Deinonychus and Velociraptor used their killing claws to pin prey, like eagles and hawks

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The sickle-shaped “killing claws” of dinosaurs like Deinonychus and Velociraptor have captured the imagination for decades. They were held aloft from the second toe, and were far bigger than the neighbouring claws. In Jurassic Park, Alan Grant tells an annoying child that the dinosaurs used their claws to disembowel their prey with slashing motions. That seems unlikely – they didn’t have a suitable cutting edge. Others have suggested that they were used for climbing onto larger prey.

But neither idea made sense to Denver Fowler from Montana State University, who has put forward a very different idea about how these animals used their infamous claws. He compared the feet of extinct dinosaurs like Deinonychus to those of living dinosaurs like eagles, hawks and other birds of prey. Both groups are known as “raptors” and Fowler thinks that they share more than their nicknames.

In his vision, which he calls the “ripper” model, Deinonychus killed small and medium-sized prey in a similar style to a hawk or eagle dispatching on a rabbit. Deinonychus leapt onto its target and pinned it down with its full body weight. The large sickle-shaped claws dug into its victim, gripping tightly to prevent it from escaping. Then, Deinonychus leant down and tore into it with its jaws. The killer claws were neither knives nor climbing hooks; they were more like anchors.

It’s a simple idea, but a potentially important one, for it casts Deinonychus’s entire body into a new light. Fowler thinks that it flapped its large feathered arms to keep its balance while killing a struggling victim. And its feet, which were adapted for grasping prey, would have given its descendants the right shape for perching on branches. Fowler says, “It really helps to make sense of the weird anatomy of these little carnivorous dinosaurs.”

Fowler’s interest began years ago when someone mentioned to him that some birds of prey have a slightly larger claw on their second toe, just like Deinonychus and its entire family – the dromaeosaurids. He soon got his chance to check that. He was going through local museum collections with colleague John Scanella, when he stumbled across a box full of bird of prey feet.

By analysing his collection, and comparing them to videos of their owners in action, Fowler showed that the shape and size of a bird of prey’s feet provides important clues about how it kills. Some use their talons as precision stabbing weapons; others use them to suffocate their prey; and yet others use them as cages.

When Fowler brought Deinonychus into the mix, he found that its feet most closely resemble those of eagles and hawks (the accipitrids) than to other birds of prey. When he looked at specific features of dromaeosaurid feet, the similarities to birds of prey stood out even further. “We saw the same parallel adaptations in the birds of prey and the fossils,” he says.

Deinonychus has a foot adapted for grasping. Its metatarsus, the bone between the ankle and toes, was surprisingly short and stocky, like that of an owl. It would have given the foot a strong grip at the expense of running speed. The fourth toe could have improved the grip even more by moving round to oppose the first one. And the joints in the foot were very mobile, all the better for resisting the struggles of prey.

This idea of Deinonychus sitting on top of small prey seems at odds with classic picture of this predator working in packs to bring down larger quarry. But again, modern birds show how the same grasping motions might have worked against big targets. Golden eagles can kill reindeer. They dig their talons deep into their victim’s back, holding on while the struggling reindeer widens its own wounds. Deinonychus might have used the same strategy to kill larger prey.

Tom Holtz Jr, who studies predatory dinosaurs, says, “Prey-riding is also common in the Galapagos hawk – there’s classic footage of them taking down marine iguanas much bigger than they are. They pin them down, and flap away as the iguanas take them for a ride.” Philip Currie from the University of Alberta also mentions the famous Mongolian “fighting dinosaurs” – a Velociraptor found in pitched battle with a Protoceratops. “It confirms that dromaeosaurids were seeking prey animals of their own approximate body size.”

Not all dromaeosaurids had a grasping foot. Earlier ones like Sinornithosaurus had a relatively long metatarsus. So did the group’s closest relatives – another lot of small hunters called the troodontids. A long metatarsus would have given them a longer stride and a faster sprint. Fowler thinks that dromaeosaurids and troodontids both evolved from fast-running ancestors. From there, they went down different paths. Troodontids took their fleet-footed adaptations even further, and chased down their prey. Meanwhile, the dromaeosaurids sacrificed speed for grasping strength, and evolved into ambush hunters with strong but slower feet.

“The paper provides a very nice explanation for aspects of dromaeosaurid anatomy that have always bothered me and others,” says Currie. “Deinonychus, Velociraptor and their relatives are often referred to as fast and agile, but their hindlimb proportions and peculiarities of their hips are inconsistent with such an interpretation.”

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Fowler thinks that his ripper model explains other odd features of dromaeosaurids beyond their feet. For example, their jaws weren’t very strong or sturdy, but you don’t need an especially strong bite or robust jaws if your prey is fully restrained by massive talons. (Hawks and eagles too have relatively weak bites for meat-eating birds).

The ripper model could also explain the weird arms of dromaeosaurids. Their hand bones suggest that the claws could exert a lot of force. However, their arms didn’t have a wide range of motion and they bore long flight feathers that might have got in the way. They look like arms that were adapted for flapping rather than for clawing or grasping.

Fowler suggests that they could have used these odd limbs in two ways. First, they could have circled them around their prey, covering it in a cloak of feathers to either hide it from rivals or trap it even further. Modern birds of prey do this – it’s called “mantling”. From within the mantle, the dinosaurs could have used small movements of their claws to pull back prey that had escaped from the feet.

Second, by flapping their arms, dromaeosaurids could have kept their balance while trying to dismember the prey between their legs. Modern raptors do exactly this – they use their wings and tail to get their body weight pressing down on top of their prey, and then to keep themselves there.

If Fowler is right, his model has important implications for the evolution of flight. The dromaeosaurids would have been very nearly ready for life in the trees. Their grasping feet, with opposable toes, could easily have adapted to grip branches as well as prey. Their flapping arms, used to balance themselves, could have adapted to help them fly. These animals were positively pre-adapted for life in the trees. Perhaps the graceful wings and perching feet of a blue tit got their start with bloody murder on the ground.

It’s an intriguing idea, which gets over the thorny question of “What use is half a wing?” Even wings that couldn’t hold dromaeosaurids aloft would have been useful for “stability flapping”. But Fowler admits that there are problems with the idea. “I’m not sure we’ve quite got there yet,” he says. “We can get to the point where you have all this vigorous flapping but taking that to a mode of locomotion – what’s the evolutionary pressure for that? We’re working on developing that part of the model.”

“As many of these things that involve behaviour and fossils, it probably extrapolates somewhat beyond the evidence but it’s hard to do anything with fossils that doesn’t suffer from that,” says Peter Makovicky, who studies raptors (the prehistoric ones). “It’s better tested than some of the other ideas that have been proposed,” he says.

Reference: Fowler, Freedman, Scannella, & Kambic. 2011. The predatory ecology 1 of Deinonychus and the origin of flapping in birds. PLoS ONE.

Images: restoration by Nobu Tomura; feet by Didier Descouens