FOWLER, D.W., FREEDMAN, E.A., SCANNELLA, J.B., & KAMBIC, R.E. (2011) The predatory ecology of Deinonychus and the origin of flapping in birds. PLoS One 6(12): e28964. doi:10.1371/journal.pone.0028964 [link to paper]
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.
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POST-PUBLICATION AUTHOR NOTES
Troodontid feet: In the paper, we note that while basal troodontids and dromaeosaurids have similarly proportioned feet (long subarctometatarsalian metatarsus & grasping foot), derived members of these clades select towards opposite directions in morphospace; dromaeosaurids evolve short robust metatarsi and maintain grasping proportions of the digits (and the D-II claw becomes especially enlarged), whereas troodontids develop a fully arctometatarsalian metatarsus, and shorten the D-IV, returning to more cursorial toe proportions. We based this on disarticulated material of multiple Troodon specimens from the Two Medicine Formation, Montana (based at the Museum of the Rockies, and other incomplete material elesewhere). I would have liked to have seen a complete derived troodontid foot to get a really good handle on the precise proportions. The recently described troodontid Talos sampsoni (Zanno et al., 2011), from the Kaiparowits Fm, Utah, preserves a wonderful complete foot that exhibits the same phalangeal proportions we observed in the Two Medicine troodontid material. The nice thing about Talos is that the foot is associated, and articulated, so we can be more confident that our prediction for derived troodontids is supported.
Balaur, a super-dromaeosaur?: Our paper also helps explain the functional anatomy of some recently described, and peculiar dinosaurs. For example, the dromaeosaur Balaur bondoc was recently described from the Late Cretaceous of Romania (Csiki et al., 2010). It 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. This is pretty strange, but based on our behaviour model, this makes sense. The short fused up metatarsus can be seen as just an extreme form of the short broad metarsus of 'normal' dromaeosaurids, fused-up for extra strength; the enlarged claw on the first digit may perform the same anchoring function as the claw on D-II. Balaur looks like it was a super-dromaeosaur; with the predatory features of normal dromaeosaurs taken to extreme measures.
New videos: A video illustrating stability flapping and foot use was kindly provided by John Norris for us to include as supp info for the new paper (you can see the original full video here). It is a few years since we gathered the video data that we used in the new paper and also the 2009 behaviour paper. Since this time, dozens if not hundreds of new videos have become available that show stability flapping / foot use behaviour in great detail. I'm providing a few links here. Note that these videos are rather grim viewing, and caution is advised.
[Superb slow-motion closeup of D-II used to latch into prey; low-vigor stability flapping] - [really good video showing stability flapping medium-vigor]
Flapping / perching etc: There have been some comments that Deinonychus / dromaeosaurs did not lead directly to birds, hence the predatory adaptations that we describe would not be directly related to similar avian features. Part of this misunderstanding of our intent is probably due to the emphasis on Deinonychus in the paper, especially the title. The RPR behaviors we describe are applicable to all dromaeosauridae (not just Deinonychus), more generally to all deinonychosauria, and potentially to the base of Paraves (hence they may be important in the origin of birds). We emphasized that basal Deinonychosauria (and to some extent, basal Paraves) exhibit physical characters consistent with RPR behaviours, and that these characters became further exagerrated in dromaeosaurs, but reduced (in some ways) in troodontids. The paper's emphasis on Deinonychus is primarily due to availability of specimens as we had three specimens of Deinonychus feet available at the Museum of the Rockies (MOR 747) all found articulated and with 3D preservation. We also included data from other dromaeosaur taxa, troodontids and more basal deinonychosaurians, Archaeopteryx, and various other paravians. The point of the paper is to emphasize exaptation and potential uses for flapping and grasping adaptations that are not flight and perching (respectively).
Art: The vivid imagery associated with the press release was specifically commissioned from artist & paleontologist Nate Carroll. I wanted something distinctive to provide a new portrait of dromaeosaurids that depicted the bloody nature of the behaviours we were describing (I said to nate, "the only colour I want to see is red"). The cartoon-y feel of the images helps draw away from the rather unpleasant fact that we are describing animals being eaten alive. It's not glamorous, 'just "nature red in tooth and claw".
Anyway, if you like the art, Nate has put it on mugs and t-shirts etc [click here].
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PRESS & MEDIA
The press release is intended for the public, and emphasizes some of the more public-friendly aspects of the research (reproduced below).
[MSU press release]
Extra images: I also made a page providing extra / larger images for press use: click here (also reproduced below; scroll down)
Press coverage (selected):
[Discover magazine: Ed Yong] - [Daily Mail, UK] - Charles Choi's article was syndicated by [Fox News], [MSNBC], and [LiveScience] amongst others - Some news sources ran with the original press release like our own [Billings Gazette, Montana] - There was also coverage by a range of blogs including [Andrea Cau's Theropoda], [Rational Conservatism], [The Bite Stuff], [Love in the time of Chasmosaurs].
xkcd webcomic: Sept 5th 2012: The research was featured in an xkcd webcomic. Thanks xkcd!
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PRESS RELEASE (Dec 2011):
BOZEMAN -- New research from Montana State University's Museum of the Rockies has revealed how dinosaurs like Velociraptor and Deinonychus used their famous killer claws, leading to a new hypothesis on the evolution of flight in birds. In a paper published Dec. 14 in PLoS 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 Deinonychus also 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.
"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."
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," 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 bodyweight."
The researchers suggest that this 'stability flapping' uses less energy than flight, making it an intermediate flapping behavior that may be 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," Fowler said. "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 behavior would be consistent with the unusual adaptations of the feet."
Another group of researchers has proposed that understanding flapping behaviors is key to understanding the evolution of flight, a view with which Fowler agrees.
"If we look at modern birds, we see flapping being used for all sorts of behaviors outside of flight. In our paper, we are formally proposing the 'flapping first' model: where flapping evolved for other behaviors first, and was only later exapted for flight by birds."
The researchers believe their new ideas will open multiple new lines of investigation into dinosaur paleobiology, and the evolution of novel anatomical structures.
"As with other research conducted at the Jack Horner paleo lab, we're looking at old paleontological questions with a fresh perspective, taking a different angle," Fowler said. "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. Revealing the selection pathways that mold and produce these structures helps us to better understand the major evolutionary transitions that shaped life on this planet."
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EXTRA PRESS IMAGES (ETC) |
