Old jaws: the bird half that includes ostriches and moas
Birds fall into two great living groups, and Palaeognathae, meaning “old jaws,” is the smaller. It holds the flightless ratites, such as ostriches and kiwis, along with the flying tinamous of the American tropics, plus moas and elephant birds that vanished within the last 1,250 years.
The name refers to a distinctive arrangement of bones in the roof of the mouth, once judged more primitive than in other birds, although some researchers now argue that the ancestor of all living birds had the other, neognath style of palate. Paleognaths are no living fossils: their DNA has kept evolving at rates similar to other birds. Today the group includes 47 species of tinamous and five kiwis among its five surviving orders, while the lithornithids, moas and elephant birds are its undisputed extinct members.
Dating the group is hard. Molecular estimates summarised by Benton in 2005 placed its origin 110 to 120 million years ago, yet firm fossils appear only from about 70 million years ago, leaving a gap of roughly 45 million years. The most basal fossil forms, such as Lithornis, come from the Northern Hemisphere, hinting at a northern origin for birds now found mostly in southern continents. A nearly complete Lithornis skeleton from early Eocene Denmark, published in 2005, has perhaps the best-preserved skull of its kind. A 2020 study found that every modern order arose only from the latest Paleocene on, starting with ostriches.
Genetics overturned old assumptions. Earlier critics suspected the ratites were an accidental mix of unrelated birds that had each lost flight, and one 1948 author thought their palate might be an embryonic trait kept into adulthood. DNA work instead shows the whole group is one natural lineage, but that flight was lost at least three separate times, so the ratites themselves do not form a single branch. Since 2010, analyses have even placed the flying tinamous closest to New Zealand’s extinct moas.
The ornithologist Pycraft coined the name Palaeognathae in 1900, arguing that a keelless breastbone could evolve independently in any bird that stopped flying. Ostriches are widely accepted as the earliest living offshoot, while the order in which the remaining groups split remains unsettled.
Source: Palaeognathae
