[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: BCF& COMMENTS ON IT (really long)



In a message dated 98-07-04 03:00:33 EDT, m_troutman@hotmail.com writes:

<<Let me point out that I mean no disrespect to George Olshevsky or any of 
the BCF proponents, this message is simply to point out some weaknesses 
or flaws of BCF and the BCF scenario.>>

This is of course not an issue of respect but of building a theory of dinosaur
phylogeny and the origin of avian flight. Certainly, BCF has weaknesses, but I
think the currently accepted "standard model," which I have playfully called
the BADD theory (for "birds are dinosaur descendants"), presently suffers from
more. Indeed, the closer BADD comes to BCF--and it has recently moved slightly
in that direction, with the discovery of feathered theropods such as
_Caudipteryx_ and _Protarchaeopteryx_ and having to consider the possibility
that these were secondarily flightless theropods--the fewer weaknesses it will
have (heh heh). BCF synthesizes the BADD model, in which birds acquire the
ability to fly as ground-dwelling, cursorial dinosaurs, with the "trees-down"
model, in which birds acquire the ability to fly as arboreal archosaurs
unrelated to dinosaurs. I simply show how birds might have originated as
arboreal archosaurs that gave rise to dinosaurs, thereby explaining the close
relationship between birds and theropods evidenced by their anatomical
similarities. Everything else is details.

<<George Olshevsky wrote regarding secondary flightlessness:

<<All the cladograms I've seen do indeed support secondary 
flightlessness in _Mononykus_, certainly. But the important thing is 
that, at this point, secondary flightlessness for many theropods is not 
>falsified< by any cladograms. There's a big difference between "not 
supported" and "falsified." Secondary flightlessness occurred repeatedly 
among Cretaceous birds (hesperornithans, _Patagopteryx_, _Gargantuavis_) 
and Cenozoic birds, and there's no reason believe that the process 
didn't also occur within Triassic and Jurassic birds groups, such as 
they were.>>

But these cladograms also support that Mononykus is within Aves far 
above the level of Archaeopteryx.  I like John Hutchinson's idea that 
alvarezosaurs are closest to the ornithurines even though I have not 
seen any evidence that this is true other than the large braincase and 
foramen magnum.>>

_Mononykus_ may or may not be "within Aves far above the level of
Archaeopteryx." (I happen to think not: the tail of _Mononykus_, if correctly
restored, is far too grave a reversal to the theropod condition for
_Mononykus_ ever to have been an avialan bird; nor are its arctometatarsalian
feet in any way avialan. Its birdlike cranial anatomy is quite likely the
result of convergence from having to accommodate a perhaps independently
enlarged braincase. I may change my mind on this, however, once I read the new
description of the _Mononykus_ skull.) This doesn't change the essence of my
remark, namely, that secondary flightlessness has been shown to yield a very
theropod-like animal.

<<It is true that there is a difference between "not 
supported" and "falsified", and so far the evidence does not support 
secondarily flightless NON-AVIAN dinosaurs.  However, just because the 
evidence at the time does not *fully* falsifiy the secondarily 
flightless hypothesis does not mean that one can scientifically justify 
supporting this hypothesis because it is not fully, 100% disproven.>>

Well, as far as I can tell, evidence from physics truly >falsifies< the idea
that birds evolved the ability to fly from the ground up. As far as I'm
concerned, it is 100%, no question disproved, even if a few diehards haven't
yet come around to thinking so. Therefore, at some point in the evolution of
avian flight, well before the appearance of _Archaeopteryx_, birds >must< have
gone up into the trees. So any ground-dwelling, cursorial descendants of those
flying, arboreal pre-archaeopterygids would, by my definition, be
>secondarily< flightless. There remains the problem of what we mean by
"flying" with regard to bird evolution, but I've been pretty clear about this
in previous posts. The dictionary defines "flight" as "passage through the
air," so if leaping from branch to branch, falling, parachuting, and gliding
were among the features of an arboreal lifestyle in pre-archaeopterygid
dinobirds, then they were technically fliers in my book, and their descendants
were secondarily flightless.

