A new species, Formica ribbeckei Radchenko & Perkovsky, sp. nov., is described based on four workers from late Eocene Rovno amber (Ukraine). It most resembles F. flori Mayr, 1868 but differs from the latter mainly by the 5-segmented maxillary palps with the preapical segment subequal in length to the apical one, and by the shorter first funicular segment. Fossil F. luteola Presl, 1822, F. trigona Presl, 1822, F. macrognatha Presl, 1822 and F. quadrata Holl, 1829 are considered incertae sedis in Formicidae. Thus, ten valid Formica Linnaeus, 1758 species (including F. ribbeckei) are known now from late Eocene European ambers. The diversity of Formica in the early and middle Eocene deposits of Eurasia and North America is considered. It is assumed that the genus Formica most likely arose in the early Eocene.

European ambers, Formica ribbeckei, new species, paleontology, syninclusions, taxonomy

The ant fauna of late Eocene European ambers (Priabonian, 33.9–37.8 Ma) is the best studied among all fossil myrmecofaunas worldwide. More than 17,500 ant specimens belonging to nearly 220 extinct species from 73 genera (34 extinct and 39 extant) and 12 subfamilies (all extant) have been discovered till now (Dlussky and Rasnitsyn 2009; Dlussky et al. 2014; Radchenko and Dlussky 2012, 2017, 2018; Perkovsky 2016, 2018; our unpublished data). These ambers are: the Baltic amber (Kaliningrad Region of Russia and Baltic coast of Poland), the Bitterfeld (= Saxonian) amber (Germany, Saxony–Anhalt), the Scandinavian (= Danish) amber (mainly from the coast of Denmark), and the Rovno amber (Rovno, Zhitomir and Volyn Regions of Ukraine). Until 2015, almost all scientifically studied inclusions in Rovno amber were mined in Klesov or Dubrovitsa (Rovno Region), and inclusions from Vladimirets and Zarechnoje Districts of the Rovno Region were added later (Martynova et al., 2019). The data on inclusions from the Zhitomir Region have so far been limited to three new genera of Diptera and Coleoptera, one species of mites, and one species of caddisfly (Fedotova and Perkovsky 2015, 2017; Perkovsky 2017; Legalov et al. 2021). Formica ribbeckei Radchenko & Perkovsky, sp. nov. and Plagiolepis klinsmanni Mayr, 1868 are the first hymenopteran species recorded from amber of the Zhitomir Region.

Sixteen formicine genera are known from late Eocene European ambers, seven of which are extinct. In total, they comprise 39 described extinct species. Of the eight extant genera of the tribe Formicini only Formica Linnaeus, 1758 is found in late Eocene European ambers, as well as three extinct genera of this tribe: Protoformica Dlussky, 1967b, Cataglyphoides Dlussky, 2008 and Conoformica Dlussky, 2008) (Dlussky 2008; Bolton 2021).

The genus Formica comprises about 180 extant species, distributed almost exclusively in the Holarctic Region (Dlussky 1967a; Radchenko 2016; Bolton 2021), and nine extinct species were known from Eocene European ambers (Dlussky 2008). In total, about 100 fossil species have been attributed to Formica so far, but more than half of these names were either synonymized or are unidentifiable and are considered incertae sedis in Formicidae, and the actual number of valid species is less than 50; all are known from various Eocene, Oligocene and Miocene deposits of Eurasia and North America (Dlussky 2008; Dlussky and Putyatina 2014; Bolton 2021; our unpublished data). The oldest Formica species were described from the early Eocene (Ypresian, ca. 50–52 Ma) deposit of Quesnel (Okanagan Highlands, Canada) (Scudder 1877) and middle Eocene (Lutetian, ca. 46 Ma) deposit of Kishenehn (Montana, United States) (LaPolla and Greenwalt 2015), and several undescribed species have been discovered in the latest Ypresian Messel (47.8 Ma) and Lutetian Eckfeld (44.3 Ma) Lagerstätten of Germany (Dlussky 2008; Dlussky and Putyatina 2014).

Taking into account the new species described below, Formica together with Plagiolepis Mayr, 1861 represent the most speciose formicine genera in amber, each containing 10 species (Dlussky 2010). Furthermore, F. flori Mayr, 1868 is one of the most common ant species in all late Eocene European ambers: about 1500 specimens were found, and its relative abundance varies in different ambers from ca. 3% to almost 11% of all collected ants (Dlussky and Rasnitsyn 2009; our unpublished data).

