2025 年 29 巻 p. 199-203
Here, we report the first discovery of Mesozoic insect fossils from the Late Cretaceous of Kyushu Island, Japan. The specimens include an isolated abdomen (Insecta, fam., gen. et sp. indet.) from the Santonian Hinoshima Formation in Kumamoto, and an isolated elytron (Coleoptera, fam., gen. et sp. indet.) from the Campanian Mitsuse Formation in Nagasaki. Although the preservation of fossils does not allow for species-level identification, these finds represent a crucial initial step toward understanding insect diversity and the composition of terrestrial ecosystems in the eastern margin of East Asia during the Late Cretaceous.
Fossil insect remains are widely distributed and abundant globally, and terrestrial insects played key roles as decomposers, predators, and pollinators in the past just as they do in the present (Grimaldi and Engel, 2005). Insect fossils are therefore important proxies for the modernization of terrestrial ecosystems throughout the Mesozoic, but they are often overlooked and have received limited scientific attention relative to other clades, partly due to their small size. In addition, the scarcity of sites yielding well-preserved or abundant specimens may also contribute to the underrepresentation of fossil insect research. In Japan, there were 20 documented Mesozoic insect fossil-bearing localities as of 2020 (Oyama et al., 2020), all of which are located on the islands of Hokkaido and Honshu, and specimens from these localities continue to be described (e.g. Aiba et al., 2023; Aiba and Inose, 2024). In contrast, no fossil insects have been reported from Kyushu and Shikoku despite the distribution of Mesozoic terrestrial deposits on both islands.
Here, we present the first records of Mesozoic insects from Kyushu. These new materials were collected from two Upper Cretaceous localities and include one specimen from the Santonian Hinoshima Formation in Kumamoto and another from the Campanian Mitsuse Formation in Nagasaki. These discoveries mark a significant first step in expanding our understanding of insect diversity and terrestrial ecology on the eastern margin of East Asia during the Late Cretaceous.
Institutional abbreviations: AMGD-IVP: Invertebrate Paleontological Collection, Amakusa Museum of Goshoura Dinosaur Island (formerly the Goshoura Cretaceous Museum), Amakusa, Kumamoto, Japan; NCDM-BENC: Collection of the Board of Education, Nagasaki City Dinosaur Museum, Nagasaki, Japan.
The insect fossils reported here were collected from two localities in Kyushu, Japan (Figure 1A). One specimen (AMGD-IVP4856) was recovered from a dark grey mudstone belonging to the lower part of the Hinoshima Formation at a locality on the western coast of the island Kugushima in Kamiamakusa, western Kumamoto (Figure 1B). The Hinoshima Formation is the basal unit of the Upper Cretaceous Himenoura Group, and it unconformably overlies Early Cretaceous granodiorites. It is notable for its rich marine fossil record, particularly mollusks such as ammonites and inoceramid bivalves that have been dated to the Santonian (Tashiro et al., 1986; Kojo et al., 2011). Tsutsumi et al. (2017) reported a U–Pb zircon age of 85.4±1.3 Ma (95% confidence level, MSWD = 0.83) from a tuff in the lower part of the Hinoshima Formation (Wadanohana [WDH] tuff sample in Tsutsumi et al., 2017), constraining it to the Santonian (Cohen et al., 2023). AMGD-IVP4856 was obtained from a stratigraphic level a few tens of meters above or below that of the WDH tuff, as indicated by the stratigraphic data reported by Kojo et al. (2011) and Tsutsumi et al. (2017).
The second specimen (NCDM-BENC101968) was collected from a blackish gray mudstone in the lower part of the Mitsuse Formation in Takahama on the Nagasaki Peninsula in the southern part of Nagasaki (Figure 1C). The lower Mitsuse Formation is distributed along the western coast of the Nagasaki Peninsula and has been believe to be a Late Cretaceous unit (Miyata et al., 2023). It is predominantly composed of fluvial conglomeratic sandstones and floodplain deposits consisting primarily of mudstones and fine-grained sandstones, and it has also produced numerous terrestrial vertebrate fossils, including hadrosauroid dinosaurs (Miyata et al., 2023). Notwithstanding the necessity for a reassessment of the stratigraphic definition and revision of the Mitsuse Formation (Miyata et al., 2023), the Takahama locality has been constrained to the middle Campanian (Cohen et al., 2023) based on a zircon U–Pb age of 79.85±0.42 Ma (2σ) reported from a tuff horizon approximately 80 meters below the stratigraphic level of NCDM-BENC101968 by Nagata et al. (2022).
AMGD-IVP4856 is a detached abdomen that was collected by Ryoji Yamada from the Kugushima locality in 2007, and it is reposited at the Amakusa Museum of Goshoura Dinosaur Island. NCDM-BENC101968 is a beetle elytron that we collected from the Takahama locality in 2017, and it is reposited at the Nagasaki City Dinosaur Museum. Both specimens were photographed using a Nikon D850 digital camera with a Macro-NIKKOR 65 mm F4.5 lens, and the photographs were processed in Adobe Photoshop. Both sides of the specimens were further documented using reflectance transformation imaging (RTI), following the methods described by Oyama et al. (2023). The corresponding RTI files are freely available via the Supplementary Online Materials of this study. Interpretative line drawings were made using Corel-Draw X8 software. In the line drawings, solid lines represent the outline of a preserved structure, and dashed lines represent damaged or unclear structures and outlines or structures on the elytron’s underside.
Class Insecta Linnaeus, 1758
Insecta fam., gen. et sp. indet.
Material.—AMGD-IVP4856, a detached abdomen exposed in ventral aspect.
