There is currently disagreement about the function of the enlarged "sickle claw" on the second toe. When John Ostrom described it for Deinonychus in 1969, he interpreted the claw as a blade-like slashing weapon, much like the canines of some saber-toothed cats, used with powerful kicks to cut into prey. Adams (1987) suggested that the talon was used to disembowel large ceratopsian dinosaurs. The interpretation of the sickle claw as a killing weapon applied to all dromaeosaurids. However, Manning et al. argued that the claw instead served as a hook, reconstructing the keratinous sheath with an elliptical cross section, instead of the previously inferred inverted teardrop shape. In Manning's interpretation, the second toe claw would be used as a climbing aid when subduing bigger prey and also as stabbing weapon.
Ostrom compared Deinonychus to the ostrich and cassowary. He noted that the bird species can inflict serious injury with the large claw on the second toe. The cassowary has claws up to 125 millimetres (4.9 in) long. Ostrom cited Gilliard (1958) in saying that they can sever an arm or disembowel a man.Kofron (1999 and 2003) studied 241 documented cassowary attacks and found that one human and two dogs had been killed, but no evidence that cassowaries can disembowel or dismember other animals. Cassowaries use their claws to defend themselves, to attack threatening animals, and in agonistic displays such as the Bowed Threat Display. The seriema also has an enlarged second toe claw, and uses it to tear apart small prey items for swallowing.
Phillip Manning and colleagues (2009) attempted to test the function of the sickle claw and similarly shaped claws on the forelimbs. They analyzed the bio-mechanics of how stresses and strains would be distributed along the claws and into the limbs, using X-ray imaging to create a three dimensional contour map of a forelimb claw from Velociraptor. For comparison, they analyzed the construction of a claw from a modern predatory bird, the Eagle Owl. They found that, based on the way that stress was conducted along the claw, they were ideal for climbing. The scientists found that the sharpened tip of the claw was a puncturing and gripping instrument, while the curved and expanded claw base helped transfer stress loads evenly.
The Manning team also compared the curvature of the dromaeosarid "sickle claw" on the foot with curvature in modern birds and mammals. Previous studies had shown that the amount of curvature in a claw corresponded to what lifestyle the animal has: animals with strongly curved claws of a certain shape tend to be climbers, while straighter claws indicate ground-dwelling lifestyles. The sickle-claws of the dromaeosaurid Deinonychus have a curvature of 160 degrees, well within the range of climbing animals. The forelimb claws they studied also fell within the climbing range of curvature.
Paleontologist Peter Mackovicky commented on the Manning team's study, stating that small, primitive dromaeosaurids (such as Microraptor) were likely to have been tree-climbers, but that climbing did not explain why later, gigantic dromaeosaurids such as Achillobator retained highly curved claws when they were too large to have climbed trees. Mackovickey speculated that giant dromaeosaurids may have adapted the claw to be used exclusively for latching on to prey.
Phillip Manning and colleagues (2009) attempted to test the function of the sickle claw and similarly shaped claws on the forelimbs. They analyzed the bio-mechanics of how stresses and strains would be distributed along the claws and into the limbs, using X-ray imaging to create a three dimensional contour map of a forelimb claw from Velociraptor. For comparison, they analyzed the construction of a claw from a modern predatory bird, the Eagle Owl. They found that, based on the way that stress was conducted along the claw, they were ideal for climbing. The scientists found that the sharpened tip of the claw was a puncturing and gripping instrument, while the curved and expanded claw base helped transfer stress loads evenly.
The Manning team also compared the curvature of the dromaeosarid "sickle claw" on the foot with curvature in modern birds and mammals. Previous studies had shown that the amount of curvature in a claw corresponded to what lifestyle the animal has: animals with strongly curved claws of a certain shape tend to be climbers, while straighter claws indicate ground-dwelling lifestyles. The sickle-claws of the dromaeosaurid Deinonychus have a curvature of 160 degrees, well within the range of climbing animals. The forelimb claws they studied also fell within the climbing range of curvature.
Paleontologist Peter Mackovicky commented on the Manning team's study, stating that small, primitive dromaeosaurids (such as Microraptor) were likely to have been tree-climbers, but that climbing did not explain why later, gigantic dromaeosaurids such as Achillobator retained highly curved claws when they were too large to have climbed trees. Mackovickey speculated that giant dromaeosaurids may have adapted the claw to be used exclusively for latching on to prey.
No comments:
Post a Comment