We obtained a piece of amber from Gulyanka village (Korosten’ District, Zhitomir Region, Ukraine) containing two ant workers assignable to Formica and P. klinsmanni. After careful investigation, somewhat unexpectedly we found that this Formica specimen differs from the common F. flori by many important features, allowing the description of a new species, F. ribbeckei Radchenko & Perkovsky, sp. nov.

The discovery of a new Formica species in three localities – Gulyanka (Zhitomir region), Klesov and Vladimirets District (Rovno Region) together with P. klinsmanni, which was previously known from Klesov, supports a similar, late Eocene age, of all these Rovno amber deposits, since reliably dated late Eocene flora is known from Gulyanka (Pimenova 1937; Sokoloff et al. 2018).

We examined all available Formica inclusions from Rovno amber (about 100), four of which belong to F. ribbeckei Radchenko & Perkovsky, sp. nov. One piece of amber contains the holotype specimen (worker) of the described species together with a worker of P. klinsmanni (Fig. 2), and three other each contain one worker of F. ribbeckei Radchenko & Perkovsky, sp. nov. The holotype and two paratype specimens are deposited in the Schmalhausen Institute of Zoology of NAS of Ukraine, Kiev (SIZK), the paratype specimen No. KH–F–057 is in the collection of M. Khomych (Ukraine).

The photographs were taken with Leica Z16 APO microscope equipped with Leica DFC 450 camera processed by LAS Core software.

Not all features of the examined specimen were properly visible and measurable, hence we measured only well visible details (accurate to 0.01 mm), particularly: HL – maximum length of the head in dorsal view, measured in a straight line from the anteriormost point of clypeus to the mid-point of occipital margin; maxHW – maximum width of the head in dorsal view just behind (above) the eyes; minHW – minimal width of the head in dorsal view at the level of clypeus; SL – maximum length of the scape measured in a straight line from its apex to the articulation with condylar bulb; OL – maximum diameter of the eye; GL – length of the gena, measured from the anterior margin of the eye to the articulation with the mandible; ML – diagonal length of the mesosoma in lateral view from the anterior margin of the neck shield to the posterior margin of the metapleuron; MH – height of the mesosoma, measured from the upper level of the mesonotum perpendicularly to the lower margin of mesopleuron; PL – maximum length of the petiole, measured from the posterodorsal margin of the petiole to the articulation with the propodeum; PW – maximum width of the petiole in dorsal view; PH – maximum height of the petiole in profile, measured from the uppermost point of the petiolar scale perpendicularly to the lowest point of the petiole; HTL – maximum length of the hind tibia; FSL1…2 – length of the funicular segments 1 and 2; FSW1– width of the funicular segment 1; MP4…5 – length of segments of the maxillary palps from 4 and 5; TL – approximate total length is calculated as the sum of HL + ML + PL + length of the gaster.

Indices: CI=HL/maxHW, HWI= maxHW/minHW, SI1=SL/HL, SI2=SL/maxHW, OI=OL/HL, GI= GL/OL, PI1=PL/HL, PI2=PL/PH, MI=ML/MH, FSI1=FSL1/FSL2, FSI2=FSL1/FSW1, MPI=MP5/MP4.

Accordingly, only ten Formica species are reliably known from late Eocene European ambers. Dlussky (2008) suggested that most of these species are quite specialized and only F. flori and F. gustawi can be considered the putative ancestors of extant Formica species. They belong to the F. fusca Linnaeus, 1758 species-group. Despite that F. ribbeckei Radchenko & Perkovsky, sp. nov. resembles F. flori, it has a reduced number of maxillary palps, and therefore we do not consider it to be one of the ancestors of the extant species of the F. fusca group.

The Formica fusca-group is considered ancestral to other extant Formica species (Dlussky 1967a; Radchenko 2016). Its representatives are characterized by a non-specialized head structure, mesosoma, petiole, maxillary palps, funicular segments, as well as a weakly developed polymorphism and a low level of social organization. Their gynes are large, which is associated with the independent founding of new colonies.

