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Back to BCF basics, part 1



I've received several very long responses to a recent posting on
the BCF versus BADD theories, responses that really require (even
longer?) responses in turn, to do them justice. To avoid sending
huge e-mail messages to the dinosaur list, I will break these up
into more manageable chunks. If you recall anything that I
haven't answered, hang in there; an answer is probably in the
works.

The first response was the following:

=================================================================

Subj:     Re: A bit about BCF theory (Lots-o-questions) 
Date:     95-11-24 03:23:48 EST 
>From:     DSmith0531@aol.com 
Sender:   dinosaur@lepomis.psych.upenn.edu 
Reply-to: DSmith0531@aol.com 
To:  dinosaur@lepomis.psych.upenn.edu (Multiple recipients of
list) 
 
In a message dated 95-11-24 02:10:10 EST, Dinogeorge@aol.com
writes: 
 
>This compromise stabilized for many millions of years (until
>the wing became well enough developed in the archosaur-to-bird
>lineage that its grasping function could be abandoned
>completely), because theropod after theropod shows up in the
>fossil record with the tridactyl manus. 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.
 
I'm a little fuzzy on this part. 
 
First, does this mean that theropods in general are offshoots
from the main line of archosaur-to-bird evolution?  Or did they
split off early, making theropods a sister group to birds?
 
Since I don't know the answer to this question, I have more
questions, depending on the answer... 
 
**CASE I : Theropods ARE offshoots of the archosaur-to-bird
line** 
 
Are tyrannosaurs, then, considered to be more bird-like than,
say, the dromaeosaurs, because the tyrannosaurs are further along
the archosaur-to-bird line, thus should be offshoots from a more
"advanced" form of proto-bird-thingy?
 
If THIS is the case, why should tyrannosaurs *lose* an extra
finger, making them didactyl, rather than having, say, the
caudal-most finger (sorry, I'm not sure which digit numbers
tyrannosaurs retained...) be the result of a fusion of two
fingers, since that seems to be the trend in building a better
bird - lose digits IV and V, then fuse the rest together.  Or did
tyrannosaurs, in fact,  have a fused digit?  If they did actually
*lose* the digit, rather than fusing two together, than what is
wrong with the BADD - or BAAM, if you prefer :o) - idea of "Well,
what the heck, why _not_ lose a couple of fingers?"  How does BCF
deal with this?

Do we see more "advanced" theropods having more-and-more
restricted arm movements, closer to the flight-stroke of volant
birds?
 
**CASE II: Theropods branched off of the archosaur-to-bird line
at some point, then continued evolving separately from birds** 
Why should theropods (e.g., tyrannosaurs, _Mononykus_) mimic the
reduction of fingers (e.g., tyrannosaurs, _Mononykus_) found in
the bird line?
 
**IN EITHER CASE** 
What exactly does the BCF version of the theropod-bird family
tree look like? 
 
Thanks for any clarification, and sorry for so many questions :o)

(What's that bit about "inquiring minds..."?) 
 
DSmith. 

=================================================================

Part One:

"I'm a little fuzzy on this part. 
 
First, does this mean that theropods in general are offshoots
from the main line of archosaur-to-bird evolution?  Or did they
split off early, making theropods a sister group to birds?
 
Since I don't know the answer to this question, I have more
questions, depending on the answer... "

Yes, theropods are offshoots from the main line of archosaur-to-
bird evolution. Each different group of theropods, however,
represents a _separate_ offshoot. In maintaining this, BCF is in
broad agreement with BADD and various cladistic analyses of the
theropods.

Here's my preliminary version of a broad classification of the
avian branch of Dinosauria. I'm still working on this:

Aves
     Basitheropoda (presently including only _Longisquama_)
     Lagosuchia
     Herrerasauria
     Ceratosauria (presently including Podokesauridae,
          Halticosauridae, and Ceratosauridae)
          Neoceratosauria (presently including Abelisauridae,
               Noasauridae, and Velocisauridae)
     Protoaviformes (presently including only _Protoavis_)
     Megalosauria (presently including Eustreptospondylidae and
          Megalosauridae)
     Carnosauria (presently including Sinraptoridae and
          Allosauridae)
     Coeluria (presently including Coeluridae, Dryptosauridae,
          and Troodontidae)
          Ornithomimosauria (presently including Harpymimidae and
               Ornithomimidae)
          Tyrannosauria (presently including Compsognathidae,
               Itemiridae, and Tyrannosauridae)
     Archaeopterygiformes (presently including Archaeopterygidae
          and Dromaeosauridae)
     Carinatae (including Alvarezsauridae and Enantiornithes)
          Oviraptorosauria (presently including Elmisauridae,
               Caenagnathidae, and Oviraptoridae)
          Avimimiformes
     Ornithurae (including Hesperornithidae, Ichthyornithidae,
          and all modern birds)

As you read down the chart, you converge on modern birds. Doubly
indented groups are major side branches from the main archo-to-
bird lineage. (I may have omitted a couple of minor families,
since I've done this from memory without having my latest listing
beside me.)

