The onset of grasses in the Amazon drainage basin, evidence from the fossil record

Judith A Kirschner,Carina Hoorn

doi:10.21425/f5fbg44827

Abstract

Poaceae (the grass family) originated in the Cretaceous, but first dominate the palynological records of the Amazon drainage basin (ADB) in the Neogene (23 to 2.5 million years ago (Ma)). However, the ecological role of grasses in the landscape during this time remains to be resolved. In this paper, we summarise the global significance of grasses and the relevance of the fossil record, and evaluate the history of the grasses in the ADB. We present a 3-stage model of the changing role of grasses, which we based on a revision of Neogene depositional environments, the palynological record, and modern grass distribution in the Neotropics. Our model comprises the following hypotheses: (H1) assumes that from c. 23 to 9 Ma western Amazonia was dominated by a megawetland (the ‘Pebas system’) that harboured large amounts of (aquatic?) grasses. In (H2) we propose that from c. 9 Ma Andean uplift prompted megafans (extremely large alluvial fans) that extended from the Andes into the lowlands. Meanwhile, the ‘Pebas’ megawetland gradually transformed into a fluvial system. In this scenario, grasses would have had a competitive advantage and were able to colonise the newly formed megafan and fluvial landscapes. Finally, in (H3) we suggest that landscape dynamics and climatic change intensified from c. 3.5 Ma, allowing for a renewed expansion of the grasses. In addition, both the fossil and molecular records suggest that from c. 5 Ma grasses were firmly established in the tropical alpine vegetation (paramo), the tropical lowland floodplains (varzeas), and savannas (cerrado). Although further study will have to confirm the precise nature of the ADB grass history, we anticipate that abiotic processes during the Neogene and Quaternary left a strong imprint in the grass phytogeography of northern South America.

Highlights

The earliest evidence for the existence of grasses (Poaceae, formerly Gramineae) dates back to the Cretaceous (Table 1; Prasad et al 2005, 2011, Strömberg 2011, Samant and Mohabey 2014, Wu et al 2018)
In this paper we addressed the question whether landscape dynamics and/or climate change could be the driving force behind increasing shares of Poaceae in the Amazon drainage basin (ADB) during the Neogene
The onset of extended Poaceae occurrence in the ADB seems to occur earlier than elsewhere on the globe, and their development is thought to be connected to Andean uplift and the formation of the Pebas megawetland system (c. 23 to 9 Ma)

Summary

Introduction

The earliest evidence for the existence of grasses (Poaceae, formerly Gramineae) dates back to the Cretaceous (Table 1; Prasad et al 2005, 2011, Strömberg 2011, Samant and Mohabey 2014, Wu et al 2018). Big knowledge gaps remain about the global expansion of grass-dominated habitats. 23 – 16 Ma), grasses started their rise to global dominance that continues until today (Kidder and Gierlowski-Kordesch 2005). Micrairoideae Chloridoideae Panicoideae Danthonioideae Pooideae Arundinoideae Aristidoideae Bambusoideae Oryzoideae Puelioideae Pharoideae Anomochlooideae Ehrhartoideae Origin Poaceae. Ages (Ma) from phylogeny (Hodkinson 2018; Prasad et al 2011). Ages (Ma) from pollen, phytoliths & cuticles (Jacobs et al, 1999; Prasad et al 2005, 2011; Poinar et al 2011, 2015; Wu et al 2018)

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