The first mainland European Mesozoic click-beetle (Coleoptera: Elateridae) revealed by X-ray micro-computed tomography scanning of an Upper Cretaceous amber from Hungary

Fossil bioinclusions in amber are invaluable source of information on the past evolution and diversity of various organisms, as well as on the paleoecosystems in general. The click-beetles, Elateridae, which originated and greatly diversified during the Mesozoic, are mostly known from the adpression-like fossils, and their diversity in the Cretaceous ambers is only poorly documented. In this study, we describe a new click-beetle based on an incomplete inclusion in ajkaite, an Upper Cretaceous (Santonian) amber from the Ajka Coal Formation from Hungary. We used X-ray micro-computed tomography scanning to reconstruct its morphology because it is deposited in an opaque piece of amber. Our results suggest that the newly described Ajkaelater merkli gen. et sp. nov. belongs to subfamily Elaterinae. It represents the first Mesozoic beetle reported from Hungary, and the first Mesozoic Elateridae formally described from mainland Europe. Our discovery supports an Eurasian distribution and diversification of Elaterinae already in the Cretaceous. The paleoenvironment of the Ajka Coal Formation agrees well with the presumed habitat preference of the new fossil taxon. The discovery of a presumably saproxylic click-beetle shed further light on the yet poorly known paleoecosystem of the Santonian present-day western Hungary.

www.nature.com/scientificreports/ a fraction of amber sites in France 15,17 , most probably due to a high proportion of the opaque amber stones, in which the inclusions can be detected only using X-ray radiographic and tomographic imaging techniques 15 . Upper Cretaceous amber-bearing sedimentary units in Hungary include the alluvial floodplain deposits of the Csehbánya Formation nearby Iharkút and the Ajka Coal Formation (Ajkacsinger) southeast from the city Ajka in the Bakony Mountains in southwestern Hungary (Fig. 1), both being Santonian in age. Despite more than a thousand of amber pieces known from the former locality, no inclusions have been found in these dominantly small (up to 3 mm), drop-like pieces so far. On the other hand, the unique type of amber from the brown coal beds of Ajka, so-called ajkaite 18 , contains many bioinclusions.
The Ajka Coal Formation, being at some places over 100 m thick 19,20 , comprises an alternation of coal beds, carbonaceous to argillaceous pelitic sediments with interbedded molluscan lumachelles, marls, and sandstone beds representing a lacustrine-palustrine sequence 21 . It has a well documented fossil flora [22][23][24][25][26] , and fauna of mollusks 27,28 and vertebrates 29 . Despite a relatively large number of available amber stones from the Ajka deposits, the record of the ajkaite inclusions is still rather scarce. Short mentions on the inclusions in ajkaite are known since the middle of the twentieth century 30,31 . Regarding the arthropods, apart from an officially unpublished MSc thesis 32 , there is only one detailed study by Borkent 33 describing two species of ceratopogonid flies. Obviously, many other ajkaite arthropods belonging to Arachnida, Diptera, Hymenoptera and Coleoptera are waiting to be formally described 34 .
The Elateridae, commonly known as click beetles, are the largest and most diverse family in the superfamily Elateroidea. Adult individuals can be usually recognized by their elongate, narrow body, structure of basal antennomeres, pro-mesothoracic clicking mechanism, acute posterior angles of pronotum, and five abdominal ventrites, of which four are connate and the last one is free [35][36][37] . Their most characteristic feature is their ability to jump into the air by rapidly sliding their prosternal process into their mesosternal cavity, with a typical clicking sound, giving their common name 35,37 . The family currently includes more than 11,000 described species worldwide 37,38 and the fossil record includes 261 species in 99 genera 39,40 . Although they are quite common in  www.nature.com/scientificreports/ various amber deposits worldwide, only a few of them have been described so far 40 . Regarding the Cretaceous click-beetles in amber, only three species have been described to date, all from the Burmese amber 41,42 .
In this study, we describe a new click-beetle inclusion in ajkaite, which represents the first Mesozoic beetle reported from Hungary as well as the first Mesozoic Elateridae described from the mainland Europe. Since this incomplete specimen is deposited in a non-transparent piece of amber, we had to use X-ray micro-computed tomography scanning to reveal its morphology. We discuss the probable ecology of a new click-beetle in connection with the Ajka Coal paleoenvironment.

