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Sturdy Arms Were Secret to the Saber-Tooth's Success

6 January 2012 3:13 pm
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NPS;AMNH (inset, skull)

Long teeth, big biceps. To prevent struggling prey from damaging their lengthy fangs (inset), saber-toothed predators (such as Smilodon, artist's concept shown) also had sturdy forelimbs to hold victims firm during a kill, a new study suggests.

As a predator, having long, saberlike fangs is an advantage only if you can keep struggling prey from snapping them in half. A new study of fossils suggests that at least three different groups of ancient hunters solved that problem the same way. They evolved sturdy forelimbs that helped them hold prey firm during the kill. And the longer the teeth were, the burlier the bones.

Modern-day cats have fangs, but these teeth are relatively short and have a circular cross section—a profile that enables them to withstand stresses applied from any direction. But several extinct varieties of saber-tooth cats, including the famed Smilodon, whose fossils are found in large numbers in California's La Brea Tar Pits, sported fearsome fangs that had either oval or bladelike cross sections. Teeth with those profiles are much more susceptible to breakage, especially if the thrashing of prey imposes side-to-side forces on the fangs, says Julie Meachen-Samuels, a paleobiologist at the National Evolutionary Synthesis Center in Durham, North Carolina.

Other groups of predatory mammals evolved saberlike teeth as well, including the Nimravidae, also known as false saber-tooth cats, which roamed Eurasia and North America between 42 million and 7 million years ago, and the so-called Barbourofelidae, which lived in Africa, Eurasia, and North America between 20 million and 5 million years ago. The skulls and physiques of these predators were remarkably similar, and the creatures occupied comparable ecological niches, Meachen-Samuels says. Although Nimravidae and Barbourofelidae had a somewhat different posture than modern cats and were slightly more flat-footed, she notes, "if you saw one of these animals from a distance, you'd think it was a cat."

Paleontologists have long debated how saber-toothed creatures killed their prey, but they've generally agreed that holding wriggling victims firm with the forelimbs was critical to ensure that the fangs didn't snap off, Meachen-Samuels says. To determine whether there was a link between fang length and the size and shape of forelimb bones, she analyzed the fossils of eight species of saber-toothed cats, five species of Nimravidae, and one species of Barbourofelidae. Then she performed a similar analysis on 13 species of modern-day cats and the extinct American lion Panthera atrox, all of which had conical fangs.

The results, published online this week in Paleobiology, reveal that the species with the longest fangs tend to have sturdier bones in their forelimbs, which, according to the trends seen in modern cats, indicates greater forelimb strength.

"This makes perfectly good sense," says Virginia Naples, a vertebrate paleontologist at Northern Illinois University in DeKalb. "If you're a predator, the best way to control your prey is to either have hypnotic power or to grab them and hang on."

Manuel Salesa, a paleobiologist at the National Museum of Natural Sciences in Madrid, says that Meachen-Samuels has found "an impressive example of convergent evolution," the process by which similar biological traits develop in unrelated species or groups.

Meachen-Samuels contends that convergent evolution is not only responsible for the link between the sizes and shapes of fangs and forelimbs in Nimravidae, Barbourofelidae, and saber-toothed cats but also produced the similar overall shapes of the bodies and skulls of these predators. "It's nice to see that something that works can evolve several times, again and again."

The new findings also may help explain how saber-toothed creatures could live side by side with their shorter-fanged relatives, a trend previously noted in several regions, says Lars Werdelin, a paleobiologist at the Swedish Museum of Natural History in Stockholm. Even though the two groups may have hunted the same prey, he says, they could have used different hunting techniques and lived in slightly different habitats, which would reduce competition among them.