Most known beetles in Rovno amber belong to the suborder Polyphaga (Bukejs et al. Reference Bukejs, Háva and Alekseev2020; Tshernyshev & Perkovsky Reference Tshernyshev and Perkovsky2021; Telnov et al. Reference Telnov, Perkovsky, Vasilenko and Yamamoto2021, Reference Telnov, Perkovsky, Kundrata, Kairišs, Vasilenko and Bukejs2022; Alekseev & Bukejs Reference Alekseev and Bukejs2022, Reference Alekseev and Bukejs2023a, Reference Alekseev and Bukejsb; Kazantsev & Perkovsky Reference Kazantsev and Perkovsky2022; Legalov et al. Reference Legalov, Nazarenko, Vasilenko and Perkovsky2022a, Reference Legalov, Vasilenko and Perkovskyb, Reference Anisyutkin and Perkovsky2023; Vitali & Perkovsky Reference Vitali and Perkovsky2022; Yamamoto et al. Reference Yamamoto, Nazarenko, Vasilenko and Perkovsky2022; Lyubarsky et al. Reference Lyubarsky, Perkovsky and Vasilenko2023; Motyka et al. Reference Motyka, Kazantsev, Kusy, Perkovsky, Yamamoto and Bocak2023; Nabozhenko & Perkovsky Reference Nabozhenko and Perkovsky2023) and a few belong to the Archostemata or Adephaga. Examples of Archostemata include Micromalthus priabonicus Perkovsky Reference Perkovsky2016 (Rovno amber) and Cupes groehni Kirejtshuk Reference Kirejtshuk2005 (Baltic and Rovno amber, Bukejs et al. Reference Bukejs, Alekseev and Kairišs2021), a new genus and species of thermophile tiger beetle (Matalin et al. Reference Matalin, Perkovsky and Vasilenko2021), an endemic sphodrine species of the extinct genus Quasicalathus Schmidt & Will, 2022 in Baltic and Rovno ambers in Schmidt (et al. Reference Schmidt, Scholz and Will2022), an extinct lebiine genus known only from Rovno amber with two species (Kirichenko-Babko et al. Reference Kirichenko-Babko, Perkovsky and Vasilenko2022a, Reference Kirichenko-Babko, Perkovsky and Vasilenkob) and the new tropical paussine (Kirichenko-Babko & Perkovsky Reference Kirichenko-Babko and Perkovsky2021) described here.
Paussine beetles are commonly known as ‘ant nest beetles’ (Skaife Reference Skaife1954) or ‘flanged bombardier beetles’ (Moore Reference Moore2006) due to their cuticular flange (flange of Coanda) located at the lateral margin of each elytron near their apex (with the exception of Metriini, see also Deuve Reference Deuve2020). The taxonomy and biogeography of paussids is fairly well examined (e.g., Darlington Reference Darlington1950; Stork Reference Stork1985; Nagel Reference Nagel1986, Reference Nagel1987b, Reference Nagel2006; Nagel et al. Reference Nagel, Robertson and Moore2017; Luna de Carvalho Reference Luna de Carvalho1989; Ball & McCleve Reference Ball and McCleve1990; Ball & Shpeley Reference Ball and Shpeley1990; Di Giulio et al. Reference Di Giulio, Fattorini, Kaupp, Taglianti and Nagel2003; Moore Reference Moore2006, Reference Moore2008; Fattorini et al. Reference Fattorini, Maurizi and Di Giulio2012, Reference Fattorini, Maurizi and Di Giulio2013; Moore & Robertson Reference Moore and Robertson2014; Robertson & Moore Reference Robertson and Moore2016; Deuve Reference Deuve2020). Currently, they are divided into five tribes: Metriini LeConte Reference LeConte1853; Ozaenini Hope Reference Hope1838; Protopaussini Gestro Reference Gestro1892; Paussini Latreille Reference Latreille1807; and Kryzhanovskianini Deuve Reference Deuve2020. Extant paussids include around 870 species, approximately 600 of which are Paussini, as the largest group of myrmecophilous beetles (Lorenz Reference Lorenz2021) and the ‘quintessential Trojan horses of the insect world’ (Moore & Robertson Reference Moore and Robertson2014, p. 1). Their geographical distribution has been discussed by Jeannel (Reference Jeannel1946), Reichensperger (Reference Reichensperger1948), Darlington (Reference Darlington1950) and Nagel (Reference Nagel, den Boer, Thiele and Weber1979). They are mainly restricted to the subtropics and tropics, with their highest diversity in the Palaeotropics (Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007).
