Bioerosion of siliceous rocks driven by rock-boring freshwater insects

Macrobioerosion of mineral substrates in fresh water is a little-known geological process. Two examples of rock-boring bivalve molluscs were recently described from freshwater environments. To the best of our knowledge, rock-boring freshwater insects were previously unknown. Here, we report on the discovery of insect larvae boring into submerged siltstone (aleurolite) rocks in tropical Asia. These larvae belong to a new mayfly species and perform their borings using enlarged mandibles. Their traces represent a horizontally oriented, tunnel-like macroboring with two apertures. To date, only three rock-boring animals are known to occur in fresh water globally: a mayfly, a piddock, and a shipworm. All the three species originated within primarily wood-boring clades, indicating a simplified evolutionary shift from wood to hardground substrate based on a set of morphological and anatomical preadaptations evolved in wood borers (e.g., massive larval mandibular tusks in mayflies and specific body, shell, and muscle structure in bivalves).


INTRODUCTION
A wide array of rock-boring animals and their bioerosion traces was described from marine environments [1][2][3] . These animals are common in recent and fossil sea beds 4 and belong to multiple invertebrate taxa such as molluscs 5,6 , crustaceans 7 , polychaetes 8 , sipunculids 9 , sea urchins [10][11][12] , sponges 13,14 , and bryozoans 2,15 . Seawater rock borers play a significant role as ecosystem engineers that may greatly increase the complexity of hardground substrates and biodiversity of associated communities 16 . Furthermore, a number of marine geological processes such as sedimentation, erosion of rock surfaces, and evolution of coastal profiles and coral reefs are greatly influenced by rock-boring animals globally [17][18][19][20][21][22] .
Conversely, rock borers are extremely rare in freshwater habitats and the relevant organisms are usually derived from marine bioeroders 23 . Until recently, macrobioerosion of rock substrates in modern freshwater environments was unknown 23,24 . There were a few reports describing bioerosion structures in freshwater mollusc shells that are largely associated with caobangiid polychaetes (Annelida: Sabellidae) 25,26 . These minute worms are an exclusively freshwater group that contains seven species boring into shells of gastropods and bivalves in Southeast Asia, India, and Sri Lanka 26,27 . Microborings in subfossil and recent freshwater bivalve shells were also discovered in North America but the producers of these boreholes remain unknown 28,29 . Other works describe microborings in freshwater mollusc shells produced by endolithic cyanobacteria in North America 30 and Argentina 31 .
Conversely, available paleoichnological data reveal that fossil macrobioerosion structures are widespread in freshwater settings globally [32][33][34] , although these settings have received little attention compared with their marine counterparts 35 . A variety of trace fossils is known to occur in freshwater deposits, including those associated with putative invertebrate macroborings 3,36,37 . Most of these ichnotaxa were described in calcareous hardgrounds 3 .
The first modern silicate rock-boring freshwater organism was discovered in the middle reaches of the Kaladan River in Myanmar in 2018 24 . It is a close relative of marine piddocks and belongs to the bivalve species Lignopholas fluminalis (Blanford, 1867) (Pholadidae). This species was known to occur exclusively in estuaries 38 but the Kaladan's population is adapted to live in fresh water 24 . The clavate borings of this species in siltstone rocks are covered by a microbial biofilm, the members of which could promote bivalve bioerosion through rock weathering by dissolving Mn-rich chlorites 39 . A year later, the second freshwater rockboring animal was described from the Philippines 40 . This unusual bivalve species, Lithoredo abatanica Shipway, Distel & Rosenberg, 2019, a member of the Teredinidae (commonly known as shipworms), drills into and ingests the limestone substrates in the lower reaches of a small river on the Bohol Island 41 .
A small body of available literature describes burrows made by recent freshwater insects in various firmgrounds such as claystones, sandstones, shales, and marls [42][43][44] . These burrows were produced by larval stages of mayflies (Ephemeroptera), nonbiting midges (Diptera: Chironomidae), and caddisflies (Trichoptera). The present study extends our knowledge on bioerosion in freshwater settings via studying a previously unknown case of a siltstone (aleurolite) boring produced by mayfly nymphs from tropical Asia (Myanmar).
