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Dinogeorge Digest #3

Date:   98-06-26 16:18:09 EDT
From:   Dinogeorge

In a message dated 98-06-26 03:03:36 EDT, Tetanurae@aol.com

<< To imply such fraud without any proof, and
 without having even examined the specimen is much worse than any comments
 George may have made about Bob Bakker and the wax skull.... >>

Good thing Feduccia isn't on the dinosaur list. He would have been timed out
for a week long ago.

Subj:   Re: secondary flightlessness
Date:   98-06-26 15:33:48 EDT
From:   Dinogeorge
To:     dannj@alphalink.com.au

In a message dated 98-06-26 03:04:03 EDT, dannj@alphalink.com.au

<< Secondly, I don't think feathered flight was necessarily so unique that
 it developed just once. Take bats and flying foxes for instance. Two
 groups that are not all that closely related, but which evolved into
 similar forms. Just because Archae had feathers and seemed capable of
 flight does not mean that it contributed in any way to the line that
 led to modern birds. >>

I have argued on this list that slightly different kinds of avian flight may
have arisen independently and convergently within Theropoda. For example,
similar feathered wings may have arisen with different digital counts in
different lineages: _Archaeopteryx_, maniraptorans, and enantiornithans with
digits 1-2-3; _Protoavis_ and modern neornithans with digits 2-3-4 (to
confound everyone who pays attention to embryological evidence); ancestral
tyrannosaurians with digits 1-2 only; and so on. Presently I think this is
rather far-fetched, but who knows?

I think the correct pattern for the evolution of small, arboreal theropods
goes something like this. An arboreal-flight-favorable mutation appears in a
species, giving it some kind of competitive advantage in its particular
environment. This engenders the appearance of dozens of daughter species that
exploit this favorable mutation, which increases the chance that >another<
arboreal-flight-favorable mutation will appear among those daughter species.
When it happens, it engenders another evolutionary burst, more daughter
species, and so forth. The better-adapted descendants drive most of their
ancestral species to extinction. Repeat lots of times at various localities
worldwide simultaneously until you have birds (and after, of course--to give
rise to the 9000+ species of extant birds). Conclusion: the Mesozoic world
was filled with hundreds, perhaps thousands of species of small, arboreal
theropods with various kinds of flight capabilities, and with dozens of
species of flightless, ground-dwelling descendants from some of these
lineages. Those few Mesozoic bird fossils that we've found so far are just
the tip of the iceberg.

I hope you don't for one minute think that the archaeopterygid of which a
mere eight specimens have been found at one locality in southern Germany just
happens to be the one and only archaeopterygid that ever existed! As with
today's birds, there were surely dozens of archaeopterygid birds in the Late
Jurassic, and scores of contemporary related families, each with its
complement of dozens of members.

<<I supose it comes down to what you define as a "bird". It is my opinion
that the best biological group descriptions are based on living animals.
An animal is not just a skeleton - it is also behaviour, appearance,
and genetics (which presumably control much of the other two). If
it were up to me to define Aves I would have it as the most recent
common ancestor of all extant birds. I wouldn't be surprised if this
was some time in the Cretaceous. That is not to say that there weren't
feathered, flying dinosaurs before that time, but in my opinion I
wouldn't necessarily call these "birds" (assuming bird=Aves).>>

To me, birds are small, feathered tetrapods (with wishbones!) that live in
trees, or any tetrapods descended from such creatures. Since this is a
diagnostic definition, not a phylogenetic one, I want to make sure the
phylogenetic definition doesn't exclude any such animals. I'm pretty sure
sauropods and ornithischians lacked feathers of any kind, but not at all so
sure with respect to theropods. So I take the phylogenetic definition of Aves
as far back as I can, to include theropod dinosaurs but exclude the sauropods
and ornithischians. Now, I (along with Bakker, Michael Cooper, and even the
late Alan Charig) think there is good evidence showing that sauropods,
prosauropods, probably segnosaurs, and ornithischians form a clade that is
the sister group within Dinosauria of the theropod/bird clade. (Others will
disagree, of course.) This supports my suggestion to define Aves as the stem
group of dinosaurs more closely related to _Megalosaurus_ than to _Iguanodon_
(and Phytodinosauria as the stem group of dinosaurs more closely related to
_Iguanodon_ than to _Megalosaurus_).

Subj:   Re: secondary flightlessness
Date:   98-06-26 15:33:43 EDT
From:   Dinogeorge
To:     dannj@alphalink.com.au

In a message dated 98-06-26 03:04:03 EDT, dannj@alphalink.com.au

<< I would have to disagree on this point (sorry George!). I think that
 flight most probably evolved quite rapidly. The earliest pterosaur
 fossils show highly specialised fliers. So too with the earliest bat
 fossils. There don't seem to be any (AFAIK) supposed "transitionary"
 forms for bats or pterosaurs. >>

You might be right about bats and pterosaurs (I don't think so, because the
fossil record is really, really lousy when it comes to preserving the small,
arboreal forms that were probably transitional between bats or pterosaurs and
their flightless ancestors), but there >are< plenty of transitional forms for
birds and pre-birds. We call them theropod dinosaurs. This fact alone
strongly supports a lengthy period of evolution of avian flight.