Most fossil paussines (20 species) are known from the late Eocene Baltic amber (Wasmann Reference Wasmann1929). These are placed in six extinct genera: Arthropterites Wasmann Reference Wasmann1925 (one species); Cerapterites Wasmann Reference Wasmann1925 (one species); Eopaussus Wasmann Reference Wasmann1926 (one species); Pleurarthropterus Wasmann Reference Wasmann1927 (=Balticarthropterus Nagel Reference Nagel1987a) (12 species); Protocerapterus Wasmann Reference Wasmann1926 (two species); and Succinarthropterus Kolbe Reference Kolbe1926 (three species) (Nagel Reference Nagel1987b; Alekseev Reference Alekseev2017; Kirejtshuk & Ponomarenko Reference Kirejtshuk and Ponomarenko2021). They probably belong to the subtribe Carabidomemnina Wasmann Reference Wasmann1928 (Paussini) (Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007). The oldest Paussini is a single species reported from the middle Eocene (44–45 Ma) Eckfeld Lagerstätte, belonging to Pleuarthropterus (Balticarthropterus) (Wappler Reference Wappler2003). Neotropical extinct paussines are described from Miocene Dominican and Mexican ambers (Nagel Reference Nagel1987b, Reference Nagel1997; Solórzano Kraemer Reference Solórzano Kraemer2006). These belong to the three extant genera in two tribes – Protopaussus Gestro Reference Gestro1892 in the Protopaussini (one species), Eohomopterus Wasmann Reference Wasmann1925 (two species) and Homopterus Westwood Reference Westwood1841 (one species), both in the Paussini. The oldest paussine Kryzhanovskiana Kataev & Kirejtshuk 2019 (originally classified as Metriini, reclassified as Kryzhanovskianini by et Deuve Reference Deuve2020) was described from earliest Cenomanian Kachin amber (Kataev et al. Reference Kataev, Kirejtshuk, Manukyan and Anokhin2019). Paussinae has 27 extinct species belonging to ten genera in three tribes described from the Cretaceous to the Neogene (Kirejtshuk & Ponomarenko Reference Kirejtshuk and Ponomarenko2021; Lorenz Reference Lorenz2021): Kryzhanovskianini (one species); Protopaussini (one species); and Paussini (25 species). Nagel (Reference Nagel1987b) and Luna de Carvalho (Reference Luna de Carvalho1989) proposed a phylogenetic classification of the fossil paussids; however, their species-level diversity has been insufficiently studied.
1. Material and methods
Ukrainian Rovno amber (Priabonian, 33.9–37.8 Ma) is the southern analogue of Baltic amber found from the north of Volyn and the Rovno and Zhitomir regions (reviewed by Perkovsky et al. Reference Perkovsky, Zosimovich, Vlaskin and Penney2010; Mitov et al. Reference Mitov, Perkovsky and Dunlop2021). It was redeposited northward to the periphery of the north-western part of the Ukrainian Crystalline Rock Massif (Perkovsky et al. Reference Perkovsky, Rasnitsyn, Vlaskin and Taraschuk2007).
This paper is based on an amber inclusion from the collection of Nikolai R. Khomych (Rovno) on long-term loan to the Schmalhausen Institute of Zoology of the National Academy of Sciences of Ukraine, Kiev (SIZK), inventory number SIZK L-956, and available for study. It was collected in the Varash district (between Voronki and nearby Luko) of the Rovno Oblast, like most of the important new taxa described from Rovno amber during the last few years (Perkovsky et al. Reference Perkovsky, Olmi, Vasilenko, Capradossi and Guglielmino2020; Golub et al. Reference Golub, Perkovsky and Vasilenko2021; Perkovsky & Nel Reference Perkovsky and Nel2021; Olmi et al. Reference Olmi, Guglielmino, Vasilenko and Perkovsky2022; Simutnik et al. Reference Simutnik, Perkovsky and Vasilenko2022 and references therein).