This contribution (1) reports on the discovery of a rock-boring freshwater insect; (2) describes this insect as a species of mayfly 1 new to science using morphological, ethological, and DNA-based evidence; (3) provides a description of recent and sub-recent bioerosion traces produced by mayfly nymphs in siliceous substrate; (4) briefly discusses ichnotaxonomic issues regarding macroborings produced by freshwater invertebrates; and (5) proposes a hypothesis on the evolutionary origin of rock-boring freshwater invertebrates.

Discovery of rock-boring freshwater insects
Siliceous rock outcrops with bioerosion traces were recorded at two sites of the Bago (historically Pegu) River, Myanmar (Figs. 1a, b, 2a-f, 3a-e, 4a-c, and 5a-e). The first site is located at the middle reaches of the river (17.6797°N, 96.2318°E, altitude 40 m), while the second site is situated at the upstream section (18.0791°N, 96.0449°E, altitude 195 m) (Fig. 1). Both sites are freshwater and are well above the upper limit of tidal influence.
At the first site, the river crosses a continuous outcrop of Miocene siltstone (aleurolite) rocks of approximately 300 m long (Fig. 2a, b). The siltstones are exposed at the right shore (Fig. 2b) and throughout the river bottom (Fig. 2a, b). Multiple insect borings densely penetrate the upper layer (up to 20-30 mm deep) of the submerged rocks. The typical boring represents an elongate tunnel-like structure, which is oriented horizontally, and shares two circular end openings (apertures) at the outer surface ( Fig.  3a-c). Numerous living mayfly nymphs were recorded inside the borings (Fig. 3d, e), whereas a number of borings were abandoned and, in some cases, filled with clay. A substance with a silky texture covers the inner surface of borings inhabited by nymphs (Fig. 3e) and, rarely, of abandoned borings (Fig. 5b). There are also elongate grooves of various depth at the rock surface, representing eroded borings having a more or less concave bottom, with the oldest ones being shallower (Fig. 3b, c). Siltstone rocks with subrecent traces of the same morphology were also recorded at the surface of the youngest river terrace, above the level of the present floodplain ( Supplementary Fig. 1).
At the upstream section, the river valley shares a characteristic V-shaped profile and is deeply incised into Miocene clay sediments 45 with numerous outcrops of siltstone rocks (Fig. 2c). The upper parts of siltstone rocks at this site were exposed, because the water level in the upstream was dropped drastically due to the lack of rainfalls at the end of the dry season (24 March 2020) (Fig. 2d). The surface of rocks was completely covered by a dense network of grooves and borings, which were morphologically similar to those recorded in the middle reaches of the river (Fig. 2e, f). At this site, all the borings were recently abandoned due to a drop in the water level, although a few fragments of dead insect larvae were collected inside some tunnels. The rock surface with eroded borings often bears a thin encrustation of an unidentified freshwater sponge (order Spongillida) (Fig. 2e). In many cases, the bioerosion surface and borings were filled with clay originated from the deposition of river suspended matter (Fig.  2f), while some smaller bioerosion beds were almost completely overlapped by alluvial clay sediments. These features indicate rather a sub-recent than recent origin, although the exact age of these traces was not estimated.
Based on morphological and genetic analyses, the rock-boring nymphs from the middle reaches of the river were found to be representatives of a mayfly species from the genus Languidipes  Hubbard, 1984 (Ephemeroptera: Polymitarcyidae). This species is new to science and is described here. Though we were unable to identify the larvae from the upstream section with certainty because of the very poor condition of available body fragments, these borings were likely produced by the same species as those in the middle reaches of the Bago River. The sub-recent bioerosion beds in the river valley should also be attributed to the dwelling activity of this mayfly species because these traces are morphologically similar (if not identical) to the borings made by modern mayfly nymphs.