<<What Triassic and Jurassic bird groups?  If you're referring to 
Protoavis as a representative of one of these phantom groups I may be 
able to understand, but I would have to say that Protoavis is of 
doubtful "origins", the braincase looks almost crocodylomorph or 
coelurosaurian and besides the heterocoelus vertebrae and the large 
renal fossae the rest of the skeleton is questionable.  Of course 
Chatterjee may be right in his interpretations but this does not fully 
prove avian status or even the presense of volant capability.  Zhou 
(1995) has commented that a dinosaur of strange habits can begin to look 
vaguely birdlike in some characters.>>

I have been through this argument many times, but it is worth repeating here.
Surely you don't think that _Archaeopteryx_ was the >only< flying, feathered
creature of the Late Jurassic? Surely you don't think that _Archaeopteryx_
sprang into existence with no evolutionary relatives or forebears, even though
the fossil record has currently presented us with no >physical< remains?
Surely you don't think, just because we have only eight _Archaeopteryx_
specimens, that the entire population of _Archaeopteryx_ at Solnhofen in the
Late Jurassic was less than millions? Regarding _Protoavis_, as Larry Witmer
notes in the introduction to Chatterjee's book, if even >one bone< in the
_Protoavis_ assemblage is avian, it shows that there were Triassic birds. Are
you prepared to state that >none< of the _Protoavis_ bones is avian?
_Archaeopteryx_ and birds arose from >something<, and in BCF the same
something (or somethings) also gave rise to theropod dinosaurs.

<<Regardless, these bird groups are for the most part purely speculative 
and cannot be taken as actual evidence (even though they sometimes 
appear to be sometimes). >>

Once we uncover real representatives of those groups, we will have a much
firmer idea of the >details< of the evolution of birds and avian flight. I
eagerly await their discovery, because this will constrain the BCF model.
Right now, I have faith that they won't >disprove< the BCF model, but you
never know, of course.

<< Or maybe you're referring to those dino/birds' such as Megalancosaurus,
Cosesaurus and Longisquama.  NO evidence points to whether or not these are a
natural group or not.>>

I never thought that these formed a natural group (clade); Basitheropoda was a
paraphyletic taxon when I proposed it in 1991. Rather, each of these genera
represents a side branch of arboreal archosaurs (or perhaps archosauriforms).
Their mere >existence< supports and justifies the idea that there could well
have been >other< arboreal archosaurs, presently unknown, that gave rise to
birds (and dinosaurs).

<<  The evidence shows that Megalancosaurus and Cosesaurus are
Prolacertiformes and that Cosesaurus is one of the closest animals to the
Pterosauria (this is Dave Peters' hypothesis, the alternative hypothesis is
virtually identical except they are not near pterosaurs).>>

Prolacertiforms are the closest diapsids to archosaurs, so it is not
surprising to find prolacertiform characters in _Megalancosaurus_ and
_Cosesaurus_, both of which I consider to be below Dinosauria in the archosaur
cladogram. Exactly where these animals fall--whether they're closer to
archosaurs than to prolacertiforms or vice versa--is not particularly relevant
to the BCF thesis. They're such charming animals that I'll expand my list of
archosaur taxa in _Mesozoic Meanderings_ #2 to include prolacertiforms, if it
turns out that basitheropods and pterosaurs are prolacertiforms instead of
archosaurs.

<<  Longisquama cannot be placed ANYWHERE, it cannot be put into the Diaspida 
confidently (Padian).>>

Here I disagree. The presence of a furcula, together with a wide-open
antorbital fenestra and dermal structures that strongly resemble feathers
(without actually being feathers), strongly supports placement of
_Longisquama_ in the archosaur cladogram as a primitive theropodomorph
dinosaur. This will be confirmed (I predict) when a complete _Longisquama_ is
discovered. Right now, the type specimen--the only existing specimen with
skeletal parts--includes only the forequarters: skull, neck, shoulders,
forelimbs, and the anterior rib cage. We need to see the pelvis, hindlimb,
feet, and tail before we can confidently exclude _Longisquama_ from
Dinosauria. And even if _Longisquama_ turns out to be not a dinosaur but an
animal that acquired a few dinosaurian features independently outside
Dinosauria, this doesn't damage BCF. It just means we have a longer wait for a
Triassic dinobird to be described.

