Pelagornis

Pelagornis is a genus of huge fossil birds. One species, P. sandersi, is one of the largest bird ever found.Morelle, Rebecca 2014. Fossil of 'largest flying bird' identified. BBC News Science & Environment.  Its wingspan was twice that of the largest Albatross. Its wingspan is estimated as about 6.4 to 7.4 metres.compare with Argentavis. The fossil of this particular species is from the Oligocene stage, about 25 million years ago (mya).

The pelagornithids are an extinct group of birds known for bony tooth-like beak projections, large size, and highly modified wing bones. There are many questions about their ecology. The bird was capable of highly efficient gliding. That suggests it had a long-range marine soaring life-style similar to that of living albatross.Ksepka, Daniel T. 2014. Flight performance of the largest volant bird. PNAS USA prepub.  

Several later species have been found, and they are not quite so large.BBC News. Ancient American bird was glider.  The whole group died out about three million years ago.

 References 

Category:Extinct birds