Rhamp/Archaeopterygian tails (long)

[Nick Longrich <longrich@stardot.com>]

        On page 83 of Wellnhoefers' _Encyclopedia_ is a Rhamphorhynchus 
with its tail bent off to the side about 35 degrees. There is also a 
picture of a juvenile Scaphognathus but it is difficult to tell in the 
jumble whether the tail is curved upwards, or to the right. (the 
cercivals are almost certainly dorsal surface upwards, and the 
back vertebrae may be) Presumably, however, 
either of these could be artifacts of decomposition and disarticulation.
     The tail of Pteranodon is not very applicable to Rhamphorhynchus. It 
does show, however, what may be the typical trait of an aerodynamic 
control surface such as a rudder or elevator- proximal flexibility and 
distal stiffening. In this case the flexibility is in the vertical, and 
the proposed control surface supported by the bony extensions of the tail 
is in the horizonal position, whether this  was a uropatagium, a surface 
connected with the wing membrane, or neither. Birds follow the same 
general pattern in using the tail as an elevator. In Archaeopterygians 
the distal tail is stiffened and the proximal flexible, in modern birds 
the distal bony stiffening has been replaced and instead the surface is 
supported by the rachi of the feathers, presumably to save weight. I 
don't have a diagram of a modern bat's skeleton handy, but the 
Icaronycteris drawing shows that the proximal vertebrae are 
much shorter relative to the distal- again, it would appear that there is  
more flexibility near  the base of the tail (assuming Icaronycteris 
actually had a uropatagium) 
        Decoupling of the caudofemoralis in pterosaurs might have 
occurred for the same reason that it occurred in Archaeopteryx (possibly 
to free the tail from the hindlimbs for use as a control surface ) so it 
might either support or argue against use of the tail for aerodynamic 
purposes depending on your argument.
        And while tail-stiffening was important in the theropods 
(tenanure = stiff-tail), with  the exception of Parksosaurus (aka 
Thescelosaurus) warreni it does not progress to anything like what
we see in Rhamphorhynchus, and in many ornithopods the stiffening 
is at the *proximal* part of the tail. Bipedal running alone does not 
seem adequate to explain this peculiar tail structure. 
        Assuming the tail to be sexual in use, this still does not 
preclude aerodynamics as the primary function. Why would immature 
rhamphorhynchs need a tail fin at all? Many birds use their tails 
for both aerodynamics and for display, as well. 
        Although the dromaeosaurs can be used as a counterexample 
(distally stiffened, proximally flexible tail, decoupled caudofemoralis) 
this requires presupposing some things about their evolutionary 
history that perhaps we should not presuppose. Dromaeosaurs are very 
strange in many ways that are yet to be completely explained. 

        nL