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Hypothesis: degrees of mesopuby
In my opinion, the conclusion that opisthopuby is primitive for
maniraptorans is erroneous. I believe the primitive state in early
coelurosaurs was somewhat (moderately to slightly) propubic, and that this
continued right on up to the maniraptoriform level and into the primitive
maniraptors (early troodonts, Achillobator, etc.). Even at the Rahonavis
and Archaeopteryx stage, it was just barely starting to shift to the
opisthopubic side of mesopuby.
The major exceptions are the segnosaurians who needed increasing
opisthopuby for an expanding gut, and then one or more clades of
dromaeosaurs also evolved in a more opisthopubic direction for reasons that
are not clear. I think that Sinovenator is one of those dromaeosaur clades
(perhaps with Sinornithosaurus?).
After we get past Archaeopteryx, opisthopuby is the rule rather than
the exception. Why oviraptors reversed back to a more propubic is anyone's
guess, but I think it is the only major reversal of this kind.
If Sinovenator is a basal troodontid, then this same kind of reversal
must have happened, but I think this is unnecessarily unparsimonious. I
believe the basal troodonts would be similar to Sinovenator, but with a
mesopubic or even slightly propubic orientation.
Of course I have looked at lots and lots of other characters in coming
to this conclusion, but that's the way it looks to me. The earliest Aves
(sensu lato), such as early troodonts would have only recently evolved fused
distal carpals (true semilunates), and would probably have unfused distal
carpals until very late in ontogeny. At the Deinonychus stage you might
have to go earlier into ontogeny to find an unfused semilunate (such as that
juvenile Deinonychus---at Yale? or Harvard?).
Well, that's all the time I have for now. If I don't get "constructive"
criticism on this, and just get "peanuts" or worse thrown at me, I will
probably go into a more extended period of lurking. We shall see.
------- Cheers, Ken
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