Dry biomes occupy~35% of the landscape in the Neotropics, but these are heavily human-disturbed. In spite of their importance, we still do not fully understand their origins and how they are sustained. The Guajira Peninsula in northern Colombia is dominated by dry biomes and has a rich Neogene fossil record. Here, we have analyzed its changes in vegetation and precipitation during the Neogene using a fossil pollen and spore dataset of 20 samples taken from a well and we also dated the stratigraphic sequence using microfossils. In addition, we analyzed the pollen and spore contents of 10 Holocene samples to establish a modern baseline for comparison with the Neogene as well as a study of the modern vegetation to assess both its spatial distribution and anthropic disturbances during the initial stages of European colonization. The section was dated to span from the latest Oligocene to the early Miocene (~24.2 to 17.3 Ma), with the Oligocene/Miocene boundary being in the lower Uitpa Formation. The early Miocene vegetation is dominated by a rainforest biome with a mean annual precipitation of 2,000 mm/yr, which strongly contrasts with Guajira's modern xerophytic vegetation and a precipitation of 300 mm/yr. The shift to the dry modern vegetation probably occurred over the past three millions years, but the mechanism that led to this change is still uncertain. Global circulation models that include the vegetation could explain the ancient climate of Guajira, but further work is required to assess the feedbacks of vegetation, precipitation, and CO 2 .
Extensive areas covered by semi-permanent water bodies were common during the Miocene in the Neotropics. These floods are noteworthy because of their possible role in promoting the high biological diversity observed today in the Amazonia. In particular, a relatively good understanding of the floods has been achieved for the Llanos basin of Colombia and western Amazonia. In these two basins the evidence suggests episodes of marine incursions and development of lacustrine systems at different times during the Miocene. Other intra-montane basins in Colombia, like the Middle and Upper Magdalena basins, also show clear evidence that water bodies covered them during that time. However, the chronostratigraphy and paleoecology of these deposits are still unclear. In this study, we use the palynological record of the Middle and Upper Magdalena valleys to establish the age of the deposits of the Barzalosa Formation, a unit that preserves a detailed record of a lacustrine system deposited during the late early Miocene in the Upper Magdalena Valley. The results indicate that the Barzalosa Formation is correlative with the lacustrine deposits of the La Cira fossiliferous horizon in the Middle Magdalena Valley. This indicates that extensive lacustrine systems covered the intramontane basins of the northern Andes during the early Miocene. Paleoecologically, the Barzalosa Formation is the result of the evolution of a lacustrine system in three phases, which show marked differences in the proportion of algae, palynological composition and sedimentary depositional sequence. Climate and tectonic processes were the most probable mechanisms controlling the evolution of the Barzalosa system.
The Nothofagus (southern beech) has a rich fossil record and a number of living species distributed exclusively in the Southern Hemisphere. Many attempts have been made to clarify the phylogenetic relationships in Nothofagus but only a few works have included fossil specimens in a phylogenetic framework for a more accurate resolution. Fossil leaves play an important role in deciphering of the evolutionary processes and are a necessary complement in phylogenetic studies. Fossils of Nothofagus have been found in sediments of Antarctica,
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