Plesions and plesiomorphons (not the same)
kinman2 at YAHOO.COM
Mon Mar 14 22:07:11 CST 2005
I believe the term plesion was first coined by
Patterson and Rosen, 1977. "Review of ichthyodectiform and other Mesozoic teleost fishes and the theory and practice of classifying fossils". Bull. Amer. Mus. Nat. Hist., 158:85172.
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. In my Pteridophyta classification, they would label Horneophytopsida, Aglaophyton, and Rhyniopsida
as three plesions (splitting off before the living Lycopsida).
In the Kinman System, it unnecessary to label Horneophytopsida and Rhyniopsida as plesions, because they are already clearly the same rank as the living taxa in Phylum Pteridophyta. I apply Plesion rank to Aglaophyton since it should probably be assigned the status of a separate Class, but noone has formally done so (and further research could conceivably put it within an already established Class (rather than one of its own). I only use Plesion rank to avoid premature naming of higher taxa. Frankly, I don't have to worry about numbers of ranks, because I already use fewer than most other workers anyway (the coding replaces the need for so many ranks).
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. The Kinman System has less need to use plesions as often, and I doubt that I would EVER need to use a plesiomorphon (if I were to use this particular plesiomorphon-type grouping, I would just code it as a paraphyletic Class Rhyniopsida sensu lato). I just have less need for such unranked taxa in my classifications, because I generally only use the mandatory ranks anyway. There's plenty of room in the margins for intermediate taxa (be they formally or informally named). I just find my system to be more stable, practical, and informative (and without many of the unpleasant side-effects of other approachs).
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