Non-technical Summary
Sexual dimorphism in surface sculpture as a result of sexual conflict in mating systems is well known in many extant diving beetles but was never recorded from fossils. We record two species of diving beetles from early Miocene Mexican amber and assign them to the hyperdiverse extant genus Copelatus. One of them, Copelatus chiapas new species, represents the first fossil diving beetle with documented sexual dimorphism in sculpture on elytra. We hypothesize that those species lived in freshwater habitats, such as swamps or pools/puddles, in a Tertiary rain forest about 20 million years ago.
Introduction
The extant genus Copelatus Erichson, Reference Erichson1832, represents the most speciose genus of the family Dytiscidae, known as predaceous diving beetles. To date, 440 species, predominantly from forested areas of the tropics, have been described (Nilsson and Hájek, Reference Nilsson and Hájek2024). Despite their diversity, Copelatus species are morphologically rather homogenous, mainly of oval and dorsoventrally weakly convex body shape and about 3–10 mm in length. Many species possess conspicuous longitudinal striae on the dorsal side of the elytron of both sexes. Copelatus species inhabit a large variety of both running and stagnant waters, including caves (Caetano et al., Reference Caetano, Bená and Vanin2013), ephemeral forest-floor depressions (Ranarilalatiana and Bergsten, Reference Ranarilalatiana and Bergsten2019), and water tanks in bromeliads (Hájek et al., Reference Hájek, Alarie, Benetti, Hamada, Springer, Hendrich, Villastrigo, Ospina Torres, Basantes and Balke2024).
At present, there are still several competing hypotheses regarding the phylogenetic placement of the subfamily Copelatinae within Dytiscidae. Morphological analyses suggest either a position of Copelatinae as the sister to all other Dytiscidae (e.g., Beutel et al., Reference Beutel, Ribera, Fikáček, Vasilikopoulos, Misof and Balke2020) or subordinated within the family, as the sister of Hydrodytinae + Hydroporinae (Miller, Reference Miller2001). The latter placement was also suggested by a recent molecular phylogenetic analysis (Baca et al., Reference Baca, Gustafson, Alexander, Gough and Toussaint2021). Alternative molecular phylogenetic analyses suggest topologies such as Copelatinae (Laccophilinae + Cybistrinae) (Désamore et al., Reference Désamore, Laenen, Miller and Bergsten2018) or Copelatinae (Matinae [Hydrodytinae + Hydroporinae]) (Vasilikopoulos et al., Reference Vasilikopoulos, Balke, Kukowka, Pflug and Martin2021).
Based on a molecular clock analysis, Copelatinae are estimated to have originated between 85 and 95 million years ago (Ma); Copelatus itself is estimated to be from about 55 Ma (Balke et al., Reference Balke, Ribera and Vogler2004; see also Désamore et al., Reference Désamore, Laenen, Miller and Bergsten2018) to around 90 Ma (Baca et al., Reference Baca, Gustafson, Alexander, Gough and Toussaint2021). This is in accordance with the few hitherto known Copelatus fossils. The oldest one is a fossil from Eocene Baltic amber; without precise locality information, its age is estimated to be circa 55–40 Ma. The other amber fossil Copelatus is known from the La Toca Formation, Dominican Republic amber (Miocene, ca. 20–14 Ma) (Miller and Balke, Reference Miller and Balke2003). There are also compressed Copelatus fossils that have been discovered: one from the Upper Rhine Graben Formation in Alsace, France (Eocene–Oligocene boundary, ca. 34 Ma) (Förster, Reference Förster1891; Fikáček et al., Reference Fikáček, Hájek and Schmied2011), and three species described from Vishnevaya Balka in Russia (Middle Miocene, ca. 16–11.6 Ma) (Říha, Reference Říha1974).
