“True Grit” Teeth Theory: Why Animals Developed Teeth 20 Million Years Early

The “true grit” teeth theory might make your teeth ache just thinking about it. For 140 years, most scientists have assumed that grass-eating animals with their highly specialized chewing teeth must have evolved together with the tough grasses they feast on. Actually, that’s wrong, according to a new study in Nature Communications headed up by University of Washington biologist Caroline Stromberg.

Scientists know that grasslands are a relatively new habitat on planet earth. A University of Utah study reported in Science Daily pushed back the development of savanna (open grassland studded with a few trees) in Africa from 2 million to more than 6 million years — but animals with the ability to chew grass have been around for far longer than that.

In Argentina, paleontologists found fossils of a great many mammals with specialized teeth suitable for chewing grass that go all the way back to 38 million years. Although it seemed way too early, the fossil evidence was so overwhelming that the scientists felt that they had to assume that the grasslands simply appeared millions of years earlier in Argentina than they did anywhere else. After all, the animals had the right teeth for the job. So if they had the teeth, they must have been eating grass, and if they were eating grass…well, there must have been grasslands.

The University of Washington study took a different approach. According to a detailed report for Phys.org News, instead of looking at animal fossils, Stromberg’s team went to Argentina to examine microscopic bits of plants material called phytoliths. The remains were not from grasses but from tropical forests. “The evolution of high-crowned cheek teeth (hypsodonty) in herbivorous mammals during the late Cenozoic is classically regarded as an adaptive response to the near-global spread of grass-dominated habitats,” she wrote.

But the classic story couldn’t have happened, since analysis of the plant matter showed that the grasslands didn’t exist. Instead, Stromberg proposed that the tough teeth developed because the animals were chewing tropical forest plants that had been contaminated by blowing volcanic ash and other grit that would wear down more fragile teeth.

When the grasslands appeared millions of years later, the herbivores “true grit” teeth were already ready for work, toughened by perhaps 20 million years of eating volcanic grit in their salad. That’s a long time to wait for a nice field of grass.

[photo courtesy Elaine Radford]