Researchers from the University of Louisville in Kentucky say the fossil is of the first known mammal of the early Late Cretaceous period of South America.
The fossil creature, named "Cronopio dentiacutus" by paleontologists, is a dryolestoid, an extinct group of mammals distantly related to today's marsupials, a university release said Wednesday.
"Cronopio" was a tiny shrew-sized mammal about 4-6 inches in length with a diet of the insects, grubs and other bugs of the time, researchers said.
It lived when giant dinosaurs roamed Earth more than 100 million years ago.
The discovery is important because mammalian skulls are very fragile, small and rarely found, the researchers said.
"Most of what we know of early mammals has been determined through teeth because enamel is the hardest substance in our bodies and survives well the passage of time; it is usually what we have left to study," Louisville paleontologist Guillermy Rougier said.
"The skull, however, provides us with features of the biology of the animal, making it possible for us to determine this is the first of its kind dating to the early Late Cretaceous period in South America," he said.
"This time period in South America was somewhat of a blank slate to us. Now we have a mammal as a starting point for further study of the lineage of all mammals, humans included."