Re: New alvarezsaurid

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

At 06:58 PM 3/30/98 -0800, you wrote:
>    I have some problems with the hypothesis that there was a cartilagenous
>carina in Avetheropoda.
        As do I, but the possibility may exist...

>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.
        I'm not convinced. A keel might have been part of a cartilagenous
"bridge" between the sternal plates. Do you have data on how strong such a
structure would be, or how strong it must be to support a keel? In any case,
this seems rather speculative.
        Out of curiosity, which avetheropods had paried sternal ossifications?

>Second, we have no evidence for it.
        And that's the real problem. Of course, we aren't likely to get
evidence one way or the other any time soon. Conservatively, we cannot say
non-avian dinosaurs had cartilagenous sternal keels. However, we also cannot
say that they did not. All we can talk about is *ossified* sternal keels,
which we have a much better chance of finding in the fossil record. That is
why I made a big deal about changing the wording. That's all.

>     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.
        But what if two members of the group have it, and evidence from most
of the others is equivocal? Nothing is as simple as it seems...

>If the basal member of the Avetheropoda had a carina, then the 
>same seems to be true in this case.
        I don't quite follow this. Are you saying that we have to find a
carina in a basal Avetheropod? Bear in mind that "basal" with respect to
tree topology does not mean underived.

>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. 
        "Diagnostic" is not the same concept as autapomorphic when you are
discussing phylogeny. Personally, I consider diagnostic characters of
secondary importance, and limited (but not trivial) utility. A character
present in the common ancestor of a clade need not be present in all of the
descendants, nor need it be found in a "basal" member of the group. 
        If snakesdiverged first from the rest of Reptilia, and we only had
one fossil snake to go on, would we consider limblessness to be present in
the reptilian common ancestor? You have to be careful when making judgements
as to whether a basal taxon is truly underived with respect to the other
members of the clade in question.
        As for the keeled sternum, we may be running into a problem of
expression and preservation of characters. It seems likely that some, if not
many theropods possessed a cartilagenous sternum (see _PDW_, Paul 1988),
which may have been ossified only late in ontogeny, or not at all. I am not
qualified to assess the implications of such an hypothesis, nor to determine
exactly how to go about testing it explicitly. However, we can avoid the
problem by limiting discussion to *ossified* sterna and sternal carinae, in
which case the characters are observable and their distributions testable.

>   No cladograms that I know of Aves consider this feature synapomorphic 
>other theropodian clades.
        I said "more *inclusive* [with respect to Avetheropoda] theropod
clades", for example Metornithes.

[Archaey with a sternal keel]
>    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.
        I'm afraid you misunderstand. I merely meant that your statement
about a lack of an ossified sternal carina in _Arcaheopteryx_ was in error,
which may affect your interpretation of the distribution of that character.

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