The key to bird flight is the fact they have a keel sternum, which servers as an enlarge anchoring point for powerful pectoral muscles. This allows them to produce enough lift for powered flight. While there are flightless birds throughout many orders of Aves,one order, Struthioniformes, better known as ratites (from the Latin ratitus for flat bottom) are strictly flightless. They consists of such well known birds as ostriches, rhea, kiwi, cassowary, and emu. Ratites lack both keel sternum and powerful pectorals. My question is this: did their ancestors once posses the power of flight and then lost it somewhere along the evolutionary pathway or did they never have the capability to begin with? Evolution is a tricky thing sometimes and what seems like the logical path is sometimes not. Chondrichthyans (sharks, rays, and ratfish) where once thought to represent a primitive stage of evolution of the vertebrate skeleton, but it appears now that their lineage branched off much latter from other vertebrates, ones that already had a mineralized skeleton. So they can be considered a deviation and not a link in the chain. Are ratites the same?
Work cited
Campbell and Reece Biology 7th edition Pearson Benjamin Cummings
(Now the other famous flightless birds, the penguin retain such structures allowing it to "fly" through the water. )
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