Introduction

Pentatomidae is the third largest family in the suborder Heteroptera Latreille, 1810 of the insect order Hemiptera, with 4,948 species, 940 genera, and nine subfamilies (Schuh and Weirauch, 2020). Its members, commonly called stink bugs, are terrestrial and mostly phytophagous insects known to be pests of crops and forests. Their fossil record is less rich and includes approximately 73 species restricted to the Cenozoic (Mitchell, 2013). Most of them are from the Eocene to Miocene lacustrine deposits distributed in Europe and North America. However, paleontological literatures on this family generally consists of old studies, and the majority of those fossil specimens have not been described (Scudder, 1890, 1891; Carpenter, 1992). Recently, Poinar and Thomas (2012) described Edessa protera Poinar and Thomas, 2012 from the Lower Miocene in Mexico. Petrulevičius and Popov (2014) described Acanthocephalonotum martinsnetoi Petrulevičius and Popov, 2014, from the Middle Eocene in Patagonia, Argentina. Wedmann et al. (2021) described Eospinosus peterkulkai Wedmann, Kment, Campos and Hörnschemeyer, 2021 from the Middle Eocene in Germany and Eospinosus greenriverensis Wedmann, Kment, Campos and Hörnschemeyer, 2021 from the Early Eocene in the USA. The fossil record from the Pliocene comes mostly from Germany, where 12 fossil species have been described (Piton and Théobald, 1935; Jordan, 1967). However, those specimens are poorly preserved and difficult to interpret morphologically. Moreover, Aiba (2019a, 2019b, 2020, 2021) described eight pentatomid fossils from the Middle Pleistocene Shiobara Group in Japan: Pentatoma semiannulata (Motschulsky, 1860), Okeanos quelpartensis Distant, 1911, Dinorhynchus dybowskyi Jakovlev, 1876, Lelia decempunctata Motschulsky, 1860, Homalogonia grisea Josifov and Kerzhner, 1978, Pentatoma metallifera (Motschulsky, 1860), Pentatoma rufipes (Linnaeus, 1758), and Menida disjecta (Uhler, 1860). All these fossils are extant species.

Figure 1.

Fossil locality: Location of the border between Nagano and Gunma prefectures, Japan. The star represents the locality, near Mt. Kabutoiwa (after Geological Survey of Japan, 1969).

In this study, we describe a well-preserved pentatomid fossil from the Upper Pliocene “Kabutoiwa Formation” of the Motojuku Group in Gunma Prefecture, Japan. The results indicate that the fossil belongs to a new genus and species of the Pliocene Pentatoma-complex.

Geological setting

The Motojuku Formation, characterized by the Miocene to Pliocene collapse basin-fill deposits, is distributed in the Arafuneyama area, northwestern part of Kanto Massif, and border area between Gunma and Nagano prefectures. The formation is mainly composed of andesitic lava, ignimbrite, and volcaniclastic rocks with minor intercalations of lake deposits (Motojuku Collapse Research Group, 1970). Due to the recent discovery of additional caldera collapse in the formation, the Motojuku Collapse Research Group (2018) granted the Motojuku Formation “group” status (the Motojuku Group) and divided it into two formations, i.e., the lower Maisawa and upper Monogatariyama formations (Figure 1). The lake deposits, called “Kabutoiwa Formation,” belong to the Maisawa Formation and are famous for yielding well-preserved fossils (e.g. Koshimizu, 1982; Ozaki, 1991). Sato (2007) reported that the ca. 3.5 Ma age (Late Pliocene) of the fossiliferous lake deposits was obtained based on K–Ar dating of overlying lava in the Maisawa Formation.

The “Kabutoiwa Formation” consists of muddy tuff laminite. Plant fossils were first reported by Yagi (1921), who later listed 33 species (Yagi, 1931). Subsequent studies have revealed 111 species of Kabutoiwa flora in 45 families and 75 genera (Suzuki, 1967; Ozaki, 1984, 1987, 1991). Ozaki (1991) suggested that the flora is characterized by typical cool temperate elements and is composed of the following two habitat communities: valley forest and slope forest. The deposits yielded well-preserved fossil leaves, which were sometimes associated with fossil insects. Yagi (1931) formed the oldest record of fossil insects, which also included photographs. Koshimizu (1982) listed 116 species of fossil insects belonging to 28 families and 13 genera of 10 orders and described 48 of these species. Kuroko (1987) reported a fossil leaf with a nepticulid mine. Recently, Tanaka and Mano (2017) and Tanaka (2021) evaluated unclassified collections of fossil insects, which are called the “Mogi collection,” deposited in the Shimonita Town Natural History Museum. Aiba et al. (2023) described a new fossil butterfly species. They concluded that the insect fauna of the “Kabutoiwa Formation” consisted of 44 families of 11 orders.