Description of the rock substrate
The rock substrate was classified as a siltstone (aleurolite) (primary grain size of 2-62 μm). Samples from the middle reaches and the upstream section of the Bago River share minor differences (Figs. 2, 3). In particular, the X-ray diffraction (XRD) analyses indicate that the rock from the middle reaches of the river consists of quartz (40 wt.%), clay minerals (17 wt.%), feldspars (14 wt.%), mica (14 wt.%), chlorite (11 wt.%), and pyrite (4 wt.%). Backscattering Electron (BSE) images show a thin-grain rock with organic matter inclusions (black) and pyrite concretions ( Supplementary Fig. 2a). The siltstone contains large (ca. 100 μm) grains of mica and chlorite. The microindentation hardness (Vickers test) ranged from 0.42 to 0.50 GPa. Based on the XRD analyses, the siltstone rock from the river's upstream consisted of quartz (59 wt.%), feldspars (19 wt.%), chlorite (10 wt.%), mica (8 wt.%), small amounts of clay minerals (3 wt.%), and calcite (1 wt.%). The BSE images show that this rock is uniformly grained, and also contains organic matter inclusions ( Supplementary Fig. 2b). The upstream siltstone contains carbonate-clay cement, which makes it more resistant to swelling. A higher content of quartz and feldspars gives elevated microhardness values (Vickers test) within a range of 0.60-0.75 GPa. The siltstone (aleurolite) studied in this work is not subjected to wetting and preserves its shape under pressure. The siltstone is cemented and can be considered as hard substrate given its hardness, cementation, and capacity to preserve its shape.

Description of the recent and sub-recent mayfly borings
The mayfly dwelling traces are described here on the basis of siltstone specimens collected from submerged rock outcrop (Figs. 4a-c and 5a-c). The upper surface of rock specimens contains apertures of the untreated borings produced by the new mayfly species, as well as a network of elongated grooves associated with its eroded borings. A typical trace has a total length of 20.1 mm and a maximum width (diameter) of 4.4 mm (Fig. 4c). It represents a horizontally oriented, banana-shaped, tunnel-like macroboring, circular in cross-section; its bottom outline is curved, while the upper outline is straight (Figs. 4c, 5c-e). It is connected with the outer surface of the rock by two circular apertures (end openings) (Fig. 5a). In some cases, one of the two apertures is not fully developed, representing a narrow, ovate or irregular hole (Fig. 5a). The inner side of the boring is rather smooth, without showing clear scratches in the examined traces (Fig. 5b). The macroboring is separated from the outer surface by a very thin and fragile rock layer. This layer could usually be traced by a few minute fractures at the rock surface (Fig. 5a). In sub-recent borings, the upper layer is often lost due to ongoing erosion processes. In such case, the partly eroded boring represents a narrow, elongated groove with parallel (sometimes slightly convex or concave) sides and concave bottom in plan view (Fig. 5b), and looks like a member of the calcareous ichnogenus Petroxestes Wilson & Palmer, 1988 (Rogerellidae) 3,46 .

Phylogenetic affinities of the rock-boring mayfly species
The cytochrome c oxidase subunit I (COI) gene sequences were obtained from five nymphs of the rock-boring mayfly (Supplementary Table 1). These sequences returned two unique and rather divergent COI haplotypes with a mean uncorrected pdistance of 2.48% (Supplementary Table 2). In some cases, such a genetic distance between COI haplotypes could indicate interspecific differences but our PTP species delimitation analysis suggested that the two haplotypes most likely belong to a single rock-boring species. The average genetic distances between the new species and two already known Languidipes taxa range from 7.83 to 8.68% that is well above an intraspecific level. In its turn, the mean COI p-distance between Languidipes corporaali (Lestage, 1922) and L. taprobanes (Hubbard, 1984) is a bit lower, 5.97%. Our phylogenetic analyses revealed that the genus Languidipes represents a well-supported clade ( Supplementary Fig. 3). The rock-boring species was recovered as sister to the wood-boring L. corporaali and L. taprobanes. Bago River]. The latter species is described here. An undescribed species was recorded from Assam, India but it is known from a single century-old female specimen 47,50,51 . The Barcode of Life Data System (BOLD; http://www.boldsystems.org) 52 contains four COI sequences of one more undescribed species from the Bago River, Myanmar.
Comments: This taxon was described as a subgenus of Povilla Navás, 1912 47 . Later, it was elevated to the full generic rank based on a set of morphological evidences 48 . Our results support this decision, as the African wood-boring mayfly Povilla adusta Navás, 1912 (the type species of the genus Povilla) is found to be distant phylogenetically from Languidipes taxa, including L. corporaali (the type species of the latter genus).
