The Fossil Record of Elateridae (Coleoptera: Elateroidea): Described Species, Current Problems and Future Prospects

Simple Summary Beetle fossils play an important role in our understanding of the origin and evolutionary history of this insect order. Despite the recently increasing rate of fossil research focused on the click-beetles (Coleoptera: Elateridae), the major group in the superfamily Elateroidea, their palaeodiversity has still remained largely understudied. In this study, we summarized current knowledge on the click-beetle fossil record with a main emphasis on the described diversity. We compiled an annotated catalogue of all described fossil species in Elateridae, assessed the reliability of their systematic placement, and discuss the current state of knowledge and prospects of research of the fossil record in the group. This study should serve as a comprehensive basis for all subsequent research dealing with the origin, early evolution and diversity of Elateridae. Abstract The Elateridae (click-beetles) are the largest family in Elateroidea; however, their relationships, systematics and classification remain unclear. Our understanding of the origin, evolution, palaeodiversity and palaeobiogeography of Elateridae, as well as reconstruction of a reliable time-calibrated phylogeny for the group, are hampered by the lack of detailed knowledge of their fossil record. In this study, we summarize the current knowledge on all described fossil species in Elateridae, including their type material, geographic origin, age, bibliography and remarks on their systematic placement. Altogether, 261 fossil species classified in 99 genera and nine subfamilies are currently listed in this family. The Mesozoic click-beetle diversity includes 143 species, with most of them described from the Jurassic Karatau, and 118 described species are known from the Cenozoic deposits, mainly from the Eocene North American Florissant Formation and European Baltic amber. Available data on the described past diversity of Elateridae suggest that almost all fossil lineages in this group are in urgent need of revision and numerous Mesozoic species might belong to different families. Our study is intended to serve as a comprehensive basis for all subsequent research focused on the click-beetle fossil record.


Introduction
The click-beetles (Elateridae) are the major family in Elateroidea, comprising more than 10,000 described species worldwide [1]. Despite the efforts of numerous studies using morphological or molecular data, the classification and phylogenetic relationships within the family remain far from fully understood [2][3][4][5][6][7][8][9][10][11]. Taking this into consideration, further development of click-beetle systematics and understanding their evolution would certainly benefit from integrating modern molecular-based methods and morphology with fossils into a combined phylogenetic approach. While our knowledge on the systematics and Remark. Representatives of Limonius and related genera have more elongated elytra and more or less campaniform or parallel-sided pronotum, whereas this species has elytra only slightly elongate, and the pronotum arcuate at sides and sinuate near posterior angles. Such body proportions and shape of thorax are usually found in Cardiophorinae and Negastriinae. Based on the image of prothorax [28], L. aboriginalis is most probably a member of Cardiophorinae; its prosternum is narrow and prosternal sutures almost parallel sided while in Negastriinae the prosternum is rather broad and prosternal sutures curved outward. However, we prefer to postpone any taxonomic changes pending a comprehensive review including the type material.  Limonius florissantensis : 515 [28]. Type material. Two syntypes (one with counterpart), sex unknown, compression fossils, No. 90,473 (USNM).
Remark. Representatives of Limonius and related genera have more elongated elytra and more or less campaniform or parallel-sided pronotum, whereas this species has elytra only slightly elongate, and the pronotum arcuate at sides and sinuate near posterior angles. Such body proportions and shape of thorax are usually found in Cardiophorinae and Negastriinae. Based on the prothorax image in Wickham [28], L. florissantensis is most probably a member of Negastriinae as its pronotum is less globular than in typical Cardiophorinae. However, the type material should be thoroughly examined before any taxonomic change can be made.
Remark. The generic attribution of this species is unclear as it was described based only on elytral characters.
Remark. Generic attribution of this species is doubtful. It might belong to Agrypnini based on the body proportions and the structure of thorax, especially as figured in Figure  6b in Heer [14]. Note, that drawing in Figure 6c in Heer [14] differs considerably in many features from Figure 6b in the same study.  Limonius praecursor : 516 [28]. Type material. Holotype, sex unknown, exoskeleton, compression fossil, MCZ 2730 and 2731 (=9417 and 10,558 in Scudder coll.) (MCZ).
Remark. Representatives of Limonius and related genera have more elongated elytra and more or less campaniform or parallel-sided pronotum, whereas this species has elytra only slightly elongate, and the pronotum arcuate at sides and sinuate near posterior angles. Such body proportions and shape of thorax are usually found in Cardiophorinae and Negastriinae. Based on the prothorax image in Wickham [28], L. shoshonis is most probably a member of Negastriinae as its pronotum is less globular than in typical Cardiophorinae. However, the type material should be thoroughly examined before any taxonomic change can be made.  Limonius volans : 517 [28]. Type material. Holotype, sex unknown, compression fossil, No. 8252 (CUB). Fossil deposit/age. USA: Colorado,Florissant Formation,Florissant,Station 14; Ma (Eocene).
