Wrestling ninjas – why sabre-toothed predators have massive arms

A cat-like animal explodes from the long grass and leaps onto an antelope. Its huge bulk drags the target to the ground and its muscled forelegs pin it down. With two long sabre-shaped canine teeth, it stabs its victim in the throat, just the once, severing its blood vessels and windpipe. Death comes quickly. The hunter could be Smilodon, a sabre-toothed cat that lived throughout North and South America, around one or two million years ago. Or, it could be a nimravid, another group of hunters that looked like cats, but belonged to a separate, closely-related family. Some of them had sabre-teeth too, and they wielded these weapons between 42 and 7 million years ago, well before Smilodon or its relatives did. Or, it could be Barbourofelis, a member of yet another group of sabre-toothed not-quite-cats, which lived between 16 and 9 million years ago. Its long sabres slipped into long grooves in its lower jaw, which looked like it was about to melt away. Julie Meachen-Samuels from the National Evolutionary Synthesis Center has shown that all three groups shared the same adaptations: massive powerful front legs for pinning their prey, and strikingly large canines for delivering precise one-hit kills. And all of them gained this combination independently. Time and again, evolution has equipped cat-like predators with the combined skills of the wrestler and the ninja. Back in 2010, Meachen-Samuels placed Smilodon fossils in an X-ray machine, and showed that it has extra-thick, reinforced bone in its upper arms, with large attachment points for its large muscles. It was a particularly butch cat. The study confirmed the idea that Smilodon used its huge teeth in a very different way to its modern relatives. Living cats use their jaws to close the throat or nose of their prey, choking them to death. Their conical canines are well-suited to the task, able to withstand forces in all directions. Sabre-teeth, while they look formidable, were actually quite fragile. They were long and flattened, rather than short and conical. If the cat’s prey struggled, its teeth would have shattered. If the teeth hit bone during a bite, they would have shattered. So Smilodon used the sabres like an assassin’s dagger rather than a swordsman’s blade, dispatching victims with quick stabs. Its big arms helped it to restrain its prey for the killing blow. Meachen-Samuels wondered if other sabre-toothed predators shared the same adaptations. She compared the skeletons of 15 species of living cats with fossils from 8 species of sabre-toothed ones, 5 nimravids and 1 barbourofelid. Across the groups, Meachen-Samuels found that the species with the most exaggerated canines also had the most robust front legs and the widest paws. Based on the dimensions of these limbs, you could predict whether one of these animals had sabre teeth with almost perfect accuracy. The sabre-teeth were clearly efficient weapons – after all, three different groups of cat-like hunters evolved them independently. But they came with a price. Meachen-Samuels says, “The teeth became so long and fragile that they needed some way to restrain their prey before they bit them. The most successful predators not only had big teeth, but had some way to protect them.” She describes the sabre-tooths as “hyper-ambush” hunters. Most modern cats are considered to be ambush predators, but they still chase their prey over short distances. “We think sabre-toothed predators like Smilodon probably chased prey even less. Beefier forelimbs would make it harder to run,” says Meachen-Samuels. “They would just hide and wait for prey to pass by and then leap out at it from cover, possibly knocking it off balance and attacking.” The nimravids could have launched their attacks from the trees, for their skeletons suggest that they were good climbers. Among modern cats, the clouded leopard also attacks from trees, has beefy front legs, and has unusually long canines – it’s the closest we still have to living sabre-tooth. (It’s less likely that Smilodon or barbourofelids were tree-based hunters for they were too bulky and their tails were too short.) History has seen a variety of other sabre-toothed animals that were very distantly related from the cats, and it would be fascinating to know if these other hunters also had beefy arms. Thylacosmilus seemed to – it looked remarkably similar to sabre-toothed cats, but it was more closely related to marsupials like kangaroos and koalas. What about the gorgonopsids? These were some of the largest predators of the late Permian era, around 260 million years ago, before the time of the earliest dinosaurs. Reference: Meachen-Samuels. 2011. Morphological convergence of the prey-killing arsenal of sabertooth predators. Paleobiology Ref: tbc Images by Ryan Somma, Rama and Dallas KrentzelMore on sabre-toothed cats

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