Languidipes    Differential diagnosis (mature nymph): The new species is morphologically similar to the congeneric L. taprobanes and L. corporaali but could be distinguished from these taxa by the following combination of characters: anteromedian projection of the head semicircular, subapical mandibular tubercle of both tusks small. L. taprobanes also shares a semicircular anteromedian projection but differs from the new species by having a left mandibular tusk with four teeth. In its turn, L. corporaali could be distinguished from the new species by having a triangular anteromedian projection of the head and much larger, rounded subapical mandibular tubercle. The apical teeth of the rock-boring species are much more massive and rounded than those of the two wood-boring taxa.
DNA  Fig. 6a). Antenna is white, its total length is nearly equal to that of head, scape bare is long, pedicel is slightly shorter and with a single long seta ( Supplementary Fig.  6b), flagellum bears multiple annuli, decreasing in length proximally ( Supplementary Fig. 4c). Labrum with rounded anterior margin and numerous bristles (Supplementary Fig. 4a). Hypopharyngeal lingua is nearly triangular, rounded apically, superlinguae are subrectangular with convex margins (Supplementary Fig. 4b).
Glossae are slightly asymmetric, shorter by 1/3 than paraglossae ( Supplementary Fig. 4d). Segment I of labial palps is oval, with a group of 25-30 strong bristles situated on dorsal surface. Segment II is asymmetrical, spoon-shaped, 2.5 times longer than broad, with slightly concave inner margin ( Supplementary Fig. 4с). Maxillae are asymmetric with a single long and narrow, acutely pointed chitinous spine ( Supplementary Fig. 4h) and two groups of long bristles: ventral and dorsal ( Supplementary Fig. 4g). Segment I of maxillary palps is oblong-shaped with a single group of short setae on outer distal corner; segment II is oval, asymmetric, about three times longer than a wide and strongly haired on inner margin ( Supplementary Fig. 4g). Mandibles are black apically, covered by long yellowish setae basally, without tooth on outer border ( Supplementary Fig. 4e, f). Mandibular incisors are reduced. Mandibular tusks are massive, robust, and stout (Fig. 6c, d). Left tusk with three (in holotype) or four (in paratypes) apical teeth, median teeth smallest, apex of all teeth rounded ( Supplementary Fig. 4e). Right tusk with two teeth of nearly triangular shape, with rounded apex, the inner tooth is smaller than the outer tooth ( Supplementary Fig. 4f). Inner margin of both tusks with a small subapical tubercle. Dorsal surface of tusks bears numerous setae and a prominent, ovate dorsal tubercle. Thorax: anterior ring of pronotum with dark gray shading and with a line of yellow setae along posterior margin; posterior ring of pronotum with a large ovate patch densely covered by yellow setae (Supplementary Fig. 6d); mesonotum and metanotum are white; pleura and sternum are whitish. Wingbuds are white with dark brown markings pattern. Legs are white, with dense yellowish setae distally ( Supplementary Fig. 5). Leg I: femora broad and well developed, with a double ventro-basal row of long filtering setae; tibio-tarsus (fused) with 3 rows of filtering setae (one on anterior face dorsally, one on outer margin, and one on inner margin) (Supplementary Fig. 5a); tarsal claw with a row of 32-35 marginal denticles (Fig. 5b). Leg II: smaller, femora thinner, with a row of short pointed bristles on dorsal margin; tibia and tarsi with row of long setae on outer (dorsal) margin, ventrally with dense shorter setae subapically and with a brush of thick setae distally; tarsus bears a row of long setae on outer margin and a group of narrow stout setae distally ( Supplementary Fig. 5c); tarsal claw without denticles (Supplementary Fig. 5d). Leg III: similar to leg II but larger, femora bears dense, short setae along inner margin, dorsal surface with a field of short, thick spines proximally; tibia bears a row of long setae on outer margin, a brush of short, thick setae on inner margin, and a distal brush of short, thick setae ( Supplementary Fig. 5e); tarsus and tarsal claw as those in Leg II ( Supplementary Fig. 5f). Abdomen is whitish, terga II-IX with a median row of long hair-like setae ( Supplementary Fig. 6e), tergum X is shaded with dark gray and bears a few hair-like setae on the surface. Gill I is uniramous (Supplementary Fig. 6f); gills II-VII are biramous, white with dark gray median line (Supplementary Fig. 6g). Paraprocts are covered with sparse pointed setae, each lobe with a distinct distal projection ( Supplementary  Fig. 6j). Cerci with rich marginal fringe ( Supplementary Fig. 6h). Paracercus is longer by about 1/4 without marginal fringe ( Supplementary Fig. 6i). Cerci and paracercus are covered with pointed scales, especially numerous on basal segments. Male and female imago and subimago. Unknown.
