KATSUYAMA, Fukui -- The Fukui Prefectural Dinosaur Museum in this central Japan city announced on Feb. 7 that it has discovered the fossilized jaw of what is believed to be one of the oldest mammals in the country.
The fossil was unearthed from a Lower Cretaceous stratum in the city of Ono, Fukui Prefecture, dating back about 127 million years ago, when dinosaurs were alive, officials said. The museum is continuing to study the specimen on the possibility it could be a new species.
The fossil is a part of the lower jawbone and has three teeth left. It measures 13.1 millimeters in length and 5.8 millimeters in height. The animal is estimated to have been some 15 centimeters long, or about the size of a mouse. It is from the Eutriconodonta order of early mammals that had molar teeth with three large cusps, which is a primitive characteristic -- and is thought to have lived in the same era as iguanodons and other dinosaurs.
The fossil was found in a stratum called the Itsuki Formation of the Tetori Group in Ono in 2019, and museum researchers and others had been studying it. Previously, a mammalian fossil found in the Kuwajima Formation of the Tetori Group in the city of Hakusan in the neighboring prefecture of Ishikawa, dating back 121 million to 130 million years, had been considered the oldest in Japan. However, the stratum where the new fossil was found dates to the almost same era as the stratum in Hakusan, and the newly found fossil may be older. The shapes of the teeth suggest the fossil is possibly of a previously unknown species.
Furthermore, the fossilized tooth of a tritylodontid -- a graminivorous mammal-like reptile that was evolving into a mammal -- was also found from the same stratum.
The fossils will be on display at the museum beginning in mid-March.
(Japanese original by Riki Iwama, Fukui Bureau)