Remark. We keep this genus tentatively under Dimini although Schimmel [91] and Schimmel and Tarnawski [131] placed it to Elaterinae without any explanation. Some characters of A. grandis, e.g., large body size, pointed last ventrite, attenuate elytral apices, transverse scutellar shield, are more typical for species of Oxynopterini rather than Dimini, though A. grandis differs from Oxynopterini in the structure of metacoxal plate and the shape of prosternal process. Unfortunately, this fossil lacks head and legs so many crucial diagnostic characters are absent.  Alaodima grandis Dolin, 1980: 76 [53]. Type material. Holotype, sex unknown, exoskeleton, compression fossil, No. 2066/2970 (part + counterpart) (PIN).
Remark. All fossil species listed under this genus should be re-examined as their body proportions and structure of prothorax resemble more Negastriinae or Cardiophorinae
Remark. This species most probably does not belong to Oxygonus as it differs considerably in the shape of prothorax (almost globular versus more or less elongate, with arcuate sides in Oxygonus) [177].
Remark. The description is based on a part of isolated elytron so the generic attribution of this species is rather problematic. However, based on the elytron reconstruction by Birket-Smith [154], it is really similar to that of Semiotus.
Remark. Fossil species assigned to this genus urgently need a revision. It is possible that most, if not all, species in fact belong to another click-beetle genera. It should be also noted, that many older authors used the name "Elater" in erroneuous way (nec Elater Linnaeus [115]), i.e., for Ampedus (for more information, see Hyslop [111]), so they actually compared these fossil species with Ampedus and not with Elater.  Elater asmodeus Zhang, 1989: 123 [58]. Type material. Holotype, sex unknown, exoskeleton, compression fossil, No. s82747 (SMJS).
Remark. This species superficially (i.e., in the body proportions and the structure of thorax) resembles Dendrometrinae rather than Elaterinae.
Remark. Cockerell [33] suggested that it is not a member of true Elater; however, he was not able to assign it to any other genus. It was listed as "Elateridae sens. l. burmitinus" by Keilbach [133]. The original description and available figure are not enough to make any conclusions about the placement of this species, and the proper study of the type specimen should be carried out in order to exclude the possibility that it is a member of Eucnemidae.  Elater canabinus Zhang, 1989: 124 [58]. Type material. Holotype, sex unknown, exoskeleton, compression fossil, No. 840,105 (SMJS).
Remark. This species is similar to Ampedini in the body proportions, the narrowed campaniform pronotum, the elongate, almost parallel-sided elytra and the slightly serrated antenna. In Elaterini, the pronotum is usually wider, elytra not parallel sided, often somewhat wedge-shaped, and antenna more serrated. However, we prefer to keep this species tentatively in Elater until the holotype is examined in detail.
Remark. This species was considered by Larsson [48] more similar to Limonius than to Elater. Type material should be studied in order to confirm the placement of this species.

Elater scudderi
Remark. Wickham [28] already mentioned that this species probably does not belong to true Elater. Indeed, it has notably arcuate pronotum, with small posterior angles which is not a character usually found in Elaterinae. The generic attribution of E. scudderi needs further investigation.
Remark. The generic attribution of this species is unclear as it was described based only on elytral characters.
Remark. The generic attribution of this species needs re-examination. Since this species has a very small body size and a rather unusual shape of body and prothorax (including a lack of the Synaptini-characteristic lateral basal indentations of pronotum), it might belong to another genus than Glyphonyx.
Remark. The description of this species is based on a part of isolated elytron and, therefore, its generic attribution is rather problematic. It was originally described in Curculionites Heer, 1847 (Curculionoidea) and only later transferred to Lissomus in Elateridae [154]. Based on the reconstruction by Birket-Smith [154], it seems that this species belongs rather to Dendrometrinae than Lissominae. Legalov [199,200] kept this species in original genus in Curculionoidea in the catalogues of fossil curculionoids. The identity of this species should be confirmed by study of the type material. 3.5.2. Tribe Protelaterini Schwarz, 1902* Protelateridae Schwarz, 1902. Type genus: Protelater Sharp, 1877: 482 [202]. For more information, including synonyms, see Kundrata et al. [94,109].
Remark. In his revision of Hypnoidinae, Stibick [164] examined Cryptohypnus exterminatus and suggested that it does not belong to Hypnoidinae. Instead, he suggested its placement in Negastriinae, close to Negastrius pectoralis group (=genus Paradonus sensu Stibick [205]), based on the "overall sculpture" and non-striate elytra. This relationship was already mentioned by Wickham [28] in the original description of C. exterminatus. Here, we formally place this species in Paradonus.
Remark. This genus needs a revision since some species differ from the type species (and also from each other) in the body proportions, the shape of antenna, thorax, etc. The systematic placement of all species should be re-evaluated since they might represent Eucnemidae, based mainly on the presence of short and broad pronotum and enlarged metacoxal plates.