Etymology: The name of the new species is derived from two words: lithos ('stone' in Greek) and phagus ('eater of' in Latin), as a reference to its rock-boring habit.
Biology: Nymphs of the new species are filter feeders, boring into siliceous (siltstone) rocks in riverine environment.
Distribution: Possible endemic to the Bago River basin, Myanmar.
Conservation: At first glance, the new species could be treated as endangered, because it shares a restricted range and inhabits a very specific freshwater environment. Currently, the Bago River basin is impacted by a variety of anthropogenic loads arising from agricultural and urban land use, deforestation, sewage inputs, garbage littering, and general habitat degradation 53 . These loads may negatively affect the rock-boring mayfly population.

DISCUSSION Macrobioerosion in fresh water and the origin of freshwater rock borers
To the best of our knowledge 23,54 , the present discovery of the new mayfly species Languidipes lithophagus sp. nov. from Myanmar represents the first example documenting macrobioerosion of hardgrounds by modern freshwater insects globally. Earlier, freshwater insect burrows were recorded in bedrock sediments of the Conecuh River, Alabama but these substrates represented firmgrounds and were subjected to wetting 44 . Furthermore, the new mayfly species appears to be the first rock-boring animal of non-marine ancestry ever reported from freshwater environments 23 . Currently, two more freshwater invertebrate species are known to produce macroborings in rocks, i.e., the piddock Lignopholas fluminalis 24,39 and the shipworm Lithoredo abatanica 40,41 . Both mollusc species are secondary freshwater members of primarily marine bivalve clades 24,40 .
Interestingly, the three rock-boring freshwater taxa of invertebrates outlined above were originated within primarily woodboring clades. Languidipes lithophagus sp. nov. belongs to a group of wood-boring mayflies and sisters to other taxa, the nymphs of which burrow in wood and bamboo 47 . Similarly, the piddock genus Lignopholas Turner, 1955 contains estuarine and freshwater bivalves, most of which bore into wood, with Lignopholas fluminalis being the only exception 38 . A putative Mesozoic stem lineage of this group, Palaeolignopholas kachinensis Bolotov et al. 55 from the mid-Cretaceous Burmese amber, also bored into wood and hardening resin of coniferous trees 55 . Finally, the shipworm Lithoredo abatanica is a unique rock-boring representative among a large monophyletic group of xylotrophic bivalves, the Teredinidae 40,[56][57][58] , although a few species in this group are considered as seagrass borers 59 .
Based on these data, we can assume that the evolutionary shift from wood to hardground substrate in fresh water is easier than that from free-living to rock-boring life style due to a set of specific preadaptations already available in wood borers. In particular, wood-boring mayfly nymphs have massive mandibular tusks 47 that could be useful to bore into lithic substrates such as siltstone. The larval mandibular tusks of Languidipes lithophagus sp. nov. are similar to those in wood-boring taxa 47 but have thicker and more rounded apical teeth (see Fig. 6c, d and Supplementary Fig. 4e, f  for detail). The wood-boring bivalves (Pholadidae and Teredinidae) also actively use mechanical pathways to make cavities and tunnels in lignic substrates, and their morphological and anatomical preadaptations (specific structure of body, shell, and muscles) 38,39,58 might facilitate an evolutionary shift to sedimentary rocks that was discovered in Lignopholas fluminalis 24,39 and Lithoredo abatanica 40,41 .