Formica radchenkoi most likely belongs to the more derived subgenus Raptiformica Forel, 1913, which includes slave-making species. Similarly, F. parexsecta Dlussky & Putyatina, 2014, described from the early Miocene deposits of Radoboj (Croatia), resembles extant species from the morphologically specialized subgenus Coptoformica Müller, 1923. Representatives of this subgenus have an emarginated occipital margin, short maxillary palps and often reduced body pubescence. Queens are quite small compared to those of the other Formica subgenera (Dlussky 1967a; Radchenko 2016). The emarginated occipital margin of the head allows an increase in the mass and strength of the mandibular muscles, which is associated with their nesting habits: workers cut grass to build nesting mounds. The small body size of gynes is associated with their biology: they are unable to independently establish a new colony and are temporary social parasites of Serviformica Forel, 1913 species (Dlussky 1967a).

Until recently, the oldest undoubted Formica species were known from the late Eocene. These are 10 species known from European amber: 8 species from Baltic amber and 7 from Scandinavian, Bitterfeld and Rovno ambers taken together. Cockerell (1920) described F. heteroptera from the Bagshot Beds in Bournemouth (Dorset, England) (Lutetian) based on a forewing, but the described features of this wing indicate that this specimen does not belong to Formica (Perfilieva, personal communication), from which it is formally excluded here. Théobald (1937) described three new Formica species and recorded impressions of F. flori (gyne) from the Upper Rhine Graben of Kleinkems (Baden–Württemberg, Germany) (latest Eocene); however, because the short antennal scape does not reach the occipital margin, this specimen differs not only from F. flori but also from all extinct and extant Formica species. The affinities of F. tripartita Théobald, 1937, F. alsatica Théobald, 1937 and F. cf. septula Théobald, 1937 with Formica from late Eocene ambers remains uncertain. Interestingly, Formica ants have not been found in the Bembridge Marls, Isle of Wight, England (late Eocene) (Dlussky and Perfilieva 2014). This could be due to unsuitable environmental conditions at Bembridge for Formica and other epigeic ants (Radchenko and Perkovsky 2021).

Two new Formica species, F. biamoensis Dlussky et al., 2015 (worker) and F. paleosibirica Dlussky et al., 2015 (male), have recently been described from the latest Eocene deposit of the Russian Far East (Bol’shaya Svetlovodnaya) (Dlussky et al. 2015). F. biamoensis resembles F. flori and F. gustawi in general appearance, but the affinity of the second species to other known Formica is uncertain. Four species have also been recorded from the United States: F. eoptera Cockerell, 1923 (described based on a forewing) from Texas, and three species have been described based on winged gynes from Florissant, Colorado (latest Eocene) by Carpenter (1930). Although Carpenter (1930) doubted the attribution of F. eoptera to Formica and suggested considering it incertae sedis in Formicidae, Bolton (2021) concurred with Cockerell (1923). Based on the original description and illustration, there is no evidence to contradict the assignment of this species to Formica, and thus, we agree with Bolton.

The oldest species, assigned to Formica, F. arcana Scudder, 1877, was described by Scudder (1877) based on a forewing from the early Eocene of Quesnel (Okanagan Highlands, Canada), but Archibald et al. (2018) doubted that this species belonged to Formica or even Hymenoptera. However, the original description of the wing venation does not contradict the main features of Formica wing venation (Perfilieva, personal communication). Another undescribed Formica species was recently recorded from the early Eocene Palana Formation (India) (Patel et al. 2019). However, the interpretation of this specimen seems to be incorrect, and based on a photograph of the imprint (loc. cit., Fig. 10) we believe that this specimem does not exactly belong to Formica and, most likely it is not an ant at all.

Recently, Dlussky (2008) and Dlussky and Perfilieva (2014) indicated that there are some Formica compression fossils from the latest Ypresian Messel (47.8 Ma) and Lutetian Eckfeld (44.3 Ma) Lagerstätten (Germany). LaPolla and Greenwalt (2015) described males of F. annosa from the Kishenehn Formation (Lutetian, ca. 46 Ma) in northwestern Montana, USA. Despite that many important diagnostic details are usually unrecognizable from imprints (in contrast to amber fossils), the described males resemble extant males of the F. fusca-group in general appearance and visible features. This observation may serve as confirmation of its ancestral position in the genus.

Extant Formica species are one of the dominant groups of ants in the temperate humid zones of the Northern Hemisphere, where they mainly inhabit forests and meadows. Some species live in semi-arid regions (steppes, Mediterranean landscapes), but in deserts and semi-deserts, they occur only in intrazonal wet places. The distribution of many species extends beyond the Arctic Circle, and species can be found at elevations of 3600 m in the Alps, 3950 m in the Pamir, 4300 m in the North American Cordillera and even up to 4800 m in the Himalayas. Some species are also distributed in the mountains of tropical regions (Mexico, Taiwan, Myanmar), and F. fusca has been introduced and naturalized in Cuba (Dlussky 1967a; Gregg 1969; Radchenko 2016).

