from CHAPTER 12: "RECOVERY DURING THE TERTIARY PERIOD"

PALAEOGNATHUS BIRDS

The origin and evolution of palaeognathus birds have been debated incessantly. The question is whether they are a natural group or merely an assemblage of unrelated forms that have followed a parallel line of evolution. Traditionally, six orders of palaeognathus birds are recognized: Tinamiformes (tinamous), Aepyornithiformes (elephant birds), Dinornithiformes (moas and kiwis), Casuariiformes (cassowaries, emus), Struthioniformes (ostriches), and Rheiformes (rheas). Of these, only tinamous can fly; the other groups are flightless.

The flightless groups are generally called ratites. The living ratites have a flat sternum, reduced wings, and a lack of ossification between the ilium and ischium around the ilioischiadic fenestra. It is generally believed that ratites descended from flying ancestors that lost their powers of flight as they evolved into medium-sized, grazing animals. This idea is supported by the discovery of Lithornis, a volant palaeognath from the Paleocene and Eocene of Wyoming (Houde and Olson 1981¹). There are ten living species of ratites, all restricted to southern continents: two species of rheas (Rhea) in South America, three species of cassowaries (Casuaris) in New Guinea, the emu (Dromaius) of Australia, three species of kiwis (Apteryx) in New Zealand, and the ostrich (Struthio) of Africa.

Some of the most spectacular ratites are now extinct: thirteen species of moas^A (Dinornithidae) of New Zealand, nine species of elephant birds^B (Aepyornithidae) of Madagascar, and eight species of mihirung birds^C (Dromornithidae) of Australia. These gigantic grazers were able to develop so successfully because of their isolation from significant competitors or predators. It is likely that their ancestors flew to these islands and then became flightless. Their fossils and eggs have been recovered from Pleistocene and Holocene localities.

The fossil record of ratites is spotty; the oldest record, Remiornis, extends back to the Paleocene of France (Martin 1992²). Another flightless form, Palaeotis from the Eocene Messel Oil Shale, is regarded as a close relative of the ostrich (Peters 1992³, 1995⁴). It shows weak development of the sternum, pectoral girdle, and wings. African ratites are known from Eocene and Oligocene deposits. The discovery of North American, European, and Mongolian ancestral ratites has zoogeographic implication; it argues against the popular notion that ratites evolved in Gondwana, a thesis based on their present distribution^D.

Recent cladistic (Cracraft 1974⁵) and biochemical (Sibley and Ahlquist 1990⁶) analyses suggest that there are two basal lineages within the palaeognathus birds--tinamous and ratites. Cracraft (1974) recognized five successive clades^E in palaeognathus birds: Palaeognathae, Ratiti, Struthiones, Struthionoidea, and Struthionidae (fig. 12.7). Cracraft's cladogram is followed in our discussion of the relationships of palaeognathus birds^F.

FIGURE 12.7
Phylogeny of palaeognathus birds (source: based on data in Cracraft 1974⁵).

NODE 1: PALAEOGNATHAE (TINAMIDAE + RATITI)
The most primitive palaeognaths are the Tinamidae. The tinamous are partridge-like birds that range from southern Mexico to the tip of South America. They have weak bills and small tails. Although tinamous have the keeled sternum essential for flight, they are clumsy fliers. They are actually most closely related to moas and kiwis and are considered to be an outgroup of the ratites. Stratigraphic range: Upper Pliocene-Holocene; living species: 47 species; size [height] : 15-50 cm.

NODE 2: RATITI (APTERYGES + STRUTHIONES)
The moas are an extinct group of large, flightless, grazing birds that once flourished in New Zealand. Excessive hunting by Maori natives caused the extermination of these birds. There are twenty^G known species of moas; Dinornis stood over 3 meters tall and probably weighed about 250 kg. The kiwis of New Zealand are the smallest and most anomalous of the ratites. They are wingless, probing birds, with their nostrils at the tip of the flexible bill. They mainly feed on insects, worms, and berries. Stratigraphic range: Upper Miocene-Holocene; living species: 3; length [height] : 30-80 cm.

NODE 3: STRUTHIONES (AEPYORNITHIDAE + STRUTHIONOIDEA)
Elephant birds are another extinct group of large flightless birds; they lived on the island of Madagascar but were exterminated by human activity. They had long and powerful legs but retained [internal] vestiges of wings. The largest bird ever to exist, Aepyornis maximus, attained a height over 3 meters. Stratigraphic range: Upper Eocene-Holocene.

