Sakhalotettix eocenicus gen. & sp. nov., the first leafhopper reported from middle Eocene Sakhalinian amber, is described and illustrated. The fossil cicadellid resembles modern Xestocephalini and Bathysmatophorini in some respects but, because of its unique combination of traits, cannot be placed with certainty in either group, or in any other modern cicadellid subfamily. It is, therefore, considered to be incertae sedis within Cicadellidae.

amber insects, Bathysmatophorinae, leafhoppers, morphology, systematic paleontology, Xestocephalini

Leafhoppers (Cicadellidae) are one of the 10 largest families of insects and are presently among the most abundant herbivores, occurring in nearly every habitat that supports vascular plants. Although the oldest representatives of the family are recorded from the Lower Cretaceous (Hamilton 1990, 1992), the fossil record of this group remains sparse and poorly documented with fewer than 100 species formally described, most based on poorly preserved compression fossils (reviewed by Dietrich and Thomas 2018). Fossil cicadellids preserved in amber are known from the Cretaceous (Poinar and Brown 2017), Eocene (Szwedo 2002, 2005, Dietrich and Gonçalves 2014, Dietrich and Thomas 2018) and Miocene (Dietrich and Vega 1995). Many of these fossils have been assigned to modern subfamilies and have been used recently to calibrate molecular time trees (Dietrich et al. 2017, Johnson et al. 2018). The fossil leafhopper described below is important because it is the first representative of the family from middle Eocene Sakhalinian amber and exhibits a combination of morphological characters not yet reported among fossil or modern Cicadellidae. It may therefore contribute to knowledge of the evolution of major lineages of this family.

Numerous amber insects were collected in the south of Sakhalin Island, Russian Far East, by an expedition of the Paleontological Institute of Academy of Science of USSR in 1972 (Zherikhin 1978). Amber occurs on a beach near the village of Starodubskoye, Dolinsk District near the Naiba River mouth, Okhotsk Sea. The same fossil resin was found nearby at the Naiba River embedded in the coal of the Naibuchi Formation (Zherikhin 1978, Kodrul 1999).

The age of Sakhalinian amber has remained controversial for a long time (Kazantsev and Perkovsky 2019, and references therein). T.M. Kodrul (1999) convincingly demonstrated using geological and paleobotanical data a middle Eocene age of the Naibuchi Formation, in which Sakhalinian amber was found in situ (see also Baranov et al. 2015, Marusik et al. 2018). Rarity of ants and composition of mosses found in Sakhalinian amber confirm swampy environments in the Sakhalinian amber forest (Ignatov and Perkovsky 2013, Radchenko and Perkovsky 2016).

Sakhalinian amber belongs to the rumanite-type. Common for such fossil resins is a high degree of polymerization of the resin itself and deformation caused by thermal metamorphism during diagenesis. Insect inclusions in rumanite-type (particularly Sakhalinian) amber are therefore often deformed and have their internal cavity filled with resin (Rasnitsyn and Quicke 2002). This makes the inclusions more difficult to observe, compared to those European Baltic and Rovno succinite-type ambers. Sakhalinian amber is characterized by the small size of the pieces and supposedly rapid loss of viscosity (Kazantsev and Perkovsky 2019).

So far, about 1250 amber inclusions of insects and arachnids have been recorded from Sakhalinian amber, with aphids and chironomids most prevalent (Batelka et al. 2019, and references therein). Very unusual for amber faunas is the rarity of adult beetles in comparison with beetle larvae (Kazantsev and Perkovsky 2019).

Leafhoppers and planthoppers compose 23% of all hemipterans in a representative collection of late Eocene Rovno amber (Perkovsky et al. 2007) and 10% of all hemipterans in a representative collection of Eocene Baltic amber (Sontag 2003), but less than 0.3% of all hemipterans in middle Eocene Sakhalinian amber, possibly due to the small size of the pieces and low viscosity of the resin (our data).

Small typhlocybine nymphs jump less actively than other Auchenorrhyncha living in open habitats, and instead move laterally when danger threatens (Olmi et al., submitted). They are common in Rovno and Baltic ambers, but absent in Sakhalinian amber. So, apparently, they were very uncommon in the Sakhalinian amber forest. The new leafhopper described below is the only Auchenorrhyncha specimen known from Sakhalinian amber and only the second hemipteran species described from this amber (Shcherbakov 2007).

The amber specimen was photographed using a Leica M165C microscope with Leica DFC 420 camera. Morphological terminology follows Dietrich (2005). Studied material is housed in the Paleontological Institute of the Russian Academy of Sciences, Moscow (PIN).

The fossil described above is moderately well preserved but several important structures, including the wings, hind legs, and abdominal terminalia are poorly visible. Nevertheless, the imperfect preservation of the specimen only partly accounts for the difficulty in placing Sakhalotettix within the current higher classification of Cicadellidae. In overall size and shape, the specimen resembles Xestocephalus Van Duzee (Aphrodinae, Xestocephalini), a modern cosmopolitan genus also recorded from lower-middle Miocene Dominican amber (Dietrich and Vega 1995). Two genera of Aphrodinae were previously described from the late Eocene (Baltic amber; Dietrich and Gonçalves 2014), but Sakhalotettix differs from these and modern aphrodines in having the gena concavely emarginate below the eye, exposing the proepisternum, and the distance between the eye and the frontoclypeus relatively broad. In the latter respects, Sakhalotettix resembles Bathysmatophorinae, a group represented by a few extant genera restricted to the Holarctic region but also recorded from late Eocene Baltic amber (Szwedo and Gebicki 1998, Szwedo 2005, Dietrich and Gonçalves 2014). However, in contrast to Sakhalotettix, all known Bathysmatophorinae are relatively large (macropterous adults > 5 mm total length) and have the ovipositor extended well beyond the pygofer apex. Unfortunately, the latter character is not visible in the holotype of Sakhalotettix eocenicus and the emarginate gena is shared with several modern subfamilies including Bathysmatophorinae, Cicadellinae, Ledrinae and Ulopinae.

Sakhalotettix is unusual in having the lateral frontal sutures well delimited ventromesad of the ocelli and extended nearly to the midline of the head. This presumably plesiomorphic trait occurs to various degrees in modern Cicadellinae, which differ in having the frontoclypeus strongly inflated and the ocelli usually situated well posterad of the anterior eye margins. Thus, based on the characters visible in the fossil, the new genus appears to have mixed affinities to at least three extant subfamilies (Aphrodinae, Bathysmatophorinae and Cicadellinae). Discovery of additional specimens with the wings and hind legs better preserved may help place the fossil with more certainty. For now, the unusual combination of plesiomorphic and apomorphic traits visible in the only specimen prevents us from placing it with confidence in any extant subfamily.

We thank Alexandr P. Rasnitsyn and Irina D. Sukacheva (both PIN) for allowing us to study the Sakhalinian amber in their care, Dmitri E. Shcherbakov (PIN) for the familial determination, Anatoly P. Vlaskin (Institute of Zoology, Kiev, Ukraine) and the late Ekaterina A. Sidorchuk (PIN) for cutting and polishing the amber specimen, and Alexandr P. Rasnitsyn and Sergej A. Simutnik (Institute of Zoology, Kiev, Ukraine) for the photos and other assistance. We also thank Jacek Szwedo for critical comments that helped improve the manuscript.