March 5, 2013
Volcanic Grit Played Key Role In Strong Tooth Development
Brett Smith for redOrbit.com — Your Universe Online
Archeologists have long assumed the evolutionary development of strong, thick-enameled teeth coincides with a mammals shift to a diet of field grasses.
“The assumption about grasslands and the evolution of these teeth was based on animal fossils,” said co-author Caroline StrÃ¶mberg, from the University of Washington. “No one had looked in detail at evidence from the plant record before. 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.”
Previous theories about Argentina, where the team of American and Argentine scientists conducted their field work, had stated the Earth´s first grasslands emerged there 38 million years ago. These theories were based largely on fossils of these specialized teeth.
However, the team found these grasslands didn´t exist when they were supposed to. In their place were tropical forests rich with palm trees and bamboo, according to the Nature study.
Specialized teeth, also called high-crowned cheek teeth, are long, with bone-like dentin and tough, layered enamel to create strong, ridged surfaces for chewing. By comparison, human teeth have short crowns and enamel on the outside of each tooth.
In Argentina, the fossil record shows mammals developed specialized teeth about 38 million years ago. StrÃ¶mberg´s previous research in North America and western Eurasia showed the emergence of grasslands coincided with the animals there evolving specialized teeth to chew those grasses.
Chewing grasses is abrasive because grasses take large amounts of silica, which forms minute structural particles inside many plants called phytoliths.
When plants decay, their unique phytoliths remain intact as part of the soil layer. Seeking to learn more about the vegetation of ancient Argentina, StrÃ¶mberg and her colleagues collected samples from the country´s Gran Barranca, meaning “Great Cliff.”
According to an analysis of the 38 million-year-old phytoliths that coincide with the development of specialized mammalian teeth, that area was covered with tropical forests, not grasslands as had been previously thought.
“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,” StrÃ¶mberg said in a statement.
According to StrÃ¶mberg, there was about a four million year lag time between the development of specialized teeth and the emergence of grasslands. Therefore, the team concluded volcanic debris must have played a role in the specialization.
Archeologists have made previous theories about other regions where animals developed specialized teeth in response to volcanic grit and dust. In Argentina, StrÃ¶mberg and her co-authors hypothesized the teeth adapted to handle the debris because of the volume of it found at the study site. In some soil layers, the volcanic layer is 20 feet thick. In other layers, fossilized roots were just starting to develop when they were buried with more ash.