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Re: cladistics question
>In a message dated 7/21/98 7:52:54 PM Eastern Daylight Time,
><< 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. >>
>I'm not familiar with the details of the above example--so I can't speak to it
>specifically--but the general point as to how cladistics deals with temporal
>issues is not clear to me either. Incomplete information about the relative
>age of specimens could lead to misleading cladistic analyses--couldn't it?
Relative age is not normally considered as prima facie evidence in a
cladistic analysis, so by itself it is not misleading. There is no rule
that states a cladogram must mirror the stratigraphic distribution of taxa.
We would hope for concordance, but lack of such is not necessarily a
problem, depending on the group.
(For what it's worth, ceratosaurs long predate most other theropods.
Ceratosaurus, a particular ceratosaur, is a late survivor.)
The problem with time has more to do with evolutionary changes occurring on
branches with no fossil record. First appearance taxa may not be good
approximations of the basal conditions for the lineage. This is why the
fossil record is so bloody important for phylogenetics. (As an example -
no one, anywhere, would think that the platypus is a good example of the
ancestral mammal. But until recently, the oldest known monotreme was
already a derived platypus.)
>As an example: say you have (all else being equal) two taxa of the same age
>with long, pointed ears and one taxa (again, all else being equal) of
>*unknown* age with long, non-pointed ears. Couldn't one interpretation of
>this be that long non-pointed ears are the pleisiomorphic state and that
>pointed ears are a synapomorphic character?
Sure, depending on the conditions of this character in related groups.
Stratigraphy would have nothing to do with it.
But, in the absence of relative
>ages, couldn't another interpretation be that the long pointed ears were a
>pleisiomorphic character of an unknown common ancestor and that the *absence
>of pointedness* was the derived condition expressed only in a single taxa?
>How do you deal with this in cladistic analysis?
With morphological data, the standard method is called outgroup comparison:
suppose the lineages thought to be close to your eared animals all had
pointed ears. This would strongly suggest that primitive ears were
There are other methods that rely on stratigraphy, commonality, or
ontogeny, but these are problematic for most situations dinosaur workers
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