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Dromaeosaurids, especially Deinonychus, are often depicted as unusually fast-running animals in the popular media, and Ostrom himself speculated that Deinonychus was fleet-footed in his original description. However, when first described, a complete leg of Deinonychus had not been found, and Ostrom's speculation about the length of the femur (upper leg bone) later proved to have been an overestimate. In a later study, Ostrom noted that the ratio of the femur to the tibia (lower leg bone) is not as important in determining speed as the relative length of the foot and lower leg. In modern, fleet-footed birds like the ostrich, the foot-tibia ratio is .95. In unusually fast-running dinosaurs like Struthiomimus, the ratio is .68, but in Deinonychus, the ratio is .48. Ostrom stated that the "only reasonable conclusion" is that Deinonychus was not particularly fast compared to other dinosaurs, and certainly not as fast as modern flightless birds.
The low foot to lower leg ratio in Deinonychus is due partly to an unusually short metatarsus (upper foot bones). The ratio is actually larger in smaller individuals than in larger ones. Ostrom suggested that the short metatarsus may be related to the function of the sickle claw, and used the fact that it appears to get shorter as individuals aged as support for this. He interpreted all these features – the short second toe with enlarged claw, short metatarsus, etc. – as support for the use of the hind leg as an offensive weapon, where the sickle claw would strike downwards and backwards, and the leg pulled back and down at the same time, slashing and tearing at the prey. Ostrom suggested that the short metatarsus reduced overall stress on the leg bones during such an attack, and interpreted the unusual arrangement of muscle attachments in the Deinonychus leg as support for his idea that a different set of muscles were used in the predatory stroke than in walking or running. Therefore, Ostrom concluded that the legs of Deinonychus represented a balance between running adaptations needed for an agile predator, and stress-reducing features to compensate for its unique foot weapon.
In his 1981 study of Canadian dinosaur footprints, Richard Kool produced rough walking speed estimates based on several track-ways made by different species in the Gething Formation of British Columbia. Kool estimated one of these track-ways, representing the ichnospecies Irenichnites gracilis (which may have been made by Deinonychus), to have a walking speed of 10.1 kilometers per hour (6 miles per hour). http://en.wikipedia.org/wiki/Deinonychus
Deinonychus cannot run as fast as expected because of :
1. Short foot-tibia ratio
2. Relative close pubis and ischium
[ 本帖最後由 Dinosaurprince 於 2009-1-3 04:59 PM 編輯 ]