>I would like to correct something here. Vertebrates did not appear
>suddenly fully formed in the fossil record. A vertebrate is defined as an
>animal that has a backbone. A vertebrate is a member of the chordata. All
>vertebrates are chordates but not all chordates are vertebrates.
I should be more careful in how I present a view that is apparently
idiosyncratic.
I am of course aware of early chordates, so-called protochordates,
hemichordates,
tunicate larvae, amphioxus, segmented worms etc. My judgment is that the
vertebrates, with their complex vertebral columns, advanced circulatory
systems,
complex appendicular skeletal structures, brains that include the major brain
structures we possess etc etc, are so different from these presumed progenitors
that one cannot propound the phylogeny you imply, not without substantial new
evidence of intermediate forms. You're accepting unproven transformations as
fact and thereby quashing attempts to model theories with greater explanatory
power than simply arranging known forms in simple-to-complex order and saying
there you go, there's your evolution.
The only evolutionary transformations we really have evidence of involve
distortion of skeletal parts and loss of parts; but the implied simple-chordate
to
vertebrate transformation is highly *elaborative*.
>To the best of my knowledge the oldest known vertebrate comes from the late
>Cambrian. Of the early invertebrate chordates, there are major differences
>between them and the later vertebrates.
Right.
>"Cephalochordates do not have a heart; blood flow is maintained by
>contractile tissue at the base of the aortic arches. The general geometry
>of the circulatory system is similar to that of primitive vertebrates, but
>there are no capillary beds, and both red blood cells and hemoglobin are
>missing from the blood. Oxygen is carried in solution in the body fluids."
>Robert L. Carroll, Vertebrate Paleontology and Evolution, (San Francisco:
>W. H. Freeman and co., 1988), p. 19
>So, to claim that vertebrates appear in modern form, fully differentiated
>at their start, ignores the latest data of which I am aware.
I didn't mean to claim that early vertebrates appear in their modern forms,
only that the complex new vertebrate anatomy and physiology come in
pretty well framed. The plumbing, the wiring, the skeletal structure are
all there. It looks like there was a macroevolutionary formative stage of
evolution to kick off the Cambrian explosion. Any resemblance here to a
creationist model is irrelevant and coincidental.
--Cliff Lundberg ~ San Francisco ~ cliff@noe.com