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

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 (páramo), the tropical lowland floodplains (várzeas), 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

Geologically recent rearrangements in central Amazonian river network and their importance for the riverine barrier hypothesis

The riverine barrier hypothesis is a central concept in Amazonian biogeography. It states that large rivers limit species distributions and trigger vicariant speciation. Although the hypothesis has explanatory power, many recent biogeographical observations addressing it have produced conflicting results. We propose that the controversies arise because tributary arrangements in the Amazon river system have changed in geologically recent times, such that large tracts of forest that were on the same side of a river at one time got separated to different sides at another. Based on topographical data and sediment dating, we map about 20 major avulsion and river capture events that have rearranged the river network in central Amazonia during the late Pleistocene and Holocene. We identify areas where past riverine barrier effects might still linger in the absence of a major river, as well as areas where such effects may not yet have accumulated across an existing river. These results call for a reinterpretation of previous biogeographical studies and a reorientation of future works to take into account the idiosyncratic histories of individual rivers

Late Miocene -Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong plateau:new evidences from foreland basin deposits along the Dunsam Chu section, Eastern Bhutan

The Shillong plateau is a unique basement-cored uplift in the foreland of the eastern Himalaya that accommodates part of the India-Eurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1,600 metres, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern Himalaya-Shillong plateau-ISM is suited to investigate effects of tectonics, climate and erosion in a mountain range-broken foreland system. This study focuses on a 2200 m-thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission-track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1 Ma in a marginal marine deltaic transitioning into continental environment after 5 Ma, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest Miocene – Pleistocene, in response to increasing partitioning of the India-Eurasia convergence into the active faults bounding the Shillong plateau

Neogene History of the Amazonian Flora: A Perspective Based on Geological, Palynological, and Molecular Phylogenetic Data

The Amazon hosts one of the largest and richest rainforests in the world, but its origins remain debated. Growing evidence suggests that geodiversity and geological history played essential roles in shaping the Amazonian flora. Here we summarize the geo-climatic history of the Amazon and review paleopalynological records and time-calibrated phylogenies to evaluate the response of plants to environmental change. The Neogene fossil record suggests major sequential changes in plant composition and an overall decline in diversity. Phylogenies of eight Amazonian plant clades paint a mixed picture, with the diversification of most groups best explained by constant speciation rates through time, while others indicate clade-specific increases or decreases correlated with climatic cooling or increasing Andean elevation. Overall, the Amazon forest seems to represent a museum of diversity with a high potential for biological diversification through time. To fully understand how the Amazon got its modern biodiversity, further multidisciplinary studies conducted within a multimillion-year perspective are needed. ▪The history of the Amazon rainforest goes back to the beginning of the Cenozoic (66 Ma) and was driven by climate and geological forces. ▪In the early Neogene (23-13.8 Ma), a large wetland developed with episodic estuarine conditions and vegetation ranging from mangroves to terra firme forest. ▪In the late Neogene (13.8-2.6 Ma), the Amazon changed into a fluvial landscape with a less diverse and more open forest, although the details of this transition remain to be resolved. ▪These geo-climatic changes have left imprints on the modern Amazonian diversity that can be recovered with dated phylogenetic trees. ▪Amazonian plant groups show distinct responses to environmental changes, suggesting that Amazonia is both a refuge and a cradle of biodiversity

Freshwater fish diversity in the western Amazon basin shaped by Andean uplift since the Late Cretaceous

South America is home to the highest freshwater fish biodiversity on Earth, and the hotspot of species richness is located in the western Amazon basin. The location of this hotspot is enigmatic, as it is inconsistent with the pattern observed in river systems across the world of increasing species richness towards a river’s mouth. Here we investigate the role of river capture events caused by Andean mountain building and repeated episodes of flooding in western Amazonia in shaping the modern-day richness pattern of freshwater fishes in South America, and in Amazonia in particular. To this end, we combine a reconstruction of river networks since 80 Ma with a mechanistic model simulating dispersal, allopatric speciation and extinction over the dynamic landscape of rivers and lakes. We show that Andean mountain building and consequent numerous small river capture events in western Amazonia caused freshwater habitats to be highly dynamic, leading to high diversification rates and exceptional richness. The history of marine incursions and lakes, including the Miocene Pebas mega-wetland system in western Amazonia, played a secondary role