Tribe Hypnomorphini
Remark. This tribe needs a revision as it most probably includes various unrelated groups.
Remark. This species strongly differs from its congeners in the presence of long sublateral carinae on pronotum, the short incision of posterior edge of pronotum, and less elongated elytra. It is probable that C. carinatus belongs to another genus.

Elaterophanes acutus
Remark. The description of this species is based on an isolated elytron and, therefore, its generic attribution is rather problematic [212].
Remark. Brodie [13] described Elater vetustus based on a compression fossil from Lilstock Formation, Apperley (Triassic) in the United Kingdom. Heer [17] described Elaterites vetustus based on a single elytron from Schambelen, Aargau (Jurassic) in Switzerland, but he attributed the species name to Brodie. It is not clear whether Heer [17] thought that the fossil from Switzerland was conspecific with the Brodie's Elater vetustus from Apperley beds of the United Kingdom, and if/why he transferred that species from Elater to Elaterites. Later, Handlirsch [26] transferred Elater vetustus Brodie, 1845 to Elaterophanes (as he did with E. socius , and he erected a new genus, Dysarestus Handlirsch, 1906, for Elaterites vetustus Heer, 1865. He suggested that it might belong to Elaterophanes. Here, we follow Carpenter [68], who classified Dysarestus in Coleoptera incertae sedis.
Remark. This genus might belong to Eucnemidae as all its species have a compact body, with a short and broad thorax, and short elytra. Unfortunately, the main diagnostic characters [99] are either absent or not well visible on the original drawings or photographs [53].
Remark. This genus needs a revision since some species differ from the type species (and also from each other) in the body size, proportions, the shape of pronotum, elytra, etc. Most species probably belong to Eucnemidae based on the compact body, with a short and broad thorax, and short elytra. Additionally, H. rasnitzyni has antennae with last three antennomeres enlarged, which is a character present in Eucnemidae rather than Elateridae. Unfortunately, the main diagnostic characters [99] are usually absent or not well visible on the original drawings or photographs [51,53].
Remark. Based on the habitus and shapes of antenna and pronotum, this species most probably belongs to Eucnemidae.
Remark. Species of this genus strongly resemble Eucnemidae in the compact body, with wide prothorax and relatively short elytra, and also Throscidae in almost trapezoidal pronotum. Unfortunately, the main diagnostic characters [99] are either absent or not well visible on the original drawings or photographs [53]. Additionally, L. innatus is most probably not congeneric with other two species.
Remark. This species differs considerably from its congeners in the large body size, shape of prosternum and hypomeron, less developed longitudinal sutures on prosternum, and more broadened metacoxal plates. Its systematic placement should be re-evaluated after the study of the type material.
Remark. This genus needs a revision since some species differ from the type species (and also from each other) in the body proportions, the shape of pronotum, elytra, etc. Most species probably belong to Eucnemidae based on the compact body, with a short and broad thorax, and short elytra. Only L. tarbinskyi Dolin, 1980 looks like a typical elaterid. Unfortunately, the main diagnostic characters [99] are usually absent or not well visible on the original drawings or photographs [53].
Remark. This species resembles Cardiophorinae in having the strongly arcuate sides of pronotum, with posterior angles short and curved inwards.
Remark: This genus might belong to Eucnemidae because its type species, L. handlirschi, along with L. minutus , have a compact body, with a short and broad thorax, and short elytra. Unfortunately, the main diagnostic characters [99] are missing in the original figures [53], and, therefore, the taxonomic decision should be postponed until the type material is studied. Additionally, L. incertus Dolin, 1980 has antennae with last three antennomeres enlarged, which is a character present in Eucnemidae rather than Elateridae. Lithoptychus carinatissimus Dolin, 1980 externally also resembles Eucnemidae but it has very conspicuous sublateral carina on each side of pronotum. Literature. Dolin (1980: 59): original description [53]; Korneev and Cate (2005: 13): checklist [120].
Remark. The systematic placement of this genus needs to be re-evaluated since its type species might actually represent Eucnemidae.
Remark. This species probably belongs to Eucnemidae due to its compact body, with a short and broad thorax, short elytra, and antennae with last three antennomeres enlarged. However, since the main diagnostic characters [99] are not well visible in the original figures [53], we prefer to postpone any taxonomic decision until the type material is examined in detail.
Remark. Generic attribution of this species needs re-examination as it strongly differs from the type species of Lithosomus in the elongated pronotum, with slightly arcuate sides (pronotum short, broad, strongly campaniform in L. erosus), more elongated elytra (elytra rather short in L. erosus), more broadened metacoxal plates which are only slightly narrowed outwards (metacoxal plates notably narrowed outwards in L. erosus), and larger punctures in elytral striae (small punctures in L. erosus).
Although the body proportions of L. longicollis somewhat resemble Cardiophorinae or Negastriinae, with almost parallel-sided prosternal sutures and only slightly broadened prosternum being more typical for Cardiophorinae, there are no reliable characters that would point us to the proper systematic placement of this species.
