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Re: Bird Origins



In a message dated 95-10-06 21:49:04 EDT, DAVEH47@delphi.com (Dave
Hardenbrook) writes:

>To George O.: I just came across an article you wrote for OMNI a couple
>years ago in which you describe your idea that the "Birds Are Dinosaur
>Descendants" theory is wrong and that in fact all dinosaurs are descendants 
>of various stages of bird evolution ("Birds Came First" theory).  What is
>the current status of the theory--How have others responded to the
>theory, how have your own opinions evolved (no pun intended) since you
>wrote the article, and I would definitely like to hear others on the
>list give their opinions (especially G. S. Paul!)?
>

Greg Paul's thoughts (1984, 1988) on _Archaeopteryx_ and its relationship to
Dromaeosauridae and Troodontidae, namely, that it was close to the ancestral
form, and that Dromaeosauridae and Troodontidae could be regarded as "giant,
flightless archaeopterygids," much as extant ratites are giant, flightless
paleognathous birds, is what inspired me to develop the BCF thesis. Indeed, a
look at the character matrix that Tom Holtz published in his paper on
arctometatarsalian theropods shows virtually no differences between
_Archaeopteryx_ and dromaeosaurids right down his list of characters; the
taxa are so closely related that they should be regarded as two families
(Archaeopterygidae and Dromaeosauridae) within the single theropod suborder
Archaeopterygiformes.

The BCF thesis is that ALL dinosaurs--not just certain groups of
theropods--were ultimately descended from small, arboreal archosaurs (which I
call dino-birds), of which one lineage (which I call the "central lineage")
represents an adaptive sequence that begins with a small, lizardlike,
probably arboreal "ancestral archosaur" (perhaps resembling _Mesenosaurus_)
and ends with any modern bird. On the way from the former to the latter, the
central lineage lost many reptilian characters and acquired the avian
characters necessary to become true birds. If you believe in the theory of
evolution, it is obvious that such a lineage must exist; the problem is to
determine the order in which the various characters were acquired. Although
this lineage seems like "directed evolution," this is merely an artifact of
our choice; innumerable lineages diverged from it stochastically over the
span of the Mesozoic, some of which developed into the large, cursorial and
graviportal forms that we call dinosaurs, most of which remained tree-bound
and were lost to the fossil record.

The evidence for arboreality in dinosaur ancestry, some of which was noted as
early as 1911 by Othenio Abel and later by other paleo-ornithologists,
includes grasping hands, long claws, pneumatic skeletons, bipedality,
feathers (as aerobrakes), progressive reduction/loss of manual digits III,
IV, and V (as the hand trended more and more toward being a wing),
retroverted first toe for grasping by the feet (useless in a cursorial
animal), stiffened tail (for use as a primitive airfoil), and maniraptoran
forelimb (a vestige of the need to fold the enlarged forelimb/wing when not
in use). None of these features is _explained_ by the BADD hypothesis ("birds
are dinosaur descendants"); rather, they simply occur ad hoc, one after
another, as exaptations that just happened by sheer chance to converge in the
cursorial "first bird," which was thus magically endowed to take off and fly.
It is the lack of any convincing explanation for the appearance and further
development of the above features within the BADD framework that I find
profoundly unsatisfying. BCF can explain the existence and order of
appearance of these characters as sequential adaptations within lineages of
small, ever more volant climbing and gliding animals.

BCF also provides a simple and convincing (to me, at any rate) explanation
for why we find all the most birdlike theropods in the fossil record _after_
the appearance of _Archaeopteryx_: the "birds" came first. One consequence of
BCF is that the reduced forelimbs of theropods are not a synapomorphy of the
group but appeared convergently dozens of times within different theropod
clades.

Note that BCF does not materially contradict _any_ well-established dinosaur
cladogram, although I happen to have developed my own, which is at variance
with the ones developed by most other workers. Even if my own cladogram of
Dinosauria proves incorrect, BCF is not closely linked to it and remains
perfectly viable within the context of any of the dinosaur cladograms I've
seen published in the past few years. The big problem with BCF for me is
trying to figure out what kind of fossil find might falsify the theory and/or
definitely distinguish it from the BADD theory. This is tough, because BCF
develops from and extends BADD by saying something about the common ancestral
forms of extant birds and extinct dinosaurs; it does not really contradict or
oppose it.

Most paleontologists still think BCF is a crock, but I believe they'll
eventually come around, once they see that the BADD hypothesis is sterile and
once they actually read _Mesozoic Meanderings_ #2. A few paleontologists have
confided to me privately that they think I'm on to something, but they're not
prepared to say so in print yet, so I won't let on to who they are here.
These things take time; BCF represents a paradigm shift of some magnitude,
and the fact that it was initiated be me rather than a "real"
paleontologist--despite the fact that I've studied dinosaurs for the past 25
years--has hindered its acceptance. Imagine how quickly other
dinosaurologists would have come around if, say, John Ostrom, Phil Currie,
Paul Sereno, or even Tom Holtz had put forth the idea first.

Two of the critters I mentioned in the article as candidates for arboreal
archosaurs on or near the central lineage have fallen by the wayside.
_Cosesaurus_ (with its incredibly shrinking antorbital fenestra) is looking
more and more like a prolacertiform, and _Megalancosaurus_, while definitely
an arboreal archosaur, has turned out to be far too specialized to be
terribly close to the central lineage. But I still hold that _Longisquama_ is
an excellent example of a primitive dino-bird, and that at least some of the
remains referred to _Protoavis_ belong to another such critter. The sheer
extent of the differences among these small forms, incidentally, is
persuasive argument for a high diversity among dino-birds in general: Try to
imagine how many transitional forms must have appeared between them.

G.O.