Re: In the Blink of an Eye

Paul Ciszek <nospam@xxxxxxxxxx> wrote:

In article <cX9Gk.1679$Ei5.577@xxxxxxxxxxxxxxxxxxxx>,
John Harshman <jharshman.diespamdie@xxxxxxxxxxx> wrote:
Paul Ciszek wrote:

2) Which, if any, phyla are known to exist in the pre-Cambrian? I thought
that molluscs were established to have existed in the pre-Cambrian, but
now I see this stuff about early-Cambrian Halkieriids possibly giving rise
to molluscs. Huh?

Several problems here. "Known to exist" is problematic because a great
many Precambrian fossils are ambiguous. Their state of preservation
isn't sufficient to tell us just what they are. There are unambiguous
sponges and cnidarians. There are supposed mollusks and arthropods, that
may be or may not be. I believe there's an inarticulate brachiopod or
two. There are burrows and tracks that are also supposed to belong to
modern phyla, but it's hard to tell.

So it sounds like sponges and cnidarians are the only existing phyla
that have a solid claim to having been around in the pre-Cambrian.

Since "phylum" is a human construct, as "Division" is in botany, this
resolves to an issue of diagnosis and conceptual mapping. We identify
certain phyla in terms of their having shared traits across the group
(usually developmental or regulatory genetic traits) and then find it
hard, in the paucity of evidence we have with fossilised specimens, to
identfify these artificial groupings in certain conditions. Much easier
to use a phylogenetic tree and retroactively infer (or retrodict, if you
like) the traits of the last common ancestors which may or may not have
been the specimens we find fossilised.

If a group turns out not to be a "natural" group - that is, it is not
descended from a common ancestor (polyphyly) or it is, but doesn't
include all descendents of that ancestor (paraphyly) then we are forced
on phylogenetic grounds to say it is not a "real" group, in which case
we cannot find it in the fossil record, for obvious reasons.
--
John S. Wilkins, Philosophy, University of Queensland
scienceblogs.com/evolvingthoughts
But al be that he was a philosophre,
Yet hadde he but litel gold in cofre

.

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