Plesions and plesiomorphons (not the same)

Curtis Clark jcclark-lists at EARTHLINK.NET
Mon Mar 14 21:01:10 CST 2005

on 2005-03-14 20:07 Ken Kinman wrote:
> In order to reduce the number of ranks needed for complex
> classifications involving fossils, this was proposed as an unnamed
> rank applied to fossil stem taxa which formed a pectinate series
> (Hennigian comb) leading up to a crown clade.

That sure sounds paraphyletic to me (and is what Wiley described).

> In my Pteridophyta
> classification, they would label Horneophytopsida, Aglaophyton, and
> Rhyniopsida as three plesions (splitting off before the living
> Lycopsida).

No, that doesn't make sense. Each alone is not a "pectinate series" (and
each might be a clade), although the three together would likely
qualify, and that was my original point.

> Anyway, Horneophytopsida + Aglaophyton + Rhyniopsida could be
> combined into a single paraphyletic Plesiomorphon
> (Paleopteridophyta???) consisting of these three holophyletic clades
> (each of which could be termed a plesion) forming a Hennigian comb
> leading up to the living (crown) Pteridophyta which begins with
> Lycopsida.

It still makes no sense--I wonder if you are misinterpreting "plesion",
although why Kluge would come up with "plesiomorphon" is still unclear.
If a plesion is a clade, I don't see why any cladist would ever see the
need to call it out as something different. But at any rate, the (often)
pectinate series of fossils basal to an excluded crown taxon is a kind
of as-yet-unresolved paraphyletic group that warrants recognition, not
as a formal taxon, but more of a "holding pattern" to avoid either
"premature cladification" (to use your words) or "reifying ignorance"
(my words) by making a formal paraphyletic taxon.

And you already know that many cladists do not agree with putting each
node going up a tree at a different rank. We are completely satisfied
with formal taxa being monophyletic, and have no expectation that a
complete cladogram could be reconstructed from the classification.

Curtis Clark        
Web Coordinator, Cal Poly Pomona                 +1 909 979 6371
Professor, Biological Sciences                   +1 909 869 4062

More information about the Taxacom mailing list