The paleontological site near Simojovel de Allende, Chiapas, Mexico, is well known for its rich Tertiary biota. The amber can be found in three lithostratigraphic units there. The oldest amber is found in the La Quinta Formation, dated to early Miocene (amber deposit began ca. 23 Ma); its fossiliferous resin came from two legume (Fabaceae) tree species of the genus Hymenaea (Calvillo-Canadell et al., Reference Calvillo-Canadell, Cevallos-Ferriz and Rico-Arce2010), whose communities developed in estuarine environments near the ancient coast, very similar to mangroves (Poinar, Reference Poinar1992; Serrano-Sánchez et al., Reference Serrano-Sánchez, Hegna, Schaaf, Pérez, Centeno-García and Vega2015). This stratum is overlaid by Mazantic Shale containing amber pieces that were originally deposited in a humid forest (Solórzano-Kraemer, Reference Solórzano-Kraemer and Penney2010). The Balumtum Sandstone is the uppermost unit that occasionally bears some amber (Serrano-Sánchez et al., Reference Serrano-Sánchez, Hegna, Schaaf, Pérez, Centeno-García and Vega2015). The insect fauna from Simojovel was revised by Solórzano Kraemer (Reference Solórzano Kraemer2007), who reported about 2400 specimens, predominantly from the order Diptera, followed by Hymenoptera and Heteroptera. Additional organisms recently described from the area represent such taxa as Blatodea (Vršanský et al., Reference Vršanský, Cifuentes-Ruiz, Vidlička, Čiampor and Vega2011), Dermaptera (Estrada-Álvarez et al., Reference Estrada-Álvarez, Núñez-Bazán, Mata-González and Vega2023), semiaquatic Heteroptera (Cifuentes-Ruiz et al., Reference Cifuentes-Ruiz, Brailovsky, Serrano-Sánchez and Vega2021), Neuropterida (Engel and Grimaldi, Reference Engel and Grimaldi2007), and curculionoid Coleoptera (Poinar and Brown, Reference Poinar and Brown2007; Peris et al., Reference Peris, Solórzano Kraemer, Peñalver and Delclòs2015; Legalov and Poinar, Reference Legalov and Poinar2016; Poinar and Legalov, Reference Poinar and Legalov2016; Legalov et al., Reference Legalov, Nazarenko and Perkovsky2019).
A review of aquatic and semiaquatic biota was published by Serrano-Sánchez et al. (Reference Serrano-Sánchez, Hegna, Schaaf, Pérez, Centeno-García and Vega2015), who mentioned a syninclusion with seven unidentified “aquatic coleopterans,” apparently belonging to the genus Copelatus, deduced from the presence of elytral striae on one specimen illustrated in dorsal view. Another unidentified species of Copelatus, in the C. consors Sharp, Reference Sharp1882a, species group, was diagnosed and illustrated by Solórzano Kraemer (Reference Solórzano Kraemer2007). Here we contribute to the known diversity of Tertiary biota in Chiapas by describing a highly characteristic new diving beetle species preserved in amber pieces from that area. Because a series of specimens from both sexes is available, we also reveal the remarkable sculptural sexual dimorphism of the new species, well known in extant species, but for the first time documented in a fossil diving beetle. In addition, we present two more specimens that cannot be assigned to any of these taxa due to poor structural visibility; both will require further investigation.
Materials and methods
The specimens were examined using a Leica M205 C stereomicroscope. Measurements were made with an ocular graticule. Photographs were taken with a Canon EOS R camera, and Mitutoyo 5× and 10× ELWD Plan Apo objectives were used for photographing the habitus. These were attached to a Carl Zeiss Jena Sonnar 3.5/135 MC. Illumination was accomplished with three LED segments SN-1 from Stonemaster. Image stacks were generated using the Stackmaster macro rail (Stonemaster), and images were assembled with Helicon Focus v. 7.61 on a MacPro 2019 with a Radeon Pro 6800X MPX GPU. We also used a Keyence VHX-6000 digital microscope equipped with a ZST 20×–2000× objective for photographing of morphological details.
The following abbreviations were used in the descriptions: TL = total length, measurement of length from clypeal margin to apex of elytra; TL−h = total length minus head length, measurement of length from anterior margin of pronotum to apex of elytra; TW = maximum width of body measured at right angle to TL.
Repositories and institutional abbreviations
Specimens examined in this study are deposited in the following collections: Staatliches Naturhistorisches Museum, Braunschweig, Germany (SNHMB) and Zoologische Staatssammlung, Munich, Germany (ZSMG).
