As herbivores, these animals had large teeth for grinding their diet of plants.

“These mammals can have enamel two to three millimeters thick. It was a lot of material to work with,” Dr. Green said.

“What we found — peptide fragments, chains of amino acids, that together form proteins as old as 18 million years — was field-changing.”

“Nobody’s ever found peptide fragments that are this old before.”

Until now, the oldest published materials are about 3.5 million years old.