ORIGINAL PAPER

The oldest micropepline beetle from Cretaceous Burmese amberand its phylogenetic implications (Coleoptera: Staphylinidae)

Chen-Yang Cai & Di-Ying Huang

Received: 20 May 2014 /Revised: 18 July 2014 /Accepted: 25 July 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract The staphylinid subfamily Micropeplinae includessmall strongly sclerotized beetles with truncate elytra leavingthe most part of abdomen exposed. Fossil micropeplines arerare and confined to Cenozoic representatives of extant gen-era. Here, we describe the oldest micropepline, Protopepluscretaceus gen. and sp. n., from the Upper Cretaceous Burmeseamber. Fluorescence microscope and confocal laser scanningmicroscopy (CLSM) were both used to reveal diagnosticfeatures of Micropeplinae and some primitive traits that placeProtopeplus very basally within Micropeplinae.

Keywords Insecta .Staphylinidae .Micropeplinae .Burmeseamber

Introduction

Micropeplinae are small (1∼3 mm long) strongly sclerotizedbeetles with truncate elytra leaving five abdominal segmentsexposed (Campbell 1968). Although this group has beentreated as a separate family (Campbell 1968; Newton 1984),it is currently widely accepted as a subfamily withinStaphylinidae (Newton and Thayer 1992). They are a smallgroup associated with moist forest floor litter, water margins,

plant debris or nests of mammals; they feed on mould sporesand hyphae (Campbell 1968; Löbl and Burckhardt 1988;Newton et al. 2000; Thayer 2005). To date, 82 extant speciesplaced in six genera have been described from theNeotropical, Nearctic, Palaearctic, Ethiopian and Oriental re-gions (Herman 2001; Thayer 2005).Micropeplinae are readilyrecognizable by antennae with nine antennomeres, the ninthbeing enlarged into an oval club; elytra with longitudinalridges and prothorax with deep excavations underneath forreception of the antennae (Newton et al. 2000).

The fossil record of Micropeplinae is rare and confined toCenozoic representatives of the extant genera. Four specieshave been known as fossils from the very end of Tertiary.Three extinct Pliocene species are attributed to the extantgenus Micropeplus Latreille, including Micropeplushoogendorni Matthews 1970 and Micropeplus hopkinsiMatthews 1970 from the Lava Camp Mine of Alaska, theUSA and Micropeplus macrofulvus Gersdorf 1976 fromWillershausen of Germany. In addition, fossils of themicropeplines from the extant species Kalissus nitidusLeConte and Micropeplus tesserula Curtis are known fromthe Pliocene of Alaska (Matthews 1970). Here, we describe anew genus and species from the earliest Cenomanian (LateCretaceous) Burmese amber from Myanmar.

Material and methods

The sole specimen is from amber deposits in the HukawngValley of northern Myanmar, currently considered to be of theearliest Cenomanian age (ca. 99 Ma; see details in Shi et al.2012). The mining is done at a hill named Noije Bum, nearTanai Village (26° 21′ 33.41″N, 96° 43′ 11.88″ E). It has beenprepared using a razor blade, polished with emery papers withdifferent grain sizes and finally lustrated with diatomite mud;then, the specimen is mounted between two microscopic

Communicated by: Sven Thatje

C.<Y. Cai :D.<Y. Huang (*)Nanjing Institute of Geology and Palaeontology, State KeyLaboratory of Palaeobiology and Stratigraphy, Chinese Academia ofSciences, 210008 Nanjing, People’s Republic of Chinae-mail: dyhuang@nigpas.ac.cn

C.<Y. Caie-mail: caichenyang1988@163.com

C.<Y. CaiUniversity of Chinese Academy of Sciences, 19AYuquanlu,100049 Beijing, People’s Republic of China

