Changes in the ammonite taxonomical diversity gradient during the late Jurassicearly Cretaceous

摘要

Aim Changes in the latitudinal gradient of taxonomic diversity of ammonites have been studied in four time slices of the late Jurassic to early Cretaceous (1) to assess the correspondence of the distribution with the general idea of a simple gradient of diversity decreasing polewards, and (2) to offer tentative interpretations of these patterns with respect to palaeoclimatic conditions. Locations The localities selected lie within the Boreal palaeohemisphere and most are concentrated between 10° and 45° N. Methods The distributions of genus-level palaeobiodiversity data have been plotted on palaeogeographical maps. Localities were selected on the basis of criteria that emphasize the accuracy of the rock dating. Palaeolatitudinal positions were obtained using software developed by one of us (BV). Data are largely derived from the literature and data sets have been built for each of the four time slices selected. Information on the palaeoenvironmental context has been included, with observations divided between four palaeoenvironmental systems: epicontinental platforms, intracratonic basins, transitional areas and distal epioceanic basins. Results The distribution plots demonstrate changes in the steepness of the latitudinal gradient of palaeobiodiversity through time. In general the lowest diversity values are recorded in mid- to high-palaeolatitudes and in epicontinental platform settings of all latitudes, whereas the highest diversities typically correspond to low-latitude basins. However, in most cases it is difficult to determine whether the poleward decrease of diversity values is a reflection of the palaeoenvironmental context or simply conforms to the diversity gradient model. All patterns are influenced by sampling, and no data were available for equatorial and high palaeolatitudes. Main conclusions Continental plate movements did not affect the shaping of latitudinal patterns of ammonite diversity and the palaeoenvironmental context is the most important controlling factor. For similar latitudes, the highest diversities occur in basin settings, suggesting that depth is an important influence on diversity distribution. Provincialism was enhanced by the sea-level fall at the end of the Jurassic that fragmented and restricted ammonite biotas. The effects of climatic fluctuations on the steepness of diversity gradients are probably significant but are currently difficult to demonstrate.