The following measurements are in millimetres:
body length (BL) – from the visible anterior margin of the head to the elytral apex;
head length (HL) – from the visible anterior part of head to the constriction of neck;
head width (HW) – maximum width of the head with eyes;
antennal club length (AL) – of the antennomeres of flagellum;
pronotal length (PL) – length along the median line from the anterior margin to the pronotal base;
pronotal width (PW) – maximum width;
elytral length (EL) – length from the basal border to the apex along the suture; and
elytral width (EW) – width across the middle of both elytra.
The material was examined with a Leica M165C stereomicroscope and photographed with an attached DFC450 C camera. Photostacking was done with Helicon Focus 6 software. Figures were prepared with Adobe Photoshop CS8. Morphological terminology follows Nagel (Reference Nagel1987b, Reference Nagel1997) and Darlington (Reference Darlington1950).
Identification was not possible to the species level, as the amber is very dark with many small inclusions of wood and air bubbles and the beetle is located near the surface, positioned such that its ventral morphology cannot be seen (mouthparts, fore coxae and forelegs).
2. Systematic palaeontology
Order Coleoptera
Family Carabidae Latreille Reference Latreille1802
Subfamily Paussinae Latreille Reference Latreille1806
Tribe Paussini Latreille Reference Latreille1806
Subtribe Carabidomemnina Wasmann Reference Wasmann1928 (Figs 1–3)
Genus ?Eohomopterus Wasmann Reference Wasmann1925
Diagnosis. With the combination of characters of the subfamily Paussinae: reduction of the pedicel (antennomere 2), robust antennae forming a distinct flagellum (antennae club), subapical elytral fold (flange of Coanda) and leg structure. Antennae as in Cerapterina: club with nine free antennomeres (3–11), pronotum without trichome-bearing clef, elytral base with carina suggesting placement in the Paussini. Belonging to the subtribe Carabidomemnina (comprises two genera, Carabidomemnus Kolbe Reference Kolbe1924 and Eohomopterus Wasmann Reference Wasmann1919) by its compact body with glabrous convex surface, flattened antennomeres, elytra with short suture and apex non-truncate, short tibia narrow basally and extended to apex (Nagel Reference Nagel1997). All Baltic amber Paussinae described by Wasmann (Reference Wasmann1928) have a non-truncate elytral apex (Nagel Reference Nagel1987b).
Figure 1 Habitus of the paussine beetle in Rovno amber, dorsal view. (a) Head, antenna and pronotum. (b) Arrow indicates the hairs (h) on the front of the head.
Figure 2 The paussine beetle in Rovno amber. (a) Base of elytra with triangular depressions (de), the elongated scutellum (sc) and two setae in the humeral region and the right middle leg in anterolateral view. (b) The femora, tibia and tarsi. Arrows indicate the setae; circles indicate the setigerous pores.
Figure 3 The paussine beetle in Rovno amber (line drawings): (a) The pronotum. (b) The antennae with 9-segmented club. (c) The right middle leg.
The Rovno specimen differs from species of Carabidomemnus by the short 4 tarsomere, the weakly expanded tarsomeres 2 and 3 (vs Carabidomemnus all tarsomeres visible), by the transversally rectangular first flagellomere (antennomere 3) (vs Carabidomemnus: relatively small, see Kolbe Reference Kolbe1928; Wasmann Reference Wasmann1928, Reference Wasmann1929) and by the intermediate antennomeres with straight apical and basal margins (Figs 1b, 3b) (vs Carabidomemnus: antennomeres 3–8 have a compact crescent shape).
