Vorompatra Lore


from

Fossil Birds

by W.E.Swinton,
Senior Fellow of Massey College,
University of Toronto

(Burgess and Sons (Abingdon) Ltd.
for the British Museum (Natural History) 1975.)

"Tertiary Birds" (pp.41-42)

By Eocene times the major division of birds into those which fly, i.e., Carinates, and those which are flightless and run, i.e., Ratites, had been made, although the evidence for the former is much greater than for the latter and it may be argued that the latter do not form a natural order. The reason for this difference in representation again lies in their respective habitats, for although there are numerous Eocene representatives of the flying birds, they are mostly those, like the pelicans, herons, cranes and gulls, which lived in watery surroundings that often led to the burying of their remains in mud and thus favored permanent preservation.

Among the flightless running birds, the emus, Aepyornis and the moas, few have relatives in the Eocene. In the British Museum (Natural History) there is a small piece of a limb bone, named Eremopezus eocaenus, from the Eocene of the Fayum (Egypt) ; it may well be from an ancestor of Aepyornis. The Aepyornis is of much later date, and comes from the Pleistocene and Holocene of Madagascar. The remains are well known but the ancestry is obscure, though the Eocene fragment referred to above, part of a tarso-metatarsus from the Oligocene of the Fayum (Stromeria fajumensis), and pieces of possibly aepyornid egg-shell named Psammornis rothschildi from the Eocene of southern Algeria, all suggest that the concentration of species in Madagascar was the last stand of birds once distributed much more widely.

Aepyornis with the smaller Mullerornis, the so-called elephant-birds, belong to a special Order, the Aepyornithiformes. Most of the members of the genus Aepyornis were large and massive but they had small skulls. Their feet normally have four toes, though the hallux is sometimes missing1. The largest species is Aepyornis titan2, a little over 3 metres (10 ft) high. The size of this great bird probably inspired some of the legends of the past such as that of the Rukh, or Roc, of Sinbad the Sailor and Marco Polo, for Arab traders have been familiar for centuries with the coast of Madagascar.

The eggs of Aepyornis are still found buried in the sand on the shores where they were laid in Pleistocene times. The real egg is certainly nothing like the dome-house that Sinbad took fifty paces to walk around. None the less, eggs of Aepyornis are always of interest, for they are not infrequently washed out of the beaches or out of the sandy soil near lakes in Madagascar and have reached many museums where their great size attracts attention. The largest specimen in the Palaeontological Department of this Museum measures 90 cm (3 ft) in its largest circumference and 75 cm in girth, so that its liquid content would be over nine litres. Some of the eggs contain the bones of embryos. Much has been made of the relative size of these eggs and of the birds that presumably laid them, but studies of eggs of other birds such as Apteryx (the kiwi), as well as of the dinosaurs, show that there is great variation in the size and nature of the shells...

"Origins of Ratites" (pp.62-68)

Now that we have seen something of the history of the birds in the Tertiary it is possible to devote attention to the origin of the Ratites. This general term is applied to those running birds, like the emu and the ostrich, which have a breast-bone (sternum) without a keel. It has been argued by several scientists that these are primitive birds that have never flown, and as stated above, Archaeopteryx (the London specimen) has been credited as being the ancestor of the Ratites whereas Archaeornis (the Berlin specimen of Archaeopteryx) was considered to be the ancestor of the Carinates.

There are several possibilities. A bird with a keel-less sternum may either be on the way to the development of a keel or may have lost it. Archaeopteryx may have been an ancestor of true flying birds with keeled breast-bones. On the other hand Hesperornis, the toothed Cretaceous diver, was certainly a form that had lost its power of flight; its breast-bone was unkeeled. There may, of course, have been birds in which a keeled breast-bone was never developed. If this supposition be granted there would be terrestrial birds that had never attained the ability to fly, but if this were so, then there would certainly be other aspects of their anatomy and physiology that would betray their terrestrial habit as clearly as the breast-bone.

