Re: Newest Phylum?

From: drosen0...@xxxxxxxxx
I think that the Linnean phyla are still considered monophyletic.

I disagree. I found several reports showing that Platyhelminthes as
traditionally defined are not monophyletic. That old phylum has
been clearly broken into at least two parts, but the relationships
between some of them aren't yet decided. So at present there is no
generally-agreed-upon list of monophyletic phyla even in this one
case. Also I recall discussions of several other traditional phyla
that have been broken up recently because they were found not to be
monophyletic.

Many classes and orders have been shown to be polyphyletic.

Yes, but moot to this point unless some of them cross phylum boundaries.
The examples you listed are all within Chordata, so they are moot here.

So asking which phyla came first probably makes sense.

If we can ever agree on a listing of them of course. Otherwise the
question isn't meaningful.

Actually Porifera are now believed to
be polyphletic with respect to other animal phyla, i.e. other
animal phyla are nested within Porifera.
That is extremely unlikely. Porifera are distinct from other
animals in that they do not have the same tissue cells as the
others. Porifera do not have muscle tissue, nerve tissue,
connective tissue, or the same type of epithial tissue.

That's not a valid argument. If Porifera is basal to the true
metazoans, then what happened is that those tissue cells evolved
once in the common ancestor of all metazoans. So the various side
branches that didn't include that one evolutionary event would be
the modern polyphletic Porifera, while the one branch that passed
through that one evolutionary event would be all the true
metazoans. Now if you have evidence that Porifera have tissue cells
*not* present in the true metazoans, some cell type that is
*common* to all Porifera, that would support your case.

I vaguely remember somebody claiming the smallest clade containing
all Porifera also included the true metazoans, i.e. true metazoans
are nested within Porifera, with the various sub-clades of Porifera
branching off the main line at various places. But I can't find
that now. The best I could find via a Google search was:

<http://en.wikipedia.org/wiki/Silicarea>
... Some scientists believe that Porifera is
polyphyletic/paraphyletic, and that some sponges, the Calcarea, are a
separate phylum which was the first to diverge from the main line of
kingdom Animalia. Silicarea is considered the next phylum to diverge
from the primary animal lineage.

That's not exactly an authorative source, and I couldn't find any
other sources to substantiate it, except several cladograms that
simply said "Polyphyletic?" alongside Porifera, nor any other
details of that alleged polyphyletic Porifera, sigh. During my
search I did come up with this interesting Web site:

<http://www.steve.gb.com/perl/lesson17/Eukarya.html>
(Radical re-doing the Linnean classification system at the higher
levels, large number of kingdoms within somewhat large number of
empires, heading toward a truly cladistic system, but still with
many unresolved questions.)

You should be sure to look at that!! It's fun reading.

Do you mean that there are phyla of protista that are nested in
Porifera?

No, that's not what I meant. I was speaking specifically of Metazoa
(or is it Eumetazoa?).

Protista not generally thought of as part of the animal kingdom
anymore, although they used to be classified as such.

Correct. Animals develop from a blastula. Nothing else does.

However, euglena and other protista have a dual animal/plant
nature.

No. They have both autotroph (photosynthetic) and heterotroph
lifestyles depending on circumstances. But they don't have animal
nature (blastula), and they don't have plant nature (embryo encased
in maternal sterile tissue, and orderly alternation of haploid and
diploid generations). Animals and plants have apparently evolved
their particular natures once each, so each is a true clade without
any question. Now it's possible that some early animal evolved to
lose its blastula form, and thus its descendents masquerade as
protists nowadays. If you know of any evidence of such, please
cite.

.

Relevant Pages

  • Re: Newest Phylum?
    ... I think that the Linnean phyla are still considered ... animal phyla are nested within Porifera. ... animals in that they do not have the same tissue cells as the others. ...
    (sci.bio.evolution)
  • Re: Newest Phylum?
    ... Which phylum is believed to have emerged most recently? ... specify whose particular list of phyla you're asking about. ... animal phyla are nested within Porifera. ...
    (sci.bio.evolution)