NaturwissenschaftenDOI 10.1007/s00114-014-1221-z

coverslips with Canada balsam as medium as described byAzar et al. (2003) (Fig. 1). The type specimen is housed in theNanjing Institute of Geology and Palaeontology, CAS,Nanjing, China. Observations and photographs were takenusing a Zeiss Discovery V20 stereo microscope and a ZeissAxio Imager 2 light microscope with a digital camera attachedrespectively. Photomicrographs with green background(Figs. 2 and 3d, f) are taken using green fluorescence as lightsource attached to a Zeiss Axio Imager 2 light microscope andusing a confocal laser scanning microscopy (CLSM) ZeissLSM 710 with ×10 objectives and using a laser at 488 nm.(Fig. 3a, b). CLSM was used to produce high resolution in-focus three-dimensional images of relatively thick sections bytargeted fluorescence (e.g. Kirejtshuk et al. 2014). The beetlein Burmese amber was imaged and constructed in 3D usingCLSM, which enables detailed observation of the beetle mor-phology without damaging the insect.

Systematic palaeontologyOrder Coleoptera LinnaeusFamily Staphylinidae LatreilleSubfamily Micropeplinae LeachGenus Protopeplus gen. n.Type species Protopeplus cretaceus sp. n., here designated.EtymologyCombination of the Greek roots “Proto-”, mean-

ing “giving rise to” and “peplus”; it is masculine in gender.Diagnosis Small, elongate-oval, glabrous; eyes protruding

laterally; antennal insertions concealed under frontal shelf;antennae with nine antennomeres and the apical antennomereforming a large club; maxillary palpi with palpomere 3 as longas palpomere 4, palpomere 4 acuminate; frons and vertexnormal, not incised; pronotum broadly explanate, with lateralcarinae and bisinuate posterior margin, pronotal median areaswith four complete longitudinal ridges; elytra relatively short,leaving six abdominal tergites exposed; elytra costate, withintervals between costae distinctly punctate; protrochantinsexposed; pro- and metacoxae contiguous; mesocoxae verynarrowly separated; abdominal tergites without carinae; pen-ultimate ventrite in male with a pair of strong longitudinal andslightly protruding ridges.

P. cretaceus sp. n.(Figures 1, 2 and 3)Etymology The epithet of the new species referred to the

Cretaceous, the age of the fossil.Material Holotype, NIGP152731.Occurrence Earliest Cenomanian (ca. 99 Ma) amber from

Hukawng Valley, Myanmar.Description Body 1.07 mm long, subunicolor reddish

brown.Head small, narrower than pronotum, 0.18 mm long and

0.23 mm wide (including eyes), with neck constriction; an-tennal insertions located before anterior margins of eyes. Eyelarge. Antenna (Fig. 3d) long, densely setose, extending

nearly to the posterior margin of pronotum; antennomere 1(a1) large, slightly elongate and more swollen at base; a2dilated, narrower than the previous one but broader than a3;a3 very long and the narrowest; a4 small and comparable inlength with antennomeres 5–8; a5 slightly longer and widerthan a4; antennomeres 6 and 7 almost in same shape and size;a8 shorter than a7; a9 oval-shaped, much more setose at apicalhalf than basal half (Fig. 3c). Antennal grooves located alonginner edges of eyes, relatively wide. Maxillary palpus rela-tively long, four-segmented, palpomere 1 (p1) small, p2 elon-gate, fusiform; p3 slightly wider than p2, as long as p2; p4elongate, narrower than p3, about the same length as p3. Gularsutures widely separated.

