The extant tropical tribe Hybocephalini is a morphologically highly derived group of the subfamily Pselaphinae (Coleoptera: Staphylinidae), which is characterized most notably by the modified squamous setae that cover various parts of the body. Ten genera and 69 extant species have been found in the Afrotropical and Oriental regions, with one species found in northern Australia. Prior to this study the evolutionary history of the tribe has been remained elusive due to the dearth of known fossils. Here, we describe the first fossil representative of Hybocephalini, Europharinodes schaufussi Yin & Cai gen. et sp. nov., based on an adult male preserved in Baltic amber (ca 45.0–38.0 Ma). Using X-ray microtomography, the anatomy including the endoskeletal structures of the head, the full pattern of foveation, and the aedeagus of the beetle were reconstructed. Europharinodes shares most derived traits that are congruent with extant members of Hybocephalini, but it also possesses plesiomorphic and autapomorphic characters unknown in living relatives. In order to constrain the phylogenetic placement of Europharinodes, we created an updated morphological character matrix to explore relationships among this genus and related groups. A monophyletic Hybocephalini was recovered by maximum likelihood and parsimony analyses, with Europharinodes being well-resolved as sister to all modern relatives in the likelihood tree. The fossil thus sheds new light on the morphological evolution of Hybocephalini and suggests a broader palaeodistribution of the tribe during the middle Eocene. The disjunct distribution of an Eocene Baltic amber species and an extant Afrotropical-Oriental distribution of the tribe is probably relictual, and was shaped by global cooling during the Eocene–Oligocene transition.

Beetles, 3-D reconstruction, Baltic amber, biogeography, Cenozoic climate cooling, Europharinodes schaufussi, phylogeny, plesiomorphic characters

Comprising more than 64,000 extant and 400 extinct species, the Staphylinidae (rove beetles) stands for the most species-rich family in the whole animal kingdom, surpassing Curculionidae (true weevils) (Yin et al. 2020; Shin et al. 2017). Among the 32 currently recognized extant subfamilies of Staphylinidae, the Pselaphinae are the second most diverse lineage (following Aleocharinae) containing over 10,000 extant species classified in 40 tribes and 1,259 described genera worldwide (Newton and Chandler 1989; Yin et al. 2019a), albeit this perhaps represents only a small fraction of their true diversity (Löbl and Kurbatov 2001; Yin 2022).

Six supertribes of Pselaphinae have been recognized and are applied to current taxonomic practice (Newton and Thayer 1995; Chandler 2001): the basal-most Faronitae, and its sister clade including the other five supertribes (Euplectitae, Goniaceritae, Batrisitae, Pselaphitae, Clavigeritae) forming the so-called ‘higher Pselaphinae’ (Parker 2016). The phylogenetic relationships among these supertribes, except for the position of Faronitae, remains unstable because most of them and many included large tribes (e.g., Trichonychini) were shown to be paraphyletic in recent taxonomic or phylogenetic studies (e.g., Chandler 2001; Parker 2016; Jałoszyński et al. 2022). This poor state of knowledge is further exacerbated by a paucity of direct (fossil) evidence pertaining to the evolutionary history of major pselaphine groups. The research of the fossil history of pselaphines experienced two stages. The first stage lasted from the 1890s to the 1920s (for a review of taxa see Spahr 1981; Newton and Chandler 1989), during which Schaufuss (1890) described a diverse pselaphine fauna from the middle Eocene Baltic amber. The Baltic Pselaphinae, like most other insect groups (Weitschat and Wichard 2010), shows affinities to extant lineages, thus providing little information on how and when the higher lineages may have originated. Furthermore, meaningful re-interpretations of this fauna become almost impossible because of Schaufuss’ limited species descriptions usual for his time, and the fact that his type materials were lost during World War II. A second surge of interest in fossil Pselaphinae started only recently, tracking an increasing discovery of the Mesozoic amber inclusions originating from northern Myanmar (Ross 2019–2021). Several stem-group members of Euplectitae and Goniaceritae were described from this deposit (Parker 2016; Yin et al. 2017; 2019a, b, 2021; Yin and Cai 2021), which shed new light on the paleobiogeography and certain trends in morphological evolution within these two supertribes. Not long ago, a critical fossil representative of stem-group Clavigeritae, Protoclaviger trichodens Parker and Grimaldi, was described (Parker and Grimaldi 2014) from the Ypresian Cambay amber (Rust et al. 2010) of western India, which shows a transitional body form foreshadowing the extreme myrmecophily of recent clavigerites before the rise of modern ants.

