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Re: The iguanodont paper
----- Original Message -----
From: "Jura" <firstname.lastname@example.org>
Sent: Thursday, December 13, 2007 6:15 AM
Scott- I disagree with your assessment of Romer et al.
Had birds been accepted as descendants of dinosaurs,
the definition of Dinosauria would not have changed at
all. The only difference would be a retraction of the
statement that dinosaurs were an evolutionary dead
Any systematists worth his salt could
tell you that snakes evolved from lizards.
(Actually, this was challenged a few times in the 1980s, based on cladistic
analyses that found one or two autapomorphies for "Sauria". Lizard paraphyly
has, of course, been considered obvious ever since, and was also considered
Yet, again, despite this, there is not
a non-lacertilian ophidian to be found in the tome.
The other way around :-)
So yeah, contra your statement, I would say that Romer
did mean to say: "yeah we like paraphyletic groups."
I would only add to it: ...because they just make things
easier to understand.
Well... that's what Romer would have said, most likely. But it boils down to
an emphasis on "important" characters that were used to delimit grades.
There was a lot of emphasis on grades.
For many purposes this is counterproductive.
For example, the traditional approach to measuring biodiversity through time
is to count taxa of a certain rank (classes, orders, families, or genera --
sometimes species) with no consideration of the problem that some of them
are paraphyletic and therefore uncountable.* How utterly significant that
today we have three amniote classes while 260 million years ago we had a
single one! What an explosion of diversity! What a time of heightened
evolutionary activity must the Triassic and maybe the Jurassic have been! In
the "not" mode.
Then comes the problem of unjustified generalizations. Amphibians cannot
tolerate saltwater; temnospondyls are amphibians; therefore temnospondyls
were unable to live in saltwater, and wherever we find a temnospondyl, we
are either looking at a freshwater (or terrestrial) sediment or at a carcass
that was washed out to sea. Complete nonsense. Instead, saltwater
intolerance is an autapomorphy of Lissamphibia. Yet, you can find the
assumption that temnospondyls and even *Ichthyostega* cannot have lived in
salty or even brackish water throughout the literature up to the end of the
20th century. Not everyone accepted it, but most did, and those who didn't
often went out of their way to emphasize what a freak, say, *Aphaneramma*
was: a marine amphibian! Shock horror!
Another common consequence of paraphyletic taxa is a phenetic approach:
plesiomorphies are treated like apomorphies. This can be seen in the
high-ranking monotypic taxa awarded to animals like *Archaeopteryx*: a bird
with teeth, a tail, and finger claws! How utterly unusual! So unusual that
it deserves its own subclass! Nonsense again. It is _normal_ for birds to
have teeth, a tail, and finger claws. The _other_ birds are the freaks.
There are even better examples out there, but I can't remember them at the
moment. I don't think I need to explain why phenetics is counterproductive
in biology... I'll do it anyway: because nothing in biology makes sense
except in the light of evolution.
* Never mind the additional problem that taxa at the same rank, except maybe
species, are _already_ uncountable.