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BFC Epiphany



I re-read George's 1995 BCF message on the list:
        http://dml.cmnh.org/1995Oct/msg00722.html

(Thanks to whoever posted that URL; I've deleted the message now so I
don't know who it is.)

I had an epiphany that now seems so obvious that I am embarrassed even
to mention it -- most of you probably realised this years ago -- but
for the benefit of anyone else who was as dumb as I was regarding BCF,
here it is.

--

George is right: there _was_ a "central lineage" leading from basal
archosaurs to birds, and every other kind of dinosaur branched off
from that lineage, making Dinosauria highly paraphyletic with respect
to the central lineage.  Here is the (simplified) cladogram to prove
it:

Archosauria
+--Crocodylia
`--Dinosauria
   +--Ornithischia
   `--Saurischia
      +--Sauropodomorpha
      |  +--Diplodocoidea
      |  `--Macronaria
      |     +--Brachiosauridae
      |     `--Titanosauria
      `--Theropoda
         +--Carnosauria
         `--Coelurosauria
            +--Tyrannosauria
            `--Aves

As you can see, this cladogram does indeed present a sequence of
paraphyletic outgroups leading to Aves: Crocodylia, Ornithischia,
Sauropodomorpha, Carnosauria and Tyrannosauria.  (For clarity and
brevity I've left out lots of other clades).

--

Now that we've established that Birds Came First, I would like to
propose my own model for dinosaur evolution: it's called Brachiosaurs
Came First, and it proposes that there _was_ a "central lineage"
leading from basal archosaurs to brachiosaurids, and every other kind
of dinosaur branched off from that lineage, making Dinosauria highly
paraphyletic with respect to the central lineage.  Here is the
(simplified) cladogram to prove it:

Archosauria
+--Crocodylia
`--Dinosauria
   +--Ornithischia
   `--Saurischia
      +--Theropoda
      |  +--Carnosauria
      |  `--Coelurosauria
      |     +--Tyrannosauria
      |     `--Aves
      +--Sauropodomorpha
         +--Diplodocoidea
         `--Macronaria
            +--Titanosauria
            `--Brachiosauridae

As you can see, this cladogram does indeed present a sequence of
paraphyletic outgroups leading to Brachiosauridae: Crocodylia,
Ornithischia, Theropoda, Diplodocoidea and Titanosauria.

Now all that remains for me to do is designate the whole of the clade
(Brachiosaurus not Crocodylus) as "stem brachiosaurs", and my work is
done.

--

Of course, the two trees above are THE SAME TREE.  I've fiddled with
the geometry, but the topology is identical.  They both show the same
evolutionary progression, but the layout is tweaked to tell a
different story.  (For some reason, most sauropod cladograms look as
though nature's goal was to produce Saltasaurus :-)

Put this way, it's obvious that the problem is simply one of
characterisation.  If you look at the basal archosaur, George can call
it a protobird and I'll call it a protobrachiosaur, and the only way
to decide which of us is "right" (if any) is by figuring out the
ancestral character states and looking at whether that basal archosaur
more closely resembled a bird or a brachiosaur.  I submit that it
didn't closely resemble either, so calling it either a protobird or a
protobrachiosaur is misleading.

(George cheats a bit here: he calls all dinosaurs stem-birds because
birds are extant, if I understand the reasoning properly.  But I
don't think that kind of blatant extantism has any place in an
enlightened society :-)

So pick a character: what we call the "avian lung", with flow-through
ventilation and air-sacs.  Since it exists in birds and also (we have
good evidence) in brachiosaurs, we can reasonably optimise it onto the
common ancestor, i.e. the most basal saurischian, and so we should
probably call it the "saurischian lung".  George would say that
brachiosaurs got the benefit of the avian lung because of brachiosaurs
being one of the paraphyletic sequence of outgroups to Aves; but I say
that birds lucked out and got the benefit of the brachiosaurid lung.

In other words: you say po-TAY-toe, I say po-TAH-toe.  (Except that,
as a good English lad, I in fact say po-TAY-to.  In fact, does
_anyone_ actually say po-TAH-toe except when singing that song?)

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor    <mike@indexdata.com>    http://www.miketaylor.org.uk
)_v__/\  "Here comes Hurst, he's got ...  There are some people on the
         pitch, they think it's all over ...  It is now!" -- Kenneth
         Wolstenholme, 1966 of course.