There is a large, monophyletic group of the so-called 'tusked burrowing mayflies', belonging to the infraorder Scapphodonta and characterized by having well-developed larval mandibular tusks 65,66 . Most of wood-boring mayflies belong to the family Polymitarcyidae (subfamily Asthenopodinae) 61 Table 3). The majority of available works are dedicated to wood-boring nymphs of the African mayfly Povilla adusta 60,[70][71][72] . In some cases, these nymphs were found to burrow into other kinds of substrate such as freshwater sponges, macrophytes, and sedge (e.g., Cyperus) stems 73,74 , as well as lithic firmgrounds 75 . Nymphs of Tortopsis sp. (Polymitarcyidae: Campsurinae) produce U-shaped burrows in hard clay deposits of the Caquetá River, Columbia 76 . Palingenia fuliginosa (Georgi, 1802) (Palingeniidae) seems to be the only nonpolymitarcyid mayfly species, the nymphs of which are known to bore into wood occasionally 67,77 . Recent pouch-like burrows recorded from firm mud in rivers of Eastern Europe were attributed to Palingenia and Polymitarcis (Ephoron) taxa 43 . In summary, wood borers appear to be more common in mayfly nymphs than firmground burrowers, whereas Languidipes lithophagus sp. nov. represents the only example of rock borers ever reported in freshwater insects. It was shown that mayflies play a number of ecosystem services such as bioturbation, bioirrigation, decomposition, nutrition for many animals, and nutrient cycling in freshwater environments 72 . Our discovery of a rock-boring mayfly species adds bioerosion of hard substrates to this extensive list. Interestingly, Languidipes lithophagus sp. nov. covers its tunnels by silky substance (see Fig. 5b for detail), as does the wood-boring species Povilla adusta from Africa 60,70 . It is unknown what is the function of this silk and whether other wood-boring mayflies produce such a substance. Borings of Lithoredo abatanica are lined with calcite tubes 41 but it is unclear whether these structures and mayfly silky tubes could be attributable to convergent evolution.

Ichnotaxonomic issues related to bioerosion in fresh water
Here, we show that mayfly nymphs may produce a specific 'banana-like' macroboring in siliceous rocks. There is a strong recommendation to avoid ichnotaxonomic descriptions of modern traces, the producers of which were recorded directly in the field 78 . Here, we follow these requirements and do not introduce an ichnotaxonomic name for the mayfly macroboring discovered at the Bago River, although some rock surfaces with sub-recent bioerosion beds, which are largely overlapped by alluvial clay sediments, were recorded at the youngest terrace in the midstream section of the Bago River. Though the tracemaker of massive bioerosion beds recorded at the upstream section of the river, as well as of those above the present floodplain levels in the midstream site, is unknown, these beds were almost certainly produced by the same rock-boring mayfly species. This conclusion is based on the morphological similarity of sub-recent macroborings and those produced recently by modern mayfly nymphs.
The macroboring produced by the modern mayfly nymphs is quite different morphologically from ichnogenera and ichnospecies that were discovered in hard substrates so far 3 . Moreover, this trace is morphologically distinct from recent mayfly burrows that were recorded in various soft and xylic substrates 43,79 . Recent and sub-recent dwelling activity of mayfly nymphs in firmgrounds, mud, and other soft sediments typically results in characteristic U-shaped burrows with long branches 42,43,54,76,80 , while Languidipes lithophagus sp. nov. produces a horizontally oriented tunnel with convex bottom and two apertures, but without clear branches. Wood-boring mayflies also produce U-shaped burrows in wood, bamboo, and other kinds of xylic substrate 47,60,70,79 .
