Tuesday, June 8, 2010

More about the secondarily flightless birds (2)

Caudipteryx is an oviraptor - a primitive secondarily-flightless bird.

"Caudipteryx (which means "tail feather") is a genus of peacock-sized theropod dinosaurs [flightless birds] that lived in the Aptian age of the early Cretaceous Period (about 124.6 million years ago). They were feathered and remarkably birdlike in their overall appearance.Caudipteryx has a short tail stiffened toward the tip, with few vertebrae, like in birds and other oviraptorosaurs.Caudipteryx had uncinate processes on the ribs, birdlike teeth, a first toe which may or may not be partially reversed and overall body proportions that are comparable to those of modern flightless birds.[1][2][5][6][3]The hands of Caudipteryx supported symmetrical, pennaceous feathers that had vanes and barbs, and that measured between 15–20 centimeters long (6–8 inches). These primary feathers were arranged in a wing-like fan along the second finger, just like primary feathers of birds and other [flightless bird] maniraptorans. No fossil of Caudipteryx preserves any secondary feathers attached to the forearms, as found in dromaeosaurids, Archaeopteryx and modern birds.[3] Either these arm feathers are not preserved, or they were not present on Caudipteryx in life. An additional fan of feathers existed on its short tail. The shortness and symmetry of the feathers, and the shortness of the arms relative to the body size, indicate that Caudipteryx could not fly."

Modern birds have markedly foreshortened tails and their body mass is centred anteriorly, near the wings1, 2, 3, 4, 5. To provide stability during powered flight, the avian centre of mass is far from the pelvis, which poses potential balance problems for cursorial birds. To compensate, avians adapted to running maintain the femur subhorizontally, with its distal end situated anteriorly, close to the animal's centre of mass; stride generation stems largely from parasagittal rotation of the lower leg about the knee joint6, 7, 8, 9, 10, 11, 12. In contrast, bipedal dinosaurs had a centre of mass near the hip joint and rotated the entire hindlimb during stride generation4, 5, 6, 7, 8, 11, 12, 13. Here we show that these contrasting styles of cursoriality are tightly linked to longer relative total hindlimb length in cursorial birds than in bipedal dinosaurs. Surprisingly, Caudipteryx , described as a theropod dinosaur14, 15, possessed an anterior centre of mass and hindlimb proportions resembling those of cursorial birds. Accordingly,Caudipteryx probably used a running mechanism more similar to that of modern cursorial birds than to that of all other bipedal dinosaurs. These observations provide valuable clues about cursoriality in Caudipteryx , but may also have implications for interpreting the locomotory status of its ancestors.

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