Oolon Colluphid
30 Mar 2009, 04:08 PM
I've just come across an interesting paper on
Forelimb posture in dinosaurs and the evolution of the avian flapping flight-stroke
Nudds and Dyke
Evolution Volume 63, Issue 4, Pages 994-1002 (January 2009)
It's pretty technical, but the long and the short of it is that, when involved in locomotion, forelimbs can either more alternately like ours, or together, and in order to have become a flapping stroke, theey need to move together, so this has implications for how theropods used their forelimbs.
Ontogenetic [developmental] and behavioral studies using birds currently do not document the early evolution of flight because birds (including juveniles) used in such studies employ forelimb oscillation frequencies over 10 Hz, forelimb stroke-angles in excess of 130°, and possess uniquely avian flight musculatures.
Living birds are an advanced morphological stage in the development of flapping flight. derived.]
To gain insight into the early stages of flight evolution (i.e., prebird), in the absence of a living analogue, a new approach using Strouhal number (St = f . A/U was used. [Erm, if they say so!]
Strouhal number is a nondimensional number that describes the relationship between wing-stroke amplitude (A), wing-beat frequency (f), and flight speed (U).
Calculations indicated that even moderate wing movements are enough to generate rudimentary thrust and that a propulsive flapping flight-stroke could have evolved via gradual incremental changes in wing movement and wing morphology.
More fundamental to the origin of the avian flapping flight-stroke is the question of how a symmetrical forelimb posture—required for gliding and flapping flight—evolved from an alternating forelimb motion, evident in all extant bipeds when running except birds.
The interesting bit is the final part, because the results suggest that the ground-up hypothesis (running - flapping - take-off) won't work as it currently stands, and that therefore the trees-down hypothesis (climb - fall - fall-with-style - glide - flap) is more likely:
Until a locomotory advantage to rudimentary flapping over asymmetrical forelimb movement can be demonstrated, cursorial hypotheses can only be viewed as plausible mechanisms for evolving more complex wing-kinematics and morphology from an already established symmetrical posture.
Or in other words, if dinobirds got airborne from the ground up, they had to evolve a limb posture that'd let them flap symmetrically first. Erm, I think.
A wingless avian precursor (i.e., a nonavian theropod) wishing to increase its running speed can either alternate its forelimbs (as a human does) or alternatively move them in a symmetrical manner (flapping) and generate thrust. These two strategies can be viewed as two alternative "adaptive peaks" (Wright 1932) representing highest "fitness" in terms of running performance (Fig. 3). If terrestrial theropods occupied the trough between these peaks, then either strategy is of course possible. If, however, they were positioned on either side of the trough, then it would be most parsimonious to expect them to adopt the strategy of the adaptive peak they are already ascending, because to adopt the other would first require a concomitant reduction in "fitness," not favored by natural selection (see Arnold et al. 2001 for an overview of evolution on phenotypic landscapes). So far research into the forelimb movements of nonavian theropods have focused on range of motion and shoulder morphology (Jenkins 1993; Gatesy and Baier 2005; Baier et al. 2007), but whether a symmetrical or asymmetric movement pattern characterized the antecedents of birds remains unclear. Initially, holding an early feathered limb out passively just produces drag (Rayner 1991a), whereas thrusting the shoulder and arm forward aids forward momentum. For example, the arms and trunk provide internal torques around the trunk's long axis and these angular impulses to the lower body are needed for the legs to alternate through stance and swing phases (Hinrichs 1990). Furthermore, amounts of thrust generated at low running speeds are extremely low (Table 2) and it is doubtful whether they would aid forward locomotion more than an asymmetric forelimb gait. It is possible to imagine how a strong competing function could have driven the evolution of forelimb movement onto the symmetrical peak (Fig. 3), such as a predatory stroke (Padian 2001) or the reduction of inertial forces during turning (Carrier et al. 2001). Existing "cursorial" flight origin hypotheses (including WAIR), however, do not currently provide this competing function.
