One of oldest mammal fossils ever found in Japan discovered

The Yomiuri Shimbun

FUKUI — One of the oldest mammal fossils ever found in Japan, dating back to the Early Cretaceous period about 127 million years ago, was discovered in stratum in Ono, Fukui Prefecture, the Fukui Prefectural Dinosaur Museum has announced.

The museum said that the fossil belongs to the group of primitive mammals of the dinosaur age called “eutriconodonta,” a name derived from their three conical teeth, and could be a previously unknown species.

The fossil is part of a lower jaw bone with three teeth and measures 5.8 millimeters in length and 13.1 millimeters in width. It was discovered in 2019 in a stratum labeled the Tetori Group Itsuki Formation in Ono.

Since it was difficult to remove the surrounding rock, the fossil was examined by a CT scan. The scan confirmed that each tooth has the three conical protrusions characteristic of triconodonts. It also showed that the jaw bone contains a linear canal called the Meckel’s groove, which is a sign of evolution from a reptile.

In Japan, at least 10 species of mammal fossils from the age of dinosaurs have been found. The oldest fossil dates back 121 million to 130 million years, and was found in the Tetori Group Kuwajima Formation in Hakusan, Ishikawa Prefecture.

The fossil discovered in the Itsuki Formation appears to be as old. “Based on its shape and other features, it is believed to be of a species not yet reported,” the museum said.

Looking at the size of the bone and teeth, the fossil is thought to be from a mouse-size mammal 16-17 centimeters in length.

At the same site, a tooth fossil from a tritylodontid, a reptile in the evolutionary stage of becoming a mammal, was also found. “These are important discoveries indicating that a variety of mammals existed at the time that dinosaurs were alive,” the museum said.

The museum plans to put the fossils found this time on display from around mid-March.