A SUGGESTION CONCERNING THE CRETACEOUS RISE TO DOMINANCE OF THE ANGIOSPERMS

Authors from Darwin onward have stressed the mystery surrounding the reasons why the angiosperms became diverse and spread rapidly into the mesic lowlands all over the world during the Lower Cretaceous (Upper Albian; 110113 m.y. BP) and then assumed worldwide dominance after the end of the Turonian (90 m.y. BP; Takhtajan, 1969, p. 130; Stebbins, 1974, p. 209; Raven and Axelrod, 1974). It is now generally thought that they originated in tropical uplands or in areas of seasonal climte within or just outside of the tropics at least 125 m.y, BP (Raven and Axelrod, 1974), and perhaps much earlier (but see Doyle and Hickey, 1976). Janzen (1970) and many others have pointed out that competition and herbivore pressure, among other factors, have led to decreased population densities among the species of plants growing in the tropics. Insect pollination is predominant, and involves several kinds of strategies that insure the accurate transfer of pollen between widely spaced individuals, as outlined by Janzen (1971). Heinrich and Raven (1972) indicated that wind-pollinated plants are largely temperate in distribution; such plants must occur in fairly dense colonies, the existence of which seems to be precluded in the tropics, for pollen transfer to be effective. When the insect-pollinated flowering plants became predominant in the tropical lowlands some 90 million years ago, they did so at the expense of the wind-pollinated and wind-dispersed gymnosperms and ferns. Once they spread under these conditions, herbivore pressure doubtless caused them to diversify rapidly (Connell and Orias, 1964; Paine, 1966; Stanley, 1973), and probably constituted a major factor in their evolution, as suggested by Ehrlich and Raven (1965), Janzen (1970), and others. Since flowering plants are favored under warm tropical conditions more than under temperate ones, by virtue of their pollination systems, their occupation of the world's lowlands in mid-Cretaceous time might reasonably be expected to have been correlated with a warming in temperature. Valentine and Moores (1972), Valentine (1973), Axelrod (1974), and others have pointed out that the major changes in environmental regime due to continental movements must have played a leading role in the major waves of diversification and extinction among organisms. Specifically, Axelrod (1970, 1974) has correlated the rise to dominance of the flowering plants with the midCretaceous expansion of epeiric seas. Milder and more equable climates spread over the lowlands of Africa and South America as their movement apart, which began 125-130 m.y. BP, progressed toward a final separation just under 90 m.y. BP (Raven and Axelrod, 1974). At the height of the Upper Cretaceous transgression of seas, starting about 110 m.y. BP, some 40% of the continents were covered by water (Hays and Pitman, 1973), this leading to a great expansion of tropical climates which gradually contracted as the seas withdrew during the remainder of the Cretaceous. A warming of about 5-6 C in marine temperatures in the equatorial Pacific from 110-90 m.y. BP has been calculated by Douglas and Savin (1974) and is reflected for the period from the Cenomanian to the Coniacian (105-88 m.y. BP) in the worldwide temperature curves summarized by Scheibnerova (1973). This period of worldwide warming in temperatures is when the angiosperms rose to dominance in the tropical lowlands (Axelrod, 1970). It is postulated that their pollination by insects, a system that doubtless evolved earlier, provided a mechanism whereby they could occur as widely dispersed populations, important in an increasingly tropical ecosystem, and thereby gain a competitive advantage over the wind-pollinated and dispersed plants that formerly occupied areas of continental scope in the tropics. Once they had spread and become diverse and dominant, they were able to respond to further shifts in climate by producing evolutionary novelties adapted to the various kinds of new adaptive zones that have appeared during the remainder of their history.