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RE: Tyrannosaurids, skippy, and sterna



On Sun, 14 Jul 2002 04:29:58  
 HPB1956 wrote:
>Steve Brusatte wrote on 07/09/2002 regarding the influence of the juvenile 
>status of Scipionyx in cladistic analysises:
>
>> Ricqles, Mateus, Antunes, and Taquet have shown that bone-specific 
>anatomical details are actually present
>> in embryos that are most likely _Lourinhanosaurus_.
>> Many characters are most definitely affected by ontogeny, but not all.
>
>That's the problem I see with juveniles in cladistic analysises. Juveniles 
>may show a mixture of already developed derived and basal characters. Some 
>basal characters may be due to the fact, that they are being transformed into 
>derived characters late(r) in ontogeny.
>
>If I understand cladistics correctly this means that juveniles may find a 
>more basal phylogenetic position due to the lack of some derived characters.
>
>Of course there's even another possibility. Evolution works also by keeping 
>juvenile characters in adults. This too would influence the phylogenetic 
>position of juveniles found in cladistic analysises.
>
>> we must not use immaturity as a sweeping "excuse." 
>
>Not "sweeping excuse" but reasonable doubts. We have to keep this in mind if 
>we try to assess the validity of the phylogenetic position of Scipionyx found 
>in a cladistic analysis.

Oh, you're certainly correct.  I wasn't talking about cladistic analyses per 
se, but about identifying characters.  Basically, the paper I cited stated that 
several unambiguous characters seem to develop early in ontogeny, especially in 
some theropods.  Therefore, despite the young age of the specimens, they can be 
identified unequivocally as theropods.

Now, inserting that information into a cladistic analysis is another matter.  
That is a situation that is tricky, especially when the only specimen you have 
of a taxon is a juvenile (as in _Scipionyx_).  I'm unsure of the best approach 
when this is the case, but caution should definitely be applied.

Steve

---
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