Faunal assemblage

Abstract: A probabilistic index of faunal similarity is proposed which compares the number of taxa common to two faunas with the number that would be expected to be in common if the taxa were distributed randomly. Departures of observed from expected numbers in common express the level of similarity or dissimilarity. The frequency of taxa in the whole data set is used to adjust for the differing probability of occurrence of taxa (cosmopolitan versus endemic). The new index can be used to determine whether similarities or dissimilarities between faunas are statistically significant. The index is tested with 1) modern biogeography of echinoids, 2) environmental distribution of modern foraminifera in Santa Monica Bay, and 3) Ordovician biogeography of nautiloids. In each case, the proposed index is more effective than traditional indexes of faunal similarity (Simpson, Jaccard, and Dice coefficients) in addition to the advantage of making possible rigorous assessment of statistical confidence. The index should also be useful in a biostratigraphic context. The computer program used for calculating the index is available from the authors.

Abstract: In the tropical Atlantic, planktonic foraminfera species are vertically distributed with highest abundances occurring in the photic zone (approximately 0–100 m). The tropical Atlantic thermocline dips from east to west and varies seasonally due to changes in the southeast and northeast trade winds. In the east, the thermocline is in the photic zone, and in the west, the well-mixed surface layer extends below the photic zone most of the year. As expected from species vertical distributions in plankton tows, the species assemblages on the seafloor are correlated to the hydrographic conditions of the overlying surface ocean layer. A new technique to reconstruct past tropical Atlantic (20°N to 20°S) photic zone hydrography and surface wind field uses faunal assemblage data from deep-sea cores. Planktonic foraminifera abundances in core tops correlate with observations of modern photic zone hydrography defined here as seasonal temperature variation and mixed layer depth. The hydrography is mathematically described using empirical orthogonal function (EOF) analysis of annual temperature range as a function of depth. Factor analysis of 29 species of planktonic foraminifera from 118 core tops produces three factors. The factors correlate to mixed layer depth and the two EOF modes. The ocean model of the Atlantic ocean produces similar map patterns of the EOF modes. Therefore the model can be used to simulate hydrographic changes to compare with faunal predicted past hydrographic changes. Since the ocean model is wind driven, this approach provides a way of evaluating the validity of estimates of past wind stress changes and the contribution of these changes to the faunal changes in the past. A double wind stress run indicates that the central and eastern equatorial and southeast regions of the study area are most sensitive to wind stress increases. Factor analysis of the foraminifera abundances from the last glacial maximum (LGM) shows that species associations change downcore and demonstrates how the methods developed in this study can be applied. Comparison of the double wind stress experiment and the LGM faunal changes indicates some areas of significant agreement suggesting that faunal changes may reflect thermocline structure response to the LGM wind field. Discrepancies may reflect the fact that uniform changes in the north and south trade wind strengths did not occur at the LGM.