Distinctive features of the Rovno amber species and comparison with known Eohomopterus species. The dorsal surface of the Rovno specimen's body is without punctures (vs finely punctate in †Eohomopterus poinari and †Eohomopterus simojovelensis), and the BL is about 6 mm long (Figs 1, 2). Its antennae are parallel and flattened, reaching the base of the pronotum in turned-back position, and the flagellum is nine-segmented (antennomeres 3–11); AL is about 1.9 mm; the width and height of the last antennomere are 0.67 mm and 0.6 mm, respectively. The first flagellomere (antennomere 3) of the Rovno specimen is transversally rectangular (Figs 1b, 3b), whereas in †Eohomopterus paulmuelleri Nagel Reference Nagel1987b and †E. simojovelensis Solórzano Kraemer Reference Solórzano Kraemer2006 it is a transverse rhomboid and in †E. poinari Nagel Reference Nagel1997 and Eohomopterus centenarius Luna De Carvalho Reference Luna de Carvalho1960 it is transverse and broadly triangular. The exterior and interior apical corners of the Rovno specimen's club articles 4 to 10 have small tooth-shaped prolongations, similar to that of †E. paulmuelleri (Nagel Reference Nagel1987b) and †E. simojovelensis (Solórzano Kraemer Reference Solórzano Kraemer2006, Figs 3, 4). Its scapus has short setae as in †E. paulmuelleri. The head is transverse with protruding eyes. Its anterior has short hairs on the front, distinguishing it from named Eohomopterus species, where this character state is known. Unfortunately, the ventral side including the mouthparts are not visible (see above).
Figure 4 Fossil records of Paussinae: Cretaceous, Kachin (Burmese) amber (BuA, black square); middle Eocene, Eckfeld, Germany (Eck, black triangle); late Eocene, Baltic amber (BaA, grey circle) and Rovno amber (RA, black star); Miocene, Dominican amber (DA, grey square) and Mexican amber (MA, black diamond).
The pronotum is slightly cordiform (slightly wider than long: PW/PL = 1.17, with rectangular posterior angles, as in Eohomopterus aequatoriensis Wasmann Reference Wasmann1899). It has a narrow lateral marginal edge and has small depressions in front of it base on both sides (Fig. 3a). In †E. poinari it is also markedly narrowed toward base (Nagel Reference Nagel1997), whereas in †E. simojovelensis it is almost rectangular.
The elytral suture is probably short, reaching the elytral middle (elytra slightly open along the suture and the outer side of right elytron is deformed) (Fig. 1a). The elytra are almost twice as long as wide (EL/EW = 1.9) and their basal edge is wider than the pronotal base. The elytral base in the area of the humeri has triangular depressions as in recent and fossil species of Eohomopterus, and behind the scutellum of each elytron there is one bristle-bearing pore (Fig. 2a) as in recent species of Eohomopterus. The setae of the seria umbilicata is not discernible, as in †E. paulmuelleri, vs in †E. poinari and †E. simojovelensis, where this is visible. The scutellum is elongate triangular (vs equilaterally triangular in †E. paulmuelleri and †E. simojovelensis) and is without punctures (Fig. 2a).
The structure of the front coxae is an important feature for identifying Eohomopterus species (Darlington Reference Darlington1950), which are distinguished by a narrow prosternal process in all members; however, in †E. poinari (Nagel Reference Nagel1997) and the Rovno specimen this is not visible. The genus Eohomopterus is characterised by the emarginated inner margin of its front tibiae, which are apically dilated and have two final spurs; however, this cannot be assessed in the Rovno specimen by preservation (see above). The tibiae of another fossil Eohomopterus are rather slender, slightly widened apically (Nagel Reference Nagel1997; Figs 3, 4; Solórzano Kraemer Reference Solórzano Kraemer2006; Fig. 2).
In the Rovno specimen, the visible part of the femora of the right foreleg is broad and short and the femora of the right fore-legs and middle-legs have short pilosity. In the Rovno specimen the middle tibia (Fig. 2b) is longer than wide (approximately 2.7 times longer than wide), flattened and its proximal end near the femora is narrow as in E. aequatoriensis.
The tarsus is slightly longer than the width of the mesotibia at the apex (Figs 2b, 3c). In the right mesotarsus (length about 0.8 mm), tarsomere 5 is longer than 1–3 combined (vs Carabidomemnus: all tarsomeres are clearly visible and not reduced), tarsomere 4 is short and tarsomeres 2–3 slightly dilated as in †E. poinari. The tarsi have long, simple and distinctly bent claws. The structure of the tarsi of † E. paulmuelleri is identical to that of E. centenarius: tarsomere 3 is more dilated than tarsomere 2; whereas in E. aequatoriensis the 2 and 3 tarsomeres are extremely strongly lobed, as in †E. simojovelensis (Solórzano Kraemer Reference Solórzano Kraemer2006).
