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Re: cladistics question

>One question that I have had with cladistics for a long time was the
>time factor.  As an example, the ceratosaurs at least used to be
>considered "basal" or "primitive" theropods, right?  Yet they didn't
>appear until tens of millions of years after the earlier more derived
>theropods, a huge expanse of time in which a great deal of evolution
>could and probably did occur.  So how can they be considered primitive
>or basal.

Basal and primitive are not the same thing.  "Basal" simply refers to the
branching order - if it belongs to a basal branch, it's basal.  Think of a
platypus - it's a basal mammal (monotremes are the basalmost branch), but
no one would argue that they're unevolved primitive things.

It was suggested to me today that this might be an example of
>paedomorphogenesis so the the ceratosaurs are actually more derived but
>retain characters from more primitive times.  What do you all think of
>this idea?
>Also I have heard of someone attempting straticladistics, merging
>cladistics data with stratigraphic data to get better answers.  Can
>anyone give me more information on this work?

The work was done primarily by Daniel Fisher and colleages at U. of
Michigan.  Unfortunately, it (a) violates one of the primary assumptions of
phylogenetic analysis (that the characters involved are heritable) and (b)
prevents one from using stratigraphy as an independent test of a phylogeny,
as it is no longer independent.


Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566