Introduction

Dictyoptera is a medium-sized group of insects with more than 10.000 described extant and fossil species, traditionally comprising cockroaches and termites (order Blattaria s. l.), mantises (order Mantodea) and the extinct family of water-walking insects (order Chresmoda) (Luo et al. 2022). Cockroaches (Blattaria s. str.) are one of the most dominant insect orders in Paleozoic and Mesozoic ecosystems (Vršanský 2008; Chen et al. 2019; Wappler and Vršanský 2021). Appearing in the Late Carboniferous (Tan 1980; Vršanský et al. 2002; Schneider and Werneburg 2006; Zhang et al. 2012), they are considered as ancestors of Isoptera, Mantodea and Chresmoda (Vršanský 2002, 2010, 2020). In the course of their 320 Ma-long evolution, cockroaches adapted to a wide range of ecosystems and developed a high degree of ecological, behavioural and morphological diversity. During the course of their evolution, there are now aquatic, pollinating, decomposing, jumping, mimicking, camouflaging, translucent, aposematic, parasitic, predatory, poisonous, eusocial, virus infection-symptomatic, holoptic, pectinate and bipectinate antennate, cavernicolous, injecting-ovipositor, brachypterous, cranefly-like and beetle-like forms (Chen and Tian 1973; Wei and Ren 2013; Šmídová and Lei 2017; Vršanský et al. 2018; Hinkelman 2020, 2021; Hinkelman and Vršanská 2020; Mlynský et al. 2019; Li et al. 2020; Sendi et al. 2020; Chen et al. 2021; Sendi 2021; Oyama et al. 2021; Kováčová 2022). Blattaria comprises about 5,000 extant and 1,500 fossil species (Liang et al. 2019; Sendi 2021; Wappler and Vršanský 2021). Extinct taxa with externally protruding ovipositors are sometimes referred to as „roaches” or „roachoids”, distinguishing them from the crown group of living true cockroaches (Li 2019; Koubová and Mlynský 2020). Fossil cockroaches are abundant, documented across numerous amber localities of various age. Most notable include North Myanmar (Vršanský et al. 2018), Baltic (Weitschat and Wichard 2002), Dominican (Gutiérrez and Pérez-Gelabert 2000; Poinar 2022) and Mexican amber (Solórzano-Kraemer 2007; Vršanský et al. 2011; Barna et al. 2019).

Alienopteridae is a unique extinct cockroach family within the superfamily Umenocoleoidea (Vršanský et al. 2018; Sendi et al. 2020). This family was described from the Late Cretaceous (Cenomanian) North Myanmar amber (Bai et al. 2016), which is one of the most species- and specimen-rich amber deposits in the world (Grimaldi et al. 2002). Alienopteridae is the only Mesozoic-type cockroach family that successfully survived the K/Pg extinction event (Vršanský et al. 2018). The fossil record of Alienopteridae ranges from Early Cretaceous (Barremian) to Middle Eocene (Ypresian/ Lutetian) with at least 21 species across 16 genera. An undescribed member of this family was also documented from the Cretaceous of Botswana by McKay (2007) (Luo et al. 2022). For a summary of the fossil records of Alienopteridae see Table 1.

Table 1 Summary of the fossil records of Alienopteridae Bai et al. (2016)
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The Cretaceous is one of the most extensively studied periods in terms of cockroach evolution, with more than 200 formally described species (Vršanský 2019; EDNA fossil insect database 2022–07-28). However, European Cretaceous ambers still remain largely unexplored. The only evaluated significant European locality is Archingeay (France) with 17 cockroach specimens in amber (Vršanský 2009).

