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Re: A bit about BCF theory



I must admit I find GO's arguments for BCF extremely interesting and worth
considering - but not necessarily convincing.  My chief concern is that (as
presented here) they are based not so much on evidence as on a sort of
socratic reasoning process, and that the results of that process do not seem
as clear-cut to me as they do to George (of course I realize that his
message was not intended as a formal scientific presentation of his
hypothesis).  I admit that I cannot do any better as I am not that
knowledgeable about the details of the fossil record and have not done bird
anatomy for years, but I would still like to raise a few counterpoints:

>The big problem is, when we
>finally do start finding good skeletal remains of small, arboreal
>maniraptorans, will we know what we are looking at?

Assuming, of course, that there WERE small, arboreal maniraptorians.  I
suspect that there were (in fact Archaeopteryx may well be one).  But I
agree that it  will be extremely difficult to place any bipedal fossil as
arboreal (unless it is extremely derived) because many of the "clues" to
arboreality in modern birds are, I suspect, awfully hard to determine from
the skeleton.  As I have said before the key bird family to look at may be
the Cracidae, which are quite arboreal but do not differ that much from
terrestrial pheasants in structure.  I do not know if anyone has done a
skeletal analysis of cracids  and other phasianids to see if there are
consistent skeletal differences between largely arboreal and largely
terrestrial forms; it might be a useful exercise.  As  far as mammals go it
might also be interesting to consider tree kangaroos, which again are not
(to my knowledge, which is limited) particularly strongly adapted skeletally
for arboreal life, tend to escape predators by leaping (from huge distances)
out of trees  and bounding off in the undergrowth, and include a
newly-discovered species that is largely terrestrial.  Which raises one
point - perhaps these creatures show modifications of the bones to withstand
shock (they  seem to be able to leap out of trees from staggering heights)
that you would not find in purely terrestrial forms.  Has anyone looked at this?

In short - do we have any idea what features of a putative bird ancestor are
truly indicative of arboreal habit, if any?

>BCF predicts that at least some of the smallest Jurassic and
>earlier maniraptorans had relatively large, grasping forelimbs.

Surely at some point the first bipedal archosaurs must have had pretty large
forelimbs, especially during the transition from quadrupedality to
bipedality; this would be true no matter which line gave rise to birds.

>Considerable diversity is to be expected, since most lineages of
>small maniraptorans never gave rise to large carnivorous forms
>but simply evolved into other small maniraptorans.

Ignorant question: what evidence is there for this?  And while we are at it
- what exactly do you mean by "small"?

>As a byproduct of the above, BCF notes that the character
>"reduced forelimbs" and its many variants, e.g., "humerus less
>than 50% of femoral length" is not a theropod synapomorphy. This
>feature recurs in dozens of theropod lineages, as indeed it
>recurs in later avian lineages throughout the Mesozoic and
>Cenozoic right up to date, whenever a lineage of flightless,
>cursorial birds branches away from a lineage of volant birds.

Storrs Olson has shown, I think pretty convincingly, that the evolution of
flightlessness from volant bird lineages (which can occur very rapidly;
there are even flightless birds that are only subspecifically differentiated
from their volant relatives, such as the Aldabra race of the White-throated
Rail Dryolimnas cuvieri) involves far more than merely forelimb reduction,
and that many of these characters can be explained as a form of
paedomorphosis.  I would be interested to know if any of the Mesozoic forms
hypothesized by some to have been derived from volant ancestors show
characters of this type.

>Did it never strike anyone as strange that, despite their
>supposed evolution as consummate grasping organs, theropod hands
>usually had only three digits? Dinosaurs plesiomorphically had a
>five-fingered (pentadactyl) manus, as shown by the preservation
>of this primitive condition in brontosaurs and ornithischians
>(i.e., phytodinosaurs). Why would the dinosaurian hand _lose two
>fingers and even three fingers_ on the theropod/avian side of the
>group? Surely five fingers are better than three for grasping and
>capturing prey.

