A fossil containing jaw bones from three animals, dating back 136 million to 140 million years and believed to be the oldest mammalian fossil discovered in the nation, has been found in Sasayama, Hyogo Prefecture, a local museum announced Thursday.

According to the Museum of Nature and Human Activities, Hyogo, located in Sanda in the prefecture, the fossilized bones will be extremely valuable to efforts to trace the process of evolution, as they belong to specimens of a genus regarded as ancestors of most mammals in existence today.

The discovery, made in the Sasayama Strata, an excavation site in the city, is the first in the world of a fossil comprising intact bone fragments of creatures from the early Cretaceous period.

The Cretaceous period extends from 144 million years ago to 65 million years ago.

Haruo Saegusa, senior researcher at the museum, said, "It's an important discovery that relates to a period where very little material has been found."

The animals' body length is estimated to have been more than 10 centimeters.

The fossil contains four sections of lower jaw bones from three small mammals. Two bone fragments--the front-right and back-right sections of the lower jaw of the same creature are about 1.2 centimeters and 1.3 centimeters long, respectively, and are well preserved.

In the two sections, eight teeth molar, premolar, and canine were identified.

Researchers said the shape of the molars indicates the mammal was either Tribotherium, a genus that scientists believe evolved to become placental or marsupial mammals, or Peramura, a genus similar to Tribotherium. Both genus are now extinct, and were believed to have been omnivorous and nocturnal and resembled mice in appearance.

The oldest mammalian fossil in the world dates back 225 million years. Two fossils from the early Cretaceous period have been found in Japan, including one in Hakusan, Ishikawa Prefecture, that dates back 130 million years, while 55 others from the same period have been found across the world.