<<  What do all these animals share?  Well, other than the fact that all have
been put forth as a possible ancestor to birds and all share a possible
arboreal lifestyle, they share nothing at all.  Sure they do have some
characters such as triangular skulls; unserrated, isodont teeth
(Megalancosaurus); a birdlike shoulder girdle (this is found in Cosesaurus, a
relative of pterosaurs, which have a birdlike shoulder; a strap-like scapula
is seen in Megalancosaurus); and feathers (Longisquama) that might link these
disparate animals together by default as possible ancestors of birds or
birdlike creatures (phantom Triassic and Jurassic "bird" groups; the elusive
and totally hypothetical 'dino/birds' that gave rise to the various groups of 
dinosaurs).  However, this would be akin to linking placental American 
flying squirrels to marsupial sugar gliders because they share a rounded 
skull; large round eyes; a gliding membrane; and similiar proportions of 
the forelimbs and hindlimbs. >>

Yes, well, if you exclude >all< the characters that they share with
archosaurs, of course "they share nothing at all" with them! Small, arboreal
tetrapods, whatever their affinities, are >extremely< rare as fossils. The
fact that _Cosesaurus_, _Megalancosaurus_, and _Longisquama_ specimens exist
at all is incredible. The fact that they're all so different from one another
indicates how diverse the arboreal archosauriforms (or basitheropods) were:
Try to imagine the possible ancestors or close relatives of
megalancosaurids--wherever you might classify them--or of _Longisquama_. As I
said once before, surely you don't think these were the >only< arboreal
archosauriforms that existed during the Triassic, that they arose
spontaneously with no relatives or forebears or descendants?

<<FINGERS

BCF predicts that if there were a variety flying forms there could be a 
variety of finger numbers (1-2-3; 2-3-4; or didactyl).  The basis of 
this is that reduction of fingers can provide a better leading edge of 
the wing for flight or gliding.  However, again this is purely 
speculative and there is no fossil evidence in support of an idea like 
this.  Embryology studies aside, there is no evidence that the 
enantiornithines, Archaeopteryx, and theropod dinosaurs had digits 1-2-3 
and ornithurine birds had digits 2-3-4.  All the available evidence, 
including Burke and Feduccia's paper, which does not provide any new 
information into the dinosaur finger debate other than that the digit 
opposite the ulna is the primary axis of cartilage condensation (digit 
IV in most tetrapods, digit III in theropods and presumeably birds), 
supports that the sauriurines (Archaeopteryx+enantiornithines) and the 
ornithurines had the same digits.  Didactyl in Compsognathus and 
tyrannosaurs does not necessarily mean that they had a common ancestor 
just as it does not mean that cuculiforms and picines hada common 
ancestor because they had a similiar, but totally convergent, according 
to all analyses starting with Bock and Miller (a non-cladistic analysis 
in 1959), zygodactyl foot. >>

BCF doesn't care how many or which digits make up the avian wing or wings. I
personally believe that 1-2-3 is correct for both modern birds (despite the
embryological evidence to the contrary, which seems highly subjective),
advanced dinobirds, and tetanuran theropods. But BCF can accommodate all kinds
of wings in its theory, with more facility than BADD. If it turns out that
modern birds all have 2-3-4 wings, it is a stake through the heart of the idea
that modern birds are maniraptorans. In BCF, it simply puts the ancestry of
modern birds among the nontetanuran ceratosaurs (which have 1-2-3-4 hands) and
indicates that virtually all known Mesozoic birds and their theropod
relatives/descendants were unrelated to modern birds. Quite unlikely, I would
think, given the present state of our knowledge of avian evolution.

<<And while on the subject of the zygodactyl foot, let me point out that 
the zygodactyl foot did not evolve for scansorial clinging as is 
commonly stated (most notably in Pat Shipman's recent book).  The 
zygodactyl foot evolved for perching according to Bock and Miller, and 
it is the ectropodactyl foot (zygodactyl in rest and perching, but 
during climbing the hallux is usually useless and digit IV deviates 
strongly laterally; seen in many woodpeckers).  A reversible digit I is 
not needed for climbing, pamprodactyl (all digits anterior) is by far 
the best scansorial foot and incidently it is found premanufactured in 
dinosaurs. >>

As long as the animal uses all four limbs for climbing, the pamprodactyl foot
>might< be more useful (who can speculate on the relative "usefulness" of foot
anatomy; how would you measure "usefulness"?); but once the forelimbs are
generally lost to climbing and become wings, the hind foot must assume most if
not all of the climbing function, and then I think the shoe is on the other
foot (brutal pun here). Then I think an opposable hallux becomes much more
"useful." Certainly an opposable hallux occurs in many, many modern arboreal
birds; this alone speaks volumes about the "usefulness" of the structure.