May I suggest that this is possibly an assumption based on our own hands
rather than those of dinosaurs?  I think that how many digits are required
may be very much related to how the hand was used.  If fine manipulation is
required, or if ONE hand is used to grasp and manipulate objects (as ours
is) then I can see George's point.  But what if the arms and hands, acting
in concert, were used as grappling hooks?  In that case the grasping
function might well have been accomplished by both arms acting together and
directed towards the midline.

 And why would the theropod hand lose those two
>fingers from the _caudal_ side of the manus: IV and V, not the
>more symmetric I and V (embryological studies of modern birds
>notwithstanding)? And note that digit III is generally the
>slenderest and most dispensable of the remaining digits, as if it
>were next in line on the way out.

Again:  if the arms were used as grapplers it might well be that the most
anterior digits would be the ones most likely to be retained as the digits
most likely to make the first and most critical contact with prey.

>BADD theory provides no explanation; it merely accepts this
>situation ("Well, what they heck, why _not_ lose a couple of
>fingers?") as a _fait accompli_.

OK, I've just suggested an explanation, for what it's worth.

> Imagine a small, arboreal form caught on the horns of
>an evolutionary dilemma: should it retain a grasping forelimb, or
>should the forelimb become more winglike and lose its grasping
>ability? The theropod manus represents a compromise between these
>two mutually exclusive evolutionary characters.

This assumes that the maniraptorian forelimb is, in fact, not an adaptive
suite of characters highly adapted to prey capture (or whatever) but the
equivalent of an amputee's stump with a prosthesis, and that it stayed that
way for millions of years through the evolution of a diversity of forms.
Maybe I'm old-fashioned but I balk at this.

What is really needed here is a detailed mechanical analysis of the
maniraptorian arm-hand complex.  If, for example, such a study were to show
that the hand was, indeed, highly adapted for grappling prey (say), and that
the pattern of digit loss and other features were what such a mechanical
analysis would predict (looking at potential lines of force, stresses etc)
then there  would be no a priori reason to assume that it had evolved for
another purpose and was evolutionarily "crippled" as a result.

This is not, I submit, equivalent to the reduction of wings in known
flightless birds (I am NOT including Monokynus as it is just too weird to
conclude much from!).  With the exception of penguins and a few species that
use their wings in display (Ostrich, Kagu), such birds have effectively lost
all function in the wing (indeed in some the wing is all but lost) and there
appears to have been no selective pressure to re-adapt it to serve some
other function (even in the presumed predatory phorusracids, which
presumably could have used a grasping limb if they had had one).  In
maniraptorians, on the other hand, the forelimb appears to have been an
important part of the animal's armaments and  would certainly have been
acted on by selection with respect to its efficiency in serving such
functions.  Given that this occurred at least through half the Jurassic and
much of the Cretaceous I find it hard to believe that such a forelimb would
have remained a "compromise".  It must have been highly functional.

> Then, all of a sudden, we
>have didactyl manus (tyrannosaurids), monodactyl manus
>(_Mononykus_), and of course volant birds, in which the three
>digits are fused into a unit. It is as if some kind of
>evolutionary threshold were crossed sometime in the Late
>Jurassic, as enough improvements occurred to the wings to allow
>the animals to forego grasping hands entirely.

The didactyl manus of tyrannosaurids, whatever its function, cannot be
viewed in isolation but as part of a shift in the prey-catching apparatus of
these dinosaurs involving the whole body - possibly a subsitution of
grasping for prey capture with powerful biting mechanisms, perhaps to subdue
larger prey that had to be killed quickly and with great force to avoid
injury to the predator.  Grasping hands may well have had little to do with
such a killing mechanism - as is true for large mammalian predators today -
note that it is usually smaller predators, taking smaller prey,that have
greater manipulative power in the forelimbs (eg raccoons).  I cannot imagine
that this process had anything to do with improved efficiency of wings (in
tyrannosaurs?  Are there ANY didactyl tyrannosaurids small enough to have
been recently derived from ancestors with wings or anything like wings?

