Re: (LONG) THOUGHTS ON BIRDS AND OTHER DINOSAURS

[Jonathon Woolf <jwoolf@erinet.com>]

GSP1954@aol.com wrote:

> (The following are some thoughts inspired by comments by Tom Holtz and
> others.)
>
> Uh-oh, I have a sense that it's deja vu all over again.
>
> You see, many moons ago, I had some outrageous notions. Why, I used to say
> that small dinosaurs were probably feathered (but that definitive evidence
> either way was not yet available), that one cannot use feathers to define
> Aves, and that one cannot assume that isolated Mesozoic feathers belong to
> birds rather than dinosaurs. Most researchers vigorously disagreed, although
> exactly why was never really clear. Seemed to have something to do with the
> notion that feathers are only found in birds, so parsimony demanded that
> feathers must be assumed to be avian unless proven otherwise. I countered that
> considering the absence of any scales or other skin for small dinosaurs, the
> matter was entirely open, parsimony was not present in either direction, and
> restoring small theropods with either feathers or scales was equally valid. In
> other words, I used a scientific methodolgy called "we don't know the heck
> what is going on yet, so let's wait for new evidence to tell us what is
> correct, and until then relax and accept all viewpoints as equally valid". You
> want to illustrate your little dinosaurs without feathers fine, with scales go
> right ahead. Now the issue is essentially settled:)

A valid point of view.  Others (like me, amateur though I am) tend to prefer the
approach of "go with the existing theory until definitive counter-evidence is
found."  Now it has been, so never again can anyone assume anything either way
about the external covering of small theropods.  After all, _Tyrannosaurus_ is a
derived coelurosaur, and none of the known samples of tyrannosaurid skin show 
any
eviudence of feathers, even _Sinosauropteryx_-type protofeathers.

<snip>

> As for cladograms and parsimony. I suspect one reason cladograms tend to plot
> Archaeopteryx closer to birds is because they do not include all the relevant
> characters. Also, avian flight characters that are lost along with flight can
> bias the results. For example, flying Confuciusornis had a long, strut-like,
> retroverted coracoid like that of modern flying birds. A flightless ostrich
> has a shorter, broader, procumbent coracoid that looks like that of a
> dinosaur. Yet the ostrich is phylogentically much closer to modern flying
> birds. In losing flight, the ratite coracoid has reverted to the dinosaur
> condition. Likewise, it is possible that the seemingly less avian - compared
> to Archaeopteryx - coracoids of say dromaeosaurs are actually secondarily
> reversed to the dinosaur condition. Scored on a cladogram, it would plot below
> the urvogel. But maybe its just secondarily flightless.
>
> So far those doing cladistic studies have completely ignored the potential
> bias introduced by the possibility of early loss of flight. Taking steps to
> negate the bias (by running the character analysis while including and
> excluding flight related characters and seeing if there is a difference)
> should be the norm.

This is a testable hypothesis.  Someone could run a new analysis of the fossil 
in
question, throwing the new ones into the mix, and see how everything sorts out.
Is anyone interested in doing this?

> As for cladistic parsimony, it is a useful but dangerous thing. If one ran a
> cladistic analysis on the skeletons of gorillas, chimps and people, the apes
> would clump together way from the human. But of course DNA analysis shows that
> chimps and humans form a clade above the level of other apes. The problem is
> not really with cladistics. A much better set of tranistional fossils,
> analyzed cladistically, would also show the correct pattern.

It's hard to tell exactly where the problem lies, actually, because as of now, 
no
one is sure what the truth is.  There is a pathetic shortage of non-hominid
hominoid fossils from the last five to ten million years.  The DNA analysis 
might
be right, or it might not be.  The morphological analysis might be right, or it
might not be.  Given some of the questions raised about DNA trees in recent 
years
(birds and mammals form a clade outside dinosaurs? excuse me?), fossils are the
only definitive evidence -- and in both the human/ape case and the dinosaur/bird
case, the fossils are not complete enough to tell for sure.

-- JSW