In summary, the Rovno amber specimen is distinguished from extant and Neogene species of Eohomopterus Wasmann Reference Wasmann1919 by the following characteristics:
the transverse rectangular antennomere 3 (1st flagellomere) (Figs 1b, 3b), vs rhomboid in †E. paulmuelleri and †E. simojovelensis and triangular in †E. poinari and E. centenarius;
the slightly cordiform pronotum (Figs 1a, b, 3a) as in E. aequatoriensis; vs rectangular with broadly rounded posterior angles in †E. simojovelensis, †E. paulmuelleri and E. centenarius;
the length of the 5th tarsomere equals that of the 1–4th tarsomeres, as in extant E. aequatoriensis (Figs 2b, 3c);
the scutellum is elongated triangular; behind it each elytron has one bristle-bearing pore (Fig. 2a) (in other fossil species, this cannot be assessed);
the body surface is without punctures (Fig. 1a) and is glabrous as in E. aequatoriensis, vs finely punctate in †E. poinari and †E. simojovelensis;
the absence the two punctures on the front of the head between the eyes;
the presence of scarce hairs on the front of the head (Fig. 1b); and
the short hairs along the edge of the pronotum and epipleurae of the elytra.
As mentioned above, some diagnostic characters cannot be assessed by preservation (e.g., mouthparts, ventral side of body, the front coxae and the forelegs). Nevertheless, based on the following characters such as its elongated shape of body, flagellum with nine parallel and flattened antennomeres, femora and tibiae of the right middle legs with short setae, tarsomere 4 is very small, the tarsomere 3 not lobed, and the base of elytra with depressions in humeral area (Figs 1, 2), the Rovno specimen is related to extant and Neogene species of Eohomopterus Wasmann Reference Wasmann1919.
3. Discussion
3.1. Coevolution of paussids with ants
An extraordinary number of myrmecophilous beetle species are known today, belonging to at least 35 families (Vander Meer & Wojcik Reference Vander Meer and Wojcik1982; Hölldobbler & Wilson Reference Hölldobler and Wilson1990; Akino Reference Akino2002; Orivel et al. Reference Orivel, Servigne, Cerdan, Dejean and Corbara2004; Mynhardt Reference Mynhardt2013); of these, the subfamily Paussinae (Adephaga, Carabidae) is one of the largest. In it species of Protopaussini, Paussini and the subtribe Physeina from Ozaenini are obligate ant predators and symbionts, possessing different adaptations for living with ants (Ball & McCleve Reference Ball and McCleve1990; Di Giulio & Vigna Taglianti Reference Di Giulio and Vigna Taglianti2001; Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007; Di Giulio et al. Reference Di Giulio, Maurizi, Rossi Stacconi and Romani2012; Maurizi et al. Reference Maurizi, Fattorini, Moore and Di Giulio2012; Parker Reference Parker2016).
According to Parker (Reference Parker2016), the Paussini likely represents one of the more ancient clades of myrmecophiles, known from late Eocene Baltic amber (36–37 million years old), little derived within Paussinae (as representatives of Carabidomemnina and Cerapterina) and dominant in the Eocene of Europa (Darlington Reference Darlington1950).
