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 fades” of the Jurassic and begin the period evolutionarily mature and well diversified. Greatest diversity in Tethys occurs in the lagoonal shales associated with the rudist or coral framework environments of the Cretaceous carbonate platforms. Their distribution was pan-tropical, extending in instances across the vast reaches of the Pacific. Three categories of Tethyan gastropods are analyzed. The first group consists of those of Jurassic ancestry. Except for the Nerineacea, these taxa are long ranging but evolutionarily conservative, showing only moderate diversification during the Cretaceous, and becoming extinct with the close of the era. The second group originates mainly during the Barremian and Aptian, reaches a climax in diversification during middle Cretaceous time, and usually declines during the latest Cretaceous, with most not lasting through the terminal event. 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.There is a trend among the Tethyan gastropod assemblages for increased provincialism with time. Early and middle Cretaceous taxa are especially widely distributed, but the latest Cretaceous is a time of restricted occurrence for many forms.Temperate Realm gastropod faunas are less diverse than those of Tethys during the Early Cretaceous. Their source is among long lived, extra-Tethys groups, but is increased, especially during major phases of transgression, by immigrants from Tethys. 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. Among the Temperate Realm assemblages loss is of generic and species level taxa, unlike the extinction of the family Actaeonellidae or the superfamily Nerineacea and a host of less prominent groups in Tethys. In essence, by the late Maastrichtian, gastropod faunas of the Temperate Realm had attained a modern faunal aspect.
The distribution of species abundance (number of individuals per species) is well documented. The distribution of species occurrence (number of localities per species), however, has received little attention. This study investigates the distribution of species occurrence for five large data sets. For modern benthic foraminifera, species occurrence is examined from the Atlantic continental margin of North America, where 875 species were recorded 10,017 times at 542 localities, the Gulf of Mexico, where 848 species were recorded 18,007 times at 426 localities, and the Caribbean, where 1149 species were recorded 6684 times at 268 localities. For Late Cretaceous molluscs, species occurrence is examined from the Gulf Coast where 716 species were recorded 6236 times at 166 localities and a subset of this data consisting of 643 species recorded 3851 times at 86 localities.Logseries and lognormal distributions were fitted to these data sets. In most instances the logseries best predicts the distribution of species occurrence. The lognormal, however, also fits the data fairly well, and, in one instance, better. The use of these distributions allows the prediction of the number of species occurring once, twice, …, n times.Species abundance data are also available for the molluscan data sets. They indicate that the most abundant species (greatest number of individuals) usually occur most frequently. In all data sets approximately half the species occur four or less times. The probability of noting the presence of rarely occurring species is small, and, consequently, such species must be used with extreme caution in studies requiring knowledge of the distribution of species in space and time.
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