"Gaaaar! I'm gonna hold you down and eat you alive! While flapping!" Photo by Dominique Pipet, via Flickr.
What did Deinonychus really do with its feet? John Ostrom initially painted the picture of a fleet-footed predator, chasing down prey animals and slashing at them with the enlarged claw on its second toe. To take it from Crichton, they were slitting bellies and dancing in spilled viscera. A few years ago, Phil Manning of the University of Manchester suggested they were more likely "climbing crampons," allowing them to cling to the panicky tenontosaurs they were attacking. In a new PLoS One paper, Denver Fowler, with Elizabeth Freedman, John Scannella, and Robert Kambic, puts forward another option: they were grasping tools for holding down smaller prey (or, in typically colorful science-speak, prey of "subequal body size").
To come to this conclusion, Fowler and team compared the feet of Deinonychus to other animals: a diverse group of extinct theropod genera as well as living birds. When compared to dinosaurs, the proportions of their feet differed strikingly from ornithomimids and alvarezsaurs, both groups whose leg proportions strongly suggest a cursorial lifestyle. They were runners. As Matt Martyniuk recently wrote in an excellent DinoGoss post, the sticky assumption that dromaeosaurs were particularly quick animals, chasing down prey, isn't really supported by the evidence. Noting that Ostrom's first ideas changed once he took full stock of the proportions of Deinonychus's metatarsus length to its tibia length, Matt writes, "Not only was Deinonychus not particularly fast, it probably could not have been nearly as fast as most other small theropods, including modern flightless birds, let alone cheetahs."
To test his hypothesis that Deinonychus was instead using its feet to hold onto prey as it killed it, Fowler compared its foot proportions to living birds who use their feet in diverse ways, including accipiters, owls, falcons, passerines, vultures, osprey, woodpeckers, and pheasants. Accipiters, AKA hawks and eagles, are adept at holding down prey, using second toes which have larger claws than the others. This second claw was the closest analog to the dromaeosaur "sickle-claw" yet found. The metatarsal bones were short and stocky like those of owls, suggesting that they were just as good at grasping small prey, if not built to constrict around prey and squeeze the life out of it the way accipiters can.
Also assisting Deinonychus in subduing and keeping a grip on its prey, Fowler suggests, is that feathered forelimbs could have been used the way accipiters use their wings, flapping to maintain stability on top of a struggling prey animal. He writes,
Even if Deinonychosauria were not capable of a full avian-like flapping ability, they may have been able to perform a rudimentary flight stroke during stability flapping. Similarly, long feathered tails are conspicuous in accipiters and aid in maneuverability and balance during stability flapping. Basal Paraves and Deinonychosauria possessed long bony tails which are shown to have been well feathered... and would have assisted balance during predation and stability flapping.This suggests another use for flapping, feathered forelimbs other than flight. The dromaeosaur would ambush its prey, hold it down by plunging its enlarged second claw into it, holding it down by its own weight, flapping to keep its balance and maintain its grip. Those long forearm feathers and fiercely clawed hands would have also been useful to "mantle" their meal, protecting it from rivals and further preventing escape. Fowler writes,
An important part of our interpretation is that ground-based predation need not necessarily be conducted at high speed. It is commonplace for extant terrestrial predators to employ surprise ambush techniques; goshawks and other forest raptor species commonly hunt on the ground, employing ambush and maneuverability as strategies, rather than outright pursuitTroodontids also figure into this, as Fowler notes their common presence in ecosystems alongside dromaeosaurs. Comparitively, the feet of troodontids were better suited for chasing down their prey. As dromaeosaurs went for larger prey, troodontids may have been specialized for hunting small mammals, for example, perhaps in darkness as suggested by their famously large eyes.
Noting that extant avian theropods such as Golden Eagles aren't afraid to go after much larger prey, Fowler writes that his research here doesn't necessarily preclude the long-inferred habit of dromaeosaurs to pick on animals their own size or larger. Had they worked up the gumption to have tenontosaur for supper, the "climbing crampon" quality of the second toe claw noted by Phil Manning would certainly come in handy for prey riding. In a sort of Cretaceous rodeo, the dromaeosaur would have leaped onto the back of a passing tenontosaur, holding on for dear life as it thrashed about until blood loss and exhaustion finally brought it down.
This research is rich with possibilities for future studies, and it will be fun to see what comes of it. There are the biomechanical implications of stability flapping to look at, as well as plenty of argument over how this hypothesis impacts the early evolution of flight. Lots of meat on this bone.
Also published in PLoS One yesterday: Scannella and Jack Horner continue to strike genera from the books, this time adding Nedoceratops to the list of ontogenetic phases of Triceratops. Another deals with how reproductive strategies affect body size in a variety of animals, including dinosaurs.