Flightless Birds Palaeognathae

The palaeognathous palate (see Box 9.2) shows primitive theropod and avian characters, but there are several synapomorphies (the extensive vomer-pterygoid joint, the elongate basipterygoid processes that meet the pterygoid). Palaeognaths also share synapomorphies in other parts of the skull (Cracraft, 1988).

Most living palaeognath groups have short fossil records, extending back to the Miocene or Pliocene only. The oldest palaeognaths, the extinct lithornithi-forms (Houde, 1986), are known from the Palaeocene and Eocene of Europe and North America (Figure 9.10(a)). Lithornithiforms have the classic palaeog-nathous palate, showing a caudal process of the palatine, and more derived forms, including modern kiwis and ostriches and the fossils of Lithornis and Palaeotis, have an additional feature, a pterygoid fossa (both features lost in the ostrich). Lithornis and relatives from North America were hen-sized birds that retained the power of flight, whereas Palaeotis was a crane-sized flightless bird.

Modern palaeognaths fall into two groups, the tina-mous, partridge-sized birds from South and Central America, and the ratites. The ratites include such well-known flightless birds as the rheas of South America, the cassowaries and emus of Australia, the kiwis of New Zealand and the ostriches of Africa. These all have reduced wings and they have lost the keel on the sternum, presumably having evolved from ancestors that could fly.

The geographical distribution of modern ratites had long suggested that the group originated in Gondwana. When Houde (1986) showed that the lithornithiforms were also palaeognaths, he argued that the early history of the group had begun in the northern hemisphere and that the ratites had reached the southern continents only 30-40 Myr ago. Molecular sequencing suggests, however, that the classic Gondwana model might be correct: Cooper et al. (2001) found evidence for deep divergences among the living genera, with the South American Rhea splitting from the others some 82 Myr ago. Whether that date is correct or not, there is no reason to reject the hypothesis that the lithornithiforms diverged from a Gondwanan ancestor and moved to the northern hemisphere in the early Tertiary (Cracraft, 2001).

Some of the most spectacular ratites are now extinct, the elephantbird, or roc, of Madagascar and the moas of New Zealand. Both groups are known by subfossil bones and fossil bones no older than the Pleistocene.

Flightless Birds The Pleistocene

Fig. 9.10 Palaeognathous birds: (a) comparison of size and form of the flighted Palaeocene and Eocene Lithornis and Paracathartes, the flightless Eocene Palaeotis and modern Casuarius (cassowary); (b) restoration of the giant flightless Dinornis from the subrecent of New Zealand. [Figure (a) modified from Houde, 1986; (b) based on a Charles R.Knight painting.]

Fig. 9.10 Palaeognathous birds: (a) comparison of size and form of the flighted Palaeocene and Eocene Lithornis and Paracathartes, the flightless Eocene Palaeotis and modern Casuarius (cassowary); (b) restoration of the giant flightless Dinornis from the subrecent of New Zealand. [Figure (a) modified from Houde, 1986; (b) based on a Charles R.Knight painting.]

There were at least ten species of moas (Figure 9.10(b)), which ranged in size from that of a turkey to heights of over 3 m. In recent molecular studies, where DNA has been recovered from their subfossil bones, it has been shown (Bunce et al., 2003) that the three 'species' of the moa Dinornis, distinguished by their size (small, medium, large) were wrongly identified. It turns out that the smaller ones were males and the larger females, and that there were in fact only two species, one on North Island and one on South Island. Moas fed on a variety of plants and, together with kiwis, flightless rails, ground parrots, geese and others, formed unique communities in the absence of mammals. After the arrival of polynesian settlers about ad 1250, it seems the moas were hunted to extinction in 100 years or less (Holdaway and Jacomb,2000).

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  • alessia zito
    Is crane a flightless bird?
    7 years ago

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