Ichnos An International Journal for Plant and Traces

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An boring in an Early wood from Bornholm, Denmark

Radek Mikuláš, Jesper Milàn, Jorge F. Genise, Markus Bertling & Richard G. Bromley

To cite this article: Radek Mikuláš, Jesper Milàn, Jorge F. Genise, Markus Bertling & Richard G. Bromley (2020): An insect boring in an Early Cretaceous wood from Bornholm, Denmark, Ichnos, DOI: 10.1080/10420940.2020.1744587 To link to this article: https://doi.org/10.1080/10420940.2020.1744587

Published online: 03 Apr 2020.

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An insect boring in an Early Cretaceous wood from Bornholm, Denmark

Radek Mikulasa, Jesper Milanb, Jorge F. Genisec, Markus Bertlingd and Richard G. Bromley† aInstitute of Geology of the Czech Academy of Sciences, Praha, Czech Republic; bGeomuseum Faxe, Faxe, Denmark; cDivision Icnologıa, Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina; dGeomuseum der WWU, Munster,€ Germany

ABSTRACT KEYWORDS An insect boring of unique shape is described from a lignitic layer within the Early Wood borings; Cretaceous; Cretaceous () Skyttegård Member of the Rabekke Formation on Bornholm. lignite; ; agrichnia Morphologically it cannot be compared to any modern or wood borings, although some structures are reminiscent of Scolytidae, Platypodidae and Lymexylonidae. Most prob- ably, however, the tracemaker was a female fungus-farming , thus producing an agrichnion.

Introduction Milan, and Noe-Nygaard 2008), the Berriasian Jydegård Significance of insect borings Formation with lagoonal and backbarrier sediments (Noe-Nygaard and Surlyk 1988), and the barrier to Terrestrial trace such as structures in palaeosols, foreshore Berriasian (Valanginian?) Robbedale wood, plant leaves and bones are of paramount import- Formation (Nielsen, Hansen, and Simonsen 1996). ance in the reconstruction of evolutionary pathways The Rabekke Formation contains two members: the and ecosystem structure (e.g., Labandeira 1998, 2013; fluviatile Homandshald Member and the overlying Wilf et al. 2006; Wappler et al. 2009). Nonetheless, they Skyttegard Member (Gravesen, Rolle, and Surlyk 1982), have received little taxonomic attention until compara- which both have their type localities in the immediate tively recently (e.g.,Wisshak, Knaust, and Bertling 2019). vicinity of the study site. The Skyttegård Member, i.e., have undergone a major radiation following the the stratigraphic level that yielded the borings described success of angiosperms (e.g., Labandeira 2013), a pro- herein, contains a relatively rich fauna. It cess at least partly recorded in their trace fossils. comprises crocodilian teeth (, Theriosuchus, Againstthisbackground,theobjectivesofthiscontribu- ; Schwarz-Wings, Rees, and Lindgren 2009), tion are to describe a new insect boring in wood and to teeth, scales and jawbone fragments of actinopterygians relate it to possible producers. We are, however, aware and sharks, postcranial remains of and to erect a new ichnotaxon on the material because at primitive (Rees, Lindgren, and Evans 2005), frag- least one-half of the sample (i.e., possible holotype) is ments of carapaces, teeth of small dromaeosaurid affected by a pyrite weathering which is capable to des- dinosaurs (Lindgren et al. 2008)aswellasatoothofa troy the sample in the horizon of few dozens of years. multituberculate mammal (Lindgren et al. 2004). In addition to the small-sized body fossils, large trace fos- Geologic setting sils are known: Surlyk, Milan, and Noe-Nygaard (2008) have described a trample-ground with abundant cross- Mesozoic sediments of Bornholm are exposed in a ser- sections of dinosaur tracks (up to 70 cm in length) and ies of faulted blocks along the western and southeastern possible lungfish estivation burrows. coasts of Bornholm, with Early Cretaceous sediments cropping out in the coastal cliffs around the town of Material and methods Arnager (Figure 1). The Nyker Group (Gravesen, Rolle, and Surlyk 1982) includes three formations: the domin- The studied specimen was recovered in 2007 as two antly terrestrial Berriasian Rabekke Formation (Surlyk, closely adjacent pieces at the base of low coastal cliffs of

CONTACT Radek Mikulas [email protected] Institute of Geology of the Czech Academy of Sciences, Praha, Czech Republic †Deceased March 24, 2018 ß 2020 Informa UK Limited, trading as Taylor & Francis Group 2 R. MIKULÁŠ ET AL.