NODE 4: STRUTHIONOIDEA (CASUARIIDAE + STRUTHIONIDAE)
Cassowaries spend much of the day hidden in the dense jungles of New Guinea and adjacent islands. They have a bony crest, or casque, on the top of the head, like a helmet, which may help to turn over loose soil or sand when they search for food. Their diet consists mainly of fruits, seeds, and berries. Emus, the world's second largest living birds, live in Australia. Both cassowaries and emus are large, flightless, cursorial birds with diminuative wings. Stratigraphic range: Pleistocene-Holocene; living species: 4; length [height] : 130-190 cm.

NODE 5: STRUTHIONIDAE (STRUTHIONINAE + RHEINAE)
The ostriches may date back to the Eocene and were widely distributed during the Neogene subperiod. Their fossils have been found in Europe, Africa, and Mongolia. Today only one species, Struthio camelus, remains. It is the largest living bird and inhabits the savanna or brushland of Africa. It can run faster than any other two-legged animal, and it lays the largest eggs of any living creature. The last ostriches in Arabia were killed during World War II. The ostriches have two toes (the 3rd and 4th), and the lateral one seems to be in the process of vanishing^H. They are large, flightless, cursorial birds with small wings^I and may grow to be 2.6 meters tall. Stratigraphic range: Eocene-Holocene; living species: 1; [height]: 180 cm.

Rheas have been restricted to South America throughout the Cenozoic. The two living species differ in size; the common rhea (approximately 1.4 meters^J) is taller than Darwin's rhea (approximately 90 cm). These flightless birds are capable of running long distances in the pampas. Rheas have small tails and only three toes; they lack an aftershaft in their loose, soft feathers. Stratigraphic range: Lower Eocene-Holocene; living species: 2; length [height] : 90-130 cm.^J

from CHAPTER 13: "BIRDS AND HUMANS"

PALAEOGNATHUS BIRDS

Since the beginning of humanity, we have worshipped birds as deities, used them as symbols of power and royalty, portrayed them in art and folklore, kept them as pets, hunted them for food and feathers, and watched them for aesthetic and recreational pleasure. Thousands of years ago our ancestors immortalized birds in numerous Stone Age cave paintings in France, Spain, Africa, India, and elsewhere. For millennia birds have enchanted us with their bright colors, striking behaviors, and melodious songs. Their awesome power of flight inspired us to conquer the air. During our long association with birds, however, we have also become their adoring assailants, causing countless species to disappear.

Birds endured two mass extinctions, one at the end of the Triassic and the second at the end of the Cretaceous, but they rebounded from these crises. Since then, the number and variety of species has progressively increased to the present peak of diversity. However, birds are now at risk because of the effects of human activity. People are directly responsible for the extinction of many wild birds. Some species have been hunted to oblivion. Others have been killed off as people destroyed natural habitats to make room for their own settlements. Human beings have also polluted the environment, which has wreaked havoc on bird diversity. These assaults started a long time ago when early humans began migrating to new frontiers and confronting pristine habitats. Extinctions of birds were more severe on islands than on continents. Edward O. Wilson (1992⁷), the famous American naturalist, named these human-induced catastrophes centinelan extinctions after the Ecuadorian ridge Centinela at the Andean foothills of Ecuador. Centinela witnessed the collapse of biodiversity and wept silently because of human interference. Today, humans seem to be creating a sixth major mass extinction^K.

EXTINCTION ON ISLANDS

Before human colonization, many islands, such as Hawaii, New Zealand, Madagascar, and the Mascarene s , supported a rich and varied endemic biota. The tropical climate and abundant rainfall gave rise to dense rain forests. Because of isolation and lack of predation, many birds on these islands became secondarily flightless. Biotic crises began thousands of years ago when early human explorers began to colonize these islands and exploit their resources. They hunted the easy prey, destroyed natural habitats, and pushed many avian species to the brink of extinction...

Centinelan extinctions were also rampant on the islands of the Indian Ocean. Madagascar is the fourth largest island in the world. A rift separated it from Africa and India during the Cretaceous, and it moved southward and then, over millions of years, evolved an exotic biota that included several species of majestic elephant birds. These robust ratites were important grazers and browsers in this tropical forest, and the largest species attained heights of over 3 meters. They resembled heavyweight ostriches with massive legs and weighed about 450 kg. Immigrants from Indonesia moved to this island in successive waves between 1,500 and 2,000 years ago, followed by African settlers. It is likely that these Malagasy settlers hunted not only the elephant birds but also their huge eggs. The eggs, about 34 cm long and 11 liters in capacity, were used for both food and containers. The beaches of Madagascar are still littered with eggshells of elephant birds. These defenseless birds became easy prey for the arriving humans. Larger animals generally had smaller populations and lower reproductive rates, making them vulnerable to overexploitation. Continued hunting and egg collecting over centuries, along with habitat destruction, led to the extinction of the largest birds that ever lived. The last living elephant bird Aepyornis was reported in 1658^L...

