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Dinosaur Digest #14



Subj:   Re: Flightless Birds (was Re: BCF ANDPDW)
Date:   98-08-05 17:31:19 EDT
From:   Dinogeorge
To:     znc14@TTACS.TTU.EDU
CC:     TomHopp, ornstn@inforamp.net
CC:     olson_c@mediasoft.net, tlford@ix.netcom.com
CC:     Dinogeorge

In a message dated 98-08-05 15:56:29 EDT, znc14@TTACS.TTU.EDU writes:

<< I am under the impression, from your Omni article and inumerable
 postings to this dinosaur list, that your "BCF" theory suggests that the
 natural history of non-avian dinosaurs consists of a successive radiations
 of a lineage of progressively more birdlike flighted arboreal archosaurs.
 You use this concept to explain the sequence of progressively more birdlike
 morpholgies you note in non-avian dinosaurs. Is this impression incorrect?>>

Yes. First come phytodinosaurs, then come the earliest, least birdlike
theropods, then the later more birdlike theropods, and so forth, from the
avian lineage (the one that leads from the dinosaur last common ancestor to
modern birds).

<<         In order to evaluate the likelihood of such a scenario, I chose to
 use our experience with flightlessness under more conventional theories. My
 point that birds seem to become flightless in depleted faunas may be not be
 true in every instance, but seems to hold in some...  if I were to make a
third generalization out of the first two, I'd say that, in order to become
flightless, a taxon
 would have to have both the motive (open niches) and the opportunity (an
 environment where a bird could start doing the terrestrial thing again
 without worrying constantly about being eaten by its ground-bound
cousins).>>

Seems reasonable to me. Don't forget that an arboreal lifestyle doesn't mean
that an animal won't also become a better runner--to deal with those times
when it's grounded for one reason or another--particularly if its forelimbs
have lost their terrestrial locomotor function and it must rely only on its
hind limbs to escape pursuit. Also don't discount possible physiological
improvements (e.g., better endothermy) within the arboreal dino-bird lineage
that could serve terrestrial descendant animals in good stead on the ground.

<<         However, we may suppose that the dinosaurs who radiated in the
first
 wave from your hypothetical arboreal lineage spent a good amount of time on
 the ground, learning to be good terrestrial dinosaurs. So why did the next
 wave succeed? Did being more birdlike make them "better" terrestrial
 carnivores (with "better" meaning able to "out-compete" the natives)? You
 have repeatedly said that this doesn't make sense! Did the first wave just
 give up and say "look, you guys are closer to being birds, so we give up". I
 could maybe buy this, if couched in less facetious terms, once. Your theory
 implies that it happened repeatedly.>>

The first time this happened, there probably wasn't enough time for much
cursorial specialization to have occurred. This was before the Late Triassic,
when the big dinosaur divergence into sauropods, prosauropods,
ornithischians, and several kinds of closely related theropods seems to have
happened very rapidly--almost simultaneously. There was nothing else like
cursorial dinosaurs around, and plenty of niches were wide open. An explosive
radiation of dinosaurs (all small and capable of rapid, tachytelic, evolution
then) established their major clades for the remainder of the Mesozoic; by
the time dinosaurs first appear in the fossil record, their major lineages
are already clearly separate.

Phytodinosaurs went on to evolve into the various mainly herbivorous groups
all by themselves with no further "input" from the mainly carnivorous
theropod branch of the dinosaur cladogram. What you want to know is why were,
say, herrerasaurians and lagosuchians >replaced< by ceratosaurians,
ceratosaurians by tetanurans, tetanurans by maniraptorans, and so forth. >If<
this is indeed what actually happened.

<<         If you're going to say that groups of terrestrial theropods
 repeatedly radiated from their flying cousins, you're going to have to
 account for why they kept surviving in a world where the members of the last
 radiation had millions of years longer to adapt to terrestriality, to being
 better "running-and-jumping-and-killing" guys in the case of theropods. >>

Well, this isn't exactly what happened. What we had were herrerasaurians and
lagosuchians first. Then ceratosaurians appeared and lived alongside the
herrerasaurians for a while. Then tetanurans appeared and lived alongside the
herrerasaurians and ceratosaurians for a while. Then maniraptorans appeared
among the tetanurans. The herrerasaurians seem to have become extinct by the
end of the Early Jurassic (or perhaps earlier); at least, we have no fossil
record of them after that. By then we had both ceratosaurians and tetanurans
side by side. The ceratosaurians didn't become extinct until the end of the
Cretaceous (as far as we know), since they seem to have survived in
Gondwanaland (e.g., India, Madagascar, S. America) until then, so they
weren't "replaced" by more derived theropods. Indeed, except for
herrerasaurians (which may have been done in by the Pliensbachian mass
extinction event, along with prosauropods), >none< of the major theropod
groups became extinct at the hands of a more birdlike group. The later groups
appear and then coexist with and among the less derived theropods. If
replacement occurs, there seems to be no particular pattern to it (e.g., why
mainly ceratosaurs in Gondwana but more advanced tetanuran
theropods--allosaurids, etc.--in the northern hemisphere?); it just happens
as a vagary of evolution. None of the more birdlike maniraptorans seems to
have displaced tyrannosaurids or ornithomimids, for example.

