New research led by the University of Washington challenges the 140-year-old assumption that finding fossilised remains of prehistoric animals with such teeth meant the animals were living in grasslands and savannahs. 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 findings have been published in an article on February 12 in the journal Nature Communications.
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 specialised teeth. But the grasslands didn’t exist. Instead there were tropical forests rich with palms, bamboos and gingers, according to Caroline Strömberg, UW assistant professor of biology and lead author:
The assumption about grasslands and the evolution of these teeth was based on animal fossils. 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 specialised 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.
caution is required when using this functional trait for habitat reconstructionIn Argentina, mammals apparently developed specialised teeth 20 million years or more before grasslands appeared, Strömberg said. This was different from her previous work in North America and western Eurasia where she found the emergence of grasslands coincided with the early ancestors of horses and other animals evolving specialised teeth. The cause and effect, however, took 4 million years, considerably more lag time than previously thought.
The idea that specialised 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, Strömberg 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, Strömberg 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 specialised teeth – were overwhelmingly from tropical forests, Strömberg said.
“In modern grasslands and savannahs 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 specialised teeth in mammals are regarded as a classic example of co-evolution, one that has occurred in various places around the world. However, as the new work shows, “caution is required when using this functional trait for habitat reconstruction,” the co-authors write.
(Source: University of Washington news release, 04.03.2013)