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The late R. E. Dickerson, in 1921, put forward the theory that tropical Tertiary molluscan faunas evolve much more slowly than do faunas from temperate regions; hence, that the percentage of Recent species in later Tertiary tropical faunas is considerably higher than in contemporaneous faunas from temperate regions. This theory, derived from analysis of a tropical fauna of inferred Miocene age from the Philippine Islands, has been frequently cited but has never been critically evaluated.
Studies of Philippine and Indonesian later Tertiary molluscan and foraminiferal faunas collected since Dickerson's time, and accurately placed stratigraphically, indicate that the Philippine molluscan faunas Dickerson believed to be of Miocene age are more probably later Pliocene in age, with a percentage of extinct species differing not very greatly from the percentages established by Lyell and Deshayes for contemporary faunas in Europe. The theory that tropical molluscan faunas evolve at a markedly different rate than those of temperate regions is therefore without basis and is probably erroneous.
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