Use of term plesiomorphic

[Thomas Yancey]

I would like some guidance on use of the term plesiomorphic when applied to
a discussion of a lineage of fossil genera with progressively younger
geologic ranges. In this example, is it appropriate to apply the term
plesiomorphic to a character that is possessed only by the oldest known
genus, then is replaced by a derived character in younger genera? I suppose
one could adopt the position that a character of the ancestor is shared by
all descendents, even if it is lost in most or all descendents, but that
seems like an inappropriate use of the term.

However, in practice, is the term used in this manner? Or does it need to
be applied in a more strict manner? Assuming that there is a range in
useage of the term, what is considered good practice vs. poor practice? In
my case, I sense that the term is being used as a substitute for the word
'primitive', which does not seem proper. It will be desirable to hear what
is the current practice in using the term in the area of descriptive
taxonomic studies.

Thanks.
T. Yancey

Thomas E. Yancey
Dept. Geology & Geophysics, Texas A&M University
College Station, Texas 77843-3115
ph: 979-845-0643
tyancey at tamu.edu