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Re: [dinosaur] ratite flightlessness evolution

> Cynthia Faux &  Daniel J. Field (2017)
> Distinct developmental pathways underlie independent losses of flight in
> ratites.
> Biology Letters 13(7): 20170234
> DOI: 10.1098/rsbl.2017.0234
> https://urldefense.proofpoint.com/v2/url?u=http-3A__rsbl.royalsocietypublishing.org_content_13_7_20170234&d=DwIBaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=JuD-CoCYRSDYV-WVGzvC54MFtPsI4NskmHEFMyk32bM&s=-8Pev-k22yGYBkeTyKH4zv_583zaXq4kZkrSCVcyRug&e=

Extra support for multiple secondary flightless events within
Palaeognathae.  The study suggests that flightlessness in ancestral
casuariids (emus + cassowaries) was acquired at small body size by
shrinking the wings.  By contrast, ancestral struthionids (ostriches)
became flightless because they grew too big to fly, and retained
larger wings; the authors propose this was also true of rheas

On the antipodean side of the world, the proposed developmental
trajectory seems to fit with the (limited) fossil evidence.  The
Oligo-Miocene crown casuariid _Emuarius_ is smaller than its extant
kin, _Dromaius_ and _Casuarius_; there is no doubt that flightlessness
is primitive for crown casuariids.  The kiwis (Apterygidae) also have
tiny wings, and among living ratites have been regarded as being
mostly closely related to casuariids.  The early Miocene stem
apterygid _Proapteryx_ is small (even smaller than extant kiwis), and
may have been volant (based on size alone).  The elephant birds
(Aepyornithidae), regarded as the sister taxon to the kiwis, also had
tiny wings.  The mid-Paleocene _Diogenornis_ from Brazil was fairly
large (~2/3 size of the greater rhea, _Rhea americana_) and had rather
large wings.  If distinct developmental pathways hold true, this
morphology makes sense if _Diogenornis_ is a stem rheid, but not if
it's a stem casuariid as has alternatively been proposed.