Systematic paleontology
Order Coleoptera Linnaeus, Reference Linnaeus1758
Suborder Adephaga Schellenberg, Reference Schellenberg1806
Family Dytiscidae Leach, Reference Leach and Brewster1815
Subfamily Copelatinae Branden, Reference Branden1884
Genus Copelatus Erichson, Reference Erichson1832
Type species
Dytiscus posticatus Fabricius, Reference Fabricius1801, by monotypy.
Copelatus chiapas new species
Figures 1–3
Copelatus chiapas n. sp., male holotype (# SNHMB.G 8199): (1) dorsal habitus; (2) ventral habitus. Scale bar = 1 mm.
Copelatus chiapas n. sp., male holotype (# SNHMB.G 8199): (1) pronotum and base of elytra; (2) metaventrite and metacoxae; (3) protarsus; (4) metatarsus. Scale bars = 0.5 mm (1, 2), or 0.25 mm (3, 4).
Copelatus chiapas n. sp., female paratypes (# SNHMB.G 8200): (1) dorsal habitus; (2) detail of elytral microsculpture. Scale bars = 1 mm (1), or 0.25 mm (2).
Type material
Holotype ♂ (SNHMB, Inventory # SNHMB.G 8199); Mexico, Chiapas State, Simojovel de Allende (MEX004). Paratypes 1♂ 5♀♀, same locality and stratigraphic interval as holotype (SNHMB, Inventory # MEX005 with two individuals = SNHMB.G 8200; MEX010 = SNHMB.G 8201; MEX PS MEX34 = SNHMB.G 8202; MEX035 with two individuals = SNHMB.G 8203).
Diagnosis
Elongate oval species; dorsal surface coarsely punctured in male, in female with dense long longitudinal striolae on pronotum and elytra; striae on elytral disc absent; appendages long, antennae filiform reaching nearly 0.4 of body length; metatarsomeres rectangular, well separated (not forming compact “scull”-like piece).
Occurrence
All specimens are from early Miocene amber collected near Simojovel de Allende, Chiapas State, Mexico. Precise location (mine) and formation are not known.
Description
Male holotype (Figs. 1, 2). Measurements (in mm): TL = 5.0; TL−h = 4.5; MW = 1.8. The whole beetle appears slightly compressed laterally; this is best seen in the compressed prosternal process, metaventrum, and metatibiae. Habitus elongate oval; sides almost parallel-sided, only slightly attenuated proximally and distally; dorsally slightly convex, ventrally flat (Fig. 1). Base of pronotum slightly broader than base of elytra; broadest posteriorly to elytral mid-length. Coloration of body brownish black; elytra with indistinct transverse orange brownish band at base. Head elongate, ~0.59× width of pronotum, almost semi-circular; anterior margin of clypeus slightly convex; antennae with antennomeres club-shaped, long and slender (antennomeres II–X ~3× longer than wide); eyes small (eye width/interocular distance ratio = 0.25), emarginate antero-laterally; reticulation consisting of fine, well-impressed isodiametric polygonal meshes; punctation consisting of coarse punctures. Pronotum transverse, broadest at posterior angles; anterior angles acute, posterior angles obtuse; sides moderately and evenly curved; reticulation imperceptible; punctation consisting of dense punctation smaller than those on head (Fig. 2.1).
Scutellar shield small, triangular. Elytra without longitudinal striae but disc in basal half covered with short longitudinal striolae (Fig. 2.1); reticulation consisting of very fine, superficially impressed polygonal meshes; meshes isodiametric, hardly perceptible in basal half, but transverse and more apparent in apical half; punctation double, large setigerous punctures present along lateral margin of elytra, fine punctures distributed on whole surface of elytra.