<<LOSS OF DIGITS

Loss of digits does not necesarily indicate arboreal habits whether in 
the actual animal or in the ancestor of the animal.  Try to tell an 
equine that its ancestors were arboreal and that is why it lost its 
digits. True, loss of digits, either manual or pedal, can be indicative 
that the animal is question came from arboreal descent or is arboreal.  
As I noted above, the hallux is frequently unused in ectropodactyl 
climbers because the extremely mobile digit IV compensates for the usual 
function of the hallux.  Digit IV, when laterally directed, resists the 
the force C, which is the force pulling the bird away from the trunk 
(there is one main force of gravity that pulls a clinging bird down, A.  
A is divided into forces B and C.  B is the inwardly  and downwardly 
pulling force.  C is the outwardly pulling force).  Any digit or 
appendage that does not support counterbalance any force acting on the 
bird is useless, in this case it is the hallux.  The lesson that this 
teaches us is that the manual digits in theropod dinosaurs probably were 
not lost due to climbing.  You see, if all were directed forward (and 
even if manual digit I was a "twist thumb" in the Bakkerian sense) then 
there would be no reason that any digits would be lost because they 
would still function like the digits II and III of an ectropodactyl 
foot, which counterbalance force B, the downward and inward force.  
Assuming that the totally hypothetical "typical" 'dino/bird' had five 
manual digits, and a possible laterally directing digit I, the other 
four digits, II, III, IV, and V, would still be functional for 
counterbalancing the force of B and perhaps even the overall downward 
force, A.  I hope that everybody understands this because without 
illustrations it is hard to understand.  The main point is that in the 
totally hypothetical "typical" 'dino/bird', ALL digits of the manus 
would be of use.  For a diagram of something similiar to this look at 
Chatterjee 1998 or better yet, look at Bock and Miller 1959.  The 
Scansorial Foot of the Woodpeckers, with Comments on the Evolution of 
Perching and Climbing Feet in Birds.  American Museum Novitates Num. 
1931.   >>

BCF does not assert that manual digit loss is a climbing adaptation. Quite the
contrary! BCF states that manual digital loss is extremely unlikely in
climbing animals, for >exactly the reasons you delineate!<. They need to have
as many fingers as possible. The question is, in BCF, "Since fingers are so
useful to climbers, and BCF says that the earliest dinobirds were climbing,
arboreal animals, Why were manual digits lost in theropods?" The BCF answer is
that the forelimbs and hands were also developing into wings, so the trailing
digits of the hand (IV and V) became a liability. They interfered with the
operation of the feathers, and they interfered with the form of the wing. So
>despite< their use in climbing, they vestigialized. NOTE: The BADD model
>cannot< account for this exact pattern of manual digital loss in ground-
dwelling cursorial bipeds, which presumably would find retaining all five
digits more "useful" for grasping and holding prey (since they were not
converting their forelimbs into wings). BADD gives >no functional reason< for
the loss of manual digits IV and V; this is just >something that happened,< ad
hoc, that turned out to be "useful" when the forelimbs were suddenly exapted
for flight.

<<Since climbing is not an acceptable reason for the elusive, totally 
hypothetical 'dino/bird' ancestors of theropods there are other 
explanations for the loss of manual digits such as predation/grasping , 
lack of use of the digits, or even a better trailing edge for a gliding 
wing (this too is rather unlikely because the two digits lost would 
still have to support the feathers.  All of these answers are likely and 
not one can be held as absolute proof of BCF or any alternative 
hypothesis. >>

You're just waving your hands here (another brutal pun); this is right where
BCF demolishes the BADD model! (And please correct your misunderstanding of
BCF with regard to manual digital loss and climbing. This is very important.)
As I just noted above, BADD provides simply >no< explanation for manual
digital loss in pre-avian theropods. It is just "something that happened" that
resulted in birds having three-digit wings. Digital loss, whether manual or
pedal, is >not< something that occurs with ease in tetrapods; consider that it
took most of the Cenozoic Era for horses to change their five-digit feet into
one-digit feet. Consider that cetaceans and pinnipeds, with their highly
modified feet, still retain all five digits; likewise ichthyosaurs,
plesiosaurs, and mosasaurs. Consider that humans still retain all five digits
in both hands and feet, the basic tetrapod condition that evolved some 350
million years ago.