Results
We describe here Ajkaelater merkli gen. et sp. nov. (Figs. 2, 3, 4, 5, Supplementary File 1) from Santonian deposits of Ajka based on an incomplete specimen embedded in amber. Based on its habitus and morphology of prothorax, we place it into the click-beetle subfamily Elaterinae, without a tribal assignment (see "Discussion" section).

Diagnosis.
Adult. Body small, oblong-ovate, presumably about 3.25 times as long as wide. Antenna slightly serrate, presumed antennomeres VI-X about twice as long as wide. Pronotum slightly wider than long when measured along midline, widest at posterior angles; lateral sides weakly rounded; posterior angles sharp, slightly divergent; sublateral carina distinct, short; lateral carina distinct, complete; both lateral and sublateral carinae divergent anterad in lateral view. Pronotosternal sutures slightly convergent posterad, then distinctly curved near procoxal cavities. Prosternum elongate and rather narrow, including prosternal process about twice as long as wide, widest near procoxal cavities; prosternal process relatively long, subparallel-sided, abruptly narrowed before apex in lateral view so that apex is on different plane than rest of prosternum. Scutellar shield longer than wide, with anterior margin slightly rounded and steeply declivous. Mesoventral cavity with sides subparallelsided for greater part of their length, narrowing slightly between mesocoxae. Elytra oblong-ovate, slightly wider than prothorax, sides slightly rounded; each elytron with distinct impressed striae formed by lines of large punctures. For more details, see the description of A. merkli gen. et sp. nov. below, Figs. 2, 3, 4, 5, and Supplementary File 1.
Etymology. Derived from the words "Ajka" (referring to the city of Ajka, the geographic origin of the fossil) and "Elater" (a genus name in Elateridae). Gender: masculine.   www.nature.com/scientificreports/ slightly excavate anteriorly (Fig. 3d). Prosternum (Figs. 2b, 3c,d) elongate, including prosternal process about twice as long as wide, in front of procoxal cavities (i.e., excluding prosternal process) about 1.30 times as long as wide, widest near procoxal cavities; anterior part not well preserved. Prosternal process (Fig. 3d) relatively long, about 0.55 times as long as prosternum in front of procoxal cavities, subparallel-sided, abruptly narrowed before apex in lateral view so that apex is on different plane than rest of prosternum (Fig. 4); apex narrowly rounded. Procoxal cavities subcircular, moderately widely separated by width of prosternal process. Scutellar shield (Figs. 2a, 3b) about 1.50 times as long as wide, anterior margin well defined, angulate, slightly rounded and steeply declivous; lateral sides sinuate, narrowed before middle and widest after middle; apex rounded. Mesoventral cavity deep, with well-defined walls; sides subparallel for greater part of their length, narrowing slightly between mesocoxae. Mesocoxal cavities narrowly separated. Elytra (Fig. 2a) oblong-ovate, slightly wider than prothorax, sides slightly rounded; each elytron with nine distinct impressed striae formed by lines of large subcircular punctures, which are separated by about diameter of a puncture; interstriae flattened, smooth. Epipleuron (Fig. 2c) well developed, wide basally, then gradually narrowed near metacoxae. Fore leg (Fig. 2b) slender, moderately long; profemur about as long as protibia, protarsomere I elongate, about 3 times as long as wide, remaining tarsomeres not preserved; remaining legs incomplete or not preserved. Immature stages unknown.

Composition and distribution. Only
Etymology. The specific name "merkli" is a patronym in honor of late Dr. Ottó Merkl (1957-2021), one of the most prominent Hungarian entomologists, the World expert in darkling beetles (Tenebrionidae), and the long-term curator of the Coleoptera Department of the Hungarian Natural History Museum.

