Current Developments in Bioerosion

Endolithic organisms that live inside rocks, shells and wood use two processes, bioerosion and embedment, to produce boring and bioclaustration residential cavities. In the trace fossil record, borings and bioclaustrations preserve the distinctive and often stereotypic activity of particular endolithic organisms, thereby linking important biological and ecological information to this special group of trace fossils. In order to use these fossils to study aspects of evolutionary paleoecology, an ecologically-significant framework is needed to logically group and subdivide hard substrate trace fossils.

A modified version of the guild concept can be applied to borings and bioclaustrations, which groups these cavities and their producers on the basis of biological affinity (or behavior), trophic level and habitat. In general, the endolithic guild is unified as a group of mostly soft-bodied organisms that produce shallow infaunal, stationary domichnia in hard substrates. A major subdivision of the endolithic guild roughly corresponds to the commonly used size-based divide between microborers and macroborers, but it is founded on the ecologicallysignificant criterion of trophic group. The autotrophic endolithic guild includes the photosynthetic microboring cyanobacteria, chlorophytes and rhodophytes, and the heterotrophic endolithic guild is dominated by macroscopic invertebrate borers and embedders, and includes bacteria, fungi and other microscopic fauna.

Primary observations on the endolithic guild suggest that (1) morphological diversity of cavities generally increased through geologic time for all endoliths, with a major Ordovician rise noted for invertebrate endoliths; (2) the class-level composition of invertebrates in the suspension feeding endolithic guild remain nearly unchanged since their rise in the Early–Middle Paleozoic until today, despite a major shift in dominant groups at the familial level that occurred in the Early Mesozoic; (3) diversity patterns within the heterotrophic endolithic guild show similar timing of onset between macroborers and embedders in the Ordovician, but differences in substrate availability may provide an ecological explanation for the decline of host-specific bioclaustrations in the Devonian relative to macroboring diversity.

The ichno-inventory is dominated by the cyanobacterial traces Eurygonum nodosum, Scolecia filosa, and Fascichnus dactylus, complemented by the common Fascichnus frutex, Planobola macrogota and Cavernula coccidia, and the chlorophyte traces Cavernula pediculata and Rhopalia catenata. Presumable borings of heterotrophs are rare albeit diverse constituents of the microboring assemblage. The ichnocoenosis composition indicates a palaeoenvironment in the shallow-euphotic zone and is in many respects ‘modern’. The fact that 17 out of the 19 recorded ichnospecies are also known to neoichnology in closely similar ichnocoenoses of today’s shallow-euphotic seas underlines the pronounced longevity of microendolithic taxa and promotes their value as deep-time palaeoenvironmental indicator.

Two thirds of the recorded ichnotaxa were reported for the first time from Carboniferous strata, thereby extending the known stratigraphic range of Cavernula pediculata, Cavernula coccidia, Rhopalia catenata, Rhopalia spinosa, Scolecia serrata, Polyactina fastigata, Saccomorpha terminalis, and Aurimorpha varia igen. n., isp. n. back into the Palaeozoic. This shows that at least 21 ichnospecies, including a high number of modern ichnotaxa, were established as early as the late Palaeozoic and reflects an increasing abundance and potential competition by chlorophyte algae and heterotroph microborers breaking the former strong dominance of endolithic cyanobacteria in microborer communities. The curve of Phanerozoic ichnospecies diversity suggests that the evolution of microbioerosion agents is to a certain degree parallel to that of the macroborers with main radiations in the Ordovician and Devonian. However, according to the current state of knowledge, the end- Permian mass extinction event seemingly had little or no impact on the diversity and evolution of microborers. Thus, diversity patterns of micorborings differ from those of marine invertebrates, most of which have a minimum in standing diversity at the Permian / Triassic boundary and a general turn-over from typical Palaeozoic to modern evolutionary faunas.

