Re: New alvarezsaurid

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

Matthew Troutman wrote:
>   The carina is not seen in basal birds such as Archaeopteryx and 
>Confuciusornis so it is not seen in the common ancestor of allosaurs and 
>birds. 
        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.
        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).
        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

Dinogeorge wrote:
>It is in _Archaeopteryx bavarica_...although it is quite reduced (secondarily?
>who knows?), to fit on the small sternum.
        Hmm... that would seem to cast some doubt on Matthew's hypothesis.

>Again, since we don't know what the common ancestor of allosaurids and birds
>looked like, we can't say whether it had a carinate sternum.
        We do not need to identify the common ancestor (indeed, in vert.
paleo., we ought not to expect to ever be able to) in order to reconstruct
its character states using parsimony. We must, of course, remind ourselves
that any such reconstructions are hypothetical, and that the expression of
the character may have been profoundly different than we see in descendant
species (as in the "climbing peton" hypothesis for the dromaeosaur sickle claw).

>If not, then the keel evolved twice (not a major problem, since it
definitely >evolved independently of birds in pterosaurs).
        Indeed, it seems a logical development for supporting chest muscles.
Of course, an intuitively pleasing adaptive story cannot supplant a
reproducable phylogenetic analysis. See Norell et al.'s response to Zhou's
criticism of the their placement of _Mononykus_ within Aves (both in AUK,
vol. 114 and 112 respectively).

        Wagner
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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                   "The truth points to itself" - Kosh