The presence of structures associated with myrmecophily in fossil beetles, such as trichomes, crassate antennae, cephalic horn, or lack of tactile setae, suggest that myrmecophily in paussines already existed in the early Palaeogene (Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007). Given the predominance of trichomes and specialised bristles, it is hypothesised that these beetles produce chemicals from specialised glands concentrated in the antennae, pronotum, elytra and pygidium (Nagel Reference Nagel, den Boer, Thiele and Weber1979; Di Giulio et al. Reference Di Giulio, Rossi Stacconi and Romani2009, Reference Di Giulio, Maurizi, Rossi Stacconi and Romani2012; Maurizi et al. Reference Maurizi, Fattorini, Moore and Di Giulio2012). This may be part of a complex and effective parasitic strategy along with acoustic mimicry (Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007). There are myrmecophilious paussines stridulatory organs known to live with stridulatory ant hosts and some paussine species with stridulatory organs are known to live with non-stridulatory ants. Adults of the ant subfamilies Ponerinae, Pseudomyrmecinae, Myrmicinae and Ectatomminae are able to produce low-frequency sounds by stridulation (Markl Reference Markl1965; Ferreira et al. Reference Ferreira, Poteaux, Delabie, Fresneau and Rybak2010), which they use for intraspecific communication and aggregation (Markl & Hölldobler Reference Markl and Hölldobler1978; Baroni-Urbani et al. Reference Baroni-Urbani, Buser and Schilliger1988; Hölldobler Reference Hölldobler1999; Hickling & Brown Reference Hickling and Brown2000; Di Giulio et al. Reference Di Giulio, Maurizi, Hlaváč and Moore2011). It appears that at least among Paussus species, anatomical differences of taxonomic value are intimately tied to host–ant interactions and therefore likely to convergently evolve very fast under strong selective pressure (Moore & Robertson Reference Moore and Robertson2014).
Probably some fossil paussines from Baltic amber like extant representatives of the subtribe Carabidomemnina (Carabidomemnus) were associated with Formicinae and Myrmicinae (Di Giulio et al. Reference Di Giulio, Fattorini, Kaupp, Taglianti and Nagel2003; Di Giulio & Moore Reference Di Giulio and Moore2004; Di Giulio Reference Di Giulio2008; Moore et al. Reference Moore, Song and Di Giulio2011; Maurizi et al. Reference Maurizi, Fattorini, Moore and Di Giulio2012). The Rovno specimen has no stridulatory organs. Based on its morphology it belongs to a ‘defiant’ species (Nagel Reference Nagel, den Boer, Thiele and Weber1979) that prey upon broods protected from attack by their adult morphology (Darlington Reference Darlington1950; Geiselhardt et al. Reference Geiselhardt, Peschke and Nagel2007).
3.2. Zoogeographical implication
Both described extant species of Eohomopterus are Neotropical: E aequatoriensis (Wasmann Reference Wasmann1899) from Ecuador; and E. centenarius from Brazil (Lorenz Reference Lorenz2021) (another species from the West Indies has not yet been formally described: Moore Reference Moore2006).
The type species E. aequatoriensis was originally assigned to the genus Homopterus Westwood Reference Westwood1841 (Wasmann Reference Wasmann1899; Nagel Reference Nagel1997). Three extinct species of this genus were described from Miocene amber: E. paulmuelleri Nagel Reference Nagel1987b and E. poinari Nagel Reference Nagel1997 in Dominican amber; and E. simojovelensis Solórzano Kraemer, 2006 in Mexican amber (Nagel Reference Nagel1987b, Reference Nagel1997; Solórzano Kraemer Reference Solórzano Kraemer2006). The distribution of Eohomopterus species including fossil records is shown in Fig. 4.
The distribution of extant Paussini is limited by tropical and subtropical regions, even where their ant hosts range further north. For example, the distribution of the paussine Ceratoderus venustus Hisamatsu Reference Hisamatsu1963 is limited to the Shikoku, Kyûshû and Yakushima islands (Maruyama Reference Maruyama2014), where the winter temperature (coldest quarter mean temperature, CQMT) is 7.4–12°C; while the ant Crematogaster vagula Wheeler Reference Wheeler1928, associated with the paussine C. venustus, is widespread in Honshu (CQMT is 6.1°C). The northern distribution limit of the Paussus favieri Fairmaire Reference Fairmaire1851 in the south of France is further south than the distribution of its ant host (LeMasne Reference LeMasne1961).
It is difficult to suggest the possible hosts for neotropical species of Eohomopterus, as the only record of an American Paussini species with ants is still apparently that of Homopterus steinbachi Kolbe Reference Kolbe1920 in a nest of the widespread dominant tropical species Dolichoderus bispinosus (Olivier Reference Olivier1792) (Dolichoderinae) (Darlington Reference Darlington1964). Two species of the bispinosus group are represented by numerous specimens in Dominican amber (Grimaldi & Agosti Reference Grimaldi and Agosti2000) and one undescribed species in Mexican amber (D. A. Dubovikoff, personal communication, 2022). Dolichoderines were dominant ants of the European amber forests (Dlussky & Rasnitsyn Reference Dlussky and Rasnitsyn2009; Perkovsky Reference Perkovsky2016), so Eocene dolichoderine species can be considered the most probable ant hosts of paussines. Most Dolichoderus Lund Reference Lund1831 species are arboreal, building their nests under bark or in dead tree branches (Dlussky & Rasnitsyn Reference Dlussky and Rasnitsyn2009).
