Cayaoa bruneti Tonni, 1979 is an early Neogene anseriform exhumed from marine sediments of the lower levels of the Gaiman Formation ("Patagoniano", Leonian Marine Stage, Chubut, Argentina). It was originally based on a partial tarsometatarsus that was not assigned to subfamily
or tribe. We re-examine the holotype and study new tarsometatarsi, and previously undescribed skeletal elements including partial femora, tibiotarsi, partial humeri and carpometacarpi. Cayaoa bruneti was a strictly foot-propelled diving bird that exhibited an extreme reduction of the
fore-limb, being probably the earliest recorded flightless duck. Similarly reduced wings are found in the distantly related anseriform Cnemiornis Owen, 1866 of subrecent deposits of New Zealand, Chendytes Miller, 1925 of the Pleistocene of California, and the moa-nalos of the
Late Quaternary of Hawaii.
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