Regarding its geochemistry, age and its preserved inclusions, the so-called ajkaite is a unique Late Cretaceous type of amber, found in western Hungary. It is known to be rich in arthropod inclusions since the middle of the twentieth century (Tasnádi Kubacska 1957). Arthropod species, formally described from ajkaite are the ceratopogonid dipterans Adelohelea magyarica Borkent, 1997 and Leptoconops clava Borkent, 1997, the hersiliid spider Hungarosilia verdesi Szabó et al., 2022, the click beetle Ajkaelater merkli Szabó et al., 2022, and the wasps Ajkanesia harmincipsziloni Szabó et Brazidec, 2022, Amissidigitus belae Szabó et Brazidec, 2022 and Spathiopteryx soosi Szabó, Brazidec et Perrichot, 2022 (Borkent 1997; Szabó et al. 2022a, b, c). Among others, undescribed specimens of Arachnida, Diptera, Hymenoptera and Coleoptera are known from museum collections. During the revision of several ajkaite amber pieces housed in museum collections across Hungary, an inclusion representing a new species of Alienopteridae, Alienopterix santonicus sp. n. is described together with its paleoecological and paleogeographical implications. This species now extends the Mesozoic record of Alienopteridae to Laurasia (previously only known from Gondwana and burmite during the Mesozoic).

Geological settings

Ajkaite is found in the Ajka Coal Formation, whose outcrops were discovered in the 1860's within the Ajka-Csinger Valley (Ajka-Csingervölgy in Hungarian) (Szabó 1871; Hantken 1878). Ajka-Csinger Valley is situated approximately four km southeast from the city of Ajka (Bakony Mountains, western Hungary) (see Fig. 1a, b).

Fig. 1
figure 1

a Locality map of Ajka and b the Ajka Coal Beds. c Map showing the position of the Ajka area within the European paleobioprovince during the Late Cretaceous (modified after Ősi et al. 2010)

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The Ajka Coal Formation occurs in the Bakony Mountains (Transdanubian Range), sometimes reaching a thickness of 120 m (Fig. 1b). Seven main coal-beds have been documented with thickness ranging between 80–360 cm (Kozma 1991). The formation is concentrated in three carbonate terrain sub-basins: Ajka, Magyarpolány-Devecser, and Gyepükaján (see Császár and Góczán 1988; Siegl-Farkas 1988). It is built up as an alternation of coal or clayey coal beds, marls, sands and sandstone beds, as well as light grey to brownish carbonaceous to argillaceous pelitic rocks with interbedded molluscan lumachelles (Haas 1983). Based on palynological and nannoplankton remains, the Ajka Coal Formation was determined to be Santonian in age (Siegl-Farkas and Wagreich 1996; Bodrogi et al. 1998; Bodor and Baranyi 2012). Apart from the inclusions discovered in ajkaite (see above), the flora and fauna were documented by numerous authors (Ősi et al. 2016; Szabó et al. 2022a and references therein). During the Late Cretaceous, the depositional area of the Ajka coal was part of the western Tethyan archipelago (Fig. 1c).

Pieces of ajkaite available today are from the coal layers and are housed in private and museum collections. Coal bed 0 was by far the richest, being the uppermost of the coal deposits, with an average thickness of 70 cm. Unfortunately, 139 years of continuous deep mining of the coal was discontinued in 2004, with little opportunity to collect further specimens. Coal bed 6, the lowest one, is the only one which is still accessible in the area of the Bocskor-trench near Ajka. However, here the coal is loose and earthy, whereas the amber is small and brittle, mostly present in the form of drops or tiny splinters.

Material and methods

The here described specimen is housed in the paleontological collection of the Supervisory Authority of Regulatory Affairs (SARA; former Mining and Geological Survey of Hungary; Budapest).

Habitus photographs were taken by Kern Optics OZL 466 stereoscopic microscope mounted with Kern Optics OCD 832 (5 MPix) microscope camera (operating software: Kern&Sohn MicroscopeVIS 2.0 Pro). Close-up photographs of the head, antennae, pronotum and the forewings were 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. For scanning electron microscopic photography modern insect specimens were gold coated. Subsequently, a Jeol, JSM-IT500HR apparatus was used for the investigations (University of Pécs, Szentágothai Research Center; Pécs). We used Adobe Photoshop Image.20 software to compose the figures. Body dimensions of the inclusion have been measured by the free version of ImageJ 1.48v.

