Cretaceous Microfloras of Brazil: Paleoenvironmental and Paleoclimatic Significance

The Cretaceous Period is conceived as one of the warmest periods in Earth history. The relationship between vegetation distribution and climate is intrinsic, each that each significantly impacts the other. Oscillations between wet and dry periods played an important role in the changes of vegetation distribution and diversity. The paleoclimatic maps from the Berriasian to the Maastrichtian of Brazil indicate a predominance of arid climates. However, palynological studies suggest that a humid belt must have appeared during the late Aptian. In Cretaceous Brazilian basins, a total of 149 palynomorph genera, assigned to bryophytes, pteridophytes (ferns), lycophytes, gymnosperms, and angiosperms, were recorded. The temporal distribution of all major plant groups shows a conspicuous increase in taxa richness during the Aptian and Albian ages. The paleoflora have been classified into three bioclimatic groups based on botanical affinities of fossil-taxa (on family level): xerophytic shrubland, lowland rainforest, and upland rainforest. The lowland rainforest flora is represented mainly by a xerophytic flora (e.g., Cheirolepidiaceae, Gnetaceae, and Ephedraceae). The wet lowland flora is the most diversified, and it is represented by bryophytes, lycophytes, pteridophytes, and angiosperms. The flora assigned herein to wet climate and upland environments mainly constituted of gymnosperms that are adapted to milder temperatures (e.g., Araucariaceae). The regional history of palynoflora is subdivided into five consecutive stages: Dicheiropollis etruscus, Tucanopollis crisopolensis, Afropollis, Classopollis/Gnetaceaepollenites/Elaterates, and Angiosperms. In spite of the great diversity of wet lowland groups, three examples with a quantitative focus are given from the Sergipe, Santos, and Pelotas basins, showing a domain of xerophytic flora. However, the abundance of the latter shows a decrease trend towards the end of the Cretaceous reflecting an increase in wetter conditions.