During the Cretaceous Period, gastropod faunas show considerable differences in their evolution between the Tethyan Realm (tropical) and the Temperate Realms to the north and south. Like Holocene faunas, prosobranch, gastropods constitute the dominant part of Cretaceous marine snail faunas. Entomotaeneata and opisthobranchs usually form all of the remainder. In Tethyan faunas the Archaeogastropoda form a consistent high proportion of total taxa but less than the Mesogastropoda throughout the period. In contrast, the Temperate faunas beginning in Albian times show a decline in percentages of archaeogastropod taxa and a significant increase in the Neogastropoda, until they constitute over 50 percent of the taxa in some faunas. The neogastropods never attain high diversity in the Cretaceous of the Tethyan Realm and are judged to be of Temperate Realm origin. Cretaceous Tethyan gastropod faunas are closely allied to those of the 'corallien facies' of the Jurassic and begin the period evolutionarily mature and well diversified. Three categories of Tethyan gastropods are analyzed. The first group consists of those of Jurassic ancestry. The second group orginates mainly during the Barremian and Aptian, reaches a climax in diversification during middle Cretaceous time, and usually declines during the latest Cretaceous. The third group originates late in the Cretaceous and consists of taxa that manage to either survive the Cretaceous-Tertiary crisis or give rise to forms of prominence among Tertiary warm water faunas. Temperate Realm gastropod faunas are less diverse than those of Tethys during the Early Cretaceous. They show a steady increase in diversity, primarily among the Mesogastropoda and Neogastropoda. This trend culminates in latest Cretaceous times when the gastropod assemblages of the clastic provinces of the inner shelf contain an abundance of taxa outstripping that of any other part of the Cretaceous of either realm. Extinction at the Cretaceous-Tertiary boundary is much less pronounced in the Temperate Realm than in the Tethys. In essence, by the late Maastrichtian, gastropod faunas of the Temperate Realm had attained a modern faunal aspect. - from Author
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Cretaceous gastropods: contrasts between tethys and the temperate provinces.