Lessons in Extinction 5
Contents Updated: Monday, September 13, 1999
Adaptations to Pollution
If the atmosphere in the late Cretaceous were gradually polluted its effects should show up in the fossil record as an adaptation of species to the pollution. Poisonous fumes or particles of dust in the air would induce the development of unusual nasal arrangements to attempt to prevent the pollutants from penetrating to the lungs.
Ankylosaurs were armored dinosaurs living at the end of the Cretaceous. They were related to a similar group called the nodosaurs which lived principally in the middle of the Cretaceous period. The earlier group had nasal passages consisting of a simple paired tube leading from the nostrils to the back of the throat. The ankylosaurs however had nasal passages stretched out into the shape of a letter ’S’ on either side of which there were additional passages forming almost a honeycomb. Teresa Maryanska has suggested that the purpose was to filter and moisten the air before it entered the lungs. Yet why should elaborate filtering systems have been necessary at the end of the Cretaceous but not apparently beforehand, even in closely related species, unless something was happening to the air?
Other species from distant parts were equally affected. Iguanodon orientalis from Mongolia had a huge bulbous nose supported by a bony arch.
Crested and non-crested hadrosaurs were contemporaneous about 75 million years ago. By the end of the dinosaurs’ reign 65 million years ago the crested hadrosaurs were particularly successful. The crests were either enormous plates or long projections having no ostensible use. The odd thing about the crests however was that they consisted of enormously extended nasal passages protected by the bone of the skull. There must have been evolutionary pressure to extend the nasal passages, and the skull had solved the problem of where to accommodate the resulting labyrinth by developing the crests. The evolutionary pressure was pollution.
The external shape of the crest did not always match the internal convolutions of the nasal passages suggesting that the external appearance was as important as the elaborate nasal extensions. Were they also visual signalling devices for courtship and mating? Hadrosaurs had acute vision judging by their well developed eye sockets and the presence of a bony ring (the sclerotic ring) to support the large eye. Several species of hadrosaurs seemed to inhabit the same territory and the visual signals could have served to distinguish them. They could have served to signal their position in the social hierarchy and probably the sex of the animal. But why did the nasal passages extend to serve these purposes unless some other cause had stimulated their development? Having started to develop a feature for one reason it is characteristic of sexual selection to make a virtue of necessity and use it for another.
David Weishampel of Florida State University has shown that the cavities could have also acted as resonance chambers for audible displays or communication. Other hadrosaurs without crests probably had inflatable sacs over their nostrils which served the same purpose and could have also served as a visual display. And such sacs would have developed as a protection against pollution in the air. Thus two distinct groups of hadrosaurs had different solutions to the same problem but one solution left obvious fossil records whereas the other has to be inferred. The development of nasal flaps and convoluted nasal passages is best explained as an evolutionary response to increasing atmospheric pollution. Once the protective measures had began to evolve these dinosaurs found that they had other uses too. That is typical of the way evolution works.
Evidence from the elaborate display apparatus of the ceratopsians and the hadrosaurs, and the signs of ritual duelling in pachycephalosaurs (and possibly ceratopsians) indicates that the dinosaurs were by this stage if not before, territorial and possessive. The pachycephalosaurs had high bony heads that they probably used in pushing contests or butting contests to assert dominance rather as sheep and goats do. The elaborate frills of the ceratopsians, besides serving a defensive purpose and as an anchor for the massive jaw muscles, were probably display devices to signal dominance and might have been brightly colored. These animals might also have engaged in ritual duelling by engaging their horns and grappling rather like rutting stags. Bakker’s contention is that such active sexual behavior is a sign of warm-blood.
Reduction in Oxygen
By measuring oxygen in microscopic air bubbles trapped in amber, fossilised tree resin, Gary Landis found oxygen levels dropped from 35 per cent, 2 million years before the end of the Cretaceous period, to 28 per cent just after the end of the Cretaceous—the time when the dinosaurs disappeared. Today, the oxygen level is only 21 per cent, but today’s animals have evolved to breathe in this atmosphere. The dinosaurs evolved to breathe a richer mixture, and were disadvantaged when the oxygen fell so much. The main criticism of the study is whether amber preserved oxygen so exactly for so long.
Even so, there is support in a study of a skeleton of Apatosaurus (Brontosaurus), one of the largest of the dinosaurs. With nostrils the size of a horse’s and no diaphragm to push air in and out of its lungs, it had a limited capacity to breathe. It was all right in an oxygen rich environment, but left it effectively with emphysema when levels fell. Faced with a cooling climate at the end of the Cretaceous, the drop in oxygen levels was a killer, albeit still a slow one. Extreme respiratory stress was the reason for the reduction in diversity of the dinosaurs in North America, which shrank from 35 genera 10 million years before the end of the Cretaceous to 12 genera at the end.
Many palaeontologists doubt the catastrophic impact theory, mainly because there was this gradual decline in the diversity of dinosaur species towards the end of the Cretaceous. Perhaps reduction in oxygen accompanied atmospheric pollution.
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