Legs with metafemora without distinct linear series of setae near anteroapical angle; all tibiae with numerous long spines; protibia distinctly broadened distally, club-shaped; all tarsi pentamerous; pro- and mesotarsomeres 1–3 strongly broadened, ventrally with four transverse rows of adhesive setae; apical pro- and mesotarsomere as long as preceding tarsomeres together; protarsal claws unequal, anterior claw slender and strongly curved, posterior claw broadened medially, straight (Fig. 2.3); metatarsomeres long, rectangular to slightly club-shaped, well separated (not forming compact “scull”-like piece), with numerous spines on edges (Fig. 2.4.); metatarsal claws equal; long natatorial setae well developed on dorsal side of pro- and mesotibia, and on both sides of metatibia and metarsomeres. Ventral side with prosternum keeled medially; prosternal process lanceolate, in most specimens artificially laterally compressed, apex obtuse; lateral parts of metaventrite (“metasternal wings”) tongue-shaped, slender; metacoxal lines closely approximated, well impressed, anteriorly abbreviated; metacoxal plates covered with long longitudinal strioles; reticulation not perceptible (Fig. 2.2); metacoxal processes rounded at posterior margin; abdominal ventrites I–II with longitudinal strioles; ventrites III–V with oblique strioles laterally; apical ventrite bordered on posterior margin.
Female
Similar to male in habitus (Fig. 3.1). Pro- and mesotarsomere 1–3 not broadened; protarsal claws equal, thin. Dimorphic in dorsal surface sculpture: one female similar to male in structure, remaining four females with pronotum and elytra (except for apical part) covered with densely distributed long longitudinal striolae (Fig. 3.2).
Variation
TL = 4.1–5.1 (mean = 4.6 ± 0.3); TL−h = 3.9–4.5 (mean = 4.2 ± 0.2); MW = 1.6–2.0 (mean = 1.8 ± 0.1).
Etymology
The species is named after the Chiapas State (Mexico), where the type locality is situated. The specific epithet is a noun in the nominative singular.
Taxonomic remarks
We can undoubtedly classify this species within the subfamily Copelatinae based on the following characters: (1) elongate habitus, (2) eyes emarginate antero-laterally, (3) exposed scutellum, (4) metacoxal lines very closely approximated, (5) metafemur without distinct linear series of setae near anteroapical angle, (6) all tarsi pentamerous, and (7) equal metatarsal claws. This subfamily is found in Central and North America represented solely by the genus Copelatus—a large and variable taxon for which no morphological synapomorphy can be postulated at present.
The intrageneric classification of Copelatus is unsatisfactory; species are clustered based on the presence of longitudinal striae on elytra into informal species groups (Sharp, Reference Sharp1882a; Guignot, Reference Guignot1961), which usually do not delineate monophyletic units (Balke et al., Reference Balke, Ribera and Vogler2004). Copelatus chiapas n. sp. possesses no regular striae on its elytra, which places it in the Copelatus hydroporoides (Murray, Reference Murray1859) species group. This group comprises 57 extant species, occurring in all zoogeographical regions, 25 of which occur in the Neotropics. In the review of Neotropical species of this group (Young, Reference Young1942), the new species keys with C. biformis Sharp, Reference Sharp1882b, from Veracruz, Mexico. However, the latter species is characterized by a broadly oval habitus, the male elytra with distinct lines of serial punctures that are deeply impressed to form two short striae, and the female elytra with four regular striae. Copelatus chiapas n. sp. also can be easily distinguished from all hitherto described fossil Copelatus species, including the one previously recorded from Simojovel (Solórzano Kraemer, Reference Solórzano Kraemer2007), because they all have elytra with distinct longitudinal striae.
Copelatus sp.
Figure 4
Diagnosis
Elongate oval species (Fig. 4.1, 4.2), TL = 3.9–4.0 mm; dorsal surface with longitudinal strioles close to pronotal basal angles (Fig. 4.3), elytra tentatively with 11 dorsal striae and submarginal stria; appendages long, antennae filiform reaching nearly 0.4 of body length; metatarsomeres rectangular, forming compact “scull”-like metatarsus.
Copelatus sp. from Mexico, Chiapas State, Simojovel de Allende: (1) dorsal habitus (# MexAmb 001); (2) dorsal habitus (# MexAmb 002); (3) detail of structure on lateral part of pronotum (# MexAmb 001). Scale bars = 1 mm (1, 2), or 0.25 mm (3).
Occurrence
All specimens are from the early Miocene amber collected near Simojovel de Allende, Chiapas State, Mexico. Precise locality (mine) and formation are not known.
Material examined
Two ♂♂, labelled: “2201/-5/29 Mexican amber, Adult, Copelatinae, Prorhinopsenius 0.9 mm, 20–15 ma” // “MEXICAN AMBER, Simojovel, Chiapas, Mexico” (ZSMG, Inventory # MexAmb 001, 002).