Remark. This genus might represent Cardiophorinae based on the globose pronotum, with short posterior angles, and a thickened and short prosternal process.
Remark. Species of this genus resemble Eucnemidae in having a broad prothorax with short pronotal posterior angles and a short prosternal process. Available figures in Dolin [53] also suggest that the type species, N. tenuis, along with N. globicollis , have a pedicel subapically attached to scape, a condition typical for Eucnemidae [99]. Basal antennomeres in remaining two species are absent in figures [53]. Systematic placement of Negastrioides needs further investigation.
Remark. This genus might belong to Eucnemidae as all its species have a compact body, with a short and broad thorax, and short elytra. Illustrations of antennae also support this hypothesis [99], especially those of P. longicornis ; however, they should be examined directly on the type material.
Remark. All species other than the type species, P. reflexicollis, resemble Eucnemidae in having a compact body, with a short and broad thorax, and short elytra. Available figures of antennae also support this hypothesis. Additionally, P. figeratus  shares almost trapezoidal pronotum, with Throscidae. Systematic position of those species should be re-evaluated after study of the type material.
Remark. This species notably differs from its congeners, especially in the body proportions and the shape of metacoxal plates.
Remark. Based on the original description and available images [75], we cannot exclude the possibility that this genus belongs to Eucnemidae due to the compact body, with a short and broad thorax. The longitudinal sutures on prosternum are obviously situated close to pronotosternal sutures, so they might be just deeply furrowed pronotosternal sutures as in some Eucnemidae. The presence of a sublateral carina on pronotum, which is a character typical for Eateridae rather than Eucnemidae, should be re-evaluated on the type material. Pollostelater baissensis Alekseev, 2011: 424 [75]. Type material. Holotype, sex unknown, exoskeleton, impression, No. 3064/7100 (PIN).
Remark. Species of this genus externally resemble Negastriinae but their placement in Elateridae should be rather confirmed by a study of type material. There are inconsistencies in original descriptions and corresponding images in Dolin [53] regarding the presence or absence of sublateral carinae in pronotal posterior angles.
Remark. Muona et al. [99] suggested that this genus may represent an unknown lineage of Eucnemidae rather than Elateridae. They based their conclusions on the available literature and figures but the type material should be studied to solve the placement of Clavelater within Elateroidea. Nevertheless, we agree with the conclusions made by Muona et al. [99].
Remark. This genus might belong to Eucnemidae as both its species have a compact body, with a short and broad thorax, and relatively short elytra [51]. The type material needs to be checked for characters distinguishing (with more or less certainty) Elateridae from Eucnemidae [99].
Remark. This genus needs a revision since some species differ from the type species (and also from each other) in the body proportions, the shape of pronotum, elytra, etc. Lithomerus cockerelli (type species) and L. brevicollis  are probably correctly assigned to Elateridae but other species from the Palearctic deposits currently assigned to this genus might belong to Eucnemidae as they have a compact body, with a short and broad thorax, and short elytra. Lithomerus wunda Martin, 2010 from Australia is most probably a member of Throscidae [99].
Remark. This species was mentioned by Dolin [53] only in the key on page 23 but the author obviously forgot to include the usual description which was otherwise available for all other new species in that paper. The holotype was figured in Figure 9 (drawing) and Plate I, Figure 3 (photograph), its collection number was mentioned in the figure legends, and the depository was specified in the introductory part of the paper [53]. The character states mentioned in the key on page 23 in Dolin [53] fulfill the requirements of Art. 13.1.1 [129] and, therefore, L. brachycollis is an available name.
Remark. Muona et al. [99] suggested that this species might belong either to Elateridae: Protagrypninae or Eucnemidae: Palaeoxeninae but they did not study the holotype.
Remark. This species is obviously not conspecific with any other Lithomerus species. Based on the figures in the original description [73], Muona et al. [99] suggested that it should be placed within Throscidae based on the body size and the shape of antenna resting in the antennal groove. They postponed its transfer to Throscidae until the holotype is examined.
Remark. We cannot exclude the possibility that this genus might belong to Eucnemidae based on a short and broad prothorax with short posterior pronotal angles. Unfortunately, the main diagnostic characters [99] are either absent or not well visible on the images [53] and so the type material should be examined to confirm the placement of Micragrypnites in Elateridae.
Remark. Muona et al. [99] examined this genus and concluded that it should be retained in Elateridae rather than transferred to Eucnemidae or Throscidae. They supported their conclusion by several morphological features, including antennomere II being attached apically to antennomere I. However, on the figures provided by Chang et al. [79], the antennomere II is attached somewhat subapically to antennomere I, and what is more, three apical antennomeres are enlarged, which is a character usually found in other clicking elateroids than Elateridae. The shape of prothorax, with broad and stout pronotum and anteriorly truncate prosternum, is also pointing to Eucnemidae. Another interesting character in this genus is a presence of sublateral carinae on pronotum, described and figured in Chang et al. [79]. These usually run from posterior angles more or less subparallel with sides. Muona et al. [99], however, wrote that "pronotal hind angles seemed to lack carina". Here, we follow Muona et al. [99], who studied the type material of the only species classified in Paraprotagrypnus, and retain this genus in Protagrypnini as originally proposed.
