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

Re: BCF in perspective (long)

In a message dated 95-12-03 16:45:48 EST, theropod@garnet.berkeley.edu (John
R. Hutchinson) writes:

> In summation, nice job
>G.O., but how about showing us a simple, inclusive version of your
>phylogeny? If you're going to convince anyone, that's the #1 thing you'll
>need; a phylogeny backed up with solid fossil evidence - not origin
>stories. BADD has a reasonably decent phylogeny, reasonably decent fossils,
>and (depending on the storyteller of course) a story that's as entertaining
>as any other; how about BCF?

Your long letter demands an equally long reply, of course. But I refer you to
my publication _Mesozoic Meanderings_ #2 for a complete BCF phylogeny of the
archosaurs as of 1991 and 1992, now sadly out of date and being rewritten for
sale (third printing) as soon as I can get the thing out. (It's almost 300
pages long now, with no figures.) Meanwhile, this little reply will have to
do until I get the time to reply in more detail.

The main objection I have to the BADD phylogeny is that it tells us nothing
about the critters _on the branches_ (not at the terminal points) of the
phylogeny. They could be dinosaurs, they could be birds, or they could be
dino-birds. Most people would interpret them as dinosaurs of some kind, but
that's where BCF comes in.

If you consider the key lineage, the one that goes from the common ancestor
of all the archosaurs, toward any particular modern bird--say, the robin--two
facts hit you in the face: (1) The archosaurs started out small (say, 10-15
cm long snout-vent length), and the robin (and many, many other birds) is
itself small. This suggests that _on that lineage_ the animals didn't get
much bigger, because then they would have had to reverse and become small
again. I'm not saying that they didn't evolve that way (by starting out
small, getting big, and then becoming small again); I'm saying we should look
at the more parsimonious alternative first. (2) The archosaurs started out
with relatively large forelimbs, and the robin (and many, many other birds)
still has relatively large forelimbs. Maybe the forelimbs shrank and then
reversed and grew big--at the same time that the animals grew big and then
became small again--but again, why not look at the more parsimonious
alternative first: that the forelimbs _didn't_ shrink and re-expand during
the evolution _of that lineage_?

If you continue in this manner with other features such as teeth, feathers,
skull shape, hind limb anatomy, forelimb/wing anatomy, and so forth, trying
the _most direct route of change_ first, then you eventually get the BCF
phylogeny. Looking at the side branches of this lineage provides some idea of
the order in which the anatomical changes along the lineage occurred. By the
way, I call this "scanalyzing," the SCAN part being an abbreviation for Scala
Naturae--the old idea of singling out the lineage from microbe to man as the
trunk of the Tree of Life.

I was quite surprised to find, after working like this with all the different
features I could see, that a coherent picture of archosaur evolution emerged,
with remarkably few reversals. A few groups, such as the earliest theropods
and the ornithischians, had to have their positions slightly modified from
the BADD phylogeny, and Saurischia turned out to be paraphyletic (so what, is
what I say, but cladists may be more concerned), but basically the phylogeny
I got with this method agreed broadly with the standard phylogeny presented
in, say, _The Dinosauria_. The closest dinosaurian phylogenies to the one I
got were first described by Michael Cooper in 1985 and by Bob Bakker in _The
Dinosaur Heresies_.

More later.