Macroborings in the Lower Ordovician Fillmore Formation, western Utah, USA, occasionally contain fossil remains of enigmatic organisms. In the most complete specimens a common morphology can be observed. The calcified body wall of the animal is vase-shaped, mimicking the shape of the boring itself. An ovoid body leads up to a neck that contains either a single or double cylinder near the aperture of the boring. The incomplete preservation of the specimens is not sufficient to identify the biological affinity of the organism at this time, but a review of potential groups is warranted. While such groups as barnacles, bivalves, mitrates, and a host of worm-like forms are potential boring inhabitants, none fit what is known of the morphology of the specimens from Utah. Regardless, recognition and future identification of these animals will lead to a greater understanding of complex hardground trophic systems during the Ordovician Bioerosion Revolution.

Euendolithic microorganisms (boring microflora) – cyanobacteria, algae and fungi – colonize all carbonate substrates in modern coral reefs and are distributed worldwide. Recent studies showed that in dead carbonate, they are important primary producers and are fed upon by various excavating invertebrate and vertebrate grazers, contributing greatly to biodestruction processes (bioerosion) and sedimentation. Additionally, it has been shown that in some live calcifying organisms, they either inflict damages to live tissues or provide a benefit to the host, depending on the euendolithic community involved (parasitic or mutualistic relationships). Based on those recent studies, the following question is raised: Are euendoliths key organisms in the functioning and maintenance of coral reefs? Reviewed literature includes studies on (1) the mechanisms used by euendoliths to penetrate into carbonate substrates (production of acids or chelating fluids; use of the products of photosynthesis / respiration and / or calcium pumps), (2) their roles in reef bioerosion and sedimentation (major roles), (3) their metabolism (important rates of production), (4) their interactions with their live hosts (symbiosis, mutualism and / or parasitism) and (5) the effects of various environmental factors such as eutrophication, sedimentation and rising atmospheric pCO2 on euendolith activities. The review concentrates on modern coral reef ecosystems.

Complex boring patterns often reflect the complexity of life cycle of the euendoliths that produce them. They are illustrated here by different stages in the development of the euendolithic ulotrichacean chlorophyte Eugomontia sacculata reconstructed on the basis of its complex trace in the shells of Mya arenariain brackish waters of the Baltic Sea at Gdansk, Poland. The trace consists of different types of boring morphologies as distinctive from one another as many traces specific to separate organisms. Because they occur associated, they may be misinterpreted as separate members of an ichnocoenosis. We propose to describe them as parts of a complex trace instead, because they are based on the genetic program of a single organism, but expressed in different proportions at different stages of its development. A new ichnospecies, Rhopalia clavigeraisp. n., is described to characterize these traces.

The destructive activity of different euendolithic cyanobacteria on marine bivalve shells typical for the Baltic Sea area, such as Mytilus edulis, Mya arenaria, Cerastoderma glaucum, and Macoma balthica, was studied in experimental settings for four months. Specifically the cyanobacterial strains of Phormidium sp., Microcoleus chthonoplastes, Anabaena flos-aquae and Synechococcus sp. – all of which not classically considered as euendolithic – were cultured under controlled conditions in the laboratory to determine their optimal environmental conditions. Each bivalve species was maintained in four experimental settings with different calcium carbonate contents in both sediment and sea water. After four months of exposure, pieces of shells were collected, thin-sectioned, and examined using scanning electron microscopy (SEM) and light microscopy. The cyanobacterial strains of Phormidium sp. and Microcoleus chthonoplastes exhibited the fastest (after two weeks) colonisation of Mya arenaria and Cerastoderma glaucum shells and formed a dense biofilm in systems with decalcified sand and carbonate-free water. Shells of Mytilus edulis were colonised only in sites where the periostracum was damaged suggesting periostracal protective properties against the destruction of calcium carbonate substrate by dissolution and bioerosion. The examination of SEM images and cross-sections revealed Phormidium sp. to be an active euendolith that produces prominent traces under laboratory conditions whereas signs of boring activity for Microcoleus chthonoplastes, Anabaena flos-aquae were inconclusive and Synechococcus sp. did not contribute to bioerosion.

