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Hypothesis: degrees of mesopuby



Dear All,
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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