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July 23, 2013

Microfossils Reveal Ancient Argentinian Landscape Different than Previously Thought

Fossilized plant silica contradicts 140-year-old assumption
Burke Museum of Natural History and Culture

Seattle – For 140 years, scientists assumed finding fossils of specialized, high-crowned cheek teeth from herbivorous mammals meant the animals were living in grasslands. A new study led by Dr. Caroline Strömberg, Burke Museum curator of paleobotany, shows these teeth aren’t always connected to grasslands, but can be an adaptation to other elements in the environment.

Eating grasses is abrasive, and animals faced with handling a lifetime of wear and tear are thought to have evolved specialized cheek teeth called high-crowned cheek teeth. They’re especially long when the animals are young, and as chewing surfaces wear away, more of the tooth emerges from the gums until the crowns are used up. Because of the presence of these teeth, scientists previously thought Patagonia, South America, was home to the earliest grasslands during the middle Eocene (approximately 38 million years ago), 20 million years before other grasslands appeared on the planet.

However, Dr. Strömberg and her colleagues, including University of Washington Biology graduate student Regan Dunn, found that the vegetation of Patagonia during this era were consistent with subtropical forests, and the adaptation of mammals with high-crowned cheek teeth may instead have been in response to volcanic ash covering the landscape’s flora.

“No one looked in detail at evidence in the plant record before,” Dr. Strömberg said. “Our findings show that you shouldn’t assume adaptations always came about in the same way; that the trigger is the same environment every time.”

Dr. Strömberg’s evidence can be found in phytoliths—microscopic plant silica. Phytoliths form inside plant cells in grasses and other plants and, when found in fossil soils and sediments, can tell paleontologists whether forest or grasslands (or something in between) grew in an area. Dr. Strömberg and Regan Dunn analyzed over 21,000 phytoliths from the Burke Museum’s paleobotany collections, which were collected during several field trips to Patagonia, Argentina. The overwhelming majority of the phytoliths came from plants that reside in subtropical forests.

The 43-to-18-million-year-old soil layers where the phytoliths were collected included large amounts of volcanic ash. Some layers were 20-feet thick. Dr. Strömberg and her co-authors hypothesize that mammals adapted specialized high-crowned cheek teeth in order to handle the abrasive grit from the ash.

Visitors can learn more about this study and the Burke Museum’s paleobotany collections in a new Why Study Evolution? display, on view now through 2015. Included in the display are 28-million-year-old fossilized mammal teeth from the Burke Museum’s paleontology collection, up-close views of microscopic phytoliths, and more.  

For interviews or high resolution photos, contact burkepr@uw.edu.

Study Information: Caroline A.E. Strömberg, Regan E. Dunn, Richard H. Madden, Matthew J. Kohn, and Alfredo A. Carlini. Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America. Nature Communications, published February 12, 2013.
Link to study: http://www.nature.com/ncomms/journal/v4/n2/full/ncomms2508.html

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Illustration by Dr. Caroline Strömberg.
Grass, palm, ginger, and dicot phytoliths. Photo courtesy of the Burke Museum.