By the end of the Mesozoic Era, an essentially modern type of flora had evolved on land. Gymnosperms, which had been the dominant plants, were gradually superseded by angiosperms, especially deciduous trees and shrubs, adapted to seasonal climates. Shortly before World War II, Cambridge biochemist Ernest Baldwin suggested that an enforced change in diet from tree ferns to deciduous plants might have caused constipation and other nutritional disorders for herbivorous dinosaurs. In 1976, Tony Swain argued that the ability of flowering plants to produce hydrolysable tannins,which are more efficient as feeding deterrents than the compounds present in lower plants, would have greatly reduced the amount of palatable food available to all herbivores. Furthermore, angiosperms synthesise many toxic alkaloids that are absent from most lower plants and are far less easily detected by extant reptiles than they are by mammals. Although these alkaloids might have been responsible for direct poisoning of some archosaurs, Swain concluded it to be more probable in the longer term that they would have interfered with several physiological activities and caused the observed thinning of ceratopsian eggshells (Sect. 12.5.1).
However, if toxic alkaloids appeared 120 mya with the first flowering plants, one wonders why the dinosaurs lingered on for a further 50 my before being struck down. Moreover, the incompatible suggestion has been made by Howard Hinton (1973) that the bright colours of many flowers - especially at the red end of the spectrum, invisible to insect pollinators - may have evolved as aposematic (warning) coloration. Mammalian herbivores lack colour vision while, as far as is known, all reptiles possess it. It is therefore possible that one of the chief selective pressures for the evolution of coloured flowers may have come from herbivorous dinosaurs. This could scarcely have occurred if they had been unable to detect the poisonous chemicals of which the flowers gave warning (Cloudsley-Thompson 1978).
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