I'm nowhere near certain about the detailed placement of some of
these groups (input would be welcomed), but I _have_ to publish
MM #2 soon. Also, lots of individual nodes aren't well resolved
because of the scarcity of specimens.

Note that I include Ornithurae _within_ Carinatae (not an error),
because the keeled sternum developed well before the pygostyle
tail. Certainly _Mononykus_ shows this!

MM #2 third printing will not list any bird taxa above
Archaeopterygiformes, but I'm planning to include Mesozoic birds
in future printings, after I accumulate and digest the necessary
references.

"Theropods" would be all the avian dinosaurs from Ceratosauria
through Archaeopterygiformes. "Maniraptorans" would be all the
avian dinosaurs from Coeluria to Ornithurae. "Tetanurans" would
be all the avian dinosaurs from Megalosauria to Ornithurae. "Non-
avian dinosaurs" would be the same as Phytodinosauria.

Back to Jeff Martz:

"**CASE I : Theropods ARE offshoots of the archosaur-to-bird
line** 
 
Are tyrannosaurs, then, considered to be more bird-like than,
say, the dromaeosaurs, because the tyrannosaurs are further along
the archosaur-to-bird line, thus should be offshoots from a more
"advanced" form of proto-bird-thingy?
 
If THIS is the case, why should tyrannosaurs *lose* an extra
finger, making them didactyl, rather than having, say, the
caudal-most finger (sorry, I'm not sure which digit numbers
tyrannosaurs retained...) be the result of a fusion of two
fingers, since that seems to be the trend in building a better
bird - lose digits IV and V, then fuse the rest together.  Or did
tyrannosaurs, in fact,  have a fused digit?  If they did actually
*lose* the digit, rather than fusing two together, than what is
wrong with the BADD - or BAAM, if you prefer :o) - idea of "Well,
what the heck, why _not_ lose a couple of fingers?"  How does BCF
deal with this?"

Tyrannosaurs form a side branch off the main lineage toward
modern birds. Since they had a more distant common ancestor with
modern birds than dromaeosaurids, they must be regarded as
farther removed; that is, dromaeosaurids would be more "advanced"
toward modern birds, although tyrannosaurs were more advanced in
other ways (e.g.: arctometatarsalian foot, didactyl manus).
Tyrannosaurs did not fuse manual digits II and III; metacarpal
III remained buried in the hand, digitless. This is the same
condition as in _Compsognathus_, and in my article on
tyrannosaurs for Gakken, I took this to mean that tyrannosaurs
and compsognathids were cursorial descendants of a lineage of
small didactyl-winged avians--an avian "evolutionary experiment"
in which the last _three_ (not just two) digits of the wing were
lost, continuing the trend begun with Lagosuchia.

I maintain that digital loss in the manus and pes does not take
place without strong selection pressure to do so. Consider
_Carnotaurus_, in which the forelimbs were _greatly_ reduced, but
which nevertheless retained all four functional digits of its
ceratosaurian ancestors. Since there was no selection pressure to
improve the wing, it vestigialized with four digits and stayed
that way. Indeed, the fourth digit is structurally distinct from
the other three, suggesting that it _enlarged_ for some as-yet-
unknown reason from the primitive reduced state. _Carnotaurus_
and its relatives evidently found some use for that digit after
all(!).

Jeff continues:

"Do we see more "advanced" theropods having more-and-more
restricted arm movements, closer to the flight-stroke of volant
birds?
 
**CASE II: Theropods branched off of the archosaur-to-bird line
at some point, then continued evolving separately from birds** 
Why should theropods (e.g., tyrannosaurs, _Mononykus_) mimic the
reduction of fingers (e.g., tyrannosaurs, _Mononykus_) found in
the bird line?
 
**IN EITHER CASE** 
What exactly does the BCF version of the theropod-bird family
tree look like?"

For an approximate version, see above. Note that theropods are
not a single group that branches off and does its own thing.
Rather, they are a bunch of little groups that branch off
individually from the lineage that is heading toward birds. This
is exactly what most cladograms of the theropods show. The
classification above differs only in a few details, and these can
always be negotiated in BCF.

Yes, as theropods head toward the maniraptoran groups, their arm
motions do become more restricted, although this effect is not as
pronounced as might be expected because in each theropod group
some of the original grasping arm motions were recovered after a
while. What we need are specimens of the BCF-conjectured small,
primitive theropods at the bases of the various groups: little
carnosaurs, little tyrannosaurs, etc. Then you'd see the
connections between them and the main lineage more clearly.

The forelimb of _Mononykus_ is a vestigial wing that has become
very specialized for some arcane purpose (like breaking into
insect colonies). It is not clear to me whether the single ungual
phalanx got there from the progression of five digits to four to
three to two that led to tyrannosaurs, or got there from three or
even four digits independently as part of becoming specialized.
We need a few ancestral mononykosaurs for examination.