The idea that specialized teeth could have evolved in response to eating dust and grit on plants and the ground is not new. In the case of Argentine mammals, Strmberg and her co-authors hypothesize that the teeth adapted to handle volcanic ash because so much is present at the study site. For example, some layers of volcanic ash are as thick as 20 feet (six meters). In other layers, soils and roots were just starting to develop when they were smothered with more ash.
Chewing grasses is abrasive because grasses take up more silica from soils than most other plants. Silica forms minute particles inside many plants called phytoliths that, among other things, help some plants stand upright and form part of the protective coating on seeds.
Phytoliths vary in appearance under a microscope depending on the kind of plant. When plants die and decay, the phytoliths remain as part of the soil layer. In work funded by the National Science Foundation, Strmberg and her colleagues collected samples from Argentina's Gran Barranca, literally "Great Cliff," that offers access to layers of soil, ash and sand going back millions of years.
The phytoliths they found in 38-million-year-old layers when ancient mammals in that part of the world developed specialized teeth were overwhelmingly from tropical forests, Strmberg said.
"In modern grasslands and savannas you'd expect at least 35 to 40 percent more likely well over 50 percent of grass phytoliths. The fact we have so little evidence of grasses is very diagnostic of a forested habitat," she said.
The emergence of grasslands and the evolution of specialized teeth in mammals are regarded as a classic exa
|Contact: Sandra Hines|
University of Washington