Dinosaurs With Killer Claws Yield New Theory About Evolution of Flight
ScienceDaily (Dec. 14, 2011) — New research from Montana State University's Museum of the Rockies has revealed how dinosaurs like Velociraptorand Deinonychus used their famous killer claws, leading to a new hypothesis on the evolution of flight in birds.
New research from Montana State University reveals how dinosaurs like Velociraptor and Deinonychus used their famous killer claws, leading to a new hypothesis on the evolution of flight in birds.
(Credit: Illustration by Nate Carroll)
In a paper published Dec. 14 inPLoS ONE, MSU researchers Denver W. Fowler, Elizabeth A. Freedman, John B. Scannella and Robert E. Kambic (now at Brown University in Rhode Island), describe how comparing modern birds of prey helped develop a new behavior model for sickle-clawed carnivorous dinosaurs like Velociraptor.
"This study is a real game-changer," said lead author Fowler. "It completely overhauls our perception of these little predatory dinosaurs, changing the way we think about their ecology and evolution."
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 behavior.
"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 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."
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," 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."
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 (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, which is odd."
Fowler thinks that 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," Fowler continued. "Velociraptor and Deinonychusalso have a very short, stout metatarsus, suggesting that they had great strength but wouldn't have been very fast runners."
The ecological implications become especially interesting when dromaeosaurids are contrasted with their closest relatives: a very similar group of small carnivorous dinosaurs called troodontids, Fowler said.
"Troodontids and dromaeosaurids started out looking very similar, but over about 60 million years they evolved in opposite directions, adapting to different niches," Fowler said. "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."
The study also has implications for the next closest relatives of troodontids and dromaeosaurids: birds. An important step in the origin of modern birds was the evolution of the perching foot.
The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds
Denver W. Fowler1*, Elizabeth A. Freedman1,John B. Scannella1, Robert E. Kambic2
1 Museum of the Rockies and Department of Earth Sciences, Montana State University, Bozeman, Montana, United States of America, 2 Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, United States of Ameria
Abstract
Most non-avian theropod dinosaurs are characterized by fearsome serrated teeth and sharp recurved claws. Interpretation of theropod predatory ecology is typically based on functional morphological analysis of these and other physical features. The notorious hypertrophied ‘killing claw’ on pedal digit (D) II of the maniraptoran theropod Deinonychus (Paraves: Dromaeosauridae) is hypothesized to have been a predatory adaptation for slashing or climbing, leading to the suggestion that Deinonychus and other dromaeosaurids were cursorial predators specialized for actively attacking and killing prey several times larger than themselves. However, this hypothesis is problematic as extant animals that possess similarly hypertrophied claws do not use them to slash or climb up prey. Here we offer an alternative interpretation: that the hypertrophied D-II claw of dromaeosaurids was functionally analogous to the enlarged talon also found on D-II of extant Accipitridae (hawks and eagles; one family of the birds commonly known as “raptors”). Here, the talon is used to maintain grip on prey of subequal body size to the predator, while the victim is pinned down by the body weight of the raptor and dismembered by the beak. The foot of Deinonychus exhibits morphology consistent with a grasping function, supportive of the prey immobilisation behavior model. Opposite morphological trends within Deinonychosauria (Dromaeosauridae + Troodontidae) are indicative of ecological separation. Placed in context of avian evolution, the grasping foot of Deinonychus and other terrestrial predatory paravians is hypothesized to have been an exaptation for the grasping foot of arboreal perching birds. Here we also describe “stability flapping”, a novel behaviour executed for positioning and stability during the initial stages of prey immobilisation, which may have been pivotal to the evolution of the flapping stroke. These findings overhaul our perception of predatory dinosaurs and highlight the role of exaptation in the evolution of novel structures and behaviours.
Citation: Fowler DW, Freedman EA, Scannella JB, Kambic RE (2011) The Predatory Ecology ofDeinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12): e28964. doi:10.1371/journal.pone.0028964
Editor: Andrew Allen Farke, Raymond M. Alf Museum of Paleontology, United States of America
Received: February 28, 2011; Accepted: November 17, 2011; Published: December 14, 2011
Copyright: © 2011 Fowler et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Support for the Museum of the Rockies graduate student fund was provided by Damaris Waggoner and David Sands. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: df9465@yahoo.co.uk
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