Saturday 25 October, 2014

Killer clawed dinosaurs yield new theory about flight in birds

Published On: Thu, Dec 15th, 2011 | Evolution | By BioNews

In a new study, scientists have described how dinosaurs like Velocriaptor and Deinonychus used their famous killer claws, which leads to new hypothesis on the evolution of flight in birds.

Denver W. Fowler, Elizabeth A. Freedman, John B. Scannella and Robert E. Kambic from the Michigan State University describe how comparing modern birds of prey helped develop a new behaviour model for sickle-clawed carnivorous dinosaurs like Velociraptor.

The study focuses on dromaeosaurids, a group of small predatory dinosaurs that include the famous Velociraptor and its larger relative, Deinonychus. Dromaeosaurids are closely related to birds, and are most famous for possessing an enlarged sickle-claw on digit two (inside toe) of the foot.

Previous researchers suggested that this claw was used to slash at prey, or help climb up their hides, but the new study proposes a different behaviour.

“Modern hawks and eagles possess a similar enlarged claw on their digit 2’s, something that hadn’t been noted before we published on it back in 2009,” Fowler, the lead researcher, said.

“We showed that the enlarged D-2 claws are used as anchors, latching into the prey, preventing their escape. We interpret the sickle claw of dromaeosaurids as having evolved to do the same thing: latching in, and holding on,” he said.

As in modern birds of prey, precise use of the claw is related to relative prey size.

“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.

“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 bird of prey feet gave clues as to the functional anatomy of their ancient relatives, toe proportions of dromaeosaurids seemed more suited for grasping than running, and the metatarsus, 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.

“A long metatarsus lets you take bigger strides to run faster; but in dromaeosaurids, the metatarsus is very short, which is odd,” he said.

According to Fowler, this indicates that Velociraptor and its kin were adapted for a strategy other than 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.

“Velociraptor and Deinonychus also have a very short, stout metatarsus, suggesting that they had great strength but wouldn’t have been very fast runners,” he said.

The study also has implications for the next closest relatives of troodontids and dromaeosaurids, the birds. An important step in the origin of modern birds was the evolution of the perching foot.

“A grasping foot is present in the closest relatives of birds, but also in the earliest birds like Archaeopteryx.

“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,” Fowler said.

The new study proposes that a similar mechanism may be responsible for the evolution of flight.

“When a modern hawk has latched its enlarged claws into its prey, it can no longer use the feet for stabilization and positioning,

“Instead the predator flaps its wings so that the prey stays underneath its feet, where it can be pinned down by the predator’s bodyweight,” he said.

The researchers suggest that this ‘stability flapping’ uses less energy than flight, making it an intermediate flapping behaviour that may be the key to understanding how flight evolved.

“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.

“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. 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 behaviour would be consistent with the unusual adaptations of the feet,” Fowler added.

The study has been published in PLoS ONE.

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