Endolithic bioerosion is difficult to analyse and to describe, and it usually requires damaging of the sample material. Sponge erosion (Entobia) may be one of the most difficult to evaluate, as it is simultaneously macroscopically inhomogeneous and microstructurally intricate. We studied the bioerosion traces of the two Australian sponges Cliona celataand Cliona orientaliswith modern technology: high resolution X-ray micro-computed tomography. Micro-CT allows non-destructive visualisation of live and dead structures in three dimensions and was compared to traditional microscopic methods. Micro-CT and microscopy showed that C. celatabioerosion was more intense in the centre and branched out in the periphery (21 vs. 9% substrate removed). In contrast, C. orientalisproduced a dense, even meshwork and caused an overall more intense erosion pattern than C. celata(48 central vs. 42% marginal substrate removed). Extended pioneering filaments were not usually found at the margins of the studied sponge erosion, but branches ended abruptly or tapered to points. Results obtained with micro-CT were similar in quality to observations from transparent optical spar under the dissecting microscope. Microstructures could not be resolved as well with micro-CT as anticipated. Even though sponge scars and sponge chips were easily recognisable on maximum magnification micro-CT images, they lacked the detail that is available from SEM. Other drawbacks of micro-CT involve high costs and presently limited access. Even though micro-CT cannot presently replace traditional techniques such as epoxy resin casts viewed by SEM, we obtained valuable information. Especially for the possibility to measure endolithic pore volumes, we regard micro-CT as a very promising tool that will continue to be optimised. A combination of different methods will produce the best results in the study of Entobia

Bioeroding sponges have historically been mystical beasts of the sea. Originally, they were classified as half cnidarian, half sponge. It required much time and scientific convincing to confirm their status as active bioeroders. The scientists Hancock and Nasonov were pioneers of their time. They recognised and defended the main concepts: the endolithic organisms are sponges, they produce the cavities they inhabit, and their activities are likely to involve chemical and mechanical processes. However, this viewpoint was often scathingly challenged, and the notion of actively bioeroding sponges was hotly disputed. Once the concept was firmly established in the mid 1900s, related research experienced a significant leap. Notably studies by Pomponi and Hatch on the ultrastructure of etching cells and their associated biochemical properties left no room for doubt: The enzymes carbonic anhydrase and acid phosphatase are associated with sponge bioerosion, providing means for mineral dissolution and digestion of organic components, thus enabling the removal of the so-called sponge chips. However, the exact etching agent remained undetected, and since 1980 research on this phenomenon significantly slowed down. Further studies predominantly focused on environmental control of bioerosion and the taxonomic value of sponge erosion traces. A recent study with electrochemical liquid ion exchange microsensors revisited the question of how sponge bioerosion is achieved and whether acid is involved. This study is presented here to conclude the summary of current knowledge in this context. Microgradients of pH and calcium ions in the tissues of Cliona celataand the non-eroding sponge Halichondria paniceafrom the North Sea were compared. The pH slightly decreased with distance into the tissue of C. celata, whereas after an initial drop it remained stable in H. panicea. Calcium concentrations in C. celataincreased slightly more with tissue depth than in H. panicea. C. celatabioerosion may be periodic (fewhours cycle) as evidenced by oscillating pH values at the sponge-substrate interface, which may explain micro-terracing in sponge scars. Nevertheless, measured pH changes were too weak to prove beyond doubt that sponge bioerosion employs acid, and further studies will be necessary to confirm present preliminary findings

This work focuses on the erosion pattern morphology of the Indonesian excavating sponge Cliona albimarginatain three different calcareous substrates (Carrara marble, branches of the coral Acropora, and the umbo of the bivalve Hippopus). The experimental data demonstrate that the sponge produced galleries that followed the preferred orientation of fibrous aragonite crystals (Acropora), or the parallel lamination within the Hippopusshell. Conversely, the homogeneous granoblastic, mosaic texture of Carrara marble was etched without preferred directions. SEM analysis showed that pit size also varies between the three substrates in relation to the respective substrate porosity.

