"Ratites" by C.A. Walker, British Museum of Natural History

Flightless ground birds, usually of large size, that include the ostriches (Struthioniformes), rheas (Rheiformes), cassowaries (Casuariiformes), kiwis (Apterygiformes), and tinamous (Tinamiformes)¹ among the living avifauna, as well as the extinct moas (Dinornithiformes) and elephant birds (Aepyornithiformes). They are all placed within the superorder Palaeognathae because of the palaeognathus condition of the palate (the bones of the palate and upper jaw are much more firmly united than is customary in birds); other common features include reduction of the forelimb skeleton, an unkeeled sternum, powerfully developed hindlimbs, and (usually) the absence of a pygostyle (tail bone, the "parson's nose"). It is doubtful whether they really form a natural group or are simply an assemblage of unrelated forms that have followed parallel lines of evolution². It has been suggested that the ratites are related through a common ancestor in South America, which migrated via Antarctica to the areas of subsequent ratite development before CONTINENTAL DRIFT fragmented the continents and moved them into their present positions. The geological record has so far failed to produce any evidence in support of this hypothesis³.

The earliest examples of ratite-type birds appear to be the ostrich-like Eleuthornis, known from the Eocene deposits of Switzerland, and two elephant birds (Eremopezus and Stromeria) which were found, respectively, in the Eocene and Oligocene sediments of northern Africa. The material is so fragmentary, however, that there must be some doubt as to the correctness of these assignments. If these early records are disregarded, the geological history of the ratites is short: there are no subsequent records before the Pliocene, although all the orders except the kiwis (which appear to be a recent development) occur in the Pleistocene⁴.

The most spectacular forms of this group are found among the extinct moas of New Zealand and the elephant birds of Madagascar. The largest moa, Dinornis maximus, reached a height of 3.5 metres (11.5ft), but not all species were as large; Anomalopteryx parva probably did not exceed 90cm (3ft). The elephant birds also produced a large species, Aepyornis maximus, which is known to have attained a height of 3 metres (9.9ft), and probably inspired some historical legends, such as the Rukh (or Roc) of Sinbad the Sailor and Marco Polo. Eggs attributable to Aepyornis are commonly found in sand dunes, where they were laid many thousands of years ago⁵, and are exceptionally large - one such example had a liquid capacity of more than 9 litres (2 gallons), which is enormous even for a bird the size of Aepyornis.

Why these two highly successful groups became extinct is not fully understood, although the moas appear to have suffered from man's arrival in New Zealand and the resulting moa-hunter culture that developed⁶. Many of the larger species, however, were probably already declining before man's involvement, possibly due to changing habitats caused by the changes in temperature during the Pleistocene.

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Notes on this text

1. Tinamous do fly, although not very well.



2. Another vote for the Ratites being a convenient grouping for look-alike birds, rather than a clade (i.e., descended of a common source).



3. One missing link in the Gondwana dispersal theory is a lack of fossils from Antarctica, which presumably the early ratites must have crossed to achieve their prehistoric distribution. Continuing research "way down under" would be expected to yield more evidence for this theory if it's correct.



4. This puzzling lack of older ratite fossils continues to undermine the Gondwana dispersal theory.



5. Some of those eggs must have been laid more recently than this, since there is increasing evidence that Vorompatra may have gone extinct only 300 years ago.



6. It is now pretty well accepted that the Maori drove the Moa to extinction by hunting. There is less evidence about the Malagasy role in the Vorompatra's demise, but no reason to rule out the possibility of a parallel situation in Madagascar.