Systematics and terminology mainly follow that of Vršanský et al. (2018, 2021a, b), following the Comstock-Needham (1896) vein systems.

Systematic Paleontology

Order Blattaria, Latreille 1810 = Blattodea Brunner von Wattenwyl, 1882 (cockroaches without termites, mantodeans and chresmodids)

Superfamily Umenocoleoidea Chen et Tian, 1973

Family Alienopteridae Bai et al., 2016

 = Aethiocarenidae Poinar et Brown, 2017

Type genus: Alienopterus Bai, Beutel, Klass, Wipfler et Zhang in Bai et al. (2016). For composition see Table 1

Stratigraphic range: Barremian—Eocene

Geographic range: Gondwana and Laurasia (by the present study)

Remarks (after Bai et al. 2006). Based on the following characters, the specimen belongs to the family Alienopteridae: filiform antennae elongated from the first antennomeres, large and strongly convex compound eyes, saddleshaped pronotum and elytrized forewings with microrectangular structures (Vršanský et al. 2021b).

Subfamily Alienopterixinae Vršanský in Vršanský et al. (2021b)

Genus Alienopterix Mlynský, Vršanský et Wang, in Vršanský et al. (2018)

Type species: Alienopterix ocularis Mlynský, Vršanský et Wang, 2018

Stratigraphic range: Cenomanian—Santonian

Geographic range: North Myanmar amber and Hungarian ajkaite amber (present study)

Remarks (after Vršanský et al. 2018). Following the fully developed forewings and the lack of separated posterior area of the pronotum, the ajkaite specimen represents the genus Alienopterix (after Vršanský et al. 2018). This genus is also more robust compared to other genera of the family, with a unique rectangular microstructure on the forewings. The present study follows Vršanský et al. (2018) on the basis of asymmetrical claws known within this genus (and all Alienopteridae but not Umenocoleidae) and lack of clavus (like in Alienopteridae, unlike Umenocoleidae), and also saddleshaped pronotum without paranotalia and without basal delimiting ridge, however, the genus is included in Umenocoleidae by Luo et al. (2022). To further discuss the correct systematic position of Alienopterix is beyond the scope of the present paper.

Alienopterix santonicus sp. n.

urn:lsid:zoobank.org:act:58D42E91-3DA5-47C1-ADE7-282CFCA0FC99

Figures 2, 3, and 4

Fig. 2
figure 2

Alienopterix santonicus sp. n. (SARA AT.10.24.1), holotype. a Habitus in dorsal view. b Habitus in ventral view. c Head in dorsal view. d Details of antennules. Scale bars: 0.5 mm (a, b), 0.2 mm (c), 0.1 mm (d)

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Fig. 3
figure 3

Alienopterix santonicus sp. n., (SARA AT.10.24.1), holotype. a Pronotum in dorsal view. b Pronotum with scutellum (sc) in ventral view. c Line drawing of the habitus in dorsal aspect, showing the arrangement of the ornamentation of the forewings. d, e Dorsal surface of the forewings, possessing the microrectangular ornamentation with microtrichiae. Scale bars: 0.25 mm (a, b), 0.5 mm (c), 120 µm (d, e)

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Fig. 4
figure 4

Alienopterix santonicus sp. n., (SARA AT.10.24.1), holotype. Line drawing of the dorsal habitus showing the hindwing venation (a) and the disruptive maculate pattern (b). Scale bar: 0.5 mm

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Description. Small, beetle-like cockroach. Head large and wide (0.4 mm long and 1.2 mm wide) with densely spaced and short microtrichiae (Fig. 2c). Epicranium 0.7 mm wide. Epicranial sutures not visible. Compound eyes very large and globular. Left eye complete (~ 0.5 × 0.3 mm in dorsal view). Ocular facets obscure, very small in diameter. Area between the eyes wide (0.6 mm). Antennomeres of the right antenna are filiform, 22 preserved. Antennomeres are almost 2.5 × as long as wide. Antennal segments are covered with short sensillae up to 0.03 mm in length (Fig. 2d).