Pronotum transverse, broadly explanate, 0.38 mm wide,0.26 mm long; widest nearly at base, anterior margin slightlyconvex in the middle, posterior margin bisinuate. Median areaof pronotum slightly convex, with four complete, nearlystraight longitudinal ridges (Fig. 3a, e). Antennal grooves onhypomera present. Prosternum transverse, with a smallprosternal process; procoxae transverse, contiguous(Fig. 3b). Prohypomera subtriangularly produced inwards(Fig. 3b). Procoxal cavities posteriorly open (Fig. 3b).Mesocoxae very narrowly separated (Fig. 3b). Metacoxaesmall, contiguous. Mesoscutellum transverse, subtriangular.Elytra 0.30 mm long, each 0.20 mm wide; elytra costate, eachelytron with a sutural costa, two complete discal costae and aslightly shorter third discal costa adjacent to humeral costa, ahumeral costa, and a curved epipleural costa; at least threeinner intervals punctate, each with two longitudinal rows ofpunctures; hind margin slightly curved. Hind wings fully devel-oped, with setae forming fringe along margin (Fig. 1). Legsslender, simple, densely setose; protrochantins exposed;procoxae transverse, protrochanters small; profemora clavate,protibiae elongate, with two oblique rows of microsetae at apicalhalf of ventral sides; protarsi apparently three-segmented, apicaltarsomere longer than basal ones combined;mesofemora clavate,mesotibiae very slender, with at least three complete rows ofmicrosetae; mesotarsi not lobed, with apical tarsomere elongate,longer than previous ones combined; metacoxae with anteriormargin strongly curved, without metacoxal plates,metatrochanters teardrop-shaped, small, metafemora slightly lon-ger than mesofemora, metatibiae slender, thinner than protibiae,with about four complete rows of microsetae, metatarsi appar-ently three-segmented, apical tarsomere much longer than basalones combined. Claws simple; empodium absent.

Abdomen broad. Abdominal segments III–VII, each withone pair of broad laterosternites, without longitudinal carinaeor basal ridges. Abdominal ventrites 1–5, each with a pair ofapically produced processes at lateral sides (Fig. 2b). Ventrites1–4, almost in the same length; ventrite 5 much longer; tergiteVII long, subtriangular.

Male. Ventrite 5 with a pair of strong longitudinal andslightly ridges on the median area (Fig. 3f). Ventrite 6

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emarginate, with a depression adjacent to the emargination(Fig. 3f). Aedeagus with sharp apex (Fig. 3g).

Discussion

Even though the Staphylinidae is one of the most diversefamilies of all animals (>59,000 described species), the newgenus Protopeplus is easily placed in Micropeplinae based onits overall body shape and the presence of several adult

autapomorphies, including antennae with nine antennomeres,apical one forming a large club and antennal grooves presenton lower surface of head and on prohypomera (Hansen 1997;Löbl and Burckhardt 1988). Micropeplinae currently com-prises six genera: Arrhenopeplus Koch, 1937; CerapeplusLöbl and Burckhardt, 1988; Kalissus LeConte, 1874;Micropeplus Latreille, 1809; Peplomicrus Bernhauer, 1928and Pseudokalissus Ryabukhin, 1990 (Herman 2001).Protopeplus gen. nov. is separated from the two remarkablegenera Kalissus and Pseudokalissus in having contiguous

Fig. 1 Protopeplus cretaceusgen. and sp. n., holotype. a Dorsalview. b Ventral view. Undernormal light. hw hind wing. Scalebars: 200 μm

Fig. 2 P. cretaceus gen. and sp.n., holotype. a Dorsal view. bVentral view. Under greenfluorescence. Scale bars: 200 μm

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procoxae, very narrowly separated mesocoxae, distinct ridges onpronotum and elytra with longitudinal rows of punctures in theintervals of costae. Protopeplus differs from Peplomicrus byhaving sharp teeth on the lateral pronotal sides, normal (notconcave) clypeus and in the absence of carinae on abdominaltergites and from Cerapeplus in having normal frons and vertex,distinct ridges on pronotum, elytra with longitudinal rows ofpunctures in the intervals between costae, narrowly separatedmesocoxae, and much shorter metatrochanters, and by lackingdense strong setae on the lateral margins of pronotum, elytra andabdomen, submarginal carinae on abdominal tergites. In super-ficial appearance, Protopeplus is somewhat similar to the speciesof remaining genera Arrhenopeplus and Micropeplus. Theyshare similar body shape, several carinae on pronotum and elytra(Campbell 1968). Compared to Arrhenopeplus, Protopeplus ismore related to Micropeplus, since the elytral intervals inMicropeplus are distinctly and coarsely punctate, whereas theintervals are impunctate and shining in Arrhenopeplus.Protopeplus differs significantly in having four distinct longitu-dinal ridges on pronotum, contiguous procoxae, narrowly sepa-rated mesocoxae and unmodified abdominal tergites.