This study explores and extends the lower size range of the sponge boring ichnotaxon Entobia, thereby introducing two new ichnospecies – E. mikraand E. nana– both reaching only a fraction of a millimetre in size. They are components of a diverse aphotic ichnocoenosis, comprising a total of 18 ichnotaxa, recorded in skeletons of the cold-water scleractinian Lophelia pertusafrom Lower Pleistocene deposits of Rhodes. Recent analogs are studied and morphometrically analysed from various aphotic, cold-temperate settings in the NE Atlantic.

The first new ichnospecies, E. mikra, is characterised by solitary or clustered, irregular cavities ranging from 60 to 335 μm in size with a mean of only 171 μm, and comprises elongate, branched and anastomosing forms. The traces posses a characteristic botryoidal microsculpture with etching scars measuring 3.6 to 8.1 μm at a mean of only 5.6 μm. The scars are locally encountered in various growth stages resembling initial rims, semi- and nearly-closed etching cells. The other new ichnotaxon, E. nana, is slightly larger with solitary or clustered cavities measuring 102 to 756 μm in diameter at a mean of 298 μm, and show a bulged to weakly botryoidal surface texture. The chambers posses varying numbers of apophyses and exploratory threads which terminate bluntly or lead to neighbouring chambers.

Both new ichnospecies are assumed to represent the work of unknown boring micro-sponges, given the lack of alternatives among potential heterotroph bioerosion agents in aphotic environments, and considering the compelling resemblance in the botryoidal microsculpture of E. mikra (recording the activity of archaeocyte etching cells) and the presence of interconnected chambers in the case of E. nana. The possibility that they are merely juvenile stages of larger entobians is recejcted because of missing intermediate growth stages and the Gaussian size class frequency distribution, mirroring normal interspecies variability and population structure.

The ultimate proof can only be drawn by histological analyses or by finding siliceous sponge spicules – if present at all in the case of the responsible sponges – allowing for a taxonomical evaluation of the trace maker and shedding new light on the actual size range and diversity of boring sponges.

The fossilised borings of endolithic sponges are generally abundant in marine sediments since the Jurassic, and sparse occurrences date back to the Lower Palaeozoic. However, the zoological identity of the boring sponge is not revealed by the morphology of the boring alone. The preservation potential of the boring is far greater than that of the spicular skeleton of the endolithic sponge. The spicules are opaline silica, which is readily soluble in seawater. Thus, while the borings are abundant and diverse, we have almost no knowledge of the bioeroding sponges that produced them.

In situations where early diagenetic cementation of the sediment occurs before the dissolution of the silica, the spicules may be preserved as casts. Such a case exists in association with hardground formation. A hardground in the Upper Cretaceous (Turonian) Chalk Rock of Buckinghamshire, SE England, yielded an intraclast containing a sponge boring referable to Entobia cretacea Portlock, 1843. Many of the chambers contain sparite-filled casts of sponge spicules. Only megascleres are present, comprising smooth oxeas about 2 mm in length and 74 μm in width, indicating an extinct species of the phloeodictyid genus Aka. No part of the original skeleton of the sponge is preserved, only very accurate external moulds of the spicules, representing little more than the ghost of the animal. But this is not unusual in palaeontology and, as it appears that this body fossil is new to science, it is herein named Aka akis sp. nov. This is only the fifth account of fossilised Aka spp. where spicule morphology and erosion traces can be observed. It is hoped that further search for spicules preserved within Entobia will allow an investigation of the endolithic sponge communities of the Mesozoic and Tertiary seas, about which almost nothing is known at present.

Polychaetes are important in the early stages of bioerosion of newly available coral substrate often as a result of coral death by disease, bleaching, Crownof- Thorns starfish attack etc., and physical destruction during storms. A succession of polychaetes are recruited to the substrate, which it is hypothesised, facilitates subsequent recruitment by sponges, sipunculans and molluscs that are dominant in ‘mature’ boring communities. Recruitment of boring polychaetes varies according to the type of substrate available, season and geographical location of substrate, and environmental factors such as light, water quality, depth and wave exposure. The mechanisms by which polychaetes bore still require further investigations.