In the fossil record, mayflies were thought to be producers of some traces belonging to the ichnogenera Rhizocorallium Zenker, 1836 [=Glossifungites Łomnicki, 1886 36 ], Fuersichnus Bromley &Asgaard, 1979, andAsthenopodichnium Thenius, 1979 35-37,43 . The first ichnogenus combines U-shaped burrows, with Rhizocorallium jenense Zenker, 1836 being its type ichnospecies, which could be produced by burrowing mayflies in soft grounds, as it often occurs in fluvial settings 36,37 . Furthermore, Rhizocorallium gingrasi (King et al. 35 ) probably represents dwelling traces of burrowing mayflies in sandstones of the Turonian age 35 . The trace fossils belonging to Fuersichnus burrows look somewhat similar to Languidipes lithophagus sp. nov. borings in general outline (i.e., a similar 'banana-like' shape) 81 . These traces were described from Triassic lacustrine beds of Greenland. Members of the ichnogenus Asthenopodichnium such as A. xylobiontum Thenius, 1979 and A. lithuanicum Uchman et al. 79 were ascribed to possible mayfly traces in fossilized xylic substrates 79,82 . Conversely, Asthenopodichnium lignorum Genise et al. 83 burrows in the Miocene fossil wood from New Caledonia were probably produced by wood rotting fungi because boring mayflies are unknown from this remote island 83 . A comparison of fossil traces with present-day burrows revealed that the link between dwelling activity of recent mayflies and Rhizocorallium, Fuersichnus, and Asthenopodichnium traces is rather problematic 43,65 . Conversely, Rhizocorallium gingrasi shares bioglyphs that are similar to the transverse microsculpture recorded in modern U-to tongue-shaped burrows of freshwater insect larvae such as mayflies and caddisflies 35 .
Narrow, elongated grooves of variable depth originated via destruction of the thin upper layer covering Languidipes lithophagus sp. nov. traces are morphologically similar to members of the ichnogenus Petroxestes (Rogerellidae) that was described from marine calcareous rocks 3,46 . Furthermore, the destructed I.N. Bolotov et al. mayfly traces strongly resemble groove-like structures that were described from the Middle Triassic Muschelkalk Group of Germany as Sulcolithos variabilis Knaust, 2020 84 . The latter ichnospecies, however, was attributed to the combined burrowing and boring activity of marine polychaetes based on records of fossil remains of these animals in the traces 84 . Our new discovery reveals that the morphology of original traces in hardground substrates could be transformed due to subsequent taphonomic processes, leading to the elimination of fragile but ichnologically important details. It was shown that taphonomic overprinting may significantly influence ichnological and even paleontological taxonomy 85 . A special term, taphotaxon, was introduced to refer to taxa, the diagnostic features of which were originated taphonomically 86 .
In summary, Languidipes lithophagus sp. nov. could be considered a regional but remarkable tracemaker of sub-recent to recent ichnofabric in siliceous successions. Occurrences of similar traces may be used as an indicator of dwelling activity of rock-boring filter-feeding insect larvae in fossil bioerosion beds preserved in fluvial settings.

Data sampling
The siltstone rock samples with macroborings were collected using a tommy-bar from two sites of the Bago River on March 22-24, 2020

Mineralogical analyses of the rock substrate
The Bago Basin is situated within an area with marine and non-marine Miocene Molasse sequences 45 . Hence, the siltstone rock deposits in the river valley were considered of the Miocene age. Scanning electron microscopy (SEM) characterization of the siltstone specimens was conducted with a SEM JSM-6480LV (JEOL Ltd., Tokyo, Japan). Polished cut samples were coated with 35 nm of carbon. The backscattered electron (BSE) images were obtained at 20 kV accelerating voltage and 0.7 nA probe current. XRD analysis of siltstone powder preparations was carried out using an ULTIMA-IV X-ray diffractometer (Rigaku, Tokyo, Japan). The equipment settings were as follows: operating mode 40 kV-15 mA; copper radiation; nickel filter; measurement range 3-65°2θ; and scanning angle step 0.05°2θ. A fixed system of focusing slits was applied. Both SEM and XRD analyses were performed at the Moscow State University, Russia. The microindentation hardness (Vickers test) of the siltstone specimens was estimated using a PMT-3M Vickers Microhardness Tester (LOMO, Russia) with 100 g load. The tester was calibrated using NaCl crystal with 10 g load. A few fragments of the siltstone were placed into briquette, were fixed with epoxy glue, and their surface was polished. Five indentations were performed on each rock fragment, and both diagonals of indentation mark were measured. The mean value was calculated on the basis of 20 measurements for each siltstone fragment.

