2025/05/22
Keio University
Hiroaki Aiba, Professor Emeritus of Keio University (part-time instructor at Keio Yokohama Elementary School), Yui Takahashi, a teacher at Keio Yochisha Elementary School, and Mr. Kotaro Saito of the Institute of Evolutionary Biology have described and reported a butterfly fossil unearthed from a stratum approximately 2.5 million years old in Shin'onsen, Hyogo Prefecture, as a new species of the genusSasakiain the family Nymphalidae. This is the second report of a new species of butterfly fossil from Japan, following the report in October 2023 of a fossil from the genusNeptis(family Nymphalidae) found in the Majisawa Formation (approximately 3.5 million years old) in Gunma Prefecture.
Insect fossils are considered rare among fossils, but butterfly fossils are particularly scarce. Only about 60 adult butterfly fossils have been discovered worldwide, of which about 40 species have been named. Moreover, more than half of these were reported in the 19th century, and only three have been discovered in the current century.
This is the world's first report of a fossil from the genusSasakia. With an estimated wingspan of 84 mm, it is the largest butterfly fossil ever discovered. It is also the world's first new species of butterfly fossil from the late Pliocene to early Pleistocene (approximately 2.5 million years ago), making it the most recent extinct fossil species of butterfly in the world and a valuable resource for studying butterfly evolution.
The fossil was discovered 37 years ago by a local high school teacher, the late Mr. Kiyoshi Kamitani, and was stored at the local museum, "Omoshiro Konchu Kasekikan" (Interesting Insect Fossil Museum). The fossil was named after its discoverer (*Sasakia kamitaniana*). While many insect fossils have been found in Shin'onsen Town, this is the first time a new species has been reported.
The results of this research were published online in the international journal of the Palaeontological Society of Japan,Paleontological Research, on May 2, 2025.
For the full press release, please see below.