Allosaurus fossil from southern Wyoming shows that the giant meat-eater died from a painful wound that appears to be unique in the fossil record: a deep stab to the crotch, delivered by the spiky tail of a stegosaur.

The injury is “emphatic and unambiguous” evidence of a fatal encounter between Allosaurus and the stocky herbivorous Stegosaurus, said paleontologist Robert Bakker, who made the find with his colleagues while studying the 150-million-year-old allosaur on display in a Wyoming museum.

Close inspection revealed that the meat-eater had suffered a “terrible wound” directly to the pubis, the distinctive boot-shaped bones that form the lower-front portion of the pelvis.

The deep, cone-shaped wound completely penetrated the solid pubic bone — a sign, Bakker said, of a powerful blow.

Bacteria and bone fragments from the attack appeared to have lingered in the wound and created what would become a fatal infection, he added.

“A massive infection ate away a baseball-sized sector of the bone,” said Bakker, of the Houston Museum of Natural Science, in a press statement.

Allosaurus pubis bone fossil
The fossil pubis bone of an Allosaurus shows the large wound (the hole on the right) thought to have been made by a spike from a stegosaur’s tail. At left is a plaster cast of the abscess that formed inside the bone, which likely proved fatal for the predator. (Courtesy Robert Bakker)

“Probably this infection spread upwards into the soft tissue attached here, the thigh muscles and adjacent intestines and reproductive organs.”

Such deadly infections are common in cases of gorings, like those by modern horned animals such as cattle or rhinos, he added.

Bakker and his team pinned the injury on a stegosaur in part because the wound matches the shape and dimensions of spikes found on the barbed tails of Stegosaurus specimens excavated from the same stratum as the victim.

What’s more, several allosaurs from other deposits have been found with obvious stegosaur wounds, although those injuries had usually healed over.

The scientists also noted that the strike was delivered with great force from below, an angle that could have been exploited most easily by Stegosaurus, which brandished its spiky tail not by swinging it from side to side, they say, but by jabbing it like a sword.

While “most dinosaur tails get stiffer towards the end,” Bakker noted, Stegosaurus benefited from a skeletal structure that actually resembles the prehensile tails of modern animals.

“The joints of a stegosaur tail look like a monkey’s tail,” Bakker said. “They have no locking joints.”

“They were built for 3-dimensional combat.”

This arrangement “offered unusually fine control for strong blows of the four-spike weapon” and allowed Stegosaurus to escape what was very likely a predatory attack.

Indeed, the analysis of the Wyoming allosaur is only the latest evidence of direct conflict among carnivorous dinosaurs and their herbivorous contemporaries, the team noted.

(Read about a famous case of dinosaur violence preserved in bone: “T. Rex Tooth Found in Dinosaur’s Tail Proves Tyrannosaurus Was a Predator, Study Says”)

An artist’s rendering depicts a defensive strike from a stegosaur that paleontologists believe caused the fatal injury. “Fossils of contemporaneous stegosaurs appear to have had unusually flexible and agile tails,” Dr. Bakker said. (Courtesy Robert Bakker)

While some prominent paleontologists have suggested that the ancient carnivores only scavenged for their food, rather than hunted it, the allosaur’s painful demise is likely proof that the meat-eater was a predator — if not always a successful one.

“Wounds inflicted by an herbivore on a carnivore would be strong evidence of active encounters,” the team writes, in a poster presentation of their findings.

“The stegosaur tail architecture persuades us that the allosaur was, indeed, the victim of a precise and powerful tail strike that penetrated through the lower pubis and caused the death of the carnivore.

“The hypothesis of a scavenging-only mode of life for allosaurs, and by extension other large carnivorous dinosaurs, is challenged by this specimen.”

(Read about another recent discovery of an ancient attack: “Tooth Found in Fossil Leg Bone Is ‘First Evidence’ of Clash Between Ancient Apex Predators“)

Bakker and his colleagues presented their findings at the meeting of the Geological Society of America in Vancouver, B.C.

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  1. Stuart McCrary

    The Allosaurus is mounted at the Glenrock Paleontological Museum in Glenrock, Wyoming

  2. Koward

    The artist’s rendition has a very probable flaw. Reptiles, at least, do not have the cloaca located on their tail but anterior to it on the torso. FWIW