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Re: Phytodinosauria status (and digit loss, and...)



> In a message dated 6/7/01 1:59:01 PM EST, david.marjanovic@gmx.at writes:
>
> << Here you IMHO run into the same problems as Feduccia -- why should the
>  forelimbs (and only they) evolve into wings in a quadruped, and why
should
>  they lose digits in an arboreal animal??? >>
>
> They can easily evolve into wings in a leaping quadrupedal arboreal
animal,
> since they are no longer entirely involved in physically supporting the
> animal but have become involved in aerodynamic control (along with the
tail)
> during the leap phase and in helping the animal to alight at the end of a
> leap

IMHO the outcome should look like one of the gliders I just mentioned in
another post, or even like a bat, or potentially even like a pterosaur, but
not like a bird.

> (furcula is the shock absorber for this).

Why do only theropods have furculae then? (From what I've read, BTW, the
"furcula" of *Longisquama* is actually the interclavicle, a bone lost in all
dinosauromorphs. Forgot the ref :-] )

> Digits were lost from the
> outside in because these digits in particular ruined the aerodynamics of
the
> feathered wing. The feathers were there long before the wing, of course,
and
> the wing itself goes back to before Herrerasaurus, which already shows
strong
> reduction of manual digit V. Evolution of the wing is the reason manual
> digits IV and V are reduced/lost in all post-sauropod dinosaurs.

My 0.02 euros' worth of thoughts on this subject (based mostly on The
Mistaken
Extinction):
Early dinosauromorphs evolved a "semi-opposable" thumb for grasping
(whatever was grasped). "Semi-opposable" means that the thumb diverges
medially a bit when extended, but its metacarpal is (unlike in e. g. humans)
immobile relatively to the others. This means that grasping occurred between
the first few fingers -- IV and V were never able to participate in this and
were therefore reduced. (*Plateosaurus* has 3 tiny phalanges on its pinkies,
however, as have some ornithischians.) This was connected to an overall
enlargement of the first 3 fingers and metacarpals, especially in theropods
where the remaining fingers are usually longer than the toes; metacarpal III
ended up at the distal end of the ulna (which has probably fooled the BAND
in their embryology studies;
http://www.dinosauria.com/jdp/archie/paulfed.html). In coelurosaurs or so mc
III seems to have acquired the ability to rotate slightly around mc II,
which may have brought it in direct opposition to the thumb and improved the
grasping ability. Maybe this saved it from reduction (except in
tyrannosaurs, which apparently didn't need it); confuciusornithids
hypertrophied this feature, probably because their IInd fingers had become
less flexible to support the primaries. Their sister group (all other known
pygostylians) slowly stopped using their hands for grasping, so their IIIrd
fingers became reduced to what they are now. II and I were indispensable for
flight.

> The hind limbs were still needed when the animals were grounded; a running
> start, for example, might have been the way they took off from the ground
> when the wings were still unable to provide all the energy for that job
> themselves.

A running start? I haven't understood much of BCF -- so far I thought it
didn't include a
ground-up scenario?

> The problem I have with the usual scenario is there is simply no way the
> small, grasping forelimbs of a Coelophysis-size theropod could have
evolved
> into wings.

Not directly, of course. The first flying theropod should IMHO have had
hands more like those of *Archaeopteryx* or, to mention something more
basal, *Ornitholestes*.

> What possible aerial function would such pre-wings of a running
> theropod have?

None =8-)

> (Or are you one of those who imagines that the wings of birds
> evolved all their aerial features on the ground,

most "aerial" features, yes

> and then as if by magic the
> animals were suddenly able to fly, never having previously touched the air
> with wings?)

Instead of the magic I prefer to fill in the (speculative like everything
else) through-the-water
scenario of Klaus Ebel that I have explained onlist half a year ago (I hope
everyone understood it :-] ) -- it's the only explanation I know why a
winged
theropod should have started to flap and take off in the first place.

> Feathered pre-wings would get in the way of any predatory
> grasping function but provide no compensating payoff.

Brooding! And later propulsion under water...
Furthermore, only finger II bore primaries, the other two were free, and
all fingers were flexible and had claws, so I don't think the effect of wing
feathers was that bad.

> And just because >I<
> may be too stupid to think of such a payoff is not an argument >for< such
> a payoff, either.

Of course not... ~:-|

> Nobody else can come up with one, either.

Well, I think there are some reasonable suggestions.

On digit loss...
The standard neodarwinistic explanation for why some organisms have lost
certain features is, AFAIK, that there is a slight advantage in losing
_everything_ because everything requires time and energy to be built up in
ontogeny and energy to be sustained later. Therefore everything will be
lost, unless the costs of losing the feature's function outweigh this
benefit.
Example -- Both acanthocephalans (sort of strange rotifers) and tapeworms
are endoparasites that take up nutrition through the skin. Tapeworms have
completely lost their guts, while acanthocephalans have only reduced it to a
sort of ligament on which the internal organs are suspended.
IMHO tyrannosauroids did use their hands for something that required 2
fingers, including a "semi-opposable" thumb, but not number III. I don't
have too many suggestions as to what that might have been...

That humans have 5 fingers per hand does IMHO not need an explanation -- the
2 million years or so that have passed since our ancestors stopped climbing
are by far not enough for digit loss. That primates in general have 5 can be
explained by arboreality where every digit that can be used in grasping is
an advantage. Re-evolving digits is practically impossible because the
formation of digits requires a very complex pattern of gene expression that
won't come back once it is degenerated.
However, humans have longer and thicker thumbs than all other apes -- this
is interpreted as an adaptation to grasping tools.