Small mammals have been gnawing on bones for calcium and protein to supplement their diet for 75 million years, according to a report on the oldest known evidence of gnawing: on dinosaur bones.

The study was published online June 16 by the journal Palaeontology.

Scientists found mammalian bite marks on the rib of a dinosaur and on the femur bone of another dinosaur, as well as on fossil bones of an aquatic reptile and a marsupial  all from the Cretaceous period.

The bones have opposing pairs of teeth marks on them, a bite attributed only to mammals during that time. They appear to have been made by multituberculates, a now-extinct group of mammals that were small and rodentlike in appearance, said Nicholas R. Longrich, the study’s lead author and a paleontologist at Yale University.

It is possible that the mammals were eating meat off the dinosaur bones, but based on the bite marks this does not seem to be the case.