The new genus shows many peculiar characters that are notfound in other genera. First, the proportion of maxillary palpus inProtopeplus is very distinctive for Micropeplinae. The reducedthird maxillary palpomere is regarded as one of the adultautapomorphies that define the subfamily (Löbl and Burckhardt1988). However, the maxillary palpomere 3 is not reduced and isabout the same length as the apical palpomere in the new genus.Second, the presence of four distinct longitudinal ridges onpronotum in Protopeplus is very remarkable for Micropeplinae.Usually, the shape of sculptures and ridges on pronotum is usefulfor defining and distinguishing extant genera. For example, inextant genera Kalissus and Pseudokalissus, the pronotal surfaceof species of the extant genera Kalissus and Pseudokalissus issmooth and lacking any ridges or depressions (Campbell 1968).In Arrhenopeplus, Micropeplus and Peplomicrus, there are aseries of closed cells delimited by raised carinae on the medianarea of pronotum (Campbell 1968). However, in the unusualgenus Cerapeplus, there are several depressions on the medianarea of pronotum and ridges that are not developed (Löbl andBurckhardt 1988). Therefore, the character that there are fourdistinct longitudinal ridges on pronotum in Cerapeplus is not

Fig. 3 Enlargements of Protopeplus cretaceus. a Pronotum and elytra. bPro-, meso- and metathorax, showing transverse, contiguous procoxaeand narrowly separated mesocoxae. c Details of apical fourantennomeres. d Left antenna and maxillary palpus. e Right half ofpronotum, with two longitudinal ridges shown. f Apical two abdominalsegments, showing male secondary sex characteristics, including

emarginate sternite VIII and the presence of a pair of strong longitudinalridges. g Same as f, showing aedeagus. a and b were taken using CLSM;d and e under green fluorescence, others under normal light. aantennomere, aed aedeagus, em emargination, mp maxillary palpomere,pr projection, ri ridge. Scale bars: 200 μm in a and b, 50 μm in c, 100 μmin others

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only important for separating it from the other genera, but alsovery significant for understanding the origin and formation of thepeculiar structure and even for phylogenetic implications. Third,the procoxae are contiguous and the mesocoxae are very nar-rowly separated in Protopeplus. Among extant micropeplinegenera, only one genus, Cerapeplus, has contiguous procoxae(Löbl and Burckhardt 1988), while the remaining genera havedistinctly separated procoxae (Campbell 1968, 1973, 1978,1989; Ryabukhin 1990). Furthermore, Recent genera have wide-ly to moderately separated mesocoxae, which is different fromthose of Protopeplus. It is probable that the contiguous procoxaerepresent an ancestral character for the Micropeplinae, and it isprobably of the same situation for the narrowly separatedmesocoxae. Fourth, the abdominal ventrite 5 of Protopeplus(male) has a pair of strong longitudinal and slightly protrudingridges on the median area. This character is absent in all theRecent micropeplines. Lastly, Protopeplus has a pair of apicallyproduced processes at lateral sides of each abdominal ventrites1–5. This character is never found in any of the extantMicropeplinae, or in the Staphylinidae.

With only five fossil micropepline species of the extantgenera Micropeplus and Kalissus from the Pliocene deposits(Matthews 1970; Gersdorf 1976), to date, no Mesozoic repre-sentatives are known. The new discovery of a remarkablegenus from about 99 million years ago stands for the oldestfossil record for this small subfamily. It suggests that theMicropeplinae has already originated during that time andpossibly can be found in other continents of that time.

Acknowledgments We are very grateful to four anonymous reviewersfor providing useful criticism and constructive suggestions that improvedthe manuscript. We thank Prof. Dany Azar for preparing the amberspecimen. This research was supported by the National Basic ResearchProgram of China (2012CB821903), Outstanding Youth Foundation ofJiangsu Province (BK 2012049) and the National Natural Science Foun-dation of China (91114201 and J1210006).

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