Using the rLAI proxy, we reconstructed a record of canopy structure from fossil phytolith assemblages from the middle Cenozoic of Argentina and spanning from 49-11 Ma (Link to paper). The record shows a gradual transition from closed, likely broadleaved forests in the late early Eocene (~49 Ma) to much more open forests and potentially palm shrubland habitats by the late Eocene until the Middle Miocene (~16 Ma) when Patagonia experienced a brief re-greening. The new rLAI archive tracks oxygen isotope records of sea surface and deep-sea temperatures, indicating that vegetation structure at this latitude was highly influenced by changes in precipitation that resulted from changes in sea temperatures. However, in contrast to the marine record, which shows a pronounced cooling event at the Eocene-Oligocene Transition (EOT ~33.9 Ma), the rLAI record suggests that major habitat changes on land occurred as early as 38 Ma with a pronounced interval of openness. This event predates the EOT by 4 million years and implies that climate change in Patagonia coincided with shallow openings of the Drake Passage and initiation of the development of the Antarctic Circumpolar Current prior to the EOT.
For herbivorous mammals, the intervals of low rLAI and eolian sedimentation are coincident with accelerated increases in the evolution of molar crown height, the first at ~38 Ma. We suggest that a combination of decreased precipitation and sparse vegetation cover changed the nature of sedimentation and pedogenesis whereby unweathered, glass-rich pyroclastic parent materials were susceptible to erosion and mobilization into animal diets. Rather than grasses as dietary abrasives driving faunal hypsodonty in Patagonia, it instead appears that a perfect storm of open, windy habitats and an abundance of swirling and reworking volcanic tephra were the main drivers of tooth evolution in South America.
Reference: Dunn, R.E., Strömberg, C.A.E., Madden, R.H., Kohn, M.J., Carlini, A.A., 2015. Linked canopy, climate and faunal evolution in the Cenozoic of Patagonia. Science 347(6219): 258-261