Discovery of a silicate rock-boring organism and macrobioerosion in fresh water

Abstract

Macrobioerosion is a common process in marine ecosystems. Many types of rock-boring organisms break down hard substrates, particularly carbonate rocks and calcareous structures such as dead corals and shells. In paleontology, the presence of rocks with boreholes and fossil macroboring assemblage members is one of the primary diagnostic features of shallow marine paleo-environments. Here we describe a silicate rock-boring organism and an associated community in submerged siltstone rock outcrops in Kaladan River, Myanmar. The rock-boring mussel Lignopholas fluminalis is a close relative of the marine piddocks, and its borings belong to the ichnospecies Gastrochaenolites anauchen. The neotectonic uplift of the area leading to gradual decrease of the sea level with subsequent shift from estuarine to freshwater environment was the most likely driver for the origin of this community. Our findings highlight that rocks with macroborings are not an exclusive indicator of marine paleo-ecosystems, but may also reflect freshwater habitats.

Fig. 1

Freshwater bioerosion site, Kaladan River, Myanmar. a Map of Myanmar with location of the downstream area of Kaladan River (red frame). b Map of the study site. The violet asterisk indicates the freshwater rock-borer’s site, and white circles indicate the levels of river water above sea level. The yellow circles indicate main towns. The maps were created using ESRI ArcGIS 10 software (www.esri.com/arcgis); the topographic base of the maps was created with Natural Earth Free Vector and Raster Map Data (www.naturalearthdata.com), Vector Map (VMap) Level 0 (http://earth-info.nga.mil/publications/vmap0.html), and ASTER GDEM (https://lpdaac.usgs.gov/node/1079) (Maps: Mikhail Yu. Gofarov). c River site with freshwater rock-borer’s ecosystem. The red arrow indicates siltstone rocks, a substrate of rock-boring bivalves. d Fragment of the black siltstone outcrop, the submerged part of which forms the basis of freshwater rock-borer ecosystem. (Photos: Olga V. Aksenova)

Fig. 2

Model of neotectonic evolution of the lower course of the Kaladan River. The long-term uplift rate R of 0.5–2 mm/year was applied (see Methods section). The model assumes a gradual shift from estuarine to freshwater environment at the rock-borer’s site due to the tectonic uplift with subsequent adaptation of the population of the brackish rock-boring species Lignopholas fluminalis to freshwater habitat. (Model: Ivan N. Bolotov, Mikhail Yu. Gofarov, Yu Wang, & J. Bruce H. Shyu)

Fig. 3

Blocks of siltstone rocks with borings and living macroinvertebrates from the freshwater bioerosion site at the Kaladan River, Myanmar. a, b Rock-boring mussels, Lignopholas fluminalis, in their clavate borings (ichnospecies: Gastrochaenolites anauchen). c Arc clams, Scaphula deltae, and empty borings filled with clay, a habitat of polychaetes Neanthes meggitti and Namalycastis indica (Nereididae). d Arc clam, Scaphula deltae, in an empty boring. e Jackknife clam, Novaculina gangetica, and sponge, Corvospongilla ultima. Scale bars = 10 mm (Photos: Olga V. Aksenova). f Longitudinal cross-section of the boring, Gastrochaenolites anauchen. Scale bar = 5 mm. (Photo: Ilya V. Vikhrev)

Fig. 4

Rock-boring and nestling macroinvertebrates associated with freshwater bioerosion site at the Kaladan River, western Myanmar. a Lignopholas fluminalis, a rock-boring species: (a1) Lateral view of an adult specimen with fully developed callum, (a2) Ventral view of apposed valves, (a3) Dorsal view of apposed valves, (a4) Anterior view, (a5) Frontal view of siphons, (a6) Lateral view of siphons, (a7) Lateral view of a young specimen, (a8) Ventral view of the anterior end, (a9) Dorsal view, (a10) Internal morphology of soft body, (a11) Frontal view of siphons (scale bars = 2 mm). b Scaphula deltae, an ark clam species: (b1) Lateral and (b2) dorsal view of a young specimen, (b3) Live clam attached by byssus in borehole, (b4) Lateral and (b5) frontal view of an adult specimen, (b6) Dorsal and (b7) ventral view of an adult specimen, (b8) Cardinal teeth of left valve, (b9) Cardinal teeth of right valve (scale bars = 2 mm). c Novaculina gangetica, a jackknife clam species: (c1) Lateral view of right and left shell valves (outside), (c2) Lateral view of right and left shell valves (inside), (c3) Shell variability, (c4) Live specimen with protruding foot and siphons (scale bars = 10 mm). d Clithon cf. reticularis, a gastropod species: (d1) Apertural view, (d2) Dorsal view, (d3) Apical view (scale bars = 2 mm). (e) Corvospongilla ultima, a sponge species: (e1) General view of a sponge body fragment (scale bar = 2 mm), (e2) Three adherent gemmules; gemmular cage covering the left gemmule is removed (scale bar = 500 µm). f Neanthes meggitti, a polychaete species: (f1) Dorsal view of complete specimen (scale bar = 2 mm), (f2) Dorsal view of the anterior end (scale bar = 0.5 mm), (f3) Ventral view of the anterior end (scale bar = 0.5 mm). g Namalycastis indica, a polychaete species: (g1) Dorsal view of animal (scale bar = 2 mm), (g2) Dorsal view of the anterior end (scale bar = 0.5 mm). (Photos: Olga V. Aksenova [a, b, d, e1, f, and g], Ekaterina S. Konopleva and Ilya V. Vikhrev [c], and Agniya M. Sokolova [e2])

Fig. 5

Ultrametric Bayesian (BEAST 2) phylogenies showing the primary marine origin of rock-boring and nestling bivalve species from the bioerosion site in the Kaladan River, including a Lignopholas fluminalis, Pholadidae (18S rRNA + 28S rRNA, 3033 bp), b Novaculina gangetica, Pharidae (COI + 16S rRNA + 28S rRNA, 2602 bp), and b Scaphula deltae, Arcidae (28S rRNA + 18S rRNA, 2450 bp, full tree includes 44 taxa but the other clade of Arcidae is not shown here: see Supplementary Fig. 2). Black numbers near nodes are BPP values inferred from BEAST/BPP values inferred from MrBayes/BS values inferred from RAxML (“—” indicates a topological difference). Pie chaps on the nodes indicate the probabilities of certain ancestral areas with respect to combined results under three different models (S-DIVA, DEC, and S-DEC) inferred from RASP v. 3.2