The geological history of the ostriches, emus and cassowaries, as has been pointed out, is brief. The nature of the birds' environment is to blame for the absence of much older material, for as we have seen or can easily deduce from the record disclosed in the preceding pages, birds of marine or shore-living habits are by far the most usually preserved. This of itself confirms the terrestrial habits of the earlier Ratites.

Many, perhaps most, birds find their food on the ground. For them flight is of value for locomotion, for the assurance of the attainment of a safe nesting place and for escape from their enemies. But if enemies of importance are absent then the need for flight in the double sense is largely gone.

The past and present history of New Zealand and of Australia shows us that land enemies such as carnivorous mammals were absent, for the Australian marsupials can be ignored in this argument. In South America the rhea can usually see its enemies in the open country and flee, as Charles Darwin was among the first to testify in A Journal of Researches. The ostrich of South ern Africa is similarly well-endowed with speed and in the neighbouring island of Mauritius, home of the dodo, there were no carnivores.

Thus land birds living in these places were able to discard flight as of no advantage to them in their economy. This abandonment led to the adaptation of other parts of their anatomy for running. The wings tended to be less important than the legs and so the former, and their muscles and attachments, grew slighter and the leg bones became longer and stouter and their mechanisms more powerful. The tail was no longer required for flight control or correction and, as this also became less important, the underlying bony structure, the pygostyle, was lost.

This all seems quite logical and only to be expected, but there are other features which may indicate a more ancient connexion between some of the running birds. These are in the structure of the palate, where the palatal bones do not come into contact with the brain-case and where the vomers are larger than usual. These are not primitive features but show a neotenous condition in which a juvenile stage in normal development has been retained in the adult largely because of changed feeding habits consequent on loss of flight and increase in size. The cassowary, emu, ostrich, kiwi and rhea, and perhaps the tinamous of South America, must be regarded as survivors in the absence of competition, just as the Australian marsupials have lived on. The study of other groups has shown that in favourable circumstances there may always have been birds ready to abandon flight and that many of these, like Diatryma and Phororhacos, became large and considerable carnivores. The remains of New Zealand moas are found either in marshy deposits, in caves, or in the kitchen middens of Maoris, and in all the countries where flightless birds, including the dodo and the solitaire, have been found, it was chiefly Man, the hunter, who directly or indirectly exterminated those that are no longer represented.

The Pleistocene and late forms of New Zealand, Australia, South America and Madagascar are united scientifically in the Superorder Palaeognathae by some authors. They are characterized by a reduced wing skeleton; a sternum with no keel; the hind legs powerfully developed; the tail with (usually) no pygostyle; no fusion of pubis and ischium distally; feathers soft and curly (another neotenous feature); palatine bones not in contact with brain-case, and vomers well developed. According to Romer's Vertebrate Paleontology, the superorder comprises the Orders Caenagnathiformes, Struthioniformes, Rheiformes, Casuariiformes, Aepyornithiformes, Dinornithiformes, Apterygiformes and Tinamiformes. Wetmore includes them all within the Superorder Neognathae or Typical Birds.

The conclusion must be that the Ratites are birds which, in Professor Owen's phrase, 'have abrogated the power of flight'. Their wing structure and their skeleton and the development of the cerebellum, which has certainly not been independently evolved, all betray their ancestry, but whether they are a natural group or merely an assemblage of forms that have each followed parallel lines of evolution is an unsolved problem.


Notes on this text

  1. Perhaps the smaller species had four toes usually, but most sources give Vorompatra a mere three.


  2. Curiously, the author uses Aepyornis titan in the text directly opposite a picture with the caption "Aepyornis maximus, an Elephant-bird" which only makes sense if they are two different species. I suspect this is an oversight, but I have not tracked down the origin of the A. titan binomial.


Chalk up another vote for Neoteny here. Swinton seems to favor Convergent Evolution as an explanation for Ratite similarities, rather than presuming them to be a natural cladistic assemblage.