Re: Sinocalliopteryx (Theropoda: Compsognathidae) ate confuciusornithids and dromaeosaurids

[David Marjanovic <david.marjanovic@gmx.at>]

> > What about the deltoideus?  That's what bats use, and apparently
>> what pterosaurs used, too. Indeed, in eumaniraptorans, the
>> deltopectoral crest is huge even in *Ichthyornis* (an animal that
>> doesn't make sense as anything but a powered flier).
>
> I'm torn on this one. If the deltoideus began as the major
> wing-elevating muscle in avialans, why (and how) did this function
> transfer to the supracoracideus in the evolution of birds? As you
> say, bats and pterosaurs still use the deltoideus for this. Why
> didn't birds stick with the deltoideus as well?

I suppose it took a while till the supracoracoideus became able to take 
over; that must have required a smaller angle between scapula and 
coracoid. Plesiomorphically, it's a forelimb protractor, not a 
rotator/abductor.

> I've heard that the  supracoracoideus provides birds with a more
> powerful upstroke than the deltoideus, and allows birds to have a
> stationary take-off directly from the ground.

That's how it works in pigeons, but not in other birds; in those that 
have been tested, the deltoideus is enough for a somewhat less smooth 
stationary take-off.

> But the fact that even fairly  derived avialans such as _Ichthyornis_
> have a major deltopectoral crest suggests that some transfer of
> function did occur: m. supracoracoideus became the principal
> upstroke muscle, and m. deltoideus major was relegated to a
> lesser/supportive function in the execution of the upstroke. This
> shift in the division of labor from deltoideus to supracoracoideus,
> with the latter taking over from the former as the major wing
> elevating muscle, might have been quite gradual. The hefty
> deltopectoral crest of _Ichthyornis_ certainly suggests that the
> deltoideus retained a more important role in wing elevation in this
> bird compared to modern birds. (But if this is so, why did
> _Ichthyornis_ have such a huge sternal keel?) By the time we get to
> the crown group (Aves), this wing-elevating function has been
> divested almost completely to the supracoracoideus.

Exactly.