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Re: Senter 2006, Confuciusornis, and humeral mobility

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

>>  Aside from _Confuciusornis_, a large, robust deltopectoral crest is
>>  characteristic of several basal birds, including _Jeholornis_,
>>  _Sapeornis_, and _Zhongjianornis_.
> And *Ichthyornis*.

_Ichthyornis_ also has a large sternum endowed with a hefty keel.  In
_Ichthyornis_, the deltopectoral crest is large (and dorsally
projecting), although it is extremely thin (based on Clarke, 2004).

> As its name says, the deltopectoral crest serves as an attachment site for
> the deltoideus, which lifts the humerus, and the pectoralis, which depresses
> it. In Neornithes, the pectoralis is broad at one end (the sternum) and
> narrow at the other (the humerus); in early birds this seems to have been
> different. A larger deltoideus may have compensated for a less well
> developed supracoracoideus apparatus (indeed, that's what bats do).

I was wondering when someone would bring this up.  Yep, this certainly
does invite comparison to bats.  In Chiroptera, the respective muscles
that elevate and depress the wing have much more equitable roles in
flapping flight.  This is unlike modern birds, in which the ventral
musculature does most of the heavy lifting (quite literally), and so
requires an extensive attachment surface.  By contrast, bats have a
rather undersized keel.