[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: vaulting pterosaur launch, questions



Michael Habib wrote:


> Actually, the degree
> to which most paleontologists (and even just biologists in general)
> misunderstand running launch has been quite surprising to me.  It is
> not really a method of improving launch speed or power - running launch
> mostly shows up in birds as a method of dealing with take off from
> compliant surfaces (ie. water). 


That's interesting, Mike.  You could probably count me among that number, 
because I thought a running take-off was considered almost canonical for early 
birds (such as _Archaeopteryx_, for example).  This was an issue addressed (and 
redressed?) by Burgers and Chiappe....  


Burgers, P. and Chiappe, L.M. (1999).  The wing of _Archaeopteryx_ as a primary 
thrust generator.  Nature 399: 60-62.


First p'graph: "Since the late 1800s, the debate on the origin of flight in 
birds has centred around two antagonistic theories: the arboreal (take-off from 
trees) and cursorial (take-off from running) models.  Despite broad acceptance 
of the idea that birds evolved from bipedal and predominantly terrestrial 
maniraptoriform dinosaurs, the cursorial model of flight origins has been less 
successful than the arboreal model. Three issues have contributed to this lack 
of success: the gap between the estimated maximum running speed of 
_Archaeopteryx_ (2 metres per second) and its estimated minimum flying speed (6 
metres per second); the high energy demands of evolving flight against gravity; 
and the problem of explaining the origin of the 'flight' stroke in an 
earthbound organism.  Here we analyse the take-off run of _Archaeopteryx_ 
through lift-off from an aerodynamic perspective, and emphasize the importance 
of combining functional and aerodynamic considerations with those of phylogeny.
  Our calculations provide a solution to the 'velocity gap' problem and shed 
light on how a running _Archaeopteryx_ (or its cursorial maniraptoriform 
ancestors) could have achieved the velocity necessary to become airborne by 
flapping feathered wings."


Sorry to switch focus from pterosaurs to birds, but how do you picture 
_Archaeopteryx_ getting itself airborne from terra firma?  By leaping from a 
standstill off the ground?   


> Proximal lever is shorter, distal levers are long. Having a short,
> stout humerus is key: it allows for massive bending and torsion
> resistance, which is pretty key for a massive quad launching animal.
> Note that big birds have short, stout femora for the same reason (but,
> again, the role is moved to the hindlimb because birds are obligate
> bipeds).


Nice point.  Also, birds have shifted stride generation from the hip to the 
femur, as part of the forward migration of the center of mass (a flight 
adaptation).  As such, the more-or-less horizontally oriented femur is exposed 
to a lot more stresses than a femur that is oriented vertically (as in 
non-avian theropods).  Hence, femur of birds is short and stout, to resist 
these forces.


Refs:


Carrano, M. T. (1998).  Locomotion in non-avian dinosaurs: Integrating data 
from hindlimb kinematics, in vivo strains, and bone morphology. Paleobiology 
24: 450-469.


Farlow, J.O., Gatesy, S.M., Holtz, T.R., Jr., Hutchinson, J.R., and Robinson, 
J.M. (2000).  Theropod Locomotion.  Am. Zool.  40: 640–663.


Gatesy, S. M. (1991).  Hind limb scaling in birds and other theropods: 
Implications for terrestrial locomotion.  J. Morph.  209: 83-96




Cheers

Tim


_________________________________________________________________
Now you can invite friends from Facebook and other groups to join you on 
Windows Live™ Messenger. Add now.
https://www.invite2messenger.net/im/?source=TXT_EML_WLH_AddNow_Now