Discussion
Recent rise of interest in research of amber fossils significantly increased the rate of discoveries of various interesting animal lineages with great importance for the understanding of their origin, evolution, and paleodiversity, as well as of the composition of past ecosystems 4 . The here reported discovery of Ajkaelater merkli gen. et sp. nov. in the Upper Cretaceous Hungarian amber is of great importance for several reasons. It is not only the first formally described beetle from ajkaite, but also the first fossil beetle described from the present-day Hungary. So far, some yet undescribed fossil beetles were found in the Miocene and Pliocene deposits of Hungary. The representatives of Dytiscidae and Staphylinidae were reported from the Late Miocene (Sarmatian) of the Tokaj Mountains 45 . Dystiscidae, Carabidae Cerambycidae, Cantharidae, Chrysomelidae, and various Curculionoidea were identified in the Pliocene alginite deposits of Pula in Bakony Mountains 46,47 . Additionally, Krzemiński et al. 46 reported Carabidae, Hydrophilidae, Silphidae, Chrysomelidae, Lagriinae, and Apioninae from the Pliocene of Gérce in western Hungary. Numerous further beetle inclusions are also known from ajkaite (personal observations of authors), however, their detailed investigation and potential formal description is a matter of future research.
Ajkaelater merkli gen. et sp. nov. also represents the first Mesozoic Elateridae described from present-day mainland Europe, and the first Elateridae reported from Santonian worldwide. Based on the fossil record, Elateridae originated either in Triassic (although those records are dubious) or, more probably, Lower Jurassic 40 , and greatly diversified later in Jurassic 39,40,48 . Mesozoic record of click-beetles is mainly composed of species described from the Asian deposits, e.g., Karabastau Formation of Kazakhstan, Zaza Formation of Russia, Yixian, Shanwang, and Jiulongshan Formations of China [48][49][50][51] , and only a few species are reported from the Mesozoic deposits of the United Kingdom 40,52 . Regarding the click-beetle inclusions in Mesozoic ambers, three described and many www.nature.com/scientificreports/ undescribed species are known from the Burmese amber 40 , and a few undescribed species were reported from the Lower Cretaceous Lebanese 53 and Spanish ambers 14,15 . The newly described species superficially resembles representatives of the tribe Oophorini (subfamily Agrypninae), which is a cosmopolitan group of click-beetles including several hundred species classified in about 20 genera 54 . The fossil record of Oophorini is known only from the Eocene of the USA and Europe 55,56 , and no fossil of this group has ever been reported from the Mesozoic 40 . Besides the general appearance, Ajkaelater gen. nov. shares with Oophorini sharp posterior pronotal angles with a distinct sublateral carina, and the compact elytra www.nature.com/scientificreports/ with distinctly impressed striae formed by lines of large punctures (Figs. 2, 3). However, these characters can be found in some other click-beetle groups as well, and the most important diagnostic characters which define Oophorini, like the shapes of head and tarsi, the hind wing venation, and the claws 35,57 , cannot be observed as these structures are not preserved in the fossil. What is more, Ajkaelater gen. nov. significantly differs from Oophorini in some taxonomically important diagnostic characters, i.e., the shape of prosternum, which is sinuate at sides, with pronotosternal sutures bent inwards (prosternum usually gradually narrowed towards the procoxal cavities in Oophorini, with pronotosternal sutures almost straight), the shape of prosternal process, which is rather stout and notably broadened near procoxae (prosternal process usually rather slender and only slightly widened near procoxae in Oophorini), and the absence of deep incisure at the base of hypomeron (incisure present in Oophorini). This combination of characters is typical for subfamily Elaterinae. Based on its habitus, shapes of antennae, pronotum and elytra, and the absence of incisures in pronotal base, Ajkaelater gen. nov. differs from most tribes of Elaterinae, and could be compared only with some Ampedini, Megapenthini, and especially Physorhinini, which sometimes superficially strongly resemble some Oophorini. However, since Ajkaelater gen. nov. lacks important structures for the tribal assignment, such as head, complete ventral mesothorax, metacoxal plates, and tarsi, we tentatively place this genus in Elaterinae as Incertae sedis. Fossil Elaterinae are known mainly from various Eocene and Miocene deposits 40 , and only a single species was described from the Mesozoic, i.e., Elater burmitinus Cockerell, 1917 from the Cretaceous Burmese amber 41 . The discrepancy between the low number of described species from the Mesozoic and the fact that Elaterinae belong to early branches of the Elateridae tree-of-life 38 can be explained by several factors, including the hard-to-observe subfamilial diagnostic characters, the misplacement of some earlier described taxa, and the general underexamination of click-beetle fossil record 39 . Indeed, Elaterinae are relatively common in Burmese amber (R. Kundrata, personal observation).
Our discovery of Ajkaelater gen. nov. in the Santonian European deposit further supports the hypothesis that Elaterinae were widely distributed and diversified in Eurasia already in the Cretaceous. Larvae of extant Elaterinae are very often saproxylic, associated with rotten, decaying wood, but some are also soil dwellers or live in leaf-litter or mosses 35,[58][59][60] . Taking this into account, we suggest that the Ajka Coal Formation paleoenvironment during the Santonian age of the Upper Cretaceous fully matched the assumed ecological needs of Ajkaelater merkli gen. et sp. nov. The depositional area of the Ajka Coal Formation was a forested swampy and lacustrine complex ecosystem with presence of both angiosperm and gymnosperm trees 61 , probably dominated by the deciduous trees of Normapolles group 62 . Hence, we suggest that the larvae of Ajkaelater merkli gen. et sp. nov. were either saproxylic, living in tree trunks or underneath the bark, or occupied a habitat with rich soil in the vicinity of swamps and lakes where the Ajka coal was formed (Fig. 5).
The discovery of Ajkaelater merkli gen. et sp. nov. not only significantly contributes to a better understanding of the palaeodiversity and evolution of the click-beetle subfamily Elaterinae in the Cretaceous Europe but also sheds further light on the yet poorly known paleoecosystem and fauna of the Santonian Ajka Coal Formation in present-day western Hungary.

