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



In a message dated 96-09-25 17:07:23 EDT, martz@holly.ColoState.EDU (Jeffrey
Martz) writes:

> 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).

"Dino-birds" is my informal term for arboreal archosaurs among whose
descendants are dinosaurs and birds. We can't do a formal taxonomy for them
when so few such forms are known. Cladistically, however, the more advanced
dino-birds (including, I believe, _Longisquama_, _Protoavis_, and even
_Archaeopteryx_) would be true dinosaurs, having diverged later than the
ancestral phytodinosaurs from the central lineage leading from the common
archosaur ancestor to modern birds. And the less advanced dino-birds
(including, I believe, _Megalancosaurus_ and its relatives, and perhaps
_Cosesaurus_), having diverged from the central lineage, would not be
dinosaurs. Some of those would be non-dinosaurian ornithodirans, for example.
 
>      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.

The current BCF reason for manual digital loss is to accommodate the
increasing size of the wing feathers, which extended backward across digits
IV and V. The presence of the extra digits hampers the airfoil and, as you
note below, adds unnecessary weight to the wing.

>      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?

In pterosaur wings, digit V was lost and digits I-III were reduced about
equally but retained a kind of grasping or even locomotor function; digit IV
was greatly elongated and held the wing, of course. In bats only one digit
per wing is not directly involved in flight, and it is reduced. You're quite
right to suggest that digital reduction would be about equal if saving weight
were the only consideration. In dinosaurs, however, the digits were lost
serially: first V, then IV, then III (in tyrannosaurians). To me the >serial
nature< of the loss is important, and suggests aerodynamic factors in
addition to weight reduction--a kind of compromise manus attempting to become
a better wing but at the same time without forfeiting too much grasping and
clinging ability at one time.

>      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)

That's the way they started out, but the [remaining] fingers of the hand
gradually elongated as the wing developed. You're exactly right to question
what a terrestrial biped might find handy about fewer, longer, more awkward
hands. The awkward manus is something theropods were stuck with after
becoming ground-dwellers. Of course, the problems created by the much more
awkward manus of hesperornithiforms, phorusrhacids, kiwis, moas, and so
forth, which vestigialized from true wings, were evolutionarily overcome in
time.

>     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.

When birds become stranded on islands and evolve into flightless forms (which
takes place quite rapidly in the absence of predators), they generally become
larger--much larger--rather than smaller (dodo of Mauritius; _Aepyornis_ of
Madagascar). That was one of the problems I had a hard time with when I was
considering island endemism. Island endemism usually acts to reduce >giant<
forms, such as the dinosaurs of Hateg, Romania and the mammoths of various
islands, but on small forms island endemism often acts as a stimulus to
evolving larger forms.

In BCF, archosaurs started out small (~50cm snout-to-tail-tip), and along the
central lineage they stayed small right up to the appearance of
_Archaeopteryx_ and the more modern birds thereafter. Large size would
generally be selected against in habitually arboreal forms because of the
falling problem, at least until the animals acquired a reliable solution to
this problem (such as really good hand-eye coordination, as in monkeys and
apes).
 
>      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.

Feather impressions are seen on most if not all of the seven (or is it eight
now?) skeletal specimens of _Archaeopteryx_, and the first specimen found is
>nothing but< a feather. Indeed, it was seeing the feather impressions in the
Dutch (Teyler) specimen (if I recall correctly off the top of my head) that
tipped Ostrom to the fact that it had been misidentified as a non-avian for
decades.

If _Compsognathus_ had feathers, they were almost certainly not the long
wing-feathers seen in _Archaeopteryx_. Downy contour feathers would probably
be it.

In BCF, _Compsognathus_ is considered a cursorial flightless descendant of a
minor radiation of didactyl-winged birds. I can imagine many reasons why a
flightless dinosaurian bird-descendant might >lose< its feathers (too hot
under them, too awkward for running, too awkward when grasping, and so
forth), but very few why such a form might gain them or enlarge them if it
had them plesiomorphically. (Maybe for display? Shade? Possible but pretty
flimsy and not compelling, I think.)