Figure 1. Geological map of Bornholm, with inset of Denmark (modified after Graversen 2009); locality marked by asterisk.

Figure 3. Present (autumn 2018) condition of the two-piece specimen after drying.

robust carton/plastic boxes. The pieces of wood were unpacked approximately a year later; then, wood was relatively hard and safe to manipulate. A marcasite/ pyrite fill present on the thinner of the two pieces of wood had remained almost unchanged; in the thicker part, however, the marcasite had disintegrated into iron sulfate(s) (probably halotrichite or rozenite) par- tially coating the surface as microscopic, colourless, needle-shaped crystals. The boring was then inspected visually and compared to descriptions in the relevant literature, focussing on European occurrences.

Results Description of the wood specimen The wooden substrate of the boring described below con- Figure 2. A: Low cliff formed by the Berriasian Skyttegård sists of a conifer stem fragment 43 mm long showing well- Member of the Rabekke Formation east of Madsegrav, preserved annual rings and fibres containing some marcas- Bornholm, looking east towards Homandshald (as seen in ite. It is preserved as two parts, which can be matched 2007); position 55.040413 N, 14.850809E. B: Sample location: along a fissure (Figure 3). This is longitudinally tangential The Skyttegård Member of the Rabekke Formation exposing a lignitic layer; condition in 2007. to the wood fibres, and the boring studied is exposed on it. The overall thickness of the sample is approximately Arnager Bay (Figure 2). It originates from the outcrop 25 mm, and it consists of three zones, listed here from studied in great detail by Surlyk, Milan, and Noe- interior to exterior: 7 to 14 mm intact wood; a fissure of 2- Nygaard (2008), which exposes the Berriasian Skyttegård 3 mm width; some 10 mm of unstructured coaly matter, Member of the Rabekke Formation. The location is situ- obviously representing the bark. ated some 500 m east of Madsegrav or 1300 m east of The inner side of the smaller (internal) is slightly the harbour of Arnager, Bornholm (Denmark). convex, bearing a knothole about 40 mm long. The At the time of recovery, the wood consistency was opposite side shows a nearly plain surface with intact mushroom-like. In order to prevent destruction owing wood. Here, the trace fossil described is pronounced to shrinkage, the specimen was slowly air-dried in by the curvature of wood fibres. ICHNOS 3

fissures as partial breaking of the stem may be due to storm damage, or secondly, the split may have occurred during embedment. It is therefore unclear whether a tracemaker took advantage of an accidental situation, or whether this is just a fortunate find facili- tated by a special taphonomic situation. In any way, a series of somewhat radial side branches has arisen from a common longitudinal tun- nel bored close to the bark, parallel to the vascu- lar elements.