As for Mononykus, well, who knows (though if I had to guess I would think
that the likeliest use for those weird forearms may have been intraspecific
combat - rather like the spikes on the wings of living jacanas).

>
>How about the peculiar nature of the large-maniraptoran forelimb?
>If you articulate the forelimb bones of _Deinonychus_ naturally,
>you find that the forelimb's motion is rather restricted: The
>hands fold with the palms toward each other, and the arm bones
>are generally incapable of pronation and supination. The
>semilunate carpal keeps the hands "flat." How is this an
>_improvement_ in the manual grasping function?

Again - if the limbs  were grapplers this would work well as the grappling
points would be directed towards each other to hold the prey (think about a
pair of ice tongs).

>The large maniraptoran predators (dromaeosaurids, troodontids,
>etc.) that evolved from such halfway-to-flight ancestors did not
>_develop_ the maniraptoran forelimb; they were "stuck with it" as
>part of their evolutionary legacy from their nearly volant
>Jurassic ancestors (which resembled _Archaeopteryx_).

See my points above; I find this very hard to accept.

>
>How about the retroverted first digit of the foot? BADD theory
>notes reduction of the hallux as an adaptation for cursoriality.
>Indeed, this is the case: in cursorial forms the tendency is to
>symmetrically _lose_ the outer digits of the foot. The evolution
>of ornithischians provides a good example of this. But the
>operative word here is "lose": This does not mean "articulate
>with the back of the adjacent digit and sit there." 

I admit that the only good reason I can think of offhand for evolving a
retroverted hallux is to produce an opposable grasping organ, and the
likeliest use for such an organ is for hanging onto a branch.  This may be a
failure of imagination on my part, though - again I would like to see an
analysis that considered whether such an arrangement might in some way
contribute to balance in a fast-moving biped.  I note that contrary to what
one might expect not all cursorial birds today show a reduced hallux - some
highly-terrestrial larks have a well-developed hallux with an extremely
long, straight claw, though what function this serves is beyond me.

It occurs to me that I may be missing something here about the distinction
between the two hypotheses - it is beginning to sound like the only
difference (as far as these arguments go) is between deriving birds from
small arboreal maniraptors  as opposed to large terrestrial ones.

>
>How about the simple rpesence of huge claws on most theropod
>hands and feet? In what other vertebrate predators are the claws
>so enormous and powerful with respect to the other phalanges?

As we have no large bipedal non-volant predators other than ourselves around
today I am not sure what this proves.  Anyway, just to answer George's
question: Hawks and owls?

> The central archosaur-to-bird lineage itself
>undoubtedly followed lots of evolutionary blind alleys, throwing
>off descendant forms, with all kinds of "almost volant but no
>cigar" adaptations, most of which became mercifully extinct but
>some of which found new roles and niches and diversified into the
>various theropod groups. But one thing the lineage did do is
>(somehow) evolve into modern birds.

I must admit that this sounds remarkably like an argument for either
directed evolution (in which I am sure GO ddoes not believe) or the kind of
argument that assumes that evolution heads in a certain direction determined
by its end product, tossing off irrelevant side branches as it goes.

Of the arguments George has given I confess the only one that really
suggests to me that the stem line for all therapods went through an arboreal
stage at some point is the reversed hallux, as it is the only one for which
no functional explanation readily suggests itself that could explain its
derivation in terrestrial forms - and even then I am not entirely convinced.
That doesn't mean I don't believe that it could have happened that way - in
fact the idea fails to bother me EXCEPT to note that having an arboreal
ancestor does not mean being a side branch on the way to bird evolution.  I
can think of at least one living mammalian species, for instance, that
evolved bipedal terrestrial locomotion as a secondary development deriving
from a suite of arboreal adaptations present in tree-living ancestors
without having taken any non-technological steps towards the evolution of
flight (at least before death).  George's model could (as presented here)
just as easily lead to the (admittedly slightly less parsimonious)
conclusion that the pathway was from an arboreal ancestor to a terrestrial
dinosaur lineage retaining features of arboreality, with some of the members
of that lineage becoming the ancestors of birds at a later date.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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