Re: New alvarezsaurid

["Matthew Troutman" <m_troutman@hotmail.com>]

>        We should probably say "ossified" sternal carina. The 
possibility
>may exist that a cartilagenous sternal keel was present in the common
>ancestor of Avetheropoda. How you would test this is beyond me.

    I have some problems with the hypothesis that there was a 
cartilagenous carina in Avetheropoda. First of all, avetheropods had two 
sternal plates, which makes it rather unlikely that a carina would 
develop because that design is not structurally strong. Second, we have 
no evidence for it.

>        As for Matthew's statement, as modified by me, it seems at 
first
>glance to be a bit hasty. All taxa within a clade do not necessarily 
have to
>posses a certain character state for it to be hypothesized as present 
in
>their common ancestor (hoary e.g. legs in tetrapods: not all tetrapods 
have
>them, but the common ancestor likely did).

     If a basal member of a group has a feature that is seen in no other 
members of the group, then it only distuingishes that genus or family 
only. If the basal member of the Avetheropoda had a carina, then the 
same seems to be true in this case. If you have a feature such as the 
carina that is seen in a basal member of a group, Ornithothoraces for 
example, and it is a consistent feature in the rest of the members in 
that group, >then< it is a diagnostic feature. 

>        I cannot offhand list the Avetheropods with ossified sterna, 
but
>some avetheropod taxa with ossified sterna appear to lack ossified 
carniae.
>The real test of whether an ossified sternal keel was present in the 
common
>ancestor of Avetheropoda is, of course, by phylogenetic analysis. 
Indeed,
>this should probably be performed as soon as possible, as I'm fairly 
sure
>several avialan cladograms consider this character state synapomorphic 
of
>more inclusive theropod clades

   No cladograms that I know of Aves consider this feature synapomorphic 
other theropodian clades.

>        Hmm... that would seem to cast some doubt on Matthew's 
hypothesis.

    Actually, it has no bearing on the hypothesis. Size and shape of the 
sternum in various bird groups is extremely variable. And in early birds 
the size is smaller than that of modern birds.

 Regards,

MattTroutman

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