fossils as roots

[Stuart G. Poss]

>From my perspective, fossil remains of scorpionfishes can not be used _a
priori_ (hope my use of Latin is correct) to root cladograms, since both
relatively specialized and relatively primitive species, (ie those
having relatively many or relatively few ingroup synapomorphies
respectively, whether or not present in the fossil) are found as
fossils.

Nonetheless, from the perspective of inferring what character states of
a cladistic character were in fact primitive, it may well be that for
some states there may be little or no difference between the state seen
in certain fossils and the ancestral state, particularly if the fossils
are taken from beds representing a time period, relatively coincident
with the time the ancestral species and decendant species diverged.
Consequently, the morphology (typically that's all there is to work with
in fossils) may be highly informative or highly misleading, depending on
the rate at which subsequent character state change occurs in the
various decendant linneages and the ability of the worker(s) to
establish the relevant context and correctly interpret available data.
Since the rates of divergence of characters are usually difficult to
establish from fossil evidence, except in well stratified series with
relatively little unconformity (and even here we may be witnessing
punctuated rather than gradual evolution), clearly a more complete
record would be more useful than an incomplete one.

In my opinion, to more fully answer this question, one would have to
know precisely what specific fossils you are working with and the
morpho-molecular properties of the character systems that have been
preserved.

Stuart Poss

Hilary Davis wrote:
>
> I'd like to hear some opinions on the use of fossils to root a cladogram
> or tree.  Is this a valid technique given the possibility that a fossil
> assemblage for a particular extant group may not be complete?
>

--
_____________________________________________________________________
Stuart G. Poss                       E-mail: sgposs at seahorse.ims.usm.edu
Senior Research Scientist & Curator  Tel: (228)872-4238
Gulf Coast Research Laboratory       FAX: (228)872-4204
P.O. Box 7000
Ocean Springs, MS  39566-7000