THE USE OF MARINE LATITUDINAL BIODIVERSITY GRADIENTS TO ELUCIDATE MESOZOIC PALEOCLIMATES

Latitudinal biodiversity gradients (LBGs) are a ubiquitous feature of life today. These gradients are often thought to be related to temperature gradients and may thus indicate paleoclimatic regimes. Theory predicts that a steeper thermal gradient will generate a steeper LBG but with diversity maxima away from the equator, whereas a more shallow thermal gradient will result in a shallower and unimodal LBG.

To test these predictions and to assess the use of biodiversity patterns for paleoclimate studies, we analyzed LBGs using Mesozoic marine occurrence data from the Paleobiology Database. Sampling-standardized patterns of diversity in latitudinal grids (rarefied diversity) and between grid-diversity (beta) were computed along latitudinal transects in the Tethys (Late Triassic, Late Jurassic), the eastern Pacific (Panthalassa) margin (Late Triassic, Early Jurassic, Cretaceous) and the Western Interior Seaway (Cretaceous).

Rarefied diversity usually exhibits an equatorial depression, a low-latitude plateau and a pronounced decline at intermediate latitudes. While this general pattern is similar to today, the major diversity decline occurred at higher latitudes, ranging from 30° in the Late Triassic eastern Pacific to 50° in the Late Jurassic Tethys. This agrees with the contention that the Mesozoic had a more shallow thermal gradient and was generally much warmer than today. The bimodal shape of the latitudinal diversity trajectory, however, is in contrast with expectations from theory. We propose that responsible for this pattern is a weakened thermal gradient but tropical temperatures similar to or higher than today are.

Beta diversity is strongly correlated with the geographic distance between grids, and peaks in beta diversity are associated with main drops in rarefied diversity. This suggests that climatically induced latitudinal changes in oceanographic features cause a decline in diversity which is also paralleled by a major compositional turnover of the fauna.