This picture shows a small dromaeosaur in side view. The forelimbs are wrapped around the prey, preventing escape as the dinosaur dispatches its victim with its teeth.
Credit: Nate Carroll
The giant killer claws of dinosaurs such as Velociraptor might have been employed much as birds of prey use similar talons — as hooks to keep victims from escaping, researchers say.
The discovery could also shed light on the origin of flight in birds, investigators added.
The raptor dinosaurs, made famous by the book and film "Jurassic Park" all possessed unusually large, curved talons on the second toes of each foot, which they held off the ground like folded switchblades. Known more formally as dromaeosaurids, they included the famous Velociraptor and its larger relative Deinonychus, and were closely related to birds.
Past studies had proposed that the sickle claws of these raptors were used to slash at prey or to help climb onto victims. Now research into modern-day birds of prey suggests a new possible killing technique — as hooks to lock onto targets.
The second toe
Scientists noted that modern hawks and eagles possess similar enlarged claws on their second toes — the "digit twos" or "D-2s." These claws "are used as anchors, latching into the prey, preventing their escape," said researcher Denver Fowler, a paleobiologist at the Museum of the Rockies in Bozeman, Mont. "We interpret the sickle claw of dromaeosaurids as having evolved to do the same thing — latching in and holding on." [In Photos: Birds of Prey]
"This strategy is only really needed for prey that are about the same size as the predator — large enough that they might struggle and escape from the feet," Fowler said. "Smaller prey are just squeezed to death, but with large prey all the predator can do is hold on and stop it from escaping, then basically just eat it alive."
"Dromaeosaurs lack any obvious adaptations for dispatching their victims, so just like hawks and eagles, they probably ate their prey alive too," Fowler said.
Other features of the feet of these dinosaurs suggest they followed what Fowler and his colleagues call "Raptor Prey Restraint" — RPR, or "ripper." For instance, the toe proportions of raptors seem more suited for grasping than running, and the metatarsus — which includes the bones between the ankles and the toes — is more adapted for strength than speed.
"Unlike humans, most dinosaurs and birds only walk on their toes, so the metatarsus forms part of the leg itself," Fowler said. "A long metatarsus lets you take bigger strides to run faster, but in dromaeosaurids, the metatarsus is very short."
All in all, Velociraptor and its kin do not seem adapted to simply running after prey.
"When we look at modern birds of prey, a relatively short metatarsus is one feature that gives the bird additional strength in its feet," Fowler said. "Velociraptor and Deinonychus also have a very short, stout metatarsus, suggesting that they had great strength but wouldn't have been very fast runners."
Such behavior intriguingly contrasts with that of their closest known relatives, a very similar group of small carnivorous dinosaurs called troodontids.
"Troodontids and dromaeosaurids started out looking very similar, but over about 60 million years, they evolved in opposite directions, adapting to different niches," said Fowler. "Dromaeosaurids evolved towards stronger, slower feet, suggesting a stealthy ambush predatory strategy, adapted for relatively large prey. By contrast, troodontids evolved a longer metatarsus for speed and a more precise, but weaker grip, suggesting they were swift but probably took relatively smaller prey."
Evolution of flapping
These findings could shed light on the evolution of flight in birds, researchers said. Such feet could have led to the evolution of flapping.
"When a modern hawk has latched its enlarged claws into its prey, it can no longer use the feet for stabilization and positioning," Fowler said. "Instead, the predator flaps its wings so that the prey stays underneath its feet, where it can be pinned down by the predator's body weight. The predator's flapping just maintains its position, and does not need to be as powerful or vigorous as full flight would require. Get on top, stay on top — it's not trying to fly away."
In much the same way, raptor dinosaurs might have flapped their feathery limbs to keep stable. [Photos of Fossil Feathers]
"We see fully formed wings in exquisitely preserved dromaeosaurid fossils, and from biomechanical studies we can show that they were also able to perform a rudimentary flapping stroke," Fowler said. "Most researchers think that they weren't powerful enough to fly — we propose that the less demanding stability-flapping would be a viable use for such a wing, and this behavior would be consistent with the unusual adaptations of the feet."
"There's an old question on the evolution of flight — 'what use is half a wing?'" Fowler told LiveScience. "I think we have provided the most complete and defensible answer."
Eventually, grasping feet could have become perching feet.
"A grasping foot is present in the closest relatives of birds, but also in the earliest birds like Archaeopteryx," Fowler said. "We suggest that this originally evolved for predation, but would also have been available for use in perching. This is what we call 'exaptation' — a structure evolved originally for one purpose that can later be appropriated for a different use."
This research could help explain the anatomy of some recently described peculiar dinosaurs. For instance, the dromaeosaur Balaur, recently unearthed in Romania, "has a very short fused-up metatarsus and a seemingly enlarged claw, not only on the second toe, as in other dromaeosaurs, but the first toe as well," Fowler said. "This is pretty strange, but based on our model, this makes sense."
The short fused-up metatarsus can be seen as an extreme form of the short broad metarsus of other dromaeosaurids, while the enlarged claws on the first toes might serve the same anchoring function as the ones on the second toes do. "Balaur looks like it was a super-dromaeosaur, with the predatory features of normal dromaesaurs taken to extreme measures," Fowler said.
The researchers believe their ideas will open new lines of investigation into dinosaur biology. New ways of looking at old anatomical structures could help solve the mystery of why the features evolved and how they were used.
"Just as you have to get beyond the idea that feet are used just for walking, so we are coming to realize that many unusual structures in modern animals originally evolved for quite different purposes," Fowler said.
The scientists detailed their findings onlineDec. 14 in the journal PLoS ONE.