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

Re: polarity of bipedality in dinosaurs (real long--TOO long)

[For those who joined late, anything with "))" is George Olshevsky, and
 anything with "}*" is Jonathan Wagner.  -- MR ]

> >))There >must< have been a tremendous amount of evolutionary
> >)) experimentation going on among the dino-birds as they became ever
> >)) more perfectly adapted to living in trees, gliding, and flying.
> {*      And that group was the dinosaurs.

     When you say "dino-birds" George, are you talking about arboreal 
dinosaurs, or a group derived from basal archosaurs that occaisionally 
produces terrestrial members that most taxonomists sink into Dinosauria 
(in other words, that Dinosauria is polyphyletic, if thats the right word).

> {* WHY?  Bats have five fingers on the wing, Pterodactyls have, I
> {* believe, four, flying squirrels have whatever squirrels have.  It's a
> {* chicken and the egg thing, really.  I can see no reason for reducing
> {* the number of fingers which can't be turned back on itself.

     I'm not sure that bats and pterosaurs are valid comparisons.  Bats 
actually expanded the size of the fingers because the flying membrane was 
spread across the digits, and pterosaurs did the same with a single digit.  
However, birds did NOT utilize the digits in this way (feathers as opposed to
a membrane), so reduction of the digits would only serve to reduce mass, a 
good thing for flight.  
     However,it is my impression that in mass reduction in this way would be 
more likely served by shrinking all digits down equally, rather than just by 
lopping off a couple digits. Pterosaurs and bats reduced ALL of the digits 
that weren't being used for flight equally (I think).  One might expext that 
given time, the redundant fingers would have ALL vanished entirely about the 
same time.  Is this bourne out by pterosaur fossils?  
     If "dino-birds" were going to retain claws for climbing, might not a 
bunch of short, clawed fingers (like squirrils have, or like pterosaurs might 
have used for climbing) be better than a couple long clawed fingers(on the 
other hand, this raises the question of what a terrestrial 
bipedal theropod might find more handy about fewer, longer fingers)  
      Also, as far as some modern clawed climbing birds getting theropod 
like clawed fingers to use for the purpose, it is not very suprising 
considering that they had to work with a (more or less) theropod like 
hand to get it, or possibly genetically reversed back to the theropod hand 
condition if such things really occur, not that three long clawed 
fingers are neccessarily better for the job than more.  They worked with 
what they had.    
> >)) of avian wings. This pattern extends naturally and automatically
> >)) to the earliest ground-dwelling, flightless birds, namely, the
> >)) theropods. Only in theropods, the wings still had claws and still
> >)) had a grasping function--and these anatomical characters and
> >)) functions exapted naturally into a highly predatory lifestyle.
> {*  Which then doesn't explain _why_they_were_reduced...

> >)) Rule." Of course I know about dwarfism, and there was even a time when I
> >)) imagined that theropods somehow underwent an extensive period of dwarf
> >)) evolution on their way to becoming birds. But I could not imagine twenty
> >)) million years of unmitigated island endemism or any other factors that

     Most of the small animals today belong to a lineage of 
millions of years of small animals.  You don't need to keep them confined 
to an island to keep them small.  If they get dwarfed as an island 
endemic species and move back to the mainland and find that it is 
advantageous in some way to stay small, they will.    

> {*  Given enough selection pressure, it won't take much time...
     Increasing the pressure does not mean they CAN evolve faster (have the 
good fortune for the right random mutatiuons to occur) just because they 

> >)) BCF explains the existence of Cretaceous birdlike dinosaurs as relics
> >)) of their Triassic and Jurassic origins. So does BADD. But what hurts
> >)) BADD is that _Archaeopteryx_ is one of the oldest maniraptorans. If BADD
> >)) were correct, we should see lots of _Velociraptor_-like theropods in the
> >)) Jurassic, but we don't. Instead, we get _Ornitholestes_ and such.]

   Who says they weren't very rare in the Jurassic and became much more 
common in the Cretaceous, like a lot of other groups (ankylosaurs and birds).

> >)) John Ostrom published a description of _Compsognathus_ in which he found
> >)) no trace of flight or contour feathers, despite the fact that the speci-
> >)) men came from the same lithographic limestone that _Archaeopteryx_ is
> {* See Paul, _PDW_ on contour feathers, which, if I recall
> {* correctly, are not preserved on Archy either.

     I also thought that MOST of the Archaeopteyx fossils from Solholfen 
didn't preserve feathers at all.  Aren't there also fewer fossils of 
Compsognathus compard to Archaeopteryx anyway?  If it did have feathers, 
it wouldn't neccessarily be very surprising if we just didn't have a 
specimen that preserved them yet. 

LN Jeff