Remarks. In the original description of the genus, Dolin erroneously used the name Praelaterium Dolin, 1973 and vice versa, Protagrypnus was used in the original description of Praelaterium. However, species names were used correctly in both cases ( [50], pp. 75,78). Species in Protagrypnus do not seem to be congeneric. They differ considerably in body size, shape of pronotum and elytra, and shape and proportions of prosternum and hypomeron. We cannot exclude the possibility that the type species belongs to Eucnemidae.
Remark. This species was examined by Muona et al. [99], who concluded that it should retain in Elateridae: Protagrypninae.
Remark. This genus might belong to Eucnemidae due to its relatively short and broad thorax, and short posterior angles of pronotum. Unfortunately, the main diagnostic characters [99] are either absent or not well visible on the original figures [53].
Remark. Based on the figure in original description, this genus might belong to Buprestidae rather than Elateridae. Therefore, the type material should be examined to confirm the proper family placement of Adocetus.
Remark. Muona et al. [99] suggested that this genus belongs to Elateridae. Brief description of Artinama does not enable its placement even to a subfamily level. According to the images in Dong and Huang [81], Artinama resembles members of tribe Agrypnini (note that the drawing in that publication does not fully correspond with the photograph).
Remark. This genus superficially resembles Selatosomini in having the frontal carina obsolete, the antenna weakly serrate, with elongated antennomere III, the pronotum rather broad, arcuate at sides and sinuate near hind angles, the posterior angles of pronotum moderately long, with sublateral carina, the prosternal lobe well developed, and elytra more or less ellipsoidal.
Remark. We cannot exclude the possibility that this genus might belong to Eucnemidae.
Remark. Wickham [28] suggested that the body form of this genus is similar to Cryptohypnus Eschscholtz, 1830(=Hypolithus Eschscholtz, 1829; Dendrometrinae: Hypnoidini) and the metacoxal plates are similar to those of Agriotes Eschscholtz, 1829 (Elaterinae: Agriotini). Based on the body size and proportions, and the more or less narrow prosternum with pronotosternal sutures almost parallel sided, this genus might belong to Cardiophorinae. However, its potential affinities to Eucnemidae should also be taken into account.
Remark. Species in this genus superficially resemble Selatosomini in the frontal carina obsolete, the antenna more or less weakly serrate, the pronotum rather broad, arcuate at sides and sinuate near posterior angles, the posterior angles of pronotum moderately long, with sublateral carina, and elytra more or less ellipsoidal.
Remark. Based on , there are more species included in the type series of C. buffoni. The specimen in Figure 10 has a well developed prosternal lobe, while specimens in Figures 4,6 and 8 have anterior portion of prosternum truncate. Additionally, the shape of elytra is also different among the figured specimens.
Remark. Muona et al. [99] examined this genus and confirmed that it belongs to Elateridae. However, based on the available morphological evidence, its position remains unclear, although it superficially resembles Selatosomini in several aspects including the morphology of head, antennae, and prothorax [84].
Remark. The description of the type species of Elateridium is based on an isolated elytron, which makes the systematic placement of this genus rather problematic. Both species described by Dunstan [39] which are listed below should be excluded from Elateridae.
the serrate antennae, large eyes, shelf-like supra-antennal carina, elongate posterior angles of pronotum, and open mesocoxal cavities. Additionally, Mionelater has a pronotum more elongated than in typical Cardiophorinae, and almost not arcuate at sides, and according to the description [46], the prosternal process in this genus is long and rather narrow, while in Cardiophorinae, it is rather short and thick. The combination of characters, such as small body size, shape of supra-antennal carina, pronotum rather large compared to elytra, and elongate posterior angles of pronotum, suggests that Mionelater might be related to Hypnoidini. However, we keep it tentatively without a subfamilial assignment until further, more detailed, study is conducted. Mionelater planatus Becker, 1963. Type material. Holotype,probably male,exoskeleton,amber inclusion,No. 12,734 (UCMP).
Remark. The placement of this genus in Elateridae is uncertain and needs further research [78,81,219].
Remark. Species of this genus do not seem to be congeneric. What is more, the type material should be studied in order to confirm that they belong to Elateridae and not to other clicking elateroid lineages, especially Eucnemidae.
Remark. Protocardiophorus jurassicus is morphologically similar to the species of Idiomerus which were transferred to Cerophytidae [77,218].
Remark. This genus needs a revision since some species differ from the type species (and also from each other) in the body proportions, the shape and structure of thorax, etc. We cannot exclude the possibility that at least some species currently classified in this genus might belong to Eucnemidae.
