Photo/Illutration A piece of fossilized bone previously believed to be of a pterosaur, right, and replicas reconstructed based on the result of a CT analysis of its interior (Naoki Shoji)

BANDO, Ibaraki Prefecture—A fossilized bone piece long hailed as coming from a pterosaur flying reptile is actually from a “suppon” softshell turtle, according to a re-examination that started with a dinner that included the four-legged delicacy.

The fossil was found in 2002 in a 72-million-year-old stratum from the late Cretaceous Period at Hiraiso Beach in Hitachinaka, Ibaraki Prefecture, and donated to the Ibaraki Nature Museum here.

When the museum sent a replica to a researcher in Germany for analysis, it was described as a “shoulder blade of a pterosaur.” That made it the first fossil of a prehistoric winged reptile found in the Kanto region.

Nicknamed “Hitachinakaryu” by locals, the fossil had been featured in sightseeing pamphlets and submitted to exhibitions outside the prefecture.

But one day in 2017, Ibaraki Nature Museum curator Taichi Kato, 32, took apart a suppon turtle during a hot pot dinner at his home after finishing a scientific paper. He noticed that its upper arm bone was similar in shape to the Hitachinakaryu fossil.

Kato borrowed a CT scanner from an optical instrument maker and found the fossilized bone had a sponge-like structure inside, which is typical of turtles. Bones of a pterosaur are usually hollow.

“I realized then and there that it was not of a pterosaur,” Kato recalled.

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A piece of fossil identified as the right upper arm bone of a large “suppon” softshell turtle from the Cretaceous Period, front, is similar in shape to that of a skeletal specimen of a modern suppon turtle, although their sizes are different. (Naoki Shoji)

The bone appeared hollow from the outside because part of the inner structure apparently drained out through a chip at the top.

Kato also noted that since 2013, three other fossilized pieces of suppon turtle shells have been found in a spot near the stratum where Hitachinakaryu was found.

After comparing its shape to that of an upper arm bone of turtles, Kato concluded in spring 2020 that the fossil was from a suppon of the Cretaceous Period.

When the chipped part is reconstructed, the fossilized right upper arm bone is about 12.5 centimeters long, indicating that the turtle had a large shell up to 70 cm in diameter.

Although Hitachinaka may have lost its pterosaur claim to fame, the turtle fossil still represents a first.

Fossils of similar-sized suppon from the Cretaceous Period have been excavated in North America, Kazakhstan and elsewhere.

But Hitachinaka is the only place in East Asia where such a fossil has been found.

“With the advancements in analysis methods, it is common to see previous views proven otherwise,” Kato said. “It serves as a lesson not to make the same mistake again, and I have a renewed determination to find a fossilized pterosaur next time.”

Hitachinaka city officials published the results of the re-examination in the February issue of its newsletter. They are correcting related entries on its website and in official publications.

The fossil is on display for a special exhibition at the Ibaraki Nature Museum through June 13.

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