Pronotum very slightly vaulted and trapezoidal/ saddleshaped with arched anterior and posterior margins, without paranotalia, wider than long (length/ width ~ 0.8/ 1 mm). Entire surface of pronotum is densely covered by short, small sensillae (Fig. 3a). Head not covered by pronotum, indicating mobility of head with a possible neck. Scutellum triangular, ~ 0.12 mm long and ~ 0.16 mm wide (Fig. 3a).

Forewings fully developed, with ellipsoidal/ lentoid outline, clavus apparently not delimited. Left forewing is 3.7 mm long and 1.3 mm wide. Forewing venation is obscure. Dorsal surface of the forewings covered by microrectangular structures, arranged in medio-laterally running rows (Fig. 3c–e). Dense and short microtrichiae on the dorsal surface of the forewings up to 0.02 mm long, following the arrangements of the microrectangular structures (Fig. 3d, e).

Left hindwing venation is partly visible through the left forewing, and well-visible from ventral view. SC slightly visible and simple, CuP simple, R1 and RS traceable, RS with at least four branches posteriorly. Pterostigma dark. Vannus (A2 veins) clear and veer-like pleated. The left hindwing also shows several mutations or deformities (sensu Vršanský 2003, 2005; Vršanský et al. 2017), at least one (and possibly up to 3) R1-RS fusions and a serious CuA-CuA fusion (Fig. 4a). Due to weak visibility, additional deformities cannot be excluded.

Cercus short and straight, number of cercomeres is obscure (10 or 11). Cercomeres with sensilla.

Symmetrical colouration on head and pronotum, asymmetrical pigmentation on forewings. Head with bow-like pattern between the eyes. The pronotum bears a complex and almost symmetrical ornamentation consisting of dark and relatively large spots, low in number (number of visible spots is 5). The forewings are ornamented by an asymmetrical pattern of irregularly shaped and arranged small maculae, sometimes fused into short stripes and rosettes (Fig. 4a).

Type material. SARA AT.10.24.1. (holotype, ?male).

Type locality. Ajka-Csingervölgy, appr. 1 km SE of the city Ajka (Bakony Mts, Hungary).

Horizon and age. Ajka Coal Formation, unknown shaft of the Ajka-Csingervölgy coal minery; Upper Cretaceous, Santonian (86.3–83.6 Ma).

Etymology. The name santonicus is derived from the Santonian age of the Ajka Coal Formation where the specimen originates from.

Differential diagnosis. Genus Alienopterix currently includes three valid species: A. mlynskyi, A. ocularis and A. smidovae. Alienopterix santonicus sp. n. clearly differs from all known species of the genus based on the combination of the following characters. Unlike in A. santonicus sp. n., epicranial sutures are visible in A. mlynskyi. Antennomeres of A. mlynskyi are less than two times as long as wide, while those of A. santonicus sp. n. are more than two times as long as wide. The species A. ocularis lacks the characteristic secondary structure of the forewing, while forewings of A. santonicus sp. n. are covered with microrectangular structures. Unlike A. santonicus sp. n., A. ocularis has a distinct, campaniform pronotum. Also, antennal segments of A. ocularis are subequal, while those of A. santonicus sp. n. are more than two times as long as wide. Antennal segments of A. smidovae are subequal and rich in short and dense sensillae, while those of A. santonicus sp. n. are more longer than wide and comparatively sparse in sensillae.