Remark. This genus needs a thorough re-examination as it is not clear whether it belongs to Elateridae [12,68]. Some authors suggested that it might belong to Artematopodidae [221,222], but it also resembles Cerophytidae by the habitus, shape of antennae, and structure of head and thorax.
Remark. This genus probably belongs to Dendrometrinae based on the morphology of prothorax.

Discussion
In this study, we summarized information on all described fossil species in Elateridae. Altogether, 261 fossil species classified in 99 genera and nine subfamilies are currently listed in this family. Nevertheless, our results show that our knowledge of click-beetle palaeodiversity varies widely with respect to systematic, spatial and temporal elements.
The highest diversity of fossil Elateridae lies in the only exclusively fossil subfamily Protagrypninae [12,51,53]. It contains 94 species in 31 genera classified in four tribes (Table A1), which is more than a third of the described species diversity of fossil Elateridae. Not surprisingly, the next most diverse subfamilies include Agrypninae (13 genera/35 spp.), Dendrometrinae (11/35) and Elaterinae (13/29), which are three most species-rich extant click-beetle subfamilies based on numbers of described species [1,228]. The remaining subfamilies are represented only by a few species each. It should be noted, however, that another 50 species are currently considered incertae sedis, without a subfamily assignment (Table A1).
Regarding the geographic origin of fossil click-beetle species, the highest diversity comes from Eurasian deposits. By far the richest locality is the famous Late Jurassic Karatau in Kazakhstan from which 100 species have been described in 29 genera [51][52][53]. Other, at least moderately rich localities, include the Jurassic Daohugou in Inner Mongolia of China (4 genera/4 spp.), the Lower Cretaceous Yixian Formation in northeastern China (7/10) and the Zaza Formation of Baissa in Siberia (2/6), and the Miocene Shanwang Formation in eastern China (2/6). In Europe, the highest diversity of click-beetles has been described from Eocene Baltic amber, which has been redeposited from its original stratigraphic positions mainly in marine sediments and fluvial deposits, and contains the most diverse assemblage of fossil insects to date, including 17 click-beetle species classified in 16 genera [47,48,229]. Other important localities include the world-famous Eocene Grube Messel Pit (2 genera/11 spp.) and the Miocene Öhningen within the Upper Freshwater-Molasse Formation in Germany (8/10). The world-famous Florissant in Colorado, USA (Eocene) is the richest deposit in North America, with 38 click-beetle species classified in 17 genera [28,29]. Five species in five genera are known from the Eocene Green River Formation in USA, and four species in three genera were described from Mexican "Chiapas" amber, which is Miocene in age [46,61]. Several fossil click-beetle lineages were reported also from South America. However, the genera Babuskaya Martins-Neto and Gallego, 2009, Cardiosyne Martins-Neto and Gallego, 2006, and Gemelina Martins-Neto and Gallego, 2006 from the Mesozoic Argentinian deposits [71,72] were recently transferred from Elateridae to Coleoptera incertae sedis [12], and two Paleocene species were described based on elytra only, and their placement in Elateridae is dubious [35]. The Australian click-beetle fossil fauna includes five species in four genera from Mesozoic deposits; however, at least four species highly probably do not belong to Elateridae (see, e.g., Muona et al. [99]). Nevertheless, considering the relatively unique and rich extant Australian click-beetle fauna [228], as well as the fact that at least some lineages were among the early splits in the elaterid phylogeny [5,11], a Mesozoic fossil record of that family in Australia would make sense. Indeed, Oberprieler et al. [98] reported a possible undescribed elaterid from the Jurassic Talbragar Fish Bed but this record needs further investigation. Thus far, there are no fossil Elateridae described from African deposits.
Our knowledge of the click-beetle fossil record should help us to better estimate the origin of the group and understand the evolutionary changes throughout its history. Based on the available data, it is possible that the Elateridae originated as early as the Triassic; however, this has to be confirmed by further research on the already described specimens and new material from Triassic deposits from around the globe. Currently, six described species are reported from the Triassic, most of them of highly doubtful family attribution, especially those from the Australian Blackstone Formation [39]. The highest diversity of fossil Elateridae is reported from the Jurassic (113 species/39 genera/five subfamilies), with the vast majority of lineages described from the rich Karatau deposit [51][52][53]. Only 24 species were described from Cretaceous localities, mainly from China and Russia, but we can expect that many more species will be discovered in Burmese amber in the near future. While 143 click-beetle species are recorded from the Mesozoic, only 118 described species are known from Cenozoic deposits. Most of them were from the Eocene Epoch, mainly due to the rich sources such as the North American Florissant Formation and European Baltic amber. It should be noted that all Mesozoic click-beetle species belong to fossil genera, i.e., those in which no extant representatives are included, with the apparently wrongly classified Burmese-amber species Elater burmitinus and "Acmaeodera" burmitina being exceptions [32,33]. On the other hand, slightly more than half of the elaterid genera known from the Paleogene (i.e., Paleocene, Eocene and Oligocene) also contain extant species, and among 12 genera known from the Neogene (Miocene) only two include exclusively fossil species.