Morphological study of the rock-boring mayfly nymphs
For the morphological study of mayfly nymphs, we applied a standard approach and terminology described by earlier authors 47,49,68,76,87 . A permanent slide with a paratype specimen (ZMMU Eph-0001) was prepared with Faure-Berlese's mounting medium. Images of the holotype of the new mayfly species were taken using a Canon EOS 7D camera with a Sigma AF 24-70 mm f/2.8 IF EX DG Aspherical HSM Canon EF lens (Canon Inc., Tokyo, Japan). The mouthparts were photographed using a stereomicroscope AXIO Zoom.V16 (Carl Zeiss, Germany). The photographs of morphological features were taken from a paratype slide (ZMMU Eph-0001) using a digital camera ToupCam 9.0 MP (Hangzhou ToupTek Photonics Co., Ltd., Hangzhou, China) attached to a light microscope Olympus CX21 (Olympus Corporation, Tokyo, Japan). Measurements of the type series were made with a stereomicroscope Leica EZ4D (Leica Microsystems GmbH, Germany).

Morphological study of the recent and sub-recent borings
First, we examined a series of modern borings that were evidently produced by mayfly nymphs in the midstream section of the Bago River. These borings were used to produce a neoichnological model for the subrecent traces. Second, siltstone specimens with similar traces collected from the sub-recent bioerosion beds were studied. The sub-recent traces were compared with those produced by recent mayflies. Siltstone specimens, recent macroborings, and sub-recent traces were photographed using a Canon EOS 7D camera with a Sigma AF 24-70 mm f/2.8 IF EX DG Aspherical HSM Canon EF lens (Canon Inc., Tokyo, Japan). Longitudinal and transverse sections of the modern borings and subrecent traces were made for morphological investigation. The morphological descriptions of borings were based on a standard approach 3,78 .

DNA extraction, PCR, and sequencing
Total genomic DNA was extracted from the ethanol-preserved tissue of insect larvae using the DNeasy Blood & Tissue Kit (Qiagen, Germany), following the manufacturer's protocol. A barcode fragment of the mitochondrial cytochrome c oxidase subunit I (COI) gene was amplified and sequenced using primers LCO1490 and HCO 2198 88  Sequence alignment, phylogenetic analyses, and DNA-based taxonomic diagnostics To reconstruct the phylogeny of boring and burrowing mayflies (Ephemeroptera), we used 23 available COI sequences. Altogether 11 sequences were generated in this study, i.e., five sequences of Languidipes lithophagus sp. nov., five sequences of Povilla adusta, and one sequence of Symbiocloeon sp. (outgroup taxon; Baetidae) (Supplementary Table 1). The other 12 sequences were obtained from NCBI's GenBank. The COI sequence dataset was aligned using the MUSCLE algorithm of MEGA7 90 and was collapsed to unique haplotypes with an online FASTA sequence toolbox, FaBox 1.5 91 . The server for IQ-TREE (W-IQ-TREE) was used to run a maximum likelihood phylogenetic analysis under TIM2+F+I +G4 evolutionary model 92 and ultrafast bootstrap algorithm (UFBoot) with 1000 replicates 93 . The Bayesian phylogeny was reconstructed with MrBayes v. 3.2.6 94 at the San Diego Supercomputer Center through the CIPRES Science Gateway 95 . Four runs, each with three heated (temperature = 0.1) and one cold Markov chain, were conducted for 30 million generations. Trees were sampled every 1000th generation. We applied a 15%-burn-in to the final tree set and reconstructed a consensus phylogeny from the remaining trees.
The DNA-based taxonomic diagnostics of the new species were as follows. First, we applied the Poisson Tree Process (PTP) algorithm using the PTP web-service (http://mptp.h-its.org) 96 to delimit the Molecular Operational Taxonomic Units (MOTUs). As an input tree, we used the IQ-TREE COI phylogeny (see above). An outgroup taxon was excluded from the input tree using a corresponding option of the PTP server. Second, we calculated uncorrected COI p-distances between Languidipes taxa with MEGA7 90 . Third, we registered fixed nucleotide substitutions in the COI gene that are characteristic for the new species in comparison with Languidipes corporaali and L. taprobanes. These substitutions were recorded with a Toggle Conserved Sites tool of MEGA7 at 50% level 90 . I.N. Bolotov et al.