The vannus of A. santonicus sp. n. is also unique. Although the vannus is not clearly preserved in any alienopterid, a veer-like pleated vannus is unknown for any Umenocoleoidea (although presumed; Vršanský, pers. comm.). The left hindwing also shows several deformities (see above). This is remarkable, as deformities occur in cockroaches from Early Cretaceous Lebanese amber (and other 127 Ma diversification points), while they were extremely rare among Late Cretaceous Myanmar amber cockroaches. This applies more to the hindwing, which bears the most of the aerodynamical weight during the stroke in this presumably well-flying organisms. Among Cretaceous Alienopteridae (and Umenocoleidae), hindwing deformities are barely known. During the whole Upper Cretaceous similar deformities were generally rare.

A. santonicus sp. n. can be distinguished from all other members of the genus by the unique maculate pattern of the dorsum—namely the pronotum and forewings.

Preservation. Body of the type specimen incomplete (length/ width 4.9/ 1.8 mm); all legs, most of the abdomen (except for seven posterior tergites and the left cercus), one of the antennae and a large part of the head including mouthparts are missing (Fig. 2a, b). Dorsally, a bubble covers the scutellum, part of the pronotum and the right forewing.

Discussion

Based on phylogenetic Bayesian network analysis, Alienopteridae experienced explosive radiation ~ 127 Ma (Vršanský et al. 2018). Until the present study, the genus Alienopterix was known to be indigenous to Myanmar: A. mlynskyi, A. ocularis and A. smidovae were discovered in the Late Cretaceous (Cenomanian) amber of Northern Myanmar (Vršanský et al. 2018, 2021b; Luo et al. 2022). Northern Myanmar amber is considered by some authors to be of Gondwanan origin (Poinar 2018). The discovery of A. santonicus sp. n. in the Santonian ajkaite amber expands the temporal and geographical distribution of the whole family and the genus as well. This genus also seems to occur in the Cretaceous ambers of Spain (L. Šmídová, unpublished observation) – thus the Mesozoic presence of Alienopteridae in Laurasia is fully validated.

Most cockroach species are regarded as omnivorous scavengers and detritivores (Sendi et al. 2020). However, according to Vršanský et al. (2018, 2021b) and Hinkelman (2020), alienopterids were most likely pollinators (of cycads, angiosperms and gymnosperms) and pollinivores. The depositional area of the Ajka Coal was characterized by swampy and lacustrine environments, dominated by angiosperms (Szabó et al. 2022a). Nonetheless, gymnosperms, in the form of Araucariaceae trunks are also reported from the formation (Rákosi L., unpublished reports; Bodor et al. 2013). As a pollinator life-style is tentatively suggested for A. santonicus sp. n., the species could have found a wide range of food sources and suitable habitats in the Ajka Coal swamp, thanks to the rich floral assemblage. Therefore, as a possible pollinator, A. santonicus sp. n. could have been playing an important role in the Ajka Coal ecosystem during the Santonian. For an artistic reconstruction of A. santonicus sp. n. see Fig. 5.

Fig. 5
figure 5

Artistic reconstruction of Alienopterix santonicus sp. n., with hypothetical greenish-reddish colouration (artwork by Márton Zsoldos)

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Metallic colouration and iridescence is fairly abundant among modern insects, including the extant cockroach genera Eucorydia, Eustegasta, Melyroidea and Pseudoglomeris (Shelford 1912; Li et al. 2018; Hinkelman et al. 2020; Yanagisawa et al. 2021). Fossil metallic cockroach species are extremely rare. This pattern occurs only in one immature cockroach from Taimyr amber (Vršanský 2019). Metallic colouration also occurs in the fossil genus Alienopterix. The dorsal surface of the forewings of A. santonicus sp. n. also bears rectangular (microrectangular) structures, built up by seemingly concave units. Similar forewing structures are present in A. smidovae and A. mlynskyi, and are also partially visible in A. ocularis (Vršanský et al. 2021b; Vršanský, pers. comm.). Following this similarity, a metallic colouration is proposed in A. santonicus sp. n. as well.

