In search for the historical roots of grasses: A review of Neogene grass history in the Amazon drainage basin
Corresponding article: Kirschner, J. A., & Hoorn, C. (2019). The onset of grasses in the Amazon drainage basin, evidence from the fossil record. Frontiers of Biogeography. DOI: 10.21425/F5FBG44827
The iconic Amazon drainage basin has attracted numerous biogeographers trying to explain its extreme biodiversity by reconstructing processes of the past. One particular aspect that remains to be resolved is the role of Poaceae (the grass family) during the Neogene. Today, grasses make up a principal component of several biomes in the Amazon basin. There is flooded grassland and savannas like the Cerrado in Brazil or the Llanos in Colombia, but also cold and temperate grasslands, such as the Páramo and the Puna in the high Andes. Here, we suggest that the rising Andes, together with environmental and climate change, favoured grass development in the Amazon drainage basin.
To investigate the biogeographical role of grasses in past Amazonia, we reviewed the literature on fossil records. Carbon dated plant remains such as pollen and phytoliths give important indications on prevailing vegetation in the past. Two records amongst our search turned out to be of particular importance. One is from western Amazonia, spanning from 18 to 10 million years ago (Jaramillo et al. 2017), witnessing the area covered with a huge wetland. The other one originates from the Amazon fan, a submarine sediment apron formed by the Amazon River, reaching back throughout the whole Neogene (23 million years – Hoorn et al. 2017).
Based on our results, we see 3 distinct phases of grass development:
Our review shows that grasses must have been a common component of the Amazon drainage basin since at least the Neogene (23 Ma). At that time, a large wetland with a fascinating fauna existed in western Amazonia. Grasses were likely a principal component in that system, similar to floodplain grasses and floating grasses along the Amazon river today. Also the rising Andes might have offered a habitat for colonising grasses.
From around 9 Ma, Andean uplift and sub Andean basin subsidence gradually transformed the wetland into a fluvial system – resulting in the transcontinental Amazon river, connecting the Andes to the sea. During that time, grasses were likely the successful pioneers of new habitats forming on Andean slopes and megafans (extremely large alluvial fans). In addition, grasses kept their space on margins and floodplains of the Amazon river.
In the last 5 million years, the fossil record indicates increased grass presence in the Amazon basin. Intensified climate change from around 3.5 million years and ongoing landscape dynamics allow another expansion of grasses. In addition to floodplains and megafans, open surfaces in the high Andes offered comprehensive habitats for grass colonisation, and in the southern part of the basin, the Brazilian Cerrado is establishing.
To test our hypotheses, we urge for improved methods to identify fossil grass pollen on subfamily level. In addition, modern pollen calibration studies will give new insights on how accurate the fossil record represents the actual vegetation. Further studies of fossil records will help to illuminate the history of grasses in the Amazon drainage basin to a greater level of detail.
Hoorn, C., Bogotá-A, G.R., Romero-Baez, M., Lammertsma, E.I., Flantua, S.G., Dantas, E.L., Dino, R., do Carmo, D.A. and Chemale Jr, F., 2017. The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin. Global and Planetary Change, 153, pp.51-65. DOI: 10.1016/j.gloplacha.2017.02.005
Jaramillo, C., Romero, I., D’Apolito, C., Bayona, G., Duarte, E., Louwye, S., Escobar, J., Luque, J., Carrillo-Briceño, J.D., Zapata, V. and Mora, A., 2017. Miocene flooding events of western Amazonia. Science Advances, 3(5), p.e1601693. DOI: 10.1126/sciadv.1601693