Material and methods
The ajkaite amber piece, contaning the here described elaterid beetle inclusion (Fig. 1c), was part of the probably largest ajkaite amber piece known to date (approximately 11 × 8 × 3 cm) (see Fig. 2a in Szabó et al. 34 ), which was donated to the Hungarian Natural History Museum by late Károly Kozma, the former chief geologist of the Ajka Coal mines. Since it was too large, dark and opaque for classic light microscopic studies, the stone was broken into numerous smaller pieces, some of which contain various representatives of Arachnida, Diptera, Hymenoptera, other Coleoptera, and numerous pieces of plant debris.
The ajkaite sample was scanned using a Bruker Skyscan 2211 nano-CT cone-beam scanner (Skyscan, Bruker, Belgium) at the University of Szeged, Hungary with X-ray source settings of 110 kV source voltage, 700 µA source current and 350 ms exposure time in micro-focus mode using 11 Mp active pixels CCD detector at 2 µm voxel resolution. The 1939 X-ray projections were collected through a 180° rotation of the sample with 0.1° angular step size in around 4.5 h. The acquired images were reconstructed by volumetric NRecon Reconstruction Software (Skyscan, Bruker, Belgium), which uses a modified Feldkamp algorithm. Artifacts which usually occur during reconstruction, such as ring artifact and beam hardening artifacts, were corrected. The 3D model was created using CTVox 3D Micro-CT Volume Rendering software (Skyscan, Bruker, Belgium). Figure 1c was taken with a QImaging MP5.0 digital microscope camera under a Nikon LV 100 polarized light microscope, and processed with Image Pro Insight 8.0 software.
Measurements of the beetle inclusion were taken using the free version of ImageJ 1.48v 63 . In Fig. 1, the map showing location of Ajka coal beds was modified after Császár and Góczán 19 , and the simplified geological section of the Ajka Subbasin was modified after Kozma 64 . The holotype is housed in the collection of the Department of Paleontology and Geology of the Hungarian Natural History Museum in Budapest, Hungary (MTM). The ZooBank LSID number for this publication is: urn:lsid:zoobank. org:pub:BAF89C8F-069E-44FA-A0CE-19A768B0E935.

Data availability
All data needed to evaluate the conclusions in the paper are present in the paper, and the video of 3D volume rendering is deposited in Zenodo (see Supplementary File 1; https:// doi. org/ 10. 5281/ zenodo. 55634 53).