In order to confirm the metallic colouration of A. santonicus sp. n., various extant iridescent taxa of Coleoptera, Mantodea and Blattodea were investigated with scanning electron microscope. Different types of microscopic surface structures were captured, however, most of them appear different from those observed in Alienopteridae (Fig. 6a–l). However, the forewing of Melyroidea magnifica Shelford, 1912, a species with distinct, metallic colouration, possesses unique microrectangular structures (as it was presented by Hinkelman et al. 2020, Fig. 2h). The microornamentation of M. magnifica is built up by concave, hexagonal units, creating a honeycomb-like pattern (Fig. 6m–p). Identical structures were observed on the dorsal surface of the elytron of Cicindela soluta Dejean, 1822 (Coleoptera: Carabidae), a brigthly coloured and metallic predatory tiger beetle (Fig. 6q–t). This type of sculpting of the dorsal forewing surface is generally very similar to that of A. santonicus sp. n., further supporting its metallic colouration. Note that the similarities between the microornamenation of the forewings of M. magnifica and the elytra of C. soluta is may be due Müllerian mimicry in Melyroidea spp.

Fig. 6
figure 6

SEM images of wing surface microstructures of various iridescent insect taxa. a, b Cetonia aurata (Linnaeus, 1761), elytron (Hungary, Budapest). c, d Limoniscus violaceus (Müller, 1821), elytron (Hungary, Bajót). e, f: Polydrusus formosus (Mayer, 1779) (Hungary, Ajka). g, h Carabus (Morphocarabus) scheidleri Panzer, 1799, yellow morphotype (Hungary, Pilisszentlélek). i, j Metallyticus splendidus Westwood, 1835, female forewing (bred in captivity, native to Asia). k, l Eucorydia yasumatsui Asahina, 1971, female forewing (bred in captivity, native to Asia). m-p Melyroidea magnifica Shelford, 1912, female forewing (Equador, Puyo; USMNH 2,039,895). q-t Cicindela soluta Dejean, 1822, elytron (Hungary, Székesfehérvár). Scale bars: A, C, E, I, M, N, Q, R: 100 µm (a, c, e, i, m, n, q, r), 50 µm (g, k, o, s), 10 µm (b, f, h, j, p, t), 1 µm (d, l)

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The metallic colouration of A. santonicus sp. n. is possibly related to its suggested pollinator life-style. Numerous important pollinator insect taxa are widely known to have iridescent metallic colouration (e.g., Jameson and Ratcliffe 2002; Jákl 2011; O’Neill et al. 2008 and references therein). Moreover, vivid metallic colouration of insects was previously interpreted as camouflage (Thayer 1909; Cott 1940; Kjernsmo et al. 2020).

The disruptive camouflage body pattern of A. santonicus sp. n. is very similar to that of the fossil umenocoleid Cratovitisma bechlyi Podstrelená in Podstrelená and Sendi (2018), from the Late Cretaceous Myanmar amber. Both species bear a similar, bow-like pattern between the eyes, which is posteriorly followed by more and more irregularly arranged maculae. This type of disruptive colouration is extremely rare in cockroaches, and reflects a specific bark niche in (sub)tropical forests (Podstrelená and Sendi 2018). Additionally bark-related life-style also increases the chance for being trapped in amber (Azar 2007).

Alienopterix santonicus sp. n. most likely benefited from its iridescent and disruptive colouration, which served as camouflage. Such metallic colouration might have served to confuse visually hunting predators, giving A. santonicus sp. n. a significant survival advantage if it was recognized on the bark surface or on flowers. Combination of iridescent (structural) and patterned (true colours) is highly unusual and as an advanced trait might be documented for the first time in history. Unfortunately and counterintuitively none of these colours can be identified with confidence at the present.