Thermal Effects

For more than 100 my before their demise, the dinosaurs enjoyed equable climatic conditions. At the end of the Mesozoic Era, however, the seasons became accentuated, as indicated by the rise of the angiosperms with their deciduous foliage and overwintering seeds. In 1965, the Canadian palaeontologist L.S. Russell proposed that winter temperatures might eventually have fallen low enough to kill such land animals as could not conserve their body warmth and did not hibernate. This might explain why the dinosaurs died out, even though they may have been tachymetabolic (with a fast rate of chemical change; Sect. 7.6), as argued by Bakker (1987) since the 1960s, while reptiles that were not endothermal were able to survive. It seems more probable, however, that the summer heat would have been more unfavourable, as argued below - a hy pothesis that fits with the recent discovery of dinosaur fossils near the poles. Russell's suggestion also fails to explain why the dinosaurs did not persist in equatorial regions, as the crocodilians did, nor why the theropods, many of which are known to have been feathered (Sect. 11.5.1), also died out.

Because chemical reactions are temperature dependent (Q10 effect), there is an evolutionary tendency towards systems that enable animals to maintain their body temperature at a constant level. It is now believed that the sail of the pelycosaur Dimetrodon (Fig. 10) may have been a device for absorbing solar heat in the cool Permian mornings, and for radiating excess heat during the hot part of the day (Sect. 7.5.2). In a similar way, according to Farlow et al. (1976), the dorsal plates of stegosaurs (Fig. 71) probably served as forced convection fins and had an important thermoregulatory function, as well as being concerned with intra-specific combat and anti-predator display (Sects. 7.5.2, 9.3.1).

The temperature at which the eggs of modern crocodilians and other reptiles are incubated affects the gender of the hatchlings. The possible influences of thermal sex determination in the K-T extinction has been discussed by Milne (1991),but it is almost unbelievable that the dinosaurs, with the gradual onset of seasons, did not evolve seasonal breeding cycles like other reptiles (Cloudsley-Thompson 1978).

Whether or not the dinosaurs were tachymetabolic (Sect. 7.6) is, I believe, largely immaterial as regards their extinction. The larger forms must have been homeothermic whether they were endothermal or not. Even the smaller species were too large to burrow and many may have been on the way to becoming birds. In contrast, modern reptiles are able to avoid inclement seasons by aestivating (summer dormancy) or hibernating in sheltered retreats. In addition, they utilise physiological thermoregulatory processes, such as emergency cooling of the body of tortoises through salivation and urination. The elongated shape and comparatively small size of most existing reptiles render prolonged homeothermy uneconomical, and even large pythons 'shiver' only when brooding their eggs (Cloudsley-Thompson 1999). The small surface-to-volume ratio of large animals, including most of the very last dinosaurs, would have rendered them susceptible to heat rather than to cold, a point overlooked by many non-biologists such as Milne (1991). Moreover, fermentation in the gut of the large herbivores would have generated a considerable amount of heat.

The thermal physiology of dinosaurs has already been discussed (Sect. 7.5). Gigantism is a useful strategy for reptiles in a stable, relatively warm climate, but thermal stresses caused by decreasing equability at the close of the Meso-zoic might well have had harmful effects on large animals. Hair would have been as disadvantageous to a large dinosaur in summer as it would be to a modern African elephant or rhinoceros. Furthermore, the work of Clemens (1986) and others in recent years has shown that the last of the dinosaurs and other extinct reptiles resided not in the tropics but in temperate regions nearer to the poles.

Why did compensation for size not prevent excess metabolic heat from being generated whether the dinosaurs were endothermic or exothermic, especially if they were as active as we now believe them to have been? It is perhaps surprising that the more advanced vertebrates of today have evolved tachymeta-bolic homeothermy rather than enzyme systems that could have functioned equally well at lower temperatures. The list of relevant problems seems endless. Nevertheless, of the many factors that undoubtedly contributed to the ultimate lack of competitiveness and demise of the dinosaurs, thermal stress may well have been one of the more significant - especially in the summer (Cloudsley-Thompson 1978).

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