Giant Birds

Archaeopteryx, the oldest known bird, lived in the Jurassic period, quite early in the time of the dinosaurs. It was only about the size of a magpie and no giant birds appeared until the Tertiary, after the dinosaurs had gone.

The teratorns were birds of prey that lived in America then. Figure 11.1 will give you an impression of their size: (b) shows the wing bones of a Californian condor, one of the largest modern birds of prey; (d) shows the wing bones of the Wandering albatross, which has the biggest wing span of all modern birds; (c) shows the wing skeleton of the best-known teratorn, Teratornis merriami: it is almost as long as the albatross wing skeleton, and rather stouter; finally, (a) is amazing. It is the humerous (the bone from the base of the wing) of Argentavis magnificens, the biggest known teratorn. It is more than twice as long as the condor humerus shown immediately below it.

Only one skeleton of Argentavis has been found (in Argentina). The wing bones are incomplete but the pieces that have been found are enough to show that the wings were strong and well developed. It seems impossible to avoid the conclusion that those huge wings were for use, that Argentavis could fly although it was far bigger than any modern flying bird.

The wing span of the condor is ten times the length of the humerus. If Argentavis was built to the same proportions its span was about 6 meters, far more than the spans of the condor (2.7 meters) or albatross (3.4 meters). Its biggest wing feathers must have been about 1.5 meters long and 200 millimeters wide. Its body mass has been estimated from the circumference of a leg bone in the same way as was done for dinosaurs, using a graph like Figure 2.5 showing tibiotarsus circumference and body mass for 324 species of modern birds. The body masses corresponding to the tibiotarsus circumferences of teratorns were read

Argentavis Magnificens
FIGURE 11.1. Wing bones of (a] Argentavis, the largest teratom; (b) a Califor-nian condor; (c) a Wandering albatross; and (d) Teratornis. From Campbell and Tonni 1 983.

off from the graph: 14 kilograms for Teratornis merriami and 80 kilograms (heavier than most men) for Argentavis.

That 80 kilograms is the best estimate that has been made of the mass of Argentavis, but there is a lot of uncertainty about it. Birds of equal mass, of different species, may have considerably different bone thicknesses. Statistical analysis of the data tells us that the conclusion should be no more precise than this: there is 95 percent probability (the odds are 19 to 1) that the mass of Argentavis lay between 37 and 166 kilograms. Even with that much uncertainty it is clear that Argentavis was much heavier than the Californian condor (about 10 kilograms) or even the Kori bustard, which reaches about 16 kilograms and seems to be the heaviest modern flying bird.

Argentavis was also much heavier than Pteranodon, the giant pterosaur (figure 8.2), which was so lightly built that its mass seems to have been no more than about 15 kilograms. However, Pteranodon had the larger wing span (7 meters. We estimated the span of Argentavis as only 6 meters). The few fragments that have been found of the even larger pterosaur, Quetzalcoatlus, suggest a span of about 12 meters and a mass of possibly about 60 kilograms. Argentavis and Quetzalcoatlus are rivals for the title of the biggest flying animal of all time.

The largest modern birds cannot fly . They are the ostrich (up to 120 kg), cassowaries (60 kg), the emu (50 kg) and the Emperor penguin (40 kg). With the exception of the penguin, these are members of the group called the ratites, which also includes the rheas and kiwis.

Ostriches and other ratites are like enormously overgrown chicks. They have tiny wings, useless for flight, and well-developed legs. They have fluffy plumage instead of the blade-like feathers of other adult birds. They also have some chick-like features in their skeletons. They are believed to have evolved by processes of development getting out of step with each other: they grow large and sexually mature while keeping a lot of juvenile features.