Although our current study is the first comprehensive overview of described fossil species in Elateridae and is intended to serve as a solid basis for all future studies of the clickbeetle fossil record, it is an annotated catalogue rather than a taxonomic revision. Therefore, it must be treated with caution and interpreted carefully. Our results clearly show that the major problem with the click-beetle fossil record lies in the highly questionable family placement of many lineages, incorrectly interpreted morphological characters for fossil higher click-beetle taxa, and dubious or sometimes clearly erroneous generic assignments of many species across the whole classification of Elateridae. Incorrect identification of Mesozoic specimens can obscure our understanding of the origin of Elateridae and greatly affect the accuracy of the dating of phylogenetic trees in various studies. Click-beetle fossils from the rich Karatau deposit were used as one of the calibration points for a recent Coleoptera phylogeny [230], and Elaterophanes was used as an important calibration point in a dated molecular phylogenetic analysis of Elateroidea [231] or even of the whole of Coleoptera [232]. Kusy et al. [10] showed that analyses using different datasets, applied models and calibrations often come to different age estimates for the major splits as well as for the origin of bioluminescence in Elateroidea.
Detailed investigation of the family placement of most Mesozoic taxa currently assigned to Elateridae is crucial for our understanding of the origin, early evolution and past diversity of the group. One of the problems is that some of the oldest fossils are known only from a single elytron or its fragment and therefore, their placement even to a (super) family remains questionable [31,37,39]. Another problem is the uncertain family placement of many compression fossils from the Mesozoic Asian deposits, mainly from China and Karatau in Kazakhstan. The systematic placement of some Chinese click-beetles was already questioned in several [86,233,234]. Recently, Muona et al. [99] studied the Mesozoic clicking Elateroidea from Chinese deposits and discussed the external characters for recognizing Eucnemidae from other clicking elateroids, especially Elateridae. They showed that only about a third, i.e., 12 of 27, described fossil click-beetle species from China can be attributed to Elateridae with more or less certainty. One species was transferred to Throscidae, six to Eucnemidae, three could be either Elateridae or Eucnemidae based on the available characters, and five could not be studied due to the unavailability of the type material. As correctly pointed out by the authors [99], this drastically changed our view of the Mesozoic clicking elateroid fauna in China. Moreover, a similar or even more dramatic situation may occur after the putative click-beetle taxa from the extremely rich Karatau deposit are re-examined in detail based on the type material. Dolin [51][52][53] focused on that deposit and reported from there an exceptionally high diversity of Elateridae, with 107 described species in 31 genera from five subfamilies. Interestingly, the vast majority species from Karatau are classified in the only fossil elaterid subfamily, Protagrypninae (Table A1). It was proposed by Dolin [51] for the earlier defined tribe Protagrypnini, which he originally described based on two genera from the Dzhil Formation in Kyrgyzstan and placed it in Agrypninae [50], and two other tribes, Desmatini and Hypnomorphini, from Karatau. He defined the subfamily based on the presence of longitudinal furrows (sutures) definining a medial field on the prosternum, a transverse suture on the mesoventrite, and the additional division of the apex of the radial cell on the hind wing [51,53]. However, it was evident even from the descriptions and illustrations that many of the species originally assigned to the click-beetle subfamily Protagrypninae may in fact represent some other clicking elateroid lineages. Indeed, Chang et al. [77] removed several species of one genus from Hypnomorphini to Cerophytidae, and many other species with potentially eucnemidor throscid-like characters should be re-examined (see Muona et al. [99] for extensive discussion on such characters, and remarks under various protagrypnine taxa in the overview of fossil species above). The diagnosis and monophyly of Protagrypninae are questionable, and especially problematic is the inclusion of Desmatini, which do not fully fit into the subfamily diagnosis as their representatives lack a clearly defined transverse suture on the mesoventrite [51,53]. They also have considerably broadened metacoxal plates, a character common in Eucnemidae ( [86,234] but see [99]). Potential transfer of a number of taxa currently listed in Elateridae to Eucnemidae or Throscidae would make sense considering the earlier origin and longer evolutionary history of eucnemids, cerophytids and throscids [230,231,235]. Indeed, many recent studies confirm a high diversity of these families in the Mesozoic fossil record [96,99,218,233,234,[236][237][238][239][240]. On the other hand, there might be some opposite cases. For example, Alekseev [74] described the monotypic genus Cretopoena from the Lower Cretaceous of Mongolia and attributed it to Eucnemidae; however, Li et al. [234] treated it as Elateriformia incertae sedis due to the lack of characters clearly separating Eucnemidae and Elateridae. Based on the general body shape and the structure of thorax this genus might indeed belong to Elateridae. Moreover, its thorax and elytra bear strong granulation which might be possible traces of scale-like setae typical for the Agrypnini. However, Cretopoena clearly differs from Agrypnini by its closed pronotosternal sutures and, therefore, its systematic position should be further investigated. Additionally, the suggested close relationship between Elateridae and recently discovered Mysteriomorphidae from the Cretaceous Burmese amber needs to be investigated using an analytical approach [241,242].
