WAS THERE A LATITUDINAL DIVERSITY GRADIENT FOR FLOWERING PLANTS DURING THE LOWER CRETACEOUS?

One of the most conspicuous patterns of life on Earth is the increase in diversity from the poles to the equator, which applies to most taxa. For flowering plants (angiosperms), it has been suggested that this latitudinal gradient has been present since the Albian (~110Ma), when angiosperms were undergoing their initial diversification. However, the small number of Lower Cretaceous studies in tropical areas has made it difficult to test this and other biogeographical patterns. The main objective of this study is to determine whether a modern-like latitudinal diversity gradient for angiosperms was present during the Aptian-Albian interval, through the comparison of quantitative data from high and tropical paleolatitudes. The data used in this study are datasets from North America and Brazil. Additionally we reconstructed the floristic composition of an Aptian-Albian tropical ecosystem from Colombia. Our preliminary analyses offer mixed results. On one hand, there are no significant differences between the number of angiosperm species among the Colombian tropical site and the high latitudes of North America (5 and 3 species in average per sample, respectively). These results would indicate the absence of a modern-like latitudinal gradient for angiosperms during this time interval. On the other hand, the Brazilian ecosystems show significantly higher number of angiosperm species than the high latitudes of North America (7 and 3 species in average per sample, respectively), which would indicate the presence of a modern-like latitudinal gradient. One possible cause of this inconsistency could be the climatic difference between the hypothesized humid west and the arid east of South America, which could affect the composition of the floras. Currently we are generating new quantitative data for other six paleotropical sites of similar age in order to make more robust latitudinal comparisons. These new data will allow us to determine more accurately the floristic composition of tropical ecosystems of Lower Cretaceous age, the patterns of tropical angiosperm radiation and diversification, the prevalent climatic conditions in which they diversified and the biogeographical patterns for other groups of plants during that period of time.