A new 3.5-million-year-old Nymphalidae fossil species discovered in Japan by Keio Yochisha Elementary School and Kagoshima University

A new butterfly fossil species (Nymphalidae, Neptis) has been discovered in Japan for the first time. The report was presented by teachers Hiroaki Aiba and Yui Takahashi of Keio Yochisha Elementary School, and Professor Yositaka Sakamaki and his research group at Kagoshima University.

Butterfly fossils are particularly rare among insect fossils, with only about 60 adult specimens found worldwide and only two have been discovered in century. Additionally, the morphology of the wing veins in this particular fossil retained some primitive features, which may provide valuable insights into inferring the evolution of butterflies. The results were published in the international journal Paleontological Research.

The fossil, discovered in the Maisawa Formation of the Upper Pliocene Motojuku Group (ca. 3.5 million years ago) in Gunma Prefecture, was collected by a student Hikaru Nishizawa at Tokyo Gakugei University for his graduation thesis nearly 40 years ago. This formation, also referred to as the Kabutoiwa Formation, has long been recognized for yielding numerous plant and insect fossils. Ken'ichi Saiki of the Natural History Museum and Institute, Chiba, entrusted the study of the specimen to Aiba.

Initially, identification was challenging due to the limited resolution of microscopes available at the time. However, recent advancements in high-resolution microscopes have unveiled previously invisible details of morphological features, such as wing veins and antennae shape. Comparisons of these forms with those of the present butterfly species proved the fossil to be a new species.

The specimen will be donated to the Gunma Museum of Natural History, which is located in the prefecture where the fossil originated. Worldwide, only 80 butterfly fossils have been reported, with 64 being adults. Only 42 species have ever been named. 25 of the species were discovered in the 19th century in regions including the Eocene strata in North America and Oligocene strata in France. Since the beginning of the 21st century, only two individuals have been found encased in Miocene Dominican amber. The fossils found to date so far come from older periods preceding the Miocene, including already extinct species, or from newer periods after the middle Pleistocene (ca. 300,000 years ago). The fossil in this study is from the Pliocene Era.

Several Pliocene butterfly specimens in Germany have been reported, but they are poorly preserved and have not been identified to the species level. The specimen of this study is the only named Pliocene butterfly fossil in the world and the most recent fossil of an extinct species.

Butterfly fossils are rarely of the whole body; most are only part of a wing. However, many parts of the fossil in this study were preserved, including the left and right antennae, head, proboscis, thorax, forewings, and hindwings. It belongs to the Nymphalidae family because of its reduced forelegs and the grooves on its antennae. Furthermore, the characteristics of the forewings and hindwings indicate that it belongs to the tribe Neptini. There are six genera in the tribe Neptini, and the Japanese species are in the genus Neptis.

Comparison of the fossil specimen with native Japanese species revealed differences in the shapes of the lower lip and forewing veins. Additionally, the specimen was compared with butterflies from five other genera and found to be morphologically distinct from them as well. In particular, the fossil has thicker CuA veins on the forewings, the overall body is slightly elongated, and the CuP vein is faintly preserved. The CuP vein is also preserved in fossils from the Eocene in the USA and is considered a primitive vein feature. The morphology of this fossil may hold the key to unraveling the evolution of the Nymphalidae and the tribe Neptini.

Journal Information
Publication: Paleontological Research
Title: A New Species of Fossil Nymphalidae (Lepidoptera, Papilionoidea) from the Upper Pliocene Motojuku Group, Gunma Prefecture, Japan
DOI: 10.2517/PR220018