Distinctive bite marks on the skulls of cat-like saber-toothed predators that once skulked about North America have revealed a nasty family secret: these felines often ambushed and killed each other.
The discovery came as a result of the accidental unearthing of a new skull of what's called a nimravid -- not a true cat, but a group of cougar-like animals with large saber-like canine teeth that lived from 32 to 34 million years ago. The skull had clear signs of being mortally bitten by another nimravid.
"The nimravid skull was found in 2010 in Badlands National Park by a girl during a Junior Ranger activity right next to the visitor center," said paleontologist Clint Boyd of the South Dakota School of Mines and Technology, who was working in the park at the time. "It had a magnificent set of bite marks on it."
The skull brought to mind another found in 1936 that also had nimravid bite marks which had long been interpreted as a rare case. But the new skull raised the question of just how common these bite marks are on nimravids.
To find out, Boyd and his colleagues gathered up as many nimravids skulls as they could from collections and took a closer look at them. This included some that were on display for the public.
"Some of the best specimens with bite marks were right in front of people," he said. "Older specimens did not show the bite marks until they were cleaned up." Some actually still had dirt in the holes made by the bite marks and others had had the holes repaired by curators unaware of their significance.
"What we found is that these bite marks are a lot more common than previously thought."
In fact the bite marks make it clear that the nimravids were attacking their competitors from behind and killing by getting one fang into an eye socket or puncturing the skull.
What's even more startling is that nimravid fang marks are not found on the skull of any of their prey, said Boyd, who is presenting his results on Oct. 30 at the meeting of the Geological Society of America in Denver. That's because they used the canines to tear out the soft tissues in the throats of their prey and would have been careful not to bang them on bone, which might have damaged their most important hunting weapon.
"Damaging their canines could be a life-threatening event," said Boyd. Yet fatal nimravid bite marks are found on a surprising 10 percent of nimravid skulls in three species of nimravids over a range of four million years.
"They're still taking into consideration not damaging their canines," said Boyd, noting how the eyes are a common target with the other canine just glancing the skull. But they are definitely taking a bigger chance when they attack their own kind.
Among other things, the discovery suggests that the typical museum mural representation of nimravids facing off in battle is probably dead wrong.
"Upper canines and lower canines can be seen in the (skulls)," he said. "So all the attacks are coming from behind. This was an ambush style attack against a competitor."
The lack of any signs of healing also means that the majority of these attacks were fatal, which rules out another old hypothesis, based on the 1936 specimen (which showed some healing), that the biting might be part of a mating behavior.
"It's very hard to get behavior from fossils," said Kurt Spearing, a researcher at Morningside College in Sioux City, Iowa, who works on fossil cats and their close relatives and was not directly involved in Boyd's work.
But in this case, he agrees that the behavior of nimravids is remarkably clear: "These guys were incredibly aggressive towards each other."