Re: NO SECONDARILY FLIGHTLESS THEROPODS

["John V Jackson" <jjackson@interalpha.co.uk>]

-Original Message--From: Matthew Troutman Date: 04 April 1998 09:16


Thankyou for setting out your argument in detail.

Your case for those K (espec. later K) theropods (lets say "dinobirds")
never having flown rests on two tenets:

1:  Flightlessness arises from neoteny.

2:  _Archaeopteryx_ shares too many features with modern birds that
     e.g. dromeosaurs don't.


1:   You quote the starling paper proving that flightlessness can be caused
by artificially induced neoteny, and you quote other examples where neoteny
appears to have caused it naturally.

As I'm sure you will agree, just because something can happen, and does
happen in some way, it doesn't follow that it has to happen in that way.  I
have to say you have greatly weakened the neoteny case by admitting that
penguins are a counterexample.  Although I can't give numbers of
individuals, there are a great many more penguin species than ratites.
Whatever the figures, penguins are a valid counterexample, and only go to
show that if a development such as flightlessness has a particular
specialised use, it may also have a particular specified mechanism - neoteny
when it suits, otherwise when it doesn't.  Your claim that penguins are a
special case supports my argument better.

>Paedomorphosis is the mechanism that makes flightless birds flightless.

That would make it pretty unique amongst adaptations.  It is more common for
genuinely parallel adaptations to have somewhat variable   mechanisms.

>It is seen in all flightless birds that we know to date.

. . . except in . . .


>The unifying character between flightless birds is that they all show
>the same consistent paedomorphic trends ( visible sutures, atrophy of
>the forelimb . . .

Some penguins have really rather long forelimbs, and I find the attempt to
use the large forelimb of dromaeosaurs as evidence against loss of flight
far from convincing.

> . . . massiveness of the . . . hindlimbs . . .

Is it surprising that animals without teeth or hand claws should develop
high speed as a defense?   But penguins have hindlimbs no bigger than
absolutely necessary for obligatory bipeds.

> . . . large orbits . . .

It would be interesting to compare the eyeball size of droms. (as a
percentage of body weight) with ostriches.  I'm not convinced by your claim
that droms had significantly smaller eyes  - but anyway, it wouldn't matter
if they did since that is part of the neoteny argument.

Protobirds are even more a special case than penguins, which brings us on
to:

2:  _Archaeopteryx_ shares too many features with modern birds that
     e.g. dromeosaurs don't.

When protobirds lost flight, they *returned* the forearm to their original
use - indeed they never really abandoned all the uses for the claws and
digits.  Birds without claws and even separate digits have to go a
completely different path - *There is now nothing much else for the arms to
do!* (except swim sometimes).

Finally, let me look at some of the body details which you say are useful
for distinguishing ancestries, and I say are inevitable by-products of loss
of flight.

Just assume for one moment that "Rahona" became flightless.  What would
happen?  Please believe me when I say that flight is an extreme adaptation
for any animal - particularly powered flight - and many adaptations in a
flying/non-flying vertebrate immediately become disadvantageous on the wrong
side of the division.

As far as the head is concerned, I see no reason to doubt the proposition
that Archy almost always ate prey much smaller than itself and didn't need
to do much heavy dismembering of large prey.  The opposite is probably true
of dromaeosaurs.  Droms' teeth had to force their way deep into flesh,
enlarging the holes in the process to help the holes merge together to form
strips.  It is quite likely that Archy (and other early birds) simply used
them to increase the friction between the prey and its jaws.  For this
reason, their teeth were relatively small and unserrated.  Droms' serrations
were probably due to re-activation of the original genetic mechanism
inherited through Archy etc from earlier times, but failing that, if they
were evolved once, they could have been evolved twice.

Archy and droms used their heads in rather different ways, and it is to be
expected that all the various detailed differences observed should have
arisen, and I am not going to attempt to justify them individually.  I am
tempted to say the importance of lightness contributed to some of these
differences, but since loss of weight in the head is somewhat less important
in birds and pterosaurs than in other parts of the body, due to striving to
put the wings back to the centre of gravity, things get a bit complex;
nonetheless I would expect to see big changes between animals with such
different lifestyles.  Bear in mind what happened to whales over a dozen
million years or so.

Incidentally, when several forms lose flight and return to their old ways
(no doubt bringing some new advantages with them) they will have a "choice"
of re-evolving the old adaptations using exactly the same mutations, or
finding some new solutions.  Wouldn't we expect to find under these
circumstances exactly the same type of confusing mosiac of partial
inheritance that troodonts and droms. show?

Oh, I'm not going to go through all the details of the arms and thorax's and
point out all the changes that will occur almost instantly as soon as loss
of flight occurs.  I would just like to draw attention to the possible use
of the sternal plates and certain other chest bones as bumpers.  It has been
said that T. rex could not run fast because it would bash the front of its
chest in when it fell over.  I believe this was a slight issue with such
flightless bipeds, and at racehorse speeds, a brittle front bumper would be
less useful than one with some give in it.  Perhaps invisible furculae can
be put down to this.  Modern birds have huge breastbones that could take it
though solid - but it's interesting to note that supermarket chickens have
the front end of their keel cartilageonous.

>GSP concluded that dromaeosaurs, since they are more "advanced"
> and share a similiar shoulder structure with flightless birds. This
>hypothesis was expanded when a recent list member called the
> "Rahona" bird a link between Archaeopteryx and dromaeosaurs.
>
>However, these two conclusions have many faults. First of all,
>dromaeosaurs have no features that can be considered more
> advanced than Archaeopteryx ( except for perhaps the sternal size
> and pubic retroversion ).

Ah well - except for them!  Pubic retroversion - a minor point?  And lets
not forget uncinate processes which really ARE useful relationship pointers
because they're almost never lost.  (No ribs found for "Rahona"
but the metatarsals, though unfused, are more constricted distally.)

> All of the best recent finds of
> dromaeosaurs and Archaeopteryx suggest that Archaeopteryx is
> more birdlike than all known dromaeosaurs.

. . .but only in easily reversed features  (e.g. the reversed distal
hallux).


I don't think the title of the thread "NO secondarily flightless theropods"
helps the case, since "none" is a big number, and surely anything that
predicts it is likely to trip up.  Also, since the early birds still
retained terrestrial adaptations such as teeth and claws they would be
better suited to flightlessness than more specialised later birds.

But of course the biggest justification for K-BCF is all the features that
are:

*  Totally understandable in terms of flight
*  Totally incomprehensible in any other reasonable terms
*  So often occuring together
*  So often implemented in very similar ways (though sometimes
      obscured by post-adaptation)
*  Absolutely always found after the first feather.

JJ