The only discernible pattern is the timing of the appearance of the
progressively more birdlike theropods, not what they may have evolved into
after finding and occupying a congenial set of niches. Also, it seems as if
at any one time, the dominant dinosaurian predators in a particular region
are also the least birdlike--which may be either because they had the longest
time to evolve away from the dino-bird lineage, or because they had indeed
occupied the large-carnivore niche the longest and were best adapted to it,
difficult to dislodge. (Sometimes an extinction event wiped out the large
predators, which were then replaced by whatever theropod lineage happened to
be available.)

Both BCF and BADD account for these successive theropod radiations with the
same kinds of cladograms; there is really little difference between the two
scenarios in systematic terms (BADD still likes saurischians, BCF doesn't;
big deal). In both BCF and BADD, the more birdlike theropods appear later
because they're descended from dinosaurs that are on their way to becoming
birds. In BADD, these dinosaurs stayed on the ground until the last possible
moment, I presume becoming "better" ground-dwelling carnivores (so why fly at
all?); in BCF, these dinosaurs were in the trees for a long time, acquiring
their birdlike characters as incremental adaptations to an arboreal/airborne
lifestyle. Every so often a lineage became permanently grounded, found a
niche or two for itself, and turned into another clade of cursorial,
terrestrial theropods.

Subj:   Re: Flightless Birds (was Re: BCF ANDPDW)
Date:   98-08-05 12:06:50 EDT
From:   Dinogeorge
To:     znc14@TTACS.TTU.EDU, tlford@ix.netcom.com
CC:     TomHopp, ornstn@inforamp.net
CC:     olson_c@mediasoft.net

In a message dated 98-08-05 11:41:09 EDT, znc14@TTACS.TTU.EDU writes:

<< I was taking George to task on
 the implication that a flying bird can do a non-avian archosaurs
 running-and-jumping-and-killing job better than the non-avian. >>

Say what? Will somebody please explain this sentence to me, including among
other things the meaning of the word "better" in this context? Then show me
where I may have said anything like this to be "taken to task" for.

Subj:   Re: OCCAM'S RAZOR & THERIZINOSAURS
Date:   98-08-03 23:35:57 EDT
From:   Dinogeorge
To:     jwoolf@erinet.com, jjackson@interalpha.co.uk

In a message dated 98-08-03 22:03:39 EDT, jwoolf@erinet.com writes, quoting
jjackson@interalpha.co.uk:

<< >
 > I don't see how you could get that meaning from what I wrote!  Paragraphs i
 > & ii are successive stages. >>

Here again, an instance of Woolfian misreading of a post. Glad to see I'm not
the only one who is being misapprehended here.

Subj:   Re: OCCAM'S RAZOR & THERIZINOSAURS
Date:   98-08-03 23:35:54 EDT
From:   Dinogeorge
To:     jwoolf@erinet.com
CC:     znc14@TTACS.TTU.EDU, cbrochu@fmppr.fmnh.org

In a message dated 98-08-03 22:02:55 EDT, jwoolf@erinet.com writes:

<< Even a stopped clock is right twice a day.  In this case, Wagner's right
that
 BCF is an orthogenetic hypothesis, _as you've explained it here_.  It seems
to
 me you're assuming that feathers evolved for flight and flight alone,
therefore
 anything that has feathers must be a flyer or an ex-flyer. >>

I like your analogy.

But BCF is not orthogenetic at all. And I have no hypothesis on the origin of
feathers, although I'm leaning toward one recently suggested in the
literature, namely, that feathers appeared as a mechanism for sulfate
excretion (through molting), and that they were subsequently exapted for
flight and/or insulation at some early point in the lineage that extends from
the archosaur LCA to birds. As I've said many, many times: Feathers (or
"pre-feathers") cannot possibly have appeared >for< flight, because avian
flight >must< have arrived after the feathers, not before. They also cannot
have appeared >for< insulation unless the animal was already an endotherm
that required insulation but didn't have it (an odd animal indeed). As I've
also said many, many times, no evolutionary hypothesis can use the future to
account for the past. I have argued until blue in the face on this very list
>against< orthogenetic hypotheses, so I cannot understand how you can claim
that BCF is an orthogenetic hypothesis.

It is appalling how my posts have been misinterpreted and misread. Please,
list members, >read what I write,< without putting your own personal agendas
and spin on my words.