Endolithic bivalves were studied to establish abundance ranking and bioerosive impact on Maldivian coral reefs. In samples of dead coral, Parapholas quadrizonata(Spengler, 1792) was by far the most numerous bivalve borer. The assemblages of variously sized and aged specimens, their spatial situation and the resulting behaviour are described and discussed. The morphology of shells and boreholes is illustrated. Intra-species competition for space may have restricted the individual growth but was not necessarily lethal. The borings show unique internal features that allow distinction from those of Gastrochaenaspecies, whereas the borehole apertures are undistinguishable between the two genera.

The Echinometridae is a diverse, largely tropical family of echinoids. Several species are active borers in shallow water, at and below the low-tide line, especially in coral reef and beachrock substrates, commonly in high-energy situations. In the Caribbean and Atlantic Ocean, Echinometra lucunter erodes two types of boring. Most commonly, the widely reported elongate grooves are produced. In these traps, drifting fragments of algae are caught on the incoming tide. Drifting algae are also caught actively by ‘chopsticks-like’ manoeuvres of the spines. Some simple gardening is undertaken on the boring walls and floor, where short algal turf and endolithic algae are harvested. Juveniles start by making simple cup-shaped borings. In some cases, however, E. lucunter retains this form of bioerosion to adulthood. Cup-shaped borings must indicate emphasis on alga-catching in highenergy environments and less on the gardening, grazing trophic style.

In the Indo – West Pacific realm, Echinometra mathaei undertakes similar trophic activities to those of E. lucunter, especially in the high-energy environment outside barrier reefs. However, in protected shoreward lagoons, another trophic activity is practiced that has largely been overlooked in echinoids. In this oceanic region, a recently identified food-source is available in back-barrier lagoons, i.e., flocs of organic particle aggregates that, like the marine snow of the open sea, are swept into the lagoon on the rising tide from the reef crest, which is subaerially exposed at low tide. Crowded E. mathaei on beachrock surfaces, each in a cup-shaped boring (Circolites isp.), show alga-catching and within-boring algal turf gardening. But in addition, marine snow is collected on the spines at each rising tide.

Echinostrephus molaris, a small echinoid that practices alga-catching on the outer side of the barrier reef, shows extreme specialisation for particle-aggregate collection in back-barrier sites. E. molaris and E. aciculatus are the only deeply boring echinoids, producing thumb-sized borings (Trypanites isp.). Very attenuated, mucus-coated aboral spines are extended from the mouth of the boring as the nutrient flux of the tidal surge climaxes. Quantities of marine snow are collected from the spines by searching tube-feet.

Conspicuous borings, belonging to the new ichnotaxon Trypanites mobilisisp. n., recorded in the bulbous spines of the echinoid Tylocidarisand in the spherical calcareous sponge Porosphaera globularis, are common in Late Cretaceous (Cenomanian) to Early Paleocene (Danian) strata of Central and NW Europe. It is suggested that the borings were produced post mortem by sipunculan worms to create a lightweight and mobile domicile, offering effective shelter and protection. This strategy, which is hitherto not known from any other animal group, represents an alternative to the conchicolous life habit. Both habits evolved almost simultaneously in sipunculans at the end of the Early Cretaceous, suggesting escalation as a response to increased predation. Moreover, it bears a number of advantages such as superior substrate availability, the avoidance of replacing the substrate during ontogeny, as well as comparatively lightweight domiciles. On the other hand, the high degree of dependence on only very few suitable specific host taxa is a major drawback of this strategy. It ultimately led to the disappearance of this mode of life in favour of the still widespread conchicolous habit at the end of the Danian when the hosts vanished together with the chalk-sea ecosystem.

