Dining on field grasses would be ruinous to human teeth, but mammals such as horses, rhinos and gazelles evolved long, strong teeth that are up to the task.
New research led by the University of Washington challenges the 140-year-old assumption that finding fossilized remains of prehistoric animals with such teeth meant the animals were living in grasslands and savannas. Instead it appears certain South American mammals evolved the teeth in response to the gritty dust and volcanic ash they encountered while feeding in an ancient tropical forest.
The new work was conducted in Argentina where scientists had thought Earth's first grasslands emerged 38 million years ago, an assumption based on fossils of these specialized teeth. But the grasslands didn't exist. Instead there were tropical forests rich with palms, bamboos and gingers, according to Caroline Strmberg, UW assistant professor of biology and lead author of an article in Nature Communications.
"The assumption about grasslands and the evolution of these teeth was based on animal fossils," Strmberg said. "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."
To handle a lifetime of rough abrasion, the specialized teeth called high-crowned cheek teeth are especially long and mostly up in the animals' gums when they are young. As chewing surfaces of the teeth wear away, more of the tooth emerges from the gums until the crowns are used up. In each tooth, bone-like dentin and tough enamel are complexly folded and layered to create strong ridged surfaces for chewing. Human teeth have short crowns and enamel only on the outside of each tooth.
In Argentina, mammals apparently developed specialized teeth 20 million years or more before grasslands appeared, Strmberg said. This was different from her previous
|Contact: Sandra Hines|
University of Washington