Systematic position of the producer Figure 4. Schematic sketch of the sample prior to fossilization (part of the thinner piece graphically removed to indicate the Among the numerous modern wood borers of various position of the boring). systematic attribution, few produce a boring pattern containing repeated rectangular branching. These few The inner surface of the larger piece corresponds belong to the Siricidae (wood wasps) and two families to its smaller counterpart just described. The exterior of beetles, the Scolytidae (bark beetles) and exhibits an intensely coalified film, which includes a Platypodidae (pinhole borers, sometimes just consid- larger “knot”. Parts of the exterior lack regular fibres ered a subfamily of Scolytidae, but see e.g., Poinar, and can be interpreted as bark remnant as well. The Vega, and Legalov 2018). knot is rounded, 9.4 mm in diameter; it height is The larvae of some wood wasps (Siricidae: 3 mm. Convexity of the knot out of the stem (Figure Urocerus) produce a pattern similar to the studied 3) suggests that the thinner piece largely represents specimen, i.e. with slightly bent tunnels running radi- inner parts of the wood. If this had contact to the ally at some distance, but they are club-shaped (e.g. bark at all, then only along its thin edge. Schwenke 1974; Brauns 1991) and thus quite different. Despite an overall morphological similarity of the Description of the boring borings, most modern Scolytidae can be excluded, based on the usually tangential orientation as well as The boring resembles a rake or two E letters placed atop details of their boring pattern: Side branches made by of each other (Figures 4 and 5). It is filled with a homo- modern Scolytidae exhibit an elongated club shape genous mass of a coaly substance randomly mixed with instead of the identical diameter seen here, they small pyrite aggregates. A 30 mm long axial tunnel is pre- branch off at a very close distance, and/or they are served on the outer edge of the wood specimen with more or less strongly curved rather than straight (e.g. approximately half its original diameter of 1.6 mm. It Ratzeburg 1837; Vite 1953; Schwenke 1974; Brauns more or less follows the course of the wood fibres, but it is 1991). The diameter of the side branch in these cases irregularly bent and slightly sinuous. Five better-preserved is similar to the space between it and the neighbour- lateral tunnels are connected to this long axis at almost ing branch, whereas in the specimen described here, right angles. As a consequence, they also run approxi- the space between the branches is very regular with mately perpendicular to the wood fibres, i.e. parallel to the three to four times their diameter. log radius. The tunnels are slightly curved, partially resem- Two scolytid tribes (ambrosia beetles) cultivating bling a C- or J-shape. Apart from those variations, the ascomycete-microbial films at the surface of their bor- side-branches are oriented roughly parallel. Their length is ings, however, exhibit a penetration pattern deviating 12 mm, 10.3 mm, 9.5 mm, 5.2 mm and 9.4 mm, with the from the other members of the family, as they do not width remaining nearly constant at 0.70 0.85 mm. brood under the bark but within the wood. In the Xyloterini (e.g. Xyloterus, Trypodendron), a radial Discussion entrance shaft is followed by a few side branches in rectangular position, which in turn carry very short, Origin of the boring almost stubby, pupal chambers oriented vertically The oblique fissure exhibiting the boring probably (e.g., Schwenke 1974; Amann 1990). Xyleborus was not produced by the tracemaker. Rather, two (Xyloborini) bores branched tunnels that either run alternative scenarios may be envisaged: Firstly, such radially into the sapwood or tangentially to it (e.g., 4 R. MIKULÁŠ ET AL.

Figure 5. Boring on a conifer tree from the Skyttegård Member of the Rabekke Formation, A: larger piece, B: smaller piece with borings accentuated; photos before drying in 2007.