The tribe Hybocephalini is a morphologically highly derived group of the supertribe Pselaphitae, characterized most remarkably by the presence of squamous setae that cover various parts of the body, combined with its dorsoventrally convex body, and greatly transverse antennomeres (Nomura 1989; Chandler 2001; Yin and Jiang 2017). To date ten genera and 69 extant species have been described from the Afrotropical and Oriental regions (Newton 2018), with one species of these genera found in northern Australia (Chandler 2001). All species of Hybocephalini are relatively rarely encountered in the field. They are collected from leaf litter samples (Nomura 1991a; Sugaya 2003), from under stones (Yin and Jiang 2017), by flight intercept traps, or more often, attracted by ultraviolet light traps (e.g., Jeannel 1950; Chandler 2001). Apharinodes sinensis Yin & Jiang was found to exhibit male wing dimorphism (Yin and Jiang 2017), suggesting complex trade-offs may exist during the life history of this species. Prior to the present study no fossil representatives of Hybocephalini have been known, complicating attempts at disentangling the evolutionary history of this tribe. Here, the first fossil hybocephaline, Europharinodes gen. nov., is described from the middle Eocene Baltic amber. Aided by X-ray microtomography, the fine anatomy of the beetle has been reconstructed, including the endoskeletal structures of the head. An updated morphological character matrix was constructed to further determine the placement of the fossil, and the biogeographic implications of the new findings are discussed. Europharinodes represents the first extinct Pselaphitae described in detail since the century-old Schaufuss species descriptions.

A monophyletic clade comprising the supertribes Pselaphitae and Clavigeritae was recovered in all analyses, with strong to moderate support (bootstrap value in ML tree = 100; pp value in MB tree = 1; bootstrap value in MP tree = 40) (Fig. 1, Supplementary material 3). Such a general relationship between these two higher groups was similarly recovered by previous studies focusing on different target groups (Parker 2016; Yin et al. 2017; Hlaváč et al. 2021) or based on different datasets (Jałoszyński et al. 2022). Within the clade, the placement of Europharinodes varies in each analysis, likely due to limited taxa sampling and insufficient character selection. The strongest support was observed in the maximum likelihood tree (Fig.), in which a monophyletic Hybocephalini (bootstrap value = 94) was recovered as sister to a lineage composed of Arhytodini + Clavigeritae. Europharinodes was well-resolved as the basal-most group of the tribe, sister to all other modern hybocephalines. A similar topology from parsimonious analysis (Supplementary material 3) recovered a weakly supported monophyletic Hybocephalini (bootstrap value = 20), and Europharinodes was included in a probably soft polytomy. Neither the position of the fossil nor the monophyly of Hybocephalini were resolved in the Bayesian tree. The fossil was included in a weakly supported clade (pp value = 0.69) together with the other hybocephalines, Caccoplectus, Rhytus, and Clavigeritae.

Aside from the results inferred from phylogenetic analyses, the position of Europharinodes was indicated also by the fossil’s unique combination of morphological traits. Within Pselaphinae the presence of squamous setae covering various parts of the body is a distinctive character state occurring in several tribes belonging to the supertribe Pselaphitae: Hybocephalini, Arhytodini, Ctenistini, Odontalgini, and Pselaphini. Europharinodes can be quickly ruled out from the latter three tribes by the small, reduced maxillary palpi that are only partially visible from the dorsal side. The members of Ctenistini often have maxillary palpomeres greatly extended laterally and with a distinct pencil-like apical projection (e.g., Li et al. 2021); Odontalgini is characterized by the long and basally pedunculate palpomeres 2 and 4, and a maximum number of foveae on the pronotum (Yin et al. 2016); and species of Pselaphini usually possess markedly extending palps that are often longer than the head and pronotum combined, and the squamous setae are usually confined to the basal impression of first visible tergite (Yin and Jiang 2016) (note: short maxillary palpi can be found in Curculionellus Westwood and a few more genera, but each genus is characteristic in the form of the palpi and other morphological characters). Both Hybocephalini and Arhytodini share the reduced size of maxillary palps, however, Europharinodes does not possess the derived characters that define Arhytodini: a) presence of ocular/gular-mandibular carinae; b) characteristic antennae modified in various forms, often greatly increased in length c) often enlarged eyes (normal size in Sabarhytus Löbl), d) femora and tibiae often with spines arranged in rows, e) variously modified tarsomeres, f) single pretarsal claws, and g) squamous setae usually confined to sulci, foveae, or articulations of body parts (Chandler and Wolda 1986; Chandler 1992, 2001; Löbl 2000).

