Paul writes:

>>Both insects and vertebrates have a fundamental segmented structure (you can
>>see this if you look at fish muscle, or the pattern of nerves on a human
>>torso, or even more obviously in insect larvae).
>>The explanation cannot be convergent evolution, but that both phyla evolved
>>from a segmented ancestor.
>
>I asked a related question here some time back, and was told the reverse--
>that what is outdated is the idea that vertebrae, ribs, six-pack-abs
>and so forth are evidence of a segmented ancestor.

I answered Paul the first time this question was raised. At that time, I
pointed out that "segmentation" was a subset of the general phenomenon of
"metamerism," and that the metamerism characteristic of the
annelids and the
arthropods was of a separate origin from that of the vertebrates.

Looking around on the web after I replied to Paul, I found that a great number
of people have come to conflate and confuse the two definitions, essentially
making metamerism and segmentation synonyms rather than one a subset of the
other. But I also found a truly nice paper that directly addresses Paul's
question, "Segmentation: mono- or polyphyletic?," by Elaine Seaver (Int. J.
Dev. Biol. 47: 583-595, 2003). It's online at:

     http://www.ijdb.ehu.es/ijdb20034778/ft583.pdf

Of importance, the operative sentences are: "Discussions of the evolution of
segmentation are complicated by the fact that there does not appear to be a
consensus on what constitutes a ‘segmental’ body plan. In addition, it is
difficult to identify groups where all members of a clade conclusively meet all
the criteria for segmentation. Generally, a distinction is made between true
segmentation and serial repetition (Willmer, 1990). Serial repetition includes
simple repeated structures and it is proposed that there is a tendency towards
such repetition in
animals."

Also by chance, just after Paul asked his question, a colleague in Australia
emailed me some pictures his group had taken of some onchyphorans. I've never
seen an onchyphoran live. They are extinct throughout North America, but
they're still abundant in Australia. These are ancient animals, characteristic
perhaps of the origins of the Euarticulata [a group that contains both the
segmented worms (Annelidae) and the Arthropoda (spiders, insects, crabs, etc.]
-- but certainly not the chordates. I put one of the images up on one of our
auxiliary servers:

     http://67.41.4.238/planipap_trio.jpg

My own personal preference for the evolutionary relationships between the
various groups under discussion is the alternative diagram shown in the middle
of the page:

     http://tolweb.org/tree?group=Bilateria&contgroup=Animals

If this diagram is correct, then clearly the appearance of metamerism in
animals is a polyphyletic invention.

Wirt Atmar
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 9/21/04 4:55:45 PM