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Re: Feduccia (was: polarity of bipedality in dinosaurs)



George Olshevsky wrote:

>Besides, there >are< fossil archosaurs that fit my criteria of what
>some dino-birds might have looked like. These include _Longisquama_,
>various megalancosaurids (or drepanosaurids), and whatever small,
>birdlike thing has gotten mixed into the type specimen of _Protoavis
>texensis_. If hard evidence for BCF, in the form of real fossil
>dino-birds (for example), were abundant, it would have become the
>standard theory long ago.
.......
>BADD and BCF are in substantial agreement that birds and theropods
>are intimately related, and agree in most of the major features of
>dinosaur-bird phylogeny.

I have just purchased, and started to read, The Origin and Evolution of
Birds by Feduccia.  I have not read much of it yet, just skimmed it, so I
can't really discuss Feduccia's ideas in detail.  However it seems to me
that he is dismissive, if not scornful, of any view that birds and theropods
are "intimately related" even if he does look to things like Longisquama and
Megalancosaurus as "protobirds" of sorts.  Some of his basic arguments
(looking past the characters themselves, which I am only doing because I
haven't read enough yet) seem to be that: 

1. the similarities between birds and maniraptorian theropods are convergent
2. animals like Velociraptor are too big, heavy and earthbound to be bird
prototypes
3.  anyway, they lived too late
4.  birds evolved from arboreal ancestors, and Velociraptor isn't the least
bit arboreal.

I probably shouldn't take him on this early in my reading, but I have some
serious problems with thes points at least.  First of all, I agree that
birds probably evolved from arboreal ancestors.  Points 2-4 strike me as
valid too.  My big problem is this:

I thnk that point 3, in a way, cancels out points 2 and 4.  Obviously things
like Velociraptor couldn't possibly be bird ancestors - but their prototypes
in (say) the early Jurassic could have been.  Since we don't know what they
were like, why could we not assume that the size and other characteristics
of Cretaceous maniraptors that bother Feduccia evolved AFTER the
proto-maniraptor/bird split?

This is, of course, speculation - but I am bothered by the idea of massive
convergences between organisms with very different lifestyles.  Feduccia
points out, rightly, that convergence can be extreme and cover a wide suite
of characters - but this usually happens among animals adapted for similar
niches (eg foot-propelled diving birds).  Why should large terrestrial
maniraptors converge in so many ways to small arboreal birds or protobirds?
I am sure that whatever one can say about creatures like Longisquama, they
look a hell of a lot less like birds than Velociraptor does in many ways.

Is Feduccia's problem that he cannot imagine an arboreal proto-maniraptor?
If so, I find this much less of a stretch than to imagine a long-bodied
quadruped like Longisquama or Megalancosaurus turning into a short-trunked
obligate biped like Archaeopteryx.  This raises the point I have made
several times here (and that no one seems to want to discuss!) that gliding
quadrupeds may be very poor models for bird ancestors, as the natural
tendency in such a creature would be to keep the trunk lengthy and the hind
limbs involved in the gliding surface in order to maximize the surface area
of the gliding membrane (Though Feduccia cites an example I have never
heard, the sakis of South America, which seem to glide on long fur extending
from the forearms. Weird.).  I find it much easier to believe in the
evolution of birds from a  functional biped developing structures on the
forearms used for upward fluttering rather than gliding.  I see no reason
why a biped (or at least a creature dependent primarily on its hind limbs
for locomotion and its front for grasping) could not be, or become, arboreal.

So why isn't it more likely that the "birdlike features" of Longisquama etc
are convergences to birds (both are arboreal) and those of maniraptors
represent true kinship, rather than the reverse?  Is it impossible to
suppose that whatever the common ancestor of birds and maniraptors was, that
it could have been a bipedal, possibly arboreal dinosaur or something very
much like one, even if it lived in the late Triassic?  Feduccia makes much
of the comparative brain sizes of birds and maniraptors and ascribes this to
arboreality in the former; if he is right, then all you have to do is
suppose that the split came either just before or shortly after the bird
ancestor became arboreal, with the convergences related to bipedality and
enlarged brain size being a special feature of the bird line.

Feduccia illustrates a hypothetical protobird on p. 92, as  long-bodied,
rather sinuous glider.  I find it hard to believe in such a creature, though
Feduccia seems convinced of it.  Am I missing something?

PS - all this has nothing to do with BCF vs BADD; I don't want to start THAT
one again!
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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