Photo/Illutration The hybodus fossil (Provided by Kuji Amber Museum)

Researchers have discovered a fossil that shares characteristics of an ancient ancestor to sharks, the first such finding in Japan.

A fossil of the blade-like part of the hybodus species was found in Kuji, Iwate Prefecture, according to the announcement by the Kuji Amber Museum on July 16.

The hybodus was an ancestor of sharks that lived between the Carboniferous Period of the Paleozoic Era to the Cretaceous Period of the Mesozoic Era, about 90 million years ago.

A characteristic of the hybodus is a blade-like protrusion in front of its two dorsal fins. The blade is believed to have been a defensive mechanism to keep predators at bay.

“While there have been findings abroad of the blade, one has never been found in Japan, so this is a significant find,” said Shinya Miyata, a curator at the Oishi Fossils Gallery of the Mizuta Memorial Museum, affiliated with Josai University in Tokyo.

“Ninety million years ago, the area where the fossil was found was likely near a river mouth, and the fossil is the first evidence that the hybodus existed in such an environment in East Asia.”

The fossil was found about 300 meters from the Amber Museum in May 2019 by a family visiting the facility. The fossil is about 20 centimeters in length.

Miyata and Ren Hirayama, a professor of paleontology at Tokyo’s Waseda University, conducted a careful analysis of the fossil and confirmed it is the blade-like part of the hybodus, based on its shallow arc, the lines found on the side and the small projection at the tip.

Based on the length of the blade, the researchers concluded the hybodus was a small animal, measuring about one meter in length.

Some modern shark species, like the Japanese bullhead, also have small blade-like parts on its dorsal fin.

After the fossil’s discovery last year, other fossils found in the general vicinity were examined a second time. That led to confirming that three teeth believed to be from a hybodus were also found nearby.

Miyata, a specialist in fish fossils, said the hybodus was the only shark species in the Mesozoic Era that had a prominent blade.