Prehistoric jaw fossil from Japan could point to a new species

Katsuyama, Fukui — The Fuiko Prefectural Museum of Dinosaurs in Katsuyama announced on February 7 that it has discovered the fossilized jawbone of what is believed to be the oldest mammal discovered in Japan.

The fossil was found in a lower Cretaceous-era stratum in Ono city, Fukui Prefecture, and dates back to 127 million years in the past, when dinosaurs still ruled the earth, an official said. The museum continues its studies with this specimen in the possibility that it is a new species.

The fossil belongs to the lower jaw with three (3) teeth. It measures 13.1 millimeters in length and 5.8 millimeters in height. The animal is estimated to have been about 15 centimeters long, about the size of a common mouse. It is from the order of Eutriconodonta, one of the first mammals to have molar teeth with 3 large cups, a primitive characteristic. It is believed that it lived during the same time as the inguanodon and other contemporary dinosaurs.

The fossil was found in a stratum called the Itsuki Formation that is part of the Tetori group in the city of Ono in 2019. Researchers at the museum have been studying it ever since. Previously, a mammalian fossil was found in the Kuwajima formation in the tetori group in Hakusan city, neighboring Ishikawa prefecture, dating from 121 million to 130 million years old, and was considered the oldest of its kind in Japan. However, the stratum where the new fossil was found dates to the same era as the Hakusan stratum, and the new fossil appears to be older than the one found there. The shape of its teeth suggest that it is a still unknown species.

A Tritylodontid tooth was also found in the same stratum, a graminivorous reptile that, due to its mammal-like shape, is believed to have evolved into a mammal.

The fossils will be on display at the museum from mid-March 2021.

(Original text in Japanese by Riki Iwama, Fukui Bureau)