New Mexico Museum of Natural History and Science

This is the skull of the oldest known mammal, Adelobasileus (uh • dell • oh • ba • SILL • ee • us), a tiny mouse-sized inhabitant of the Late Triassic forests. Discovered by NMMNHS paleontologists in 1989, the skeletal structure of the ear region identifies the skull as mammalian. Adelobasileus first evolved during the Triassic.