Forelimb posture in dinosaurs and the evolution of the avian flapping flight-stroke
Nudds and Dyke
Evolution Volume 63, Issue 4, Pages 994-1002 (January 2009)
It's pretty technical, but the long and the short of it is that, when involved in locomotion, forelimbs can either more alternately like ours, or together, and in order to have become a flapping stroke, theey need to move together, so this has implications for how theropods used their forelimbs.
Ontogenetic [developmental] and behavioral studies using birds currently do not document the early evolution of flight because birds (including juveniles) used in such studies employ forelimb oscillation frequencies over 10 Hz, forelimb stroke-angles in excess of 130°, and possess uniquely avian flight musculatures.
Living birds are an advanced morphological stage in the development of flapping flight. derived.]
To gain insight into the early stages of flight evolution (i.e., prebird), in the absence of a living analogue, a new approach using Strouhal number (St = f . A/U was used. [Erm, if they say so!]
Strouhal number is a nondimensional number that describes the relationship between wing-stroke amplitude (A), wing-beat frequency (f), and flight speed (U).
Calculations indicated that even moderate wing movements are enough to generate rudimentary thrust and that a propulsive flapping flight-stroke could have evolved via gradual incremental changes in wing movement and wing morphology.
More fundamental to the origin of the avian flapping flight-stroke is the question of how a symmetrical forelimb posture—required for gliding and flapping flight—evolved from an alternating forelimb motion, evident in all extant bipeds when running except birds.
The interesting bit is the final part, because the results suggest that the ground-up hypothesis (running - flapping - take-off) won't work as it currently stands, and that therefore the trees-down hypothesis (climb - fall - fall-with-style - glide - flap) is more likely:
Until a locomotory advantage to rudimentary flapping over asymmetrical forelimb movement can be demonstrated, cursorial hypotheses can only be viewed as plausible mechanisms for evolving more complex wing-kinematics and morphology from an already established symmetrical posture.
Or in other words, if dinobirds got airborne from the ground up, they had to evolve a limb posture that'd let them flap symmetrically first. Erm, I think.
A wingless avian precursor (i.e., a nonavian theropod) wishing to increase its running speed can either alternate its forelimbs (as a human does) or alternatively move them in a symmetrical manner (flapping) and generate thrust. These two strategies can be viewed as two alternative "adaptive peaks" (Wright 1932) representing highest "fitness" in terms of running performance (Fig. 3). If terrestrial theropods occupied the trough between these peaks, then either strategy is of course possible. If, however, they were positioned on either side of the trough, then it would be most parsimonious to expect them to adopt the strategy of the adaptive peak they are already ascending, because to adopt the other would first require a concomitant reduction in "fitness," not favored by natural selection (see Arnold et al. 2001 for an overview of evolution on phenotypic landscapes). So far research into the forelimb movements of nonavian theropods have focused on range of motion and shoulder morphology (Jenkins 1993; Gatesy and Baier 2005; Baier et al. 2007), but whether a symmetrical or asymmetric movement pattern characterized the antecedents of birds remains unclear. Initially, holding an early feathered limb out passively just produces drag (Rayner 1991a), whereas thrusting the shoulder and arm forward aids forward momentum. For example, the arms and trunk provide internal torques around the trunk's long axis and these angular impulses to the lower body are needed for the legs to alternate through stance and swing phases (Hinrichs 1990). Furthermore, amounts of thrust generated at low running speeds are extremely low (Table 2) and it is doubtful whether they would aid forward locomotion more than an asymmetric forelimb gait. It is possible to imagine how a strong competing function could have driven the evolution of forelimb movement onto the symmetrical peak (Fig. 3), such as a predatory stroke (Padian 2001) or the reduction of inertial forces during turning (Carrier et al. 2001). Existing "cursorial" flight origin hypotheses (including WAIR), however, do not currently provide this competing function.