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Re: What's a plesion?

In a classification of families (within an Order), a plesion is basically a distinctive genus which doesn't fit in well in any particular family, but hasn't been named a separate family yet either. Sometimes careful study, especially when additional material is found, will make it possible to assign that genus to a particular established family or to a separate family of its own.
The plesion is favored by many cladists who use Linnean ranks, but don't like monotypic families. It is especially good for genera that seem distinctive, but are poorly known. particularly when they form a cladistic series in a stem leading up to a crown clade.
Likewise, if you are classifying the Orders within a Class, you can classify a distinctive genus or family as a plesion, but eventually it might be assigned its own separate Order.
--------Cheers, Ken
P.S. When I get a cladistic series with lots of plesions (plesia?) in a cluster, that is when I start seriously consider a paraphyletic grouping (especially when the groups jump around a lot in different cladistic analyses). That is why I have gone back to a broader paraphyletic Hypsilophodontidae.
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From: "aspidel" <aspidel@infonie.be>
Reply-To: aspidel@infonie.be
To: "The Dinosaur Mailing List" <dinosaur@usc.edu>
Subject: What's a plesion?
Date: Sun, 19 Aug 2001 15:02:38 +0200

Hi dear dino-list members,

I'm also getting slowly into cladistics and classification, it's important to understand the evolution of dinosaurs, as I draw them.

So... What's a plesion?

Thanks in advance, friendly - LJB. 


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