Although the general morphology of Europharinodes is largely congruent with modern Hybocephalini (for tribal diagnosis see Chandler 2001: 495), the fossil exhibits a combination of characters found in different recent genera: heavy squamous setae that cover most areas of the body (Figs 2B, 3A, C, D) (Apharinodes Raffray), four-segmented maxillary palpi (Fig. 3H, I) (Mestogaster Schmidt-Göbel), antennal club formed by enlarged and unmodified three apical antennomeres (Fig. 2C, F) (Hybocephalus Motschulsky, Hybocephalodes Raffray, Filigerinus Jeannel, Filigerodes Jeannel, Mestogaster), projecting paratergites (Figs 3A, 4G) (Mecochelia Motschulsky), and visible tergites subequal in length along the midline (Figs 3A, 4G) (Apharinodes, Filigerinus, Filigerodes, Mecochelia, Stipesa Sharp) (Raffray 1890; Jeannel 1950, 1951; Nomura 1989; Chandler 2001; Yin and Jiang 2017). Other than a mosaic morphological combination, Europharinodes also possesses two presumably plesiomorphic characters which are unknown or rarely presented in modern species. The first is the presence of two subequal pretarsal claws on all tarsi (Fig. 2G), in contrast to one major anterior claw plus a reduced, posterior (accessary) claw in all extant species. The second, revealed by X-ray microtomography, is the dorso-ventrally symmetric aedeagus with an enlarged, bulbous basal capsule, a short, non-protruding endophallus, and large, elongate parameres. All extant members of Hybocephalini except for Apharinodes and a few Stipesa have a tubular aedeagus with a strongly constricted basal capsule (e.g., Jeannel 1950) similar to that of the genus Pseudophanias Raffray belonging to the tribe Tmesiphorini (Yin et al. 2015; Yin 2019; Inoue and Nomura 2020; Inoue et al. 2020); while Apharinodes has the aedeagus with a moderately bulbous basal capsule, but the endophallus is asymmetric and often protruding, and the parameres are short and reduced in size (Nomura 1989; Yin and Jiang 2017). Notably, Europharinodes bears a pair of antero-hypomeral foveae (Fig. 4B, C; ahf) on the prothorax, and a pair of anterolateral foveae (not shown in the figure) on the mesoventrite, two presumably autapomorphic characters that are absent among modern Hybocephalini but variously present in other pselaphine groups. Europharinodes also have more elongate, subquadrate antennomeres, opposed to the moderately to strongly transverse antennal segments in most extant species. The combination of the above morphological characters readily separates Europharinodes from all extant genera. Combining the morphological evidence and the results of our phylogenetic analyses present above, Europharinodes is tentatively postulated as a stem-group member of Hybocephalini.