Geologically recent rearrangements in central Amazonian river network and their importance for the riverine barrier hypothesis

The riverine barrier hypothesis is a central concept in Amazonian biogeography. It states that large rivers limit species distributions and trigger vicariant speciation. Although the hypothesis has explanatory power, many recent biogeographical observations addressing it have produced conflicting results. We propose that the controversies arise because tributary arrangements in the Amazon river system have changed in geologically recent times, such that large tracts of forest that were on the same side of a river at one time got separated to different sides at another. Based on topographical data and sediment dating, we map about 20 major avulsion and river capture events that have rearranged the river network in central Amazonia during the late Pleistocene and Holocene. We identify areas where past riverine barrier effects might still linger in the absence of a major river, as well as areas where such effects may not yet have accumulated across an existing river. These results call for a reinterpretation of previous biogeographical studies and a reorientation of future works to take into account the idiosyncratic histories of individual rivers.</p

The Cenozoic history of palms:Global diversification, biogeography and the decline of megathermal forests

10.1111/geb.13436GLOBAL ECOLOGY AND BIOGEOGRAPHY313425-43

The evolutionary history of the Central Asian steppe-desert taxon Nitraria (Nitrariaceae) as revealed by integration of fossil pollen morphology and molecular data

The transition from a greenhouse to an icehouse world at the Eocene-Oligocene Transition (EOT) coincided with a large decrease of pollen from the steppe-adapted genus Nitraria. This genus, now common along the Mediterranean coast, Asia and Australia, has a proposed coastal origin and a geographically widespread fossil record. Here we investigated the evolution, taxonomic diversity and morphological disparity of Nitraria throughout the Cenozoic by integrating extant taxa and fossil palynological morphotypes into a unified phylogenetic framework based on both DNA sequences and pollen morphological data. We present the oldest fossil pollen grain of Nitraria, at least 53 Myr old. This fossil was found in Central Asian deposits, providing new evidence for its origin in this area. We found that the EOT is an evolutionary bottleneck for Nitraria, coinciding with retreat of the proto-Paratethys Sea, a major global cooling event and a turnover in Central Asian steppe vegetation. We infer the crown age of modern Nitraria spp. to be significantly younger (Miocene) than previously estimated (Palaeocene). The diversity trajectory of Nitraria inferred from extant-only taxa differs markedly from one that also considers extinct taxa. Our study demonstrates it is therefore critical to apply an integrative approach to fully understand the plant evolutionary history of Nitrariaceae.publishedVersio

Sporopollenin chemistry and its durability in the geological record: an integration of extant and fossil chemical data across the seed plants.

Sporopollenin is a highly resistant biopolymer that forms the outer wall of pollen and spores (sporomorphs). Recent research into sporopollenin chemistry has opened up a range of new avenues for palynological research, including chemotaxonomic classification of morphologically cryptic taxa. However, there have been limited attempts to directly integrate extant and fossil sporopollenin chemical data. Of particular importance is the impact of sample processing to isolate sporopollenin from fresh sporomorphs, and the extent of chemical changes that occur once sporomorphs enter the geological record. Here, we explore these issues using Fourier transform infrared (FTIR) microspectroscopy data from extant and fossil grass, Nitraria (a steppe plant), and conifer pollen. We show a 98% classification success rate at subfamily level with extant grass pollen, demonstrating a strong taxonomic signature in isolated sporopollenin. However, we also reveal substantial chemical differences between extant and fossil sporopollenin, which can be tied to both early diagenetic changes acting on the sporomorphs and chemical derivates of sample processing. Our results demonstrate that directly integrating extant and late Quaternary chemical data should be tractable as long as comparable sample processing routines are maintained. Consistent differences between extant and deeper time sporomorphs, however, suggests that classifying fossil specimens using extant training sets will be challenging. Further work is therefore required to understand and simulate the effects of diagenetic processes on sporopollenin chemistry