RUSSIAN 

term «plesiomorphon»

Citation from the book by N.J. Kluge 2004 "The Phylogenetic System of Ephemeroptera" (p.15-16):

Phylogenetic status of taxon

 It is generally accepted to divide all taxa into holophyletic (the term introduced by Ashlock 1971 instead of "monophyletic" sensu Hennig), paraphyletic (the term introduced by Hennig) and polyphyletic (the term introduced by Haeckel) ones*. Phylogenetic (cladistic) principle of systematics declares that all taxa should be holophyletic only, but never polyphyletic or paraphyletic. However, besides evidently holophyletic, paraphyletic and polyphyletic taxa, there are many those which phylogenetic status is not clarified yet, and they exist in all classifications together with holophyletic ones. Some authors call them "plesions", to distinguish from holophyletic taxa; in the preliminary text of this revision, the term "plesion" was also used in this meaning. Dr J.G. Peters paid my attention to the fact that in modern literature the term "plesion" is used in various meanings, and helped to find its original definition.

Indeed, Patterson and Rosen (1977)** who introduced the term "plesion" (in plural "plesions"), applied it for taxa of any phylogenetic status, including holophyletic ones, and used it to represent a phylogenetic tree as a sequence of taxa rather than a hierarchy of taxa: in their classification, several plesions written one after another, mean that this is a pectinate portion of phylogenetic tree, in which each plesion is a sister group of all those that succeed it. In this sense, the term "plesion" indicates systematic position of the taxon and substitutes traditional Linnaean ranks, but does not characterize the taxon itself. In the ancient Greek, the word "plesion" (plural "plesia") means neighbour in direct and indirect meanings, that is close to the meaning of the term "plesion" proposed by Patterson and Rosen. In the same paper (Patterson & Rosen 1977:163) it was suggested to put into quotation marks name of a taxon which is "nonmonophyletic, or ... no longer contains its type genus, or both". In order to avoid confusion, it would be better not to use the term "plesion" and quotation marks to signify paraphyletic taxa.

Instead, here is suggested a new term "plesiomorphon" (in English plural "plesiomorphons", but not Greek "plesiomorpha"): plesiomorphon is a taxon characterized by plesiomorphies only; thus, its holophyly is not proven, and possibly (or probably) this taxon is paraphyletic, but its paraphyly is also not proven yet. Plesiomorphon can be a taxon of any size and systematic position in ranking or non-ranking classification, living or fossil, with any name (ranking, hierarchical, circumscriptional or other).

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* For detailed explanation - see Kluge 2000 and English translation of its Chapter I.2.

** Patterson, C. & Rosen, D.E. 1977. Review of ichthyodectiform and other Mesozoic teleost fishes and the theory and practice of classifying fossils. — Bull. Amer. Mus. Nat. Hist. 158 (2): 85–172.