Escherich 1923; Vite 1953; Amann 1990; Brauns stem of Lymexylon is “Lymexyl-”(see also Wheeler 1991). The pattern produced by Xyleborus dispar 1986). Article 29.1 of the International Code of (Fabricius) is quite similar to the one observed in the Zoological Nomenclature (ICZN (International fossil specimen, but the side branches are very straight Commission for Zoological Nomenclature) 1999), how- in the former, exactly parallel, equally spaced and ever, explicitly allows formation of a family name based rather short. All Scolytidae may therefore be excluded on the nominative of its type genus, and Article 29.5 as producers of the study specimen. demands maintenance of a family name spelling in pre- Female beetles of the Platypodidae bore branched vailing usage, even in an etymological incorrect way. radial tunnels into the sapwood (e.g., Escherich 1923; Thus Barber (1952)waswrongto“emend” the family Schwenke 1974), frequently exhibiting stepwise progress name of Fleming (1821).] with rectangular offshoots (so-called “Leiterg€ange” [¼ Certain similarity can be found between the herein- ladder galleries] of German authors), a pattern that is described borings and probable beetle borings in bog oak almost identical to Xyloterini. This is true also for the trunks (Jach et al. 2018). These authors depicted both pupal chambers linked to this system; they also are too branching and unbranching galleries occurring on radial regular and short to be considered a modern analogue ruptures of oak trunks; they, however, do not correspond of the fossil borings from Bornholm. to the comb-like plan of the find from Bornholm. If there was no longitudinal tunnel, the boring pat- In essence, no modern analogue of the fossil boring tern observed might also correspond to Lymexylonidae is known to us, although a fungus-farming beetle is (ship-timber beetles; Hylecoetus, Lymexylon)aspro- the most probable producer. Among them, the boring ducers; their tunnels are very similar in orientation, pattern of xyloterine scolytids is closest to the fossil shapeandevensizebyrunningmoreorlessparallelto one, though sufficiently different to preclude identifi- each other in various orientation to the wood fibre (e.g. cation of this beetle group as the tracemaker. Nußlin€ 1922;Escherich1923;Vite 1952,Brauns1991). [Nomenclatural note: The name Lymexylonidae was Ecology of the producer established by Fleming (1821)basedonLymexylon Fabricius 1792.Barber(1952) explained that it should The boring pattern of a main shaft with multiple, be changed it to , as the correct genitive short, almost rectangularly set branches is only seen ICHNOS 5 in two beetle families, the Platypodidae and the xylo- geometrically simple structures. None of these avail- terine Scolytidae. Both groups share an endoxylic life- able ichnogenera (see Wisshak, Knaust, and Bertling style, i.e. the wood forms both substrate for 2019) is close to the Bornholm specimen, as none ascomycete cultivation and shelter for larvae and comprises a longitudinal tunnel and radial galleries. adults. For this judgement, the orientation of the tun- nels towards the wood is unimportant, especially the Conclusions question whether the beetle started boring in the bark or at the fissure. A wood boring from the Berriasian of Bornholm, In both cases, female adults carry fungal spores in Denmark, exhibits a hitherto unreported pattern. special pouches (mycangia) to inoculate the fresh/liv- Neither modern nor fossil records exist of borings con- ing sapwood. They are mixed xylo- and myceto- sisting of a longitudinal (vertical in upright stem) tun- phages, whereas the larvae feed on the ambrosia nel and side rather straight branches running radially fungal film, at least during their first weeks (Vite into the heartwood. Comparing this specimen with the 1952; Schwenke 1974). Later, they may expand exist- structures produced by modern xylophagous beetles ing tunnels to pupal chambers, but this is not seen in shows that a female fungus-feeding (“ambrosia”) beetle the case studied here. Regardless of the possibly is the most probable producer. At a general scale, this doubtful attribution to a certain family, the boring has report shows that wood borings may add valuable been produced by a female beetle laying the founda- information to our understanding of evolu- tion for its ambrosia culture. The boring described tion, but that the field is grossly under-studied. here therefore qualifies as an agrichnion (cf. Vallon, Rindsberg, and Bromley 2016). Acknowledgement Several wood boring beetles are known from the Early Cretaceous as body fossils, and due to the excel- All the co-authors are indebted to Richard Bromley for his deep knowledge, sense of humour and love of the nature. lent preservation of tree- related insects in amber, They thank the official Ichnos reviewers, Alfred Uchman ambrosia beetles are even over-represented in the fos- and Lothar H. Vallon, for numerous useful comments. sil record (e.g., Poinar, Vega, and Legalov 2018). The body fossil record therefore well matches the trace fossil record. Disclosure statement No potential conflict of interest was reported by the author(s). Comparison with other fossil occurrences

Various fossil wood borings attributed to beetles have Funding been described (for a partial review, see Labandeira, Lepage, and Johnson 2001), many of them clearly rep- RM thanks the Czech Science Foundation for the support (No 18-05935S). The paper is financially supported also by resenting scolytid galleries with tangential radiating Research Plan of the Institute of Geology of the Czech tunnels and hence with an orientation differing from Academy of Sciences, No RVO 67985831. the sample studied here. A boring most similar in pat- tern but dissimilar in orientation of the tunnels has been reported from the German middle . Kierst References and Wiesner (1975) refer this trace fossil with an iden- tical penetration depth, size and branching pattern to Amann, G. 1990. Kerfe des Waldes, 343. 10th ed. Augsburg: the Bostrychidae, based on these criteria. Certain simi- Naturverlag. “ larity (i.e., occurrence of borings both in radial and in Barber, H. S. 1952. Notes on Telegeusis and Some Relatives (Coleoptera: Lymexylidae.).” The Pan-Pacific Entomologist subsurface positions towards the wood sample) can be 28: 163–170. recognized also between the Bornholm lignite boring Brauns, A. 1991. Taschenbuch Der Waldinsekten. 4th ed., and the find determined by Rajchel and Uchman 860. Stuttgart – Jena: Fischer. (1998; Oligocene, Poland). However, the material of Escherich, K. 1923. Die Forstinsekten Mitteleuropas; Zweiter Rajchel and Uchman (1998) lacks the typical E – Band, Spezieller Teil, Erste Abteilung, 663. Berlin: Parey. Fabricius, J. C. 1792. “Determinatio Generis Ips branching as observed in the material described herein. Affiniumque.” Actes de la SocieteD’Histoire Naturelle de Few insect wood borings have been formally Paris 1: 27–35. named; some names refer to pupal chambers, some to Fleming, J. 1821. “Insecta.” Supplement to the Encyclopaedia tangentially radiating scolytine tunnels, some to Britannica 5: 41–56. 6 R. MIKULÁŠ ET AL.

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