But one circumstance that results in digital loss is cursoriality; in some
(but by no means all) tetrapod lineages that are persistently cursorial, the
outer digits of manus and pes became gradually vestigialized and lost. Indeed,
when such digital loss is identified in a lineage, it is generally taken as
prima facie evidence of cursoriality in that lineage. The functional reason
for this is that the lost digits are superfluous; they add extra weight to the
limbs without providing a compensatory functionality. The pattern of cursorial
digital loss, however, is to make the feet axially symmetric. In avians,
manual digital loss is markedly >asymmetric.< BADD fails to answer the
question of why manual digital loss in theropods occurred at all, let alone in
such a specifically asymmetric way.

Asymmetry of limb elements in tetrapods often stems from adaptation to flight.
Airfoils must be asymmetric, because it is the difference between the airflow
over the top and bottom surfaces that provides the lift; a symmetric wing
would have the same kind of airflow over top and bottom and thus generate
near-zero lift. Wing asymmetry occurs everywhere in the wing: in the feathers
themselves as well as in the shapes and arrangement of the skeletal elements.
Theropods had highly asymmetric hands, thicker at the "leading" edge (digit
I), thinner at the trailing edge (digits III-V, with IV and V vestigialized or
nonexistent), and BCF accounts for this in a perfectly natural manner. The
BADD model, in which the forelimbs must preacquire their asymmetry for some
reason unrelated to flight, has tremendous difficulty explaining it.

<<MIRACLES

One part of the BCF scenario is that there are no "miracles" within it.  
In normal theropod phylogeny bipedalism was gained, partial  decoupling 
of the forelimbs, and then a forelimb stroke that evolved either for 
predation or other means that turned into the flight stroke.  It has 
been quiped that excepting this would be akin to accepting miracles.  
Dinogeorge finds it more likely that flight evolved from creatures 
without the help of "miracles" (most others call them exaptations). >>

Exaptations are not miracles, neither in BCF nor in any other evolutionary
theory. Feathers, perhaps acquired for sulfate excretion, become exapted for
insulation, display, flight, you name it, in BCF and in BADD. A >miracle<
would be the accumulation of twenty or thirty avian characters for twenty or
thirty different reasons other than for flying, only to find that this
particular combination of twenty or thirty characters enabled the animal to
fly. How can adaptations to a cursorial lifestyle suddenly also be perfectly
suited for a volant lifestyle? If you want to have a whale evolve from
mesonychid carnivores, you put them in the water >first<; they don't evolve
flippers and >then< go into the water! Likewise with birds: Put them in the
air, or at least, up in the trees, >first<. Then let them evolve all their
adaptations for flying. Why is this not perfectly obvious??

<<Going by the definition of miracle given, I can sight MANY. MANY, MANY 
instances within accepted (and virtually proven) parts of avian 
evolution that can be defined as "miracles".  The evolution of the 
reversed hallux and the anisodactyl foot through perching which then got 
exapted for downward tree-creeping in nuthatches.  The evolution of the 
zygodactyl foot for perching and then the evolution of lateral movement 
in digit IV during climbing making the ectropodactyl foot for scansorial 
climbing that became the modified pamprodactyl foot of the extinct 
ivory-billed woodpecker.  The evolution of a keratinous sheath over the 
mandibles, loss of teeth, evolution of a beak and the adaptive radiation 
of the varieties of bird beaks such as the finch bill, the hawk bill, 
and the flamingo bill. And so on, and so on, and so on.  Under the 
definition of "miracle" given by G.O., these can be considered miracles 
unless there are problems with these well established, tested and proven 
sequences. The "miraculous" evolution of features that led to flight in
theropods are what most all call exaptations. >>

You misunderstand my definition of a miracle. To me, a miracle would be the
spontaneous boiling of a cup of water because all the water molecules
happened, by chance, to align their thermal motions for a measurable interval
of time. This is the same kind of miracle, though actually far less likely
(because there are lots more molecules in a cup of water than anatomical
features in a tetrapod skeleton), as the kind BADD wants us to accept: that
twenty or thirty anatomical features, unrelated to flying, accumulate in a
single animal ("align themselves") and thus allow it to fly. I am not calling
exaptations miracles; I am calling the serial alignment of twenty or thirty
unrelated exaptations a miracle.

