

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_.

And *Ichthyornis*.

 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.)