‘Bonus’ reptile fossil identified as new species — 25 years after display in east Japan

A fossil of the head of Amphicotylus milesi, a new species believed to be an ancestor of crocodiles, is photographed at Fukushima Museum in Aizuwakamatsu, Fukushima Prefecture. (Mainichi/Kengo Miura)

AIZUWAKAMATSU, Fukushima — A fossil believed to be of an ancestor of the crocodile, dating back some 155 million years — displayed and stored at the Gunma Museum of Natural History in east Japan since 1996 — has been revealed to be a new species.

This photo shows a fossilized skeleton of Amphicotylus milesi, a new species believed to be the ancestor of crocodiles. (Photo courtesy of the Gunma Museum of Natural History)

The fossil of the approximately 3-meter-long reptile came as an “extra” item when the Gunma Prefectural Government purchased a dinosaur fossil unearthed from the same stratum in the United States.

Yuji Takakuwa, director of the museum’s geological research section, was surprised by the discovery which came a quarter of a century after the 1993 unearthing, and described it as “a great bonus.”

A team from Fukushima Museum in the Fukushima Prefecture city of Aizuwakamatsu and other organizations researched and analyzed the fossil, and their findings were published in the British science journal Royal Society Open Science on Dec. 8. The fossil is of a new species of Goniopholididae, which are believed to be the ancestors of crocodiles, and was named Amphicotylus milesi, after a researcher who was involved in the excavation.

Team leader Junki Yoshida, 30, a curator at Fukushima Museum, began studying the crocodile fossil at the Gunma Museum of Natural History in 2017 when he was a graduate student.

According to Takakuwa, there were no crocodile experts in Japan, so the natural history museum in the Gunma Prefecture city of Tomioka had apparently continued to display the fossil without conducting detailed research on it.

The fossil was unearthed in 1993 in Wyoming, from the same stratum as several other fossils, including the dinosaur Camarasaurus. Yoshida and his team compared similar fossils excavated in North America and other regions, and noticed features such as its wide mouth tip and nostrils, which proved that it was a new species.

Additional bone research revealed that the new species had tongue valves that blocked its throat to prevent it from drinking water, just like present-day crocodiles. This is the first indication that an ancestor of terrestrial crocodiles may have already adapted to water in the late Jurassic period.

Until now, the natural history museum had displayed the fossil as a “Goniopholididae fossil,” with only the head permanently displayed at the foot of the full-body skeleton of the Camarasaurus. The main attraction was the dinosaur, but the reptile fossil had been well preserved because those who unearthed it had requested that it be stored together with the Camarasaurus, on the grounds that they were found together.

Takakuwa said, “I knew it was an important fossil because it contains almost all the bones in its body. I am very happy to know that it is a new species.” The museum immediately replaced the display title with the scientific name of the new species on Dec. 8.

At a news conference, Yoshida said, “The baton of my research has been passed on. There are still many mysteries, so I want to continue my research.”

The Gunma Museum of Natural History will exhibit the whole skeleton of Amphicotylus milesi from Jan. 22, 2022. Fukushima Museum plans to exhibit the fossil from fiscal 2022 onward.


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