After the family placements of Mesozoic (and also younger) taxa are investigated in detail, it would be important to classify all fossil species to the proper genera and subfamilies. This will, however, be hampered not only by the lack of visible diagnostic characters on fossil specimens but also by the constantly changing definitions of the higher taxa and apparently unstable suprageneric classification of Elateridae [1,6,11,228]. The presence and character of the putative median plate-like structure on the prosternum laterally defined by furrows or sutures in Protagrypninae was discussed and questioned by Muona et al. [99] who concluded based on a study of fossil and extant specimens of clicking elateroids that it might be a place which is abruptly lower than the surrounding portions of the prosternum. The second important character, i.e., the transverse suture on the mesoventrite, which mainly defines Protagrypnini and Hypnomorphini, may in fact represent the line between the mesoventrite body and the depressions on the anterior edge of the mesoventrite and mesanepisternum, which are common in clicking elateroids and for which we use the terms "procoxal rests" [243] or "anterior articulating surface" [244]. The secondarily divided radial cell in the hind wing venation needs further investigation as this character is usually difficult to observe in compression fossils. Among the Cenozoic click-beetle fossils, those which are in the most urgent need of revision are species described by Scudder [19] and Wickham [27,28], especially those from the Green River and Florissant Formations [245], and species described by Heer from Öhningen [14], for which similarly inaccurate generic attributions were also reported in other beetle families [246,247]. Of special interest are the waste-basket genera such as Elater or Ctenicera. Further, maximum effort should be put into the study of the genera incertae sedis which include 50 species from various geological ages (Table A1), and also into description of the as-yet formally undescribed click-beetle fossils reported from various deposits [229,248,249], as they may provide further information on the diversity of the main click-beetle lineages.
Last but not least, Elateridae students should pay special attention to the study of fossils included in amber deposits. Although the research of beetles (and other taxa) from various ambers is nowadays very popular [92,188,189,[250][251][252] and scientists were even able to describe within a short time span several new beetle families based on amber material [241,[253][254][255][256][257], the diversity of click-beetles in fossilized plant resins has been highly understudied. A study of the Elateridae diversity in amber is of great importance due to the three-dimensional preservation of specimens which allows us to compare the fossil fauna with extant specimens in much greater detail than in the case of compression fossils [188,229]. With the use of modern techniques such as micro-CT, researchers are able to reconstruct the morphology of a particular beetle even when the imporant diagnostic characters are obscured by opaque bubbles or suboptimal body position [94,242,258]. Regarding the clickbeetle diversity in amber, only a few formally undescribed Elateridae were reported from Cretaceous Lebanese and Oligocene/Miocene Dominican ambers [54,95,188,253]; personal observations of authors]. Becker [46] and Zaragoza Caballero [61] described four species from the Miocene Mexican "Chiapas" amber. Cretaceous Burmese amber contains a high number of Elateridae from various lineages [249,259]; personal observations of authors] but only two species were described by Cockerell [33] (although one originally in Bupestidae) and a single species by Otto [96]. Elateridae were among the most-represented beetle families in Eocene Baltic amber [30,48]; hence it is not surprising that most click-beetle species were described from that amber, mainly due to the work by Iablokoff-Khnzorian [47]. However, the vast majority of click-beetles known from all ambers remain undescribed.

Conclusions
An understanding of the origin, evolution, and past diversity of click-beetles is hampered by the lack of detailed knowledge on their fossil record. We summarized the current knowledge on all described fossil species in Elateridae, and assessed each species based on its description and available illustrations to conclude whether its position in Elateridae and its generic attribution can be considered reliable or not. Our results suggest that the Triassic records based largely on isolated elytra are mostly dubious and may belong to different beetle families, and numerous Jurassic and Cretaceous lineages currently listed in Elateridae might belong to Eucnemidae or Throscidae. The vast majority of the fossil click-beetle species are in urgent need of revision, and the incertae sedis genera should be investigated to correctly assign them to subfamilies and tribes. We can expect many more lineages to be discovered mainly from the more and more intensively studied amber inclusions, especially from Eocene Baltic amber and Cretaceous Burmese amber, which both include a relatively high proportion of elaterid fossils.
Author Contributions: Conceptualization, R.K.; investigation, R.K., G.P., A.S.P. and J.H.; writingoriginal draft, R.K. and G.P.; supervision, R.K.; project administration, R.K.; writing-review and editing, R.K., G.P., A.S.P. and J.H.; funding acquisition, R.K., J.H. and A.S.P. Results of this study were partly used in the unpublished bachelor thesis of G.P. supervised by R.K. All authors have read and agreed to the published version of the manuscript.