For the first time, the endoskeletal structure of the head of a fossil Pselaphinae was partially reconstructed thanks to the application of X-ray microtomography technology. The tentorium (Fig. 2E; ta) of Europharinodes is more generalized and corresponds more to the ground plan of free-living groups such as Batrisitae (Nomura 1991b; Luo et al. 2021) than to Clavigeritae, of which the tentorium is distinctly reduced in size (Jałoszyński et al. 2020) as an adaptation to extreme myrmecophily. Surprisingly, the tentorium of Europharinodes shows a conspicuous difference from that of Pselaphus heisei Herbst (Pselaphitae: Pselaphini) which lacks anterior arms (Beutel et al. 2021). The dorsal tentorial arms of Europharinodes derived immediately from the inner wall of the large, distinct vertexal foveae (dorsal tentorial pits) (Fig. 3E; vf), and extending ventrally to connect posterior tentorial arms fusing with the inner wall of the widely separated gular foveae (posterior tentorial pits) (Fig. 3E; gf). The tentorium divergent anteriorly near the base of posterior arms to form curved and thin anterior arms, which extend anteriorly and end at the inner wall of the clypeus; the tentorial bridge is absent. Other inner structures of the head except for the pharynx cannot be clearly examined. The pharynx (Fig. 2E; ph) is the anterior-most part of the foregut (Beutel et al. 2013). In Europharinodes it extends from the anatomical mouth (which cannot be clearly seen) posteriorly to approximately the anterior 1/3 of prothoracic length. The posterior half of the pharynx is roughly the same width as the anterior half, and the pharyngeal lumen in cross-section appears to be suboval to slightly cordiform. The generalized pharynx, combined with the form of the mandibles, may indicate predacious habits of Europharinodes, albeit the biology of its modern relatives remains largely, if not at all, unknown. For the endoskeletal structures of the other main body segments, not much except for the three pairs of furcae can be observed. The profurcal arms are widely separated at the base, and in anterior view, each arm is slightly curved laterally. Only parts of the mesofurca and the basal part of the metafurca remain, but it would not be unreasonable to postulate that their overall morphology was generally concordant with that of modern Pselaphitae (e.g., Yin et al. 2013).

Different theories explaining the seemingly contradictory co-existence of thermophilic and temperate insect lineages in Baltic amber (Wheeler 1914), termed as the ‘Wheeler’s dilemma’ (Archibald and Farrell 2003), have been proposed. Among them, the theory of an equable terrestrial palaeoclimate of the amber forest with temperate summer and mild winter of low-temperature seasonality (Archibald and Farrell 2003; Archibald et al. 2010) appears to be broadly favored by recent researchers (Wedmann et al. 2010; Perkovsky 2016; Alekseev 2017; Bogri et al. 2018; Schmidt et al. 2019), and corroborated by studies of floras (e.g., Sadowski et al. 2017). Here we adopt Archibald and Farrell (2003)’s explanation as the most convincing theory regarding the ‘mixed’ fauna from Baltic amber. On the other hand, discoveries of present-day arthropod groups showing affinities with Afrotropical or Oriental regions in Baltic amber have been well-documented in published literature (e.g., Larsson 1978; Engel 2001), and a similar biogeographic connection also holds true for Hybocephalini. The present-day Hybocephalini are exclusively found in the Afrotropical and Australo-Oriental regions, with a higher species diversity occurring in sub-Saharan Africa (Fig. 6B). Notably, most species, if not all, are difficult to collect in the field and are rarely represented in museum collections. Insufficient available data, both morphological and molecular, prevented any meaningful studies tracing the origin and historical biogeography of this group. In this regard, the discovery of the first fossil member of the tribe is significant. The new fossil provides evidence that members of the stem-group Hybocephalini had already been well-established in Europe by the middle Eocene, which have since become extinct. During the Eocene–Oligocene transition, the global climate dramatically transformed from a ‘greenhouse’ to an ‘icehouse’ (Zachos et al. 2001; Hren et al. 2013), with an increase of seasonality (Eldrett et al. 2009; Mosbrugger et al. 2005; Utescher et al. 2015). Although dating the origin of Hybocephalini at this time appears to be premature due to the scarcity of paleontological data, this tribe may have been relatively widespread before the Eocene, and the northern Europe perhaps served as an important migration route for African-Asian spreading of the group. Following the Eocene–Oligocene Transition, the European representatives may have experienced significant declines in diversity and eventually become extinct, leaving the tropical survivors arriving in the Recent and forming isolated distributions as we observe today.

The original micro-CT data are available from the Zenodo repository (https://doi.org/10.5281/zenodo.6058375).

The authors have declared that no competing interests exist.

We are grateful to the two anonymous reviewers who provided critical comments on an earlier version of the manuscript, which improved the paper. We are thankful to Su-Ping Wu for technical help with micro-CT reconstruction. Financial support was provided by the National Natural Science Foundation of China (grant no. 31872965), the Second Tibetan Plateau Scientific Expedition and Research project (grant no. 2019QZKK0706), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant nos. XDB26000000 and XDB18000000) and the Science and Technology Commission of Shanghai Municipality (grant no. 19QA1406600).