"crown taxa"

Ken Kinman kinman at HOTMAIL.COM
Tue Apr 23 20:08:48 CDT 2002


     I rarely use the phrase "crown taxa" myself, but I believe it was first
used by paleonotologists for a node-based clade where the node is the common
ancestor of all the living taxa.  Since I now include Order Diadectiformes
in the amniote clade, I would regard diadectiforms as stem amniotes and all
other amniotes as "crown group" amniotes.
      This isn't necessarily a good idea, such as grouping living amphibians
(caecilians, salamanders, and frogs) into a crown group "Lissamphibia".  It
may well turn out that some of the living groups arose from temnospondyls
and some from lepospondyls, so the crown group "Lissamphibia" will have to
be abandoned or considerably enlarged in content.  It once was just a debate
about whether lissamphibians evolved from temnospondyls or from
lepospondyls, but if it really was a "mixed bag", amphibian systematics is
going to be a cladistic and nomenclatural quagmire (unless you use the
Kinman System, where the ordinal OTUs will be largely unchanged, but just
recoded).
     Anyway, the "crown group" concept of paleontology (sensu stricto) was
apparently borrowed and expanded by neontologists, especially in describing
"crown" eukaryote groupings (as Brian Tindall noted).  This is an even
messier usage of crown groups, and there are several eukaryotic "crown
groups" floating around.  One is the plant-animal-eumycotan clade, while
others are more inclusive (as in the "Tree of Life" project).
     Perhaps paleontological crown groups should be distinguished from the
broader neontological usage (as with eukaryotes)?  But I just wonder if the
concept is worth bothering with at all.  Especially when you have cases like
the microsporidians---which were originally excluded from even the broadest
eukaryote "crown group", and now have been transferred to Eumycota (and thus
part of a much more exclusive "crown group").  Long branch attraction is
just one of many homoplastic pitfalls, and even newer forms may come along
with whole genomes as well (who knows?).  I think I'll stick with my modular
approach to classification.
            --------  Ken Kinman
P.S.  Reptilian systematics will probably be soon facing a similar problem
(to that developing with amphibians, as mentioned above), if it turns out
that turtles are saurians that are secondarily anapsid.  In that case, the
"crown group" reptiles would shrink even more in content (at least for the
strict cladists who do not regard primitive synapsids as reptiles).  Is it
any wonder Norman Platnick was bashing node-based taxa and those vertebrate
paleontologists who have put all of their eggs into the phylocode basket?
*****************************************
On Apr 23, 2002, B.J.Tindall wrote:
>I was looking for a definition of "crown taxa" since I was not sure whether
>I understood the term correctly and couldn't find anything conclusive.  My
>initial impression was that "crown taxa" were those organisms living today,
>but I have seen the term used in such a way that it would mean that crown
>taxa are vertebrates and higher plants etc., whereas protists would not be
>considered to be crown taxa. Could anyone provide more information?



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