The biggest extinct birds are also ratites. They are the moas, which lived in New Zealand, and the elephant birds, in Madagascar. The biggest moa is Dinornis maximus, 3.5 meters tall (twice the height of an average man, figure 11.2). The biggest elephant birds looked very similar and were about 3 meters tall. There were also some giant birds that were not ratites. Diatryma, a wicked-looking predator that lived in North America, was about 2 meters tall (figure 11.2). It lived quite early in the Cenozoic era but the moas and elephant birds are more recent. Indeed, the moas survived in New Zealand until after the Maoris arrived.

I have made scale models of moas and used them to estimate the

Cenozoic Birds
FIGURE 11.2. Dinornis maximus (the largest moa), Diatryma steini, and an adult man, all to the same scale.

masses of the living birds, in much the same way as I estimated the masses of dinosaurs (chapger 2). I modeled the main features of the skeleton in wire and then added clay to represent the flesh, making models that represented the birds as if they had been plucked. I measured the volumes of the models and used them to calculate the masses of the plucked birds, assuming that their densities were the same as the density of a plucked goose carcase (which I measured). Finally, I added an allowance for the feathers, which I assumed to be the same fraction of body mass as in turkeys and kiwis. The result for a big Dinornis was 240 kilograms, about the same as a large tiger. The biggest elephant birds were stouter, although they were a little shorter, and may have been nearly twice as heavy.

Though Dinornis was the biggest moa, the one that fascinates me is Pachyornis elephantopus, shown in figure 11.3. Its splendid name means "fat bird with elephant's feet," and seems very suitable. Its leg bones are amazingly thick. Compare it with the ostrich, drawn beside it to the same scale. I estimate this moa's mass as 130 kilograms (from measurements on a model) and the ostrich, whose skeleton is illustrated, as only 68 kilograms, but even so the moa bones look disproportionately thick.

Appearances can be deceptive, so I measured the moa's bones and calculated strength indicators in the same way as for dinosaurs (chapter 4). The results are shown in table 11.1. The value for the tibiotarsus (shin bone) is about the same as for an ostrich, and those for the other two bones are twice as high as for the ostrich. To be consistent with my line of argument in chapter 4, I should conclude that Pachyornis was at least as athletic as ostriches, but I find that hard to believe. Ostriches are exceedingly fast runners, probably faster than the African antelopes. Should I conclude that moas were also exceedingly fast? I cannot believe that they were, with those clumsy-looking legs.

The key to the problem may be that moas seem to have had no need to run. They fed on plants, as remains of their stomach contents show, and so had no need to run to capture food. There seem to have been no big predators in New Zealand, until the Maoris arrived, so there was nothing to run away from. (That is to say, there was nothing to run away from while they were evolving. They seem to have been easy prey to the Maoris, who hunted them to extinction.) It was not the same for the dinosaurs: the flesh-eating dinosaurs had to run to catch prey and the plant-eating ones had to run to escape. This difference between moas and dinosaurs may justify a different interpretation of their strength indicators.

My idea involves safety factors. Suppose an engineer is designing a small bridge to carry a maximum load of ten tonnes. He would be an optimist if he calculated the thickness of steel that could just support ten tonnes without breaking, and ordered steel that thick. Any reputable engineer would allow a safety factor: he might design the bridge to be able to carry twenty tonnes although he expected the maximum load to be only ten. The reason for this is that neither load nor strength can be predicted precisely. An unexpectedly large load may arrive, or the steel may be substandard, and in either case a bridge that was expected to be strong enough may fail. The bigger the safety factor the less likely this is to happen.

Giant Ostrich

FIGURE 11.3. Skeletons of (a| Pachyornis and (b) an ostrich, to the same scale. From Alexander 1 983a.

FIGURE 11.3. Skeletons of (a| Pachyornis and (b) an ostrich, to the same scale. From Alexander 1 983a.

TABLE 11.1. Strength indicators [Z/W\, see p. 53) for leg bones of an ostrich and the moa Pachyornis elephantopus.

Strength indicator (square meters per giganewton) for: Femur Tibiotarsus Tarsometatarsus ostrich moa

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  • Fletcher
    When giant birds were extinct?
    8 years ago

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