Re: secondary flightlessness (wordy)

[Dinogeorge@aol.com]

In a message dated 98-06-24 16:47:12 EDT, th81@umail.umd.edu writes:

<< Under phylogenetic-driven hypotheses, all that would be required to
convince
 me that _Caudipteryx_, _Protarchaeopteryx_, _Mononykus_, _Velociraptor_,
 _Allosaurus_, _Apatosaurus_, and/or _Triceratops_ were secondarily
 flightless is to demonstrate that the most parsimonius phylogeny places
 these taxa within a clade whose ancestral state is unambiguously volant.
 This is the same standard which evolutionary biologists working on other
 groups of organisms employ. >>

I, for one, don't think _Apatosaurus_ and _Triceratops_ were secondarily
flightless. (Sheesh!) They probably, however, had a common arboreal ancestor
with birds, way back in the Middle Triassic somewhere, when the first great
cladistic split occurred in Dinosauria.

Certainly life would be a lot simpler if it were always the case that we had a
phylogenetic bracket. But in the evolution of avian flight--as in the
evolution of avian endothermy, by the way--we do not. If we did, the case
would already be closed in favor of theropod secondary flightlessness, and
there would be no debate and no need for my attempts to convince anyone. (I
am, of course, eagerly awaiting the discovery of a small, 50-cm-long Early
Jurassic carnosaur with large forelimbs and feather impressions...)

We certainly know that avian flight evolved at >some< point in the evolution
of birds from theropods. The problem is, when? So far, we keep turning up
these Cretaceous birdlike theropods that can't fly, but since we know that
something like avian flight was already present in the Late Jurassic
_Archaeopteryx_, we know that a volant lifestyle was already being exploited
millions of years earlier. It seems most natural to me to conclude--or at
least to entertain the hypothesis!--that the flightless Cretaceous forms
descended from Jurassic fliers. This is because I believe that avian flight
took tens or scores of millions of years to evolve, slowly and
incrementally--allowing for the existence of thousands of potential flying
ancestral species--and because I think it is extraordinarily unlikely that
avian flight developed from the ground up, working on large, heavy animals
>against< gravity rather than on small, lightweight animals >with< it.

<<However, as Alan Brush & others were talking about in the post-press
conference milling about time, there are gradations in what we mean by
"fliers", and it may be that one or both of the new Chinese forms had the
ability to get off the ground for short pulses of airborn transport.
Whether we call this "flight" is debatable.>>

As I have noted in previous communications, I follow the dictionary definition
of flight, namely "passage through the air." The kind of flying that birds do
now, which is powered (by flapping) and unstable (for maneuverability) and
which I term "ornithoptering," is only one kind of flight. There are also
leaping, falling, parachuting, gliding, and powered stable flight (perhaps we
can call this "archaeoptering"), and possibly other kinds of flight that we
cannot visualize because there are no living animals that exploit them. There
are also gradations of flight within each of these categories. "Flight" is a
continuum that extends from falling to the kinds of frenetic orrnithoptering
that hummingbirds and swifts do.

Ornithoptering is so physiologically demanding and so contingent on the
possession of numerous specific anatomical features that it must have evolved
through lots of intermediate stages of less demanding kinds of flight: A
slightly better wing here, a slightly lighter foot there, somewhat stronger
pectoral muscles here, and so on. All cladists understand that the phylogeny
of theropods--as descendants of such intermediate flying forms--can provide a
roadmap to the evolution of avian flight. The problem is, I think, to perceive
the appearance of the various avian features (such as furcula, lost manual
digits, retroverted hallux, opisthopubic pelvis, stiffened tail, hollow bones,
bipedal stance, endothermy and/or aerobic metabolism, maniraptoran forelimb,
semilunate carpal, fused carpometacarpus, keeled sternum, and so forth) among
flightless theropod groups not as the random-chance accumulation of characters
but as incremental improvements in the context of the evolution of avian
flight.