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Origin of mammalian diaphragm

From: Ben Creisler

A new online paper:

Tatsuya Hirasawa & Shigeru Kuratani (2013)
A new scenario of the evolutionary derivation of the mammalian
diaphragm from shoulder muscles.
Journal of Anatomy (advance online publication)
DOI: 10.1111/joa.12037

The evolutionary origin of the diaphragm remains unclear, due to the
lack of a comparable structure in other extant taxa. However, recent
researches into the developmental mechanism of this structure have
yielded new insights into its origin. Here we summarize current
understanding regarding the development of the diaphragm, and present
a possible scenario for the evolutionary acquisition of this uniquely
mammalian structure. Recent developmental analyses indicate that the
diaphragm and forelimb muscles are derived from a shared cell
population during embryonic development. Therefore, the embryonic
positions of forelimb muscle progenitors, which correspond to the
position of the brachial plexus, likely played an important role in
the evolution of the diaphragm. We surveyed the literature to
reexamine the position of the brachial plexus among living amniotes
and confirmed that the cervico-thoracic transition in ribs reflects
the brachial plexus position. Using this osteological correlate, we
concluded that the anterior borders of the brachial plexuses in the
stem synapsids were positioned at the level of the fourth spinal
nerve, suggesting that the forelimb buds were laid in close proximity
of the infrahyoid muscles. The topology of the phrenic and
suprascapular nerves of mammals is similar to that of subscapular and
supracoracoid nerves, respectively, of the other amniotes, suggesting
that the diaphragm evolved from a muscle positioned medial to the
pectoral girdle (cf. subscapular muscle). We hypothesize that the
diaphragm was acquired in two steps: first, forelimb muscle cells were
incorporated into tissues to form a primitive diaphragm in the stem
synapsid grade, and second, the diaphragm in cynodonts became
entrapped in the region controlled by pulmonary development.