A Formica flori Mayr, 1868 worker ant preserved in the same piece of Baltic amber as Eopaussus balticus Wasmann Reference Wasmann1926 (Wasmann Reference Wasmann1929, Figs 35, 36) was redetermined. The general appearance of the body and the individual morphological structures visible in the photograph (long scapuses, the position of the eyes, the structure of the mesosome and petiole) indicate that the ant shown in the photograph is more likely to belong to the species Yantaromyrmex geinitzi (Mayr Reference Mayr1868) and not Formica (D. A. Dubovikoff, personal communication, 2023).
The climate of the Rovno amber forest was warmer than that the Baltic amber forest (Mänd et al. Reference Mänd, Muehlenbachs, McKellar, Wolfe and Konhauser2018; Perkovsky Reference Perkovsky2018; Sokoloff et al. Reference Sokoloff, Ignatov, Remizowa, Nuraliev, Blagoderov, Garbout and Perkovsky2018; Yamamoto et al. Reference Yamamoto, Nazarenko, Vasilenko and Perkovsky2022; Anisyutkin & Perkovsky Reference Anisyutkin and Perkovsky2023; Jenkins Shaw et al. Reference Jenkins Shaw, Perkovsky, Ślipiński, Escalona and Solodovnikov2023), a probable reason for the absence in Baltic amber of some cryophobic Rovno taxa with extant Western Hemisphere distributions; for example, the brentid genus Toxorhynchus Scudder Reference Scudder1893 and Caulophilus Wollaston Reference Wollaston1854 (Nazarenko et al. Reference Nazarenko, Legalov and Perkovskу2011; Bukejs & Legalov Reference Bukejs and Legalov2020) and probably the cerambycid Poliaenus europaeus Vitali & Perkovsky Reference Vitali and Perkovsky2022 as well (Vitali & Perkovsky Reference Vitali and Perkovsky2022).
The confident relationship of the Rovno specimen with Eohomopterus, as well as the presence of the Dominican amber Protopaussus pristinus Nagel Reference Nagel1997 (extant Protopaussini are limited to the Oriental Region tropics and subtropics), testify to transarctic migrations of their hosts in the early Palaeogene, similar to the intercontinental dispersal of giant cryophobic ants from extinct subfamily Formiciinae (Archibald et al. Reference Archibald, Johnson, Mathewes and Greenwood2011, Reference Archibald, Mathewes and Aase2023).
4. Conclusion
The presence of Neotropical Carabidomemnina in Rovno amber as well as of Protopaussini in Dominican amber is of significant interest as a probable result of transarctic migrations of host ants in the early Eocene. This unexpected Rovno amber record, like that of the new tribe of wingless pincer wasps (Olmi et al. Reference Olmi, Guglielmino, Vasilenko and Perkovsky2022) highlights the significance of the fossil biota of the Rovno amber forest.
5. Acknowledgements
The authors are grateful to Nikolai R. Khomich (Rovno, Ukraine) for an opportunity to study the material, to Alexandr P. Rasnitsyn (A.A. Borissiak Paleontological Institute, Moscow, Russia) for helpful discussion, Dmitry A. Dubovikoff (St. Petersburg State University, Russia) for important advices and unpublished data, Joshua Jenkins Shaw J. (Natural History Museum of Denmark, Copenhagen, Denmark), Bruce Archibald (Beate Biodiversity Museum, University of British Columbia, Canada) for editing and Roman Babko (Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Kyiv, Ukraine) for drawings. The authors thank anonymous reviewers for improving the overall quality of the manuscript.
6. Financial support
Evgeny Perkovsky is supported by a grant from scholarship programme Scholars at Risk Ukraine jointly funded by the Villum Foundation, Carlsberg Foundation and Novo Nordisk Foundation.
7. Competing interests
The authors declare none.
