Fossils preserving a prehistoric casualty reveal that small-brained, plant-eating stegosaurs were deadly fighters capable of killing carnivores in spike-to-tooth combat. Even predatory allosaurs with their enormous jaws filled with long, serrated, back-curving teeth can fall victim to the stegosaurian martial arts.
Researchers working in late Jurassic sediments in the Brushy Basin Member of the Morrison Formation in Albany County, Wyoming, unearthed the pubis bone of an allosaur that showed evidence of a fatal stab wound shaped exactly like the conical tail spike of stegosaurs that have previously been dug up in this same layer.
The stego tail spike passed all the way through the allosaur’s bone, causing an abscess. “A massive infection ate away a baseball-sized sector of the bone,” Robert Bakker from the Houston Museum of Natural Science says in a news release. The infection likely spread upwards into the attached soft tissue, the thigh muscles, and maybe even the intestines and reproductive organs nearby.
Because their tail spikes point outward and backward, for a stegosaur to strike a vertical blow that would inflict a wound like this, it would have had to sweep its tail under the allosaur and then twist it up through the lower pelvis.
Wounds like these have been seen in rodeo cowboys and horses when they’re gored by longhorn cattle. Herbivores like rhinos and buffalo also defend themselves against predators with horns on their heads. Stegos were similarly well-armed, except they wielded their spiky weapons on their tails. Since the team didn’t find any signs of healing, they think the allosaur fell victim to herbivore defense.
According to previous work, stego tails are more dexterous that most other dino tails, which get stiffer towards the end. Stegosaurs, on the other hand, had massive muscles at the base of the tails, and they boasted flexibility and fine muscle control all the way to the tip. They also had no locking joints, and their functional accessory joints (called zygapophyses) continued all the way down the tail as well. “The joints of a stegosaur tail look like a monkey's tail,” Bakker says. All these tail specializations provided power and precision in the direction of the spike trajectory. “They were built for three-dimensional combat,” he adds.
The team presented their findings at the meeting of the Geological Society of America in Vancouver this week.
Images: Robert Bakker