<<USE OF FORELIMBS "AFTER FLIGHT"

One main part of the BCF theory is that after flight (parachuting, 
gliding, powered flight) the forelimbs of an animal would turn back to 
normal because they lack the specializations that modern bird forelimbs 
do.>>

They wouldn't "turn back to normal," they would >lose< whatever flying
function they had acquired but would >retain< the grasping function that they
hadn't yet lost. Pre-maniraptoran theropod forelimbs derive from a stage in
evolution when they had >both< a grasping, climbing function >and< a
rudimentary flying function. The more winglike the forelimb, the less likely
it would be to still have some kind of grasping function that could be exapted
for predation (or whatever) in a cursorial, flightless form. Hence we have
such Cretaceous forms as _Mononykus_, with those bizarre single-clawed hands:
flightless animals descended perhaps from ancestors with highly winglike
forelimbs.

<<  Of course again, there is no evidence that theropods are derived 
from "flying", totally conjectural 'dino/birds'.  And really, there is 
no evidence that the theropods came from flighted ancestors; I cannot 
think of one instance where a gliding animal lost its "flight", so it is 
purely conjectural to assume what will happen once this happens.  In 
volant, powered flying ancestors the forelimbs are usually useless, 
probably due to paedomorphosis.  I'm sure you all remember what 
paedomorphosis is, and I still consider it the only factor that causes 
teresstrial flightlessness.>>

It could well be the easiest route to flightlessness in fully volant,
ornithoptering birds. But there is no reason to state that it is the >only<
way for flightlessness to evolve. This is unfounded dogma, plain and simple,
for which you have no support from the fossil record. Certain features of
ratite birds are paedomorphic, but other features are peramorphic, and still
others are neither. If paedomorphosis were the >exclusive< means of acquiring
flightlessness, then ratite birds would >exactly< resemble giant chicks--and
of course they do not. Ratite hind limbs are certainly not paedomorphic, for
example, and neither were theropod hind limbs. Indeed, I can't quite grasp the
relevance of your comments concerning paedomorphosis, both here and in
previous posts, to BCF.

<<  Hypermorphosis was put forth as a flight losing feature by Darren Naish,
but ratites hardly seem overdeveloped in maturity (of course, neither did
their ancestors to an extent, but not to the extent of modern ratites) and all
of the features given as hypermorphic features are better described as
paedomorphic features and some do not contradict the paedomorphosis argument
(such as the well-developed aftershaft of ratites, many juvenile birds like
the hoatzin have well-developed aftershafts).  Of course, that's just my
humble opinion, feel free to contradict me.  Anyway, back to forelimbs.
Phorusrhacoids are the only birds with fully functional forelimbs 
throughout life, but their forelimbs were modified to a point not seen 
in any theropod.  So there!>>

BCF doesn't state that a grasping function >cannot< be reacquired by the
forelimbs, only that it is unlikely. If phorusrhacoids had grasping forelimbs,
this is fine, and certainly does nothing to negate the idea that they were
secondarily flightless(!). Indeed, it suggests that some dinobirds were even
better fliers that one might give them credit for--and that their descendant
theropods could have "reacquired" grasping forelimbs well after that function
had actually been lost in their ancestral dinobirds.

<<FINALE

I have a lot more to say but I'll quit here.  As I have said before, I 
mean no disrespect to anybody mentioned above, I simply am pointing out 
the flaws of BCF.  I do subscribe to some of the points of BCF (birds 
came from arboreal ancestors) but a good portion of it.  I think I have 
said enough for now.>>

Likewise, I'm sure. Thanks for the opportunity to correct some possible
misunderstandings of BCF.