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Re: Senter 2006, Confuciusornis, and humeral mobility
Aside from _Confuciusornis_, a large, robust deltopectoral crest is
characteristic of several basal birds, including _Jeholornis_,
_Sapeornis_, and _Zhongjianornis_.
According to one hypothesis, a large deltopectoral crest in certain
basal birds at least partly compensated for the lack of an elongate,
keeled sternum. The implication is that the prominent deltopectoral
crest offered an alternative strategy to power the wing during
flapping flight. However, no supporting evidence was presented. To
my mind, the large deltopectoral crest indicates that the forelimb
was used for some non-flight function, such as trunk-climbing.
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).
None of this rules out trunk-climbing, but that would require a big
humeral retractor muscle, too.
(BTW, the earliest limbed vertebrates had a separate deltoid tubercle
and pectoral crest.)