Regan E. Dunn

Paleoecology & Paleobotany

Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

Strömberg, C.A.E., Dunn, R.E., Madden, R.H., Kohn, M.J., Carlini, A.A., 2013. Nature Communications (12 February 2013). DOI: 10.1038/ncomms2508 (Link)

Southern South America has long been thought to be the cradle of grassland evolution. This assumption is based on the observation that fossil South American herbivores had “grazer”–grass eating– tooth morphologies (hypsodonty) around 38 Ma, while on other continents this same tooth evolution didn’t take place until 20 million years later. Hypsodonty is an adaptation to resist excessive tooth wear in animals as longer teeth significantly extend the life of a grass-eating mammal. Phytoliths in grasses have long been proposed as a significant source of dental abrasives in grazing animals’ diets. To test this so-called “Early Grasslands” hypothesis, my PhD advisor Caroline Strömberg (U Washington) and colleagues Richard H. Madden (U Chicago), Alfredo Carlini (CONICET, Museo de La Plata, Argentina), Matt Kohn (Boise State U) and I sampled phytolith-bearing strata at Gran Barranca, Argentina.

C. Strömberg and I analyzed <50 phytolith samples from rocks 42-18 Ma and found something surprising…grasses though present, were always found in relatively low abundances compared to palms and forest indicator phytoliths. Thus, we rejected the “Early Grasslands” hypothesis.

Screen Shot 2015-08-30 at 5.44.48 PM
Figure excerpted from Strömberg et al., 2013

We concluded that grasses are not required for hypsodonty to evolve in herbivorous mammals. Instead, we suggested that other factors may have contributed to this evolutionary pattern in South America including grit ingestion from volcanic ash.

Dickie Book

To read more about hypsodonty in mammals and the role of Earth surfaces processes, visit this link which is a great book written by my colleague Richard H. Madden. It is also available through, and hopefully at your university’s library.

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