The Cenozoic origin and spread of grass-dominated ecosystems
Grasslands today cover 1/4 of Earth’s land surface,
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and provide habitat and food for millions of animals, including humans. The global spread of grass-dominated vegetation was undoubtedly one of the most profound ecosystem change since the extinction of the dinosaurs. What do we know about this major transition in Earth history?


As it turns out, most of what we know has come from the fossil record of mammals. Hoofed mammals (ungulates) living in open, dry

Grassland in southwestern Montana, USA. Photo courtesy of J. Nicol.

grasslands and savannas today tend to have very high-crowned (hypsodont) teeth to deal with abrasion from silica-rich grasses and grit. As a result, hypsodonty is generally viewed as an adaptation to
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feeding in grasslands, and the evolution of large, herbivorous mammals with high-crowned teeth on all major continents during the Cenozoic is traditionally thought to mark the near-global expansion of open, grass-dominated habitats (Jacobs et al. 1999).


However, other lines of evidence (fossil soils, plant macrofossils and palynofloras; Jacobs et al. 1999) provide different dates for this ecological change and, as a result, the timing of the spread of grasslands remains controversial (Strömberg 200

Cheek tooth crown height. Low-crowned (brachydont) human tooth and high-crowned (hypsodont)) horse tooth. See Strömberg (2006).

2). The issue is complicated by the fact that plant macrofossils, pollen and spores are extremely rare in the well-oxidized Cenozoic deposits associated with mammalian faunas.


The goal of our research is to document the Cenozoic spread of open, grass-dominated habitats, focusing on understanding: a) When did open, grass-dominated habitats spread? b) How did climatic and tectonic changes influence this transition? and c) How did changes in vegetation relate (spatially and temporally) to the evolution of hypsodonty in large, herbivorous mammals? We
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address these questions by studying a previously non-utilized source of direct paleobotanical data, plant silica (phytolith) assemblages that we extract from Eocene-Miocene fossil soils and sediment. We consider these phytolith data in the context of available isotopic, climatic, paleobotanical and sedimentological data to determine the relative timing, rate, and nature of climate, vegetation, and faunal changes and propose underlying mechanisms.


Several lines of evidence indicate that the tempo and mode of grassland ecosystem evolution differed among continents. To explore this variation, we examine the paleobotanical records in several parts of the world: North America (Great Plains, Pacific Northwest), Europe, Asia, and South America.

Miocene (17 million-year-old) grass epidermis phytolith from Nebraska, USA. Scale bar = 10 micrometers.  

By comparing the results for these different areas, we hope to gain a better understanding of regional vs. global drivers in grassland ecosystem evolution.


Results


The work so far has resulted in important revisions of previously accepted scenarios concerning:

  1. Cenozoic vegetation change in North America

  2. Cenozoic vegetation change in western Eurasia

  3. Cenozoic vegetation change in southern South America

  4. evolution within grasses (Poaceae)


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