Scientists from the University of Chile have discovered a 74-million-year-old fossil of a tiny, mouse-sized mammal named Yeutherium pressor in Chilean Patagonia. This mammal lived during the Upper Cretaceous period alongside dinosaurs. The finding provides new insights into evolutionary transitions and represents the third species from this era identified in Chile. The fossil, a jawbone, is the smallest known mammal specimen from the region and contributes to understanding mammalian diversity during the time of dinosaurs.