In the pattern of extinction among island birds, large flightless birds were the main victims because of overkill. These birds, having never before seen human beings, were unafraid of people and were easily approached and killed. Flightless rails were also severely threatened on these islands. Many of the extinct species come from six groups: ratites, rails, waterfowl, pigeons, crows, and birds of prey. The clearing and destruction of forests for agriculture fragmented the habitat of native species, contracted their range, reduced their resources, and caused their populations to dwindle. There was nowhere left to feed, hide, or escape. The surviving island birds are, for the most part, mere relics of a rich avifauna that vanished not long ago.

References

1. Houde, P. and S.L. Olson. 1981. Paleognathous carinate birds from the early Tertiary of North America. "Science" 214:1236-1237.
2. Martin, L.D. 1992. The status of the late Paleocene birds "Gastornis" and "Remiornis". In Campbell, "Papers in Avian Paleontology", 97-108.
3. Peters, D.S. 1992. Messel birds: a land-based assemblage. In "Messel: An Insight into the History of Life and of the Earth", 137-151. Edited by S. Schaal and W. Ziegler, Oxford, England: Clarendon Press.
4. ____________ 1995. "Idiornis tuberculata" n. spec., ein Weiterer ungewöhnlicher Vögel aus der Grube Messel (Aves: Gruiformes: Cariamidae: Idiornithinae). "Courier Forsch, Inst. Senckenberg" 181:107-120.
5. Cracraft, J. 1974. Phylogeny and evolution of the ratite birds, "Ibis" 116:494-521.
6. Sibley, C.G., and J.E. Ahlquist. 1990. "Phylogeny and Classification of Birds: A Study in Molecular Evolution." New Haven: Yale University Press.
7. Wilson, E.O. 1992. "The Diversity of Life." New York: Norton.

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Notes on this text

1. Mike Dickison maintains a site called the Moa Pages, a great source of information. Check out his page of common mistakes, one of which is that the number of moa species recognized has been parsimoniously reduced from 13 to 11 in recent years.



2. Nine species of elephant birds? Ever notice how no two sources agree on the number?



3. Were the Mihirungs ratites? If you adhere to the monophylous view that the ratites are all descended from a common non-volant ancestor, it remains to be proven (by all means, let me know if there have been findings that closely tie the Mihirungs to the card-carrying ratites). If, however, you believe the ratite assemblage to be loosely grouped, based on derived characteristics which resemble each other through convergent evolution (as I do), then by all means they are ratites, as long as they display a keelless sternum. The Mihirungs are a relatively recent addition to the Class Aves, so a lot remains to be learned about them.



4. Very good point, since North America, Europe, and Mongolia were in Laurasia. Maybe they flew there, in exactly the same way that ratites can't (my apologies to Douglas Adams).



5. A "clade" is a group of related organisms including their common ancestry, together with all intermediate species. It may also be defined as a branching, the point at which one species bifurcates to form two. It's the fundamental building block of the taxonomic study of Cladistics.



6. Note that Cracraft's work predates the discovery of the Dromornithids, which is why they are not discussed here. A pity; perhaps the author will remedy that next time.



7. "...twenty known species of moa"!? I'd've sworn he said 13 earlier, for which reason I included a note about there only being 11.



8. It has been proposed that the two toes may dwindle to a single toe in time, just as the five-toed Hyracotherium graduated to the modern single-toed Horse.



9. Ostrich wings are too small to bear the bird aloft, but it's all relative. The wings are rather large, all things considered, and are deployed prominently for courtship display.



10. Another proofreading oversight, I think: "approximately 1.4 meters" equates to 140 cm. rather than 130. At least, it's supposed be just that easy--that's the major selling point of the Metric System, after all.



11. For more information on the previous five, click here.



12. This must be de Flacourt's account, but there is no evidence that he saw the bird: he was probably reporting second-hand from native sources. Vorompatra may have already gone extinct by this time, albeit recently, since the oral accounts were still there for the French Governor to hear.