Materials and methods

The studied specimen was collected by Mr. Hikaru Nishizawa during a bachelor's thesis approximately 40 years ago. The specimen was preserved in grayish fine muddy tuff. However, its counterpart is missing. The studied specimen is deposited in the Gunma Museum of Natural History under repository number GMNHPI-6322. The material was examined using a Leica M205 C microscope (Leica Corporation, Wetzlar, Germany). Photographs were taken using a Leica MC170HD Macro-scope and measurements with the Leica Application Suite version 4.1.3. Photographs were sharpened and adjusted in contrast and tonality using Adobe Photoshop^TM version CS6 (Adobe Systems Incorporated, San Jose, CA, USA). The terminology used to describe the specimen followed that of Tsai et al. (2011).

Systematic paleontology

Figure 2.

Tetrapentatoma nishizawai gen. et sp. nov., holotype. A, habitus, dorsal view; B, head, dorsal view; C, ommatidia of compound eye; D, humeral angle; E, serrated anterolateral margins of pronotum; F, left antenna; G, right antenna; H, hind tarsus. I–IV: antennomere I–IV; t1–t3: first to third tarsomeres.

Figure 3.

Line drawing of Tetrapentatoma nishizawai gen. et sp. nov., holotype. I–IV: antennomere I–IV; tIII–VIII: tergite III–VIII. Scale bar: 5.0 mm.

Discussion

The studied specimen was identified to belong to the family Pentatomidae as indicated by typical characteristics of the family such as the broadly oval-shaped body, 3-segmented tarsi, ocelli positioned basal to compound eye, developed pronotum, and large scutellum (Schuh and Slater, 1995; Takai and Ishikawa, 2012; Rider, 2015; Schuh and Weirauch, 2020).

Nevertheless, the studied fossil is characterized by four-segmented antennae in contrast to the typical five-segmented antennae of Pentatomidae. Only the subfamilies Cyrtocorinae Distant, 1880 and Serbaninae Leston, 1953 of Pentatomidae have four-segmented antennae (Rider et al., 2018) although this fossil is clearly distinguishable from them since the former has two-segmented tarsi and an oddly shaped body, whereas the latter exhibits an armor-like shape (Schuh and Slater, 1995; Rider et al., 2018).

Another exception is Peromatus Amyot and Serville, 1843 (Edessinae Amyot and Serville, 1843), which also has four-segmented antennae (Amyot and Serville, 1843; Rider, 2006). The genus Peromatus is a small taxon, which is only limited in Central and South America and includes only eight species (Amyot and Serville, 1843; Rider, 2006). The present fossil is somewhat similar to Peromatus, which has a broad oval body. Peromatus is characterized by a mesosternal carina lower than the metasternal carina (Amyot and Serville, 1843); unfortunately, the fossils were not preserved in the ventral view. However, many other features of this genus do not match those of the present fossil. Based on the observations of Amyot and Serville (1843), Rider (2006), Melo et al. (2017), and Stephen (2020), the studied fossil can be distinguished from Peromatus by considering the following characteristics:

Therefore, this fossil cannot be classified in Peromatus. Based on these results, we conclude that this fossil is not consistent with any other genera and species of Pentatomidae and should thus be defined as a new genus and species of Pentatomidae.

Some of the subfamilies and tribes within Pentatomidae (e.g. Pentatominae and Pentatomini) are polyphyletic, and their classification by morphology is confusing (Roca-Cusachs et al., 2022), thus making it difficult to determine the subfamily or tribe of this new genus fossil.

On the other hand, recent studies (Tsai and Rédei, 2014; Rédei and Tsai, 2021) have considered seven genera in Pentatomini, namely Pentatoma Olivier, 1789, Acrocorisellus Puton, 1886, Bifurcipentatoma Fan and Liu, 2012, Ramivena Fan and Liu, 2010, Lelia Walker, 1867, Priassus Stål, 1868, and Zhengica Rédei and Tsai, 2021 as Pentatoma-complex based on male genital and other morphological characters. These morphological characters, typical for the Pentatoma-complex, include large-oblong bodies, possessing anteriorly finely denticu-late pronotum, produced humeri and scutellum covering half of the abdomen (Rédei and Tsai, 2021). The fossil's morphology is consistent with the above characteristics; therefore, it is reasonable to consider it as a member of the Pentatoma-complex.

Pentatoma-complex is a monophyletic group consisting of species that have been shown to be closely related as revealed by phylogenetic analysis (Roca-Cusachs et al., 2022). Pentatoma, a representative member of the Pentatoma-complex, is the type genus of Pentatomini and Pentatominae. This indicates that the higher taxa of this fossil can be included in Pentatomini and Pentatominae.

Pentatoma-complex is distributed only in East Asia (Tsai and Rédei, 2014; Rédei and Tsai, 2021). The discovery of fossils of Pentatoma-complex, Tetrapentatoma nishizawai gen. et sp. nov. from the Late Pliocene, is important evidence that Pentatoma-complex appeared earlier than the Late Pliocene in East Asia. And this is the oldest fossil record of them.