Subj:   Re: BCF ANDPDW
Date:   98-08-03 17:17:22 EDT
From:   Dinogeorge
To:     TomHopp

In a message dated 98-08-03 13:34:01 EDT, TomHopp@aol.com writes:

<< You conclude that a miracle is required, after stating a preposterous set
 of requirements.  Who said the cursorial ancestor was large?  YOU DID.  Who
 said the transition to flight was sudden? YOU DID.  WHO said the suite of
 characters were ALL flight related?  YOU DID, (vide supra). >>

Sorry, I have only repeated what others have stated in the literature; I
didn't make any of this "preposterous" stuff up. I'm glad to see that not
everyone holds to those ideas.

If BADD theory is correct, then dinosaurs did not become airborne or arboreal
fliers until their wings had already attained a certain level of
flightworthiness for reasons other than flying. At what level would this be?
Remember, the farther back in time and evolutionary development this level
gets pushed, the closer BADD comes to being BCF. There may even come a time
when BADD=BCF. The BADD I'm talking about is the one that says, "Since we
have no evidence of flying birds prior to _Archaeopteryx_, only running
dinosaurs, flight must have begun very shortly before _Archaeopteryx_."

By the way, the transition from wings for brooding to wings for flying, while
easier and therefore more likely than the transition from forelimbs for
grasping prey to wings for flying, is still nowhere near as easy as the
transition from wings for flying to wings for flying. It is entirely possible
that wings that evolved for flying became exapted for brooding during their
evolution--that's certainly a hypothesis that makes sense--but to have wings
evolve >for brooding< and >later< become exapted for flight is putting the
cart before the horse.

If wings originally evolved >for brooding<, without use in any kind of
flying, why did the hand >lose< its outer two digits? Surely a wider span
would have been of signal use in brooding. Indeed, one might almost expect a
bird with brooding wings to evolve >extra< manual digits over and above the
basal dinosaur complement of five, to help cover more area. And why encumber
the hands with feathers at all, when other parts of the anatomy--e.g., the
sides of the body and the belly--might well be more appropriate? Brooding
occupies only a small portion of a bird's life, whereas flying is something a
bird does many times a day, practically every day. Why select for brooding
when selection for improved flying would clearly carry more benefit for the
animal? Finally, secondary flightlessness has occurred convergently in dozens
of extant species, so one might expect secondary flightlessness to have
occurred during the Mesozoic, perhaps with greater frequency because Mesozoic
birds weren't quite as advanced fliers as birds are today. So how prevalent
is wing-brooding among extant birds?

We're in the business of making up Just So Stories here, so your Just So
Story has to be more compelling than everyone else's Just So Story.

Subj:   Re: BCF ANDPDW
Date:   98-08-03 00:43:35 EDT
From:   Dinogeorge
To:     m_troutman@hotmail.com

In a message dated 98-08-02 23:31:20 EDT, m_troutman@hotmail.com writes:

<<The problem here is that I do not subscribe to BCF :-)>>

Too bad, your loss (heh heh).

<< I do not believe that you can explain almost every single feature of
dinosaurian evolution by the arboreal habits of the ancestral "dino/birds".
>>

Good. Neither do I, and I don't know why you would think I did. Once
theropods came down out of the trees as bipedal predators, they continued to
evolve in ways that favored this new lifestyle. Some theropods were
handicapped by features that had evolved for climbing, perching, and/or
flying, but these little problems were generally overcome or capitalized upon
in one way or another. Hollow bones to lower weight? They remained hollow,
but the bone became harder and stronger to compensate. Large, trenchant claws
for clinging to trees? They became larger claws for killing prey. Large,
retroverted pedal digit I at the base of the foot for perching? It became a
small dew claw, still at the base of the foot but out of the way for walking
and running. And so on.

Clearly, other interpretations of these features are >possible<. You can
always say that everything evolved in theropods "for predation," or "for
running," or whatever. There is no way to disprove these reasons except
perhaps with a series of time machines. But these >do not compel< flight.
They do not explain the evolution of flight, they merely >allow< it, and only
weakly at that--as a kind of afterthought. This is >just not good enough<.
The existence of avian flight, or any kind of vertebrate flight for that
matter, must be the result of a consistent and persistent selection pressure;
it is an energetically costly lifestyle that requires some pretty specific
and extreme specializations by the flying organisms. Entrusting one's life to
the vagaries of air currents, wind, and weather, being held aloft in a
virtual fall by nothing more than a few feathers or a web of skin, is
dangerous business. We know this because, when the obvious mechanisms of
selection pressure are removed (such as predators), flightlessness almost
inevitably ensues, even though a flightless bird finds itself at a
considerable evolutionary disadvantage if the selection pressure mechanisms
are suddenly restored.

In other words, animals need to be "pushed" into evolving flight; they won't
just evolve flight spontaneously because they happen to have some of the
anatomical equipment that would allow them to do so. When the "push" is
removed, they stop flying. That's why it's so much easier to get a running
theropod from a flying dino-bird than it is to get a flying dino-bird from a
running theropod, and why BCF works.