Ecologically, Formica is a keystone genus (together with Lasius Fabricius, 1804 and Myrmica Latreille, 1804) in the temperate zone of the Holarctic. Many species (e.g. members of the subgenera Formica s. str., Coptoformica, some Serviformica) dominate in ant communities (Dlussky 1967a; Zakharov et al 2013; Zakharov 2018). All Formica species are epigeic ants; they build nests in the ground, often with soil mounds, or make “ant-hills” from small pieces of branches, conifer needles or grass. They actively forage not only on the ground but also in the grass and on trees and bushes where they collect honeydew of aphids and other homopterans and prey on a variety of invertebrates.

It can be assumed that the extinct Formica species had a similar lifestyle. Homopteran insects were common by the late Eocene and are present in European ambers, and numerous syninclusions of aphids and ants have been recorded (Perkovsky 2006, 2007, 2008, 2009, 2010, 2011a, b; Perkovsky and Wegierek 2018). Most likely, workers of one of the most numerous amber ants, F. flori (as well as other Formica species), were trapped in resin during foraging on tree trunks when collecting honeydew.

The mixed character of the Baltic amber ant fauna, including both extant temperate and tropical genera, was already mentioned by Wheeler (1910, 1915). The same can be said about all other late Eocene European ambers (e.g. see Dlussky and Rasnitsyn 2009). The composition of amber fauna can be explained by the climate of the late Eocene, which was equable and has no current analogues (Radchenko and Perkovsky 2021 and references therein).

The ant genera and some species found in late Eocene European ambers that are classified as Holarctic (temperate) are: Formica, Lasius, Temnothorax Mayr, 1861 and Camponotus mengei Mayr, 1858, but Gesomyrmex Mayr, 1868, Yantaromyrmex Dlussky et Dubovikoff, 2013, Dolichoderus robustus Dlussky, 2002 and Bradoponera Mayr, 1868 are classified as tropical (Dlussky 2002a; Dlussky and Rasnitsyn 2009; Dubovikoff 2012; Radchenko and Perkovsky 2021). We cannot attribute the extinct taxa Ctenobethylus Brues, 1939, Monomorium mayrianum Wheeler, 1915 and Plagiolepis klinsmanni to either Holarctic or tropical groups (Radchenko and Perkovsky 2021).

More than 60 ant syninclusions have been found in late Eocene European ambers, and about 20 contain Formica (Dlussky, pers. comm. 2013; Radchenko and Perkovsky 2021). Wheeler (1915) reported three Formica syninclusions: F. flori with Ctenobethylus goepperti (Mayr, 1868), F. flori with Camponotus mengei and F. horrida with Temnothorax gracilis (Mayr, 1868) (names of species and genera are given according to modern taxonomy). Fifteen more Formica syninclusions are now known from Baltic, Bitterfeld and Rovno ambers, and only four include Holarctic species: three with Lasius schiefferdeckeri Mayr, 1868 and one with C. mengei. Five syninclusions represent mixed Holarctic and tropical fauna: one F. flori with Gesomyrmex hoernesi Mayr, 1868, one with D. robustus, one with Bradoponera meyeri Mayr, 1868, one with Yantaromyrmex geinitzi (Mayr, 1868) and one F. gustawi with Y. geinitzi. Finally, six pieces of amber contain F. flori with ants of unknown geographic affiliation: four with C. goepperti, one with M. mayrianum and one with P. klinsmanni.

The genus Formica most likely arose in the early Eocene, diversified in the late Eocene, and its species became dominant in ant communities. Then they continuously evolved to the present and have now become the dominant group of ants in the temperate zone of the Northern Hemisphere.

We are sincerely grateful to Mr Hans-Werner Ribbecke (Mecklenburg–Vorpommern, Germany) for the gift of a holotype specimen of the described species, to Dr Ksenia S. Perfilieva (Moscow State University, Russia) for useful advices, to Dr Kateryna V. Martynova (SIZK, Kiev, Ukraine) for taking images of the holotype specimen, to Dr Sarah C. Crews (California Academy of Sciences, USA) for improving of the language and comments to the ms, and to Dr Julian Katzke and unknown reviewer for valuable comments. This work was supported by a grant NRFU No. 2020/02/0369 (for A. G. Radchenko).