Ratite distribution in the Palaeocene
(Once again thanks to Ron Blakey, NAU Geology, for the free use in desecrating the maps)
A basic map of ratite “urheimats” during the earliest Cenozoic: red = ostriches and kin, indigo = “lithornithids”, green = rheas, yellow = casuariforms, dark red/crimson = moas and tinamous and brown = kiwis and elephant birds.
A Laurasian origin for Paleognathae has been proposed in several recent studies, in contrast to the old idea that ratites are gondwannan vicariants. The two most basal clades, “lithornithids” and ostriches, certainly are dominant during the Paleogene of Laurasia and predate gondwannan ratite fossils. Lithornithids are thought to have had a circumpolar distribution and are found in European and North American sites as recently as the mid-Eocene. Ostriches are also found in all of Laurasia but have diversified into several clades as they lost the ability to fly early on: Geranoididae in North America, Palaeotiidae in Europe and Eogruidae in Asia. Of these, it seems eogruids might be the most likely ancestors of modern ostriches, which would later invade Africa and India in the Miocene.
Rheas have a poor fossil reccord, but putative members occur in the Paleocene and Eocene of South America and Antarctica. They don’t seem to have expanded beyond these landmasses.
Tinamous are known from the fossil reccord prior to the Pliocene, but moas have been in New Zealand most likely since the Paleocene, as the known Miocene forms are already flightless and specialised. They might have crossed Antarctica or flat out flown across the Pacific.
Casuariforms are best known from Australia but the earliest representative, Diogenornis, comes from South America. As Diogenornis still has relatively large wings its unclear if it and emus/cassowaries lost the ability to fly independently or if they crossed Antarctica into Australia (or vice versa).
Kiwis and elephant birds have the most drastic geographic distance. Given hw specialised elephant birds are I’m assuming they might have been on Madagascar since the Paleocene, so the still volant kiwi ancestors might have crossed the Indian Ocean with India as a stepping island, then Australia before arriving to New Zealand in the Miocene. Or they could have both come from either Asia and migrated southwards or Antarctica and inversely migrated northwards. The absence of Australian fossils is of particular interest.
This leaves Africa as the only continent without indigenous ratites prior to the Miocene. Eremopezus and “aepyornithid-like” forms might be the original African ratites, but for now their relations to the rest of Aves remain unknown.