Occurrence.—Kugushima locality, Kumamoto, Japan. Santonian, lower part of the Upper Cretaceous Hinoshima Formation.
Description.—Abdomen consisting of five flattened ventrites, broad, widely oval, gradually narrowed posteriorly, apical sternite carinated, has wide, rounded apex, 6.18 mm in length and 6.18 mm in width (Figure 2). Abdominal ventrite I transversely wide, its anterior margin strongly bisinuate, intercoxal process of metacoxae rather large, triangular, fully separating metacoxae, higher than anterior angles of sternite I, as wide at base as one-third of maximal width of sternite I. Ventrites II–IV also transversely wide but gradually taper posteriorly; ventrite V rather small, semicircular. Metacoxal rests very narrow, their length equal to one-fourth of sternite I maximum length (suggesting rather short, presumably transversely broad metacoxae). Sternites III–V with narrow laterosternites. Sternites separated by thin, simple sutures.
Remarks.—AMGD-IVP4856 is identified as part of an arthropod abdomen, specifically belonging to Insecta. Based on the presence of distinct segmentation and the characteristic anterior margin of abdominal ventrite I, it is probably referrable to the orders Coleoptera or Hemiptera. However, the specimen exhibits features more consistent with a coleopteran abdomen, as it lacks the ‘wavy’ margins typical of the sternites of hemipterans. Additionally, hemipteran abdomens often display a vertical genital opening slit, which is absent in this specimen. Among coleopterans, AMGD-IVP4856 is most similar to the abdomens of curculionoids (Caridae, Curculionidae), buprestoids (Burpestidae), and some tenebrionoids (Tenebrionidae, Ciidae). Due to the limitations of identifying isolated abdomens, further classification is not feasible at this stage.
Order Coleoptera Linnaeus, 1758
Coleoptera fam., gen. et sp. indet.
Material.—NCDM-BENC101968, a left elytron with slightly damaged base.
Occurrence.—Takahama locality, Nagasaki, Japan. Middle Campanian, lower part of the Upper Cretaceous Mitsuse Formation.
Description.—Elytron 10.92 mm in length, 6.63 mm in width (Figure 3); gradually narrowing posteriorly, became weakly sharp apex; dorsum of elytron smooth with at least two visible longitudinal striae present dorsally, one at the elytral suture and the other stria presents only central part of it; epipleural carina present at right margin. Carinated surface with punctate sculpturing, possibly organized into irregular rows. Epipleural margin with long, straight locking flange on the underside or ‘lateral furrow locking with abdominal pleura’ (Hammond, 1979). Thin stria running alongside epipleural and sutural margins. Elytral apex bears a pair of semioval impressions on the underside.
Remarks.—NCDM-BENC101968 is characterized by the hardened and simple veins typical of the elytra of coleopterans. However, the isolated elytron is impossible to identify which beetle lineage expecting some one having characteristic features such as superfamily Cupedoidea. Further classification requires the discovery of additional specimens.
Developing of the subelytral space is one of the key apomorphies of Coleoptera leading to their evolutionary success. Tightly locked closed space between the elytra, mesothorax, metathorax, and abdomen is formed from different types of ridges of different lengths on the underside of the elytra, which goes into recesses and furrows on the thorax and abdomen, resembling a zip-lock mechanism. Many different kinds of locking structures are found among the extant families of Adephaga and Polyphaga (see list of taxa possessing this character in Prokin and Sazhnev, 2024). Extinct beetles, such as †Schizocoleidae, †Phoroschizidae (=†Schizophoridae), and some †Rhombocoleidae and †Triaplidae are also characterized by a locking elytral flange (or ‘schiza’) near the epipleual margin.
According to scheme of locking flange evolution (Fedorenko, 2009), this character state in NCDM-BENC101968 is at rather advance stage (3rd stage sensu Fedorenko, 2009), being less than half of the elytral length, reduced basally, and apically accompanied by paired apical impressions. Such structure could have bore microsculpturing or setation in vivo, which could in turn suggest air-intaking behavior in which the beetle would stick its abdominal apex through the water surface to breathe (e.g. as in extant dytiscids). However, there is no unambiguous evidence that NCDM-BENC101968 belongs to an aquatic beetle.
We report the first Mesozoic insects from Kyushu, Japan. These discoveries include an indeterminate insect abdomen from Kumamoto and an indeterminate coleopteran elytron from Nagasaki. These new specimens contribute not only to the understanding of insect-bearing localities from the Mesozoic but also to the paleogeography of the terrestrial ecosystems in Japan. However, due to the poor preservation of both specimens, their alpha taxonomy remains unresolved. Better-preserved, preferably complete specimens are needed for further taxonomic assessment.
We are grateful to two anonymous reviewers for their useful and constructive comments, and to the editorial board of Paleontological Research for handling the reviewing process. Ryoji Yamada (Hikawa, Kumamoto, Japan) kindly donated AMGD-IVP4856 to AMGD. We are also thankful to Olivier Béthoux (CR2P) for useful discussions. This work was supported by JSPS Postdoctoral Fellowship for Research Abroad (No. 202360114) to N.O.
Supplementary data 1, RTI data of NCDM-BENC101968 (side1 and side2) from the Takahama locality.
N.O. and E.Y. lead the study and are primarily responsible for the taxonomic assessments of insect fossils. K.M., H.Y., D.N. and S.K. contributed to the fieldwork for NCDM-BENC-101968. H.Y. found NCDM-BENC101968. K.H., H.K. and K.M. contributed to the fieldwork for AMGD-IVP4856. All the authors contributed to writing the manuscript.
