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

David Marjanovic <david.marjanovic@gmx.at> wrote:

> 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'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.  But I've yet to see this spelled out in the

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.

> Assuming they were capable of powered flight in the first place -- why?

Yes, that was my point.  If confuciusornithids were incapable of
flapping flight (because the humerus could not be raised above the
dorsum), then a ground-level take-off is impossible.  These avialans
would need height in order to launch.  That (probably) meant climbing
trees.  This would explain the large, recurved manual claws
confuciusornithids (on two of the digits, anyway).  Otherwise,
confuciusornithids might have spent most of their time on the ground -
where they were occasionally caught by _Sinocalliopteryx_.