Remarks
The two additional male specimens cannot be assigned to a species known from Mexican amber due to poor structural visibility. Based on the tentative presence of 11 dorsal elytral striae and a submarginal stria, they may belong to the Copelatus erichsoni Guérin-Méneville, Reference Guérin-Méneville1847, species group—one of the largest groups worldwide—containing currently 145 species, 35 of them occurring in Latin America (Nilsson and Hájek, Reference Nilsson and Hájek2024). Further investigation would require μCT scanning as well as comparison with the other existing Copelatus species from Chiapas amber.
Discussion
Based on a review of aquatic and semiaquatic fauna from the La Quinta Formation, Serrano-Sánchez et al. (2015, p. 243) characterized the Miocene landscape in Simojovel as an “estuarine environment similar to modern mangrove communities.” This opinion was corroborated by subsequent studies on various arthropod groups, predominantly various crustaceans such as Brachyura (Serrano-Sánchez et al., Reference Serrano-Sánchez, Guerao, Centeno-García and Vega2016), Copepoda (Huys et al., Reference Huys, Suárez-Morales, Serrano-Sánchez, Centeno-García and Vega2016), and Ostracoda (Matzke-Karasz et al., Reference Matzke-Karasz, Serrano-Sánchez, Pérez, Keyser, Pipík and Vega2019), and most recently insects (Heteroptera) as well (Cifuentes-Ruiz et al., Reference Cifuentes-Ruiz, Brailovsky, Serrano-Sánchez and Vega2021). In contrast, studies of predominantly terrestrial fossils from the later Mazantic Shale Formation suggest the presence of a humid forest (Solórzano-Kraemer, Reference Solórzano-Kraemer and Penney2010; Estrada-Álvarez et al., Reference Estrada-Álvarez, Núñez-Bazán, Mata-González and Vega2023).
The precise formation is not known for our Copelatus specimens. However, a mangrove habitat with brackish water and regular fluctuation of water level is rather an unsuitable environment for most aquatic beetles; and in fact, we are not aware of any diving beetles that live in mangroves today. We suggest freshwater habitats, such as swamps or pools/puddles in a rain forest—the environment documented from the study area (e.g., with palynological studies; Graham, Reference Graham1999)—as the most probable place where the newly described Copelatus lived.
Sexual dimorphism, which is well known in the majority of diving beetle groups, is interpreted as the result of sexual conflict. Because multiple prolonged matings may reduce overall fitness in females, they are thought to resist mating attempts under certain circumstances, while males are thought to attempt to force copulation. In terms of morphology, the most apparent adaptations in the males are the expansion of male pro- and, to a lesser extent, mesotarsomeres, with the presence of adhesive setae on the ventral side, and sometimes the modification of protarsal claws. Female adaptations most frequently comprise modification of the cuticle sculpture on the pronotum and elytra to create various striolae, striae, or grooves, or reticulate patterns. Those structures appear to interfere with the male grasping device, giving the females greater control over the decision to mate. In many Dytiscinae species with modified elytra, females are dimorphic, with some modified and others not, and the proportion of modified females appears to vary among populations (for review of Dytiscidae sexual systems, see Miller and Bergsten, Reference Miller, Bergsten and Yee2014).
Although we expect that sexual dimorphism exists in extinct diving beetles, their limited fossil records prevent us from confirming this. Here, we document sexual dimorphism in the newly described Copelatus species. The male has distinctly broadened pro- and mesotarsi with adhesive setae on their ventral side, and a modified, medially broadened posterior protarsal claw. The female is dimorphic; one preserved female is identical to the male in its dorsal surface sculpture, while the other four females have the surface of their pronotum, and elytra covered with densely distributed, long longitudinal striolae. While this is the first fossil diving beetle for which a distinct sexual dimorphism has been documented, we expect additional examples will be found in the fossil record as more new species are discovered.
Acknowledgments
We are obliged to R.G. Beutel and one anonymous reviewer for valuable comments on the manuscript. The work of J. Hájek was supported by the Ministry of Culture of the Czech Republic (DKRVO 2024-2028/5.I.a, National Museum, 00023272).
Declaration of competing interests
The authors declare none.
Open access