The parasitic foraminifer Hyrrokkin sarcophagapredominantly infests the cold-water coral Lophelia pertusaand the co-occurring bivalve Acesta excavata, showing a commensal or parasitic behaviour. It occurs also on some other corals (e.g., Caryophyllia sarsiae), bivalves (e.g., Delectopecten vitreus) and sponges (Geodiasp.), typically in aphotic environments. The aim of the study is to describe its traces from various host substrates, to characterise its parasitic behaviour and to map the geographical distribution of the genus Hyrrokkin. Epoxy-resin casts of H. sarcophagatraces in A. excavata C. sarsiae, D. vitreusand L. pertusa,and of H. carnivoratraces in A. excavata, were SEM analysed. The boring pattern is in all cases characterised by a shallow groove of up to 7 mm in diameter (max. 2 mm deep), from which ‘whip’-shaped extensions protrude vertically into the substrate. In A. excavatathe foraminifer can penetrate the entire valve to the mantle cavity, producing a thick shaft of fused ‘whips’. This parasitic attack is answered by a strong callus formation of the mollusc. One individual foraminifer can repeatedly bypass the organic-rich callus, resulting in a thick aragonite pinnacle. The trace surface texture is xenoglyph and changes with the penetrated host-microstructures. This is especially obvious on deeply penetrating trace portions (e.g., ‘whip’-shaped filaments) and is a strong indication for a chemical penetration mode (etching). The trace of Hyrrokkin is described as Kardopomorphos polydioryxigen. n., isp. n. On the substrates without the shaft, related to parasitic behaviour, Hyrrokkinmight feed directly on adjacent external host tissue. H. sarcophagais known along the North Atlantic continental margin from polar to subtropical latitudes and H. carnivoraoccurs on the continental margin of Mauritania, Congo and Guinea. In the Mediterranean we could document the parasitism of H. sarcophagafrom Last Glacial A. excavata.Traces or detached foraminifer tests occur in Early Pleistocene cold-water coral deposits on Sicily and Rhodes. Recent H. sarcophagahas not been observed above 11°C and is scarce near 5°C water temperature. Hyrrokkinsp. was reported from the Canadian Pacific on fossil sponges and was observed on Acesta patagonicain the Beagle Channel (Chile).

Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that are a valuable tracer of sub-surface microorganisms. Two morphologically distinct granular and tubular ichnofossils are produced. The ‘Granular form’ consists of individual or coalescing, spherical bodies with average diameters of ~0.4 μm. The ‘Tubular form’ are straight, sometimes ranched, to curving and spiralled tubes with average diameters of 1-2 μm and lengths of up to ~200μm. A biogenic origin for these structures is confirmed by: the concentration of DNA that binds to biological stains in recent examples; enrichments in C, N and P along their margins in both recent and ancient examples; and systematic C isotope shifts measured upon disseminated carbonate in the surrounding glass. The constructing microorganisms are thought to include heterotrophs and chemolithoautotrophs that may utilise Fe and Mn from basaltic glass as electron donors and derive carbon sources and electron acceptors from circulating fluids. These microbial ichnofossils are found at depths of up to 550 metres in the oceanic crust in the glassy rims of pillow basalts and interpillow breccias. A diverse spectrum of ichnofabrics is created by overlapping phases of granular and tubular bioerosion; banded abiotic dissolution; and the precipitation of phyllosilicates, zeolites and iron-oxy-hydroxides. The resulting ichnofabrics have been documented from in situ oceanic crust spanning the youngest to the oldest oceanic basins (0 to 170 Ma). Their geological record extends to include meta-volcanic glass in oceanic crustal fragments from Phanerozoic to Proterozoic ophiolites. Examples infilled by the mineral titanite (CaTiSiO₄) have also been found in Palaeo- to Mesoarchaean pillow basalts from the Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of Western Australia. Direct ²⁰⁶Pb / ²³⁸U radiometric dating of the Australia examples has confirmed their Archaean age and thus they represent the oldest candidate ichnofossils on Earth.

Microbial alteration is an important pathway for bone degradation. Organisms involved in bioerosion of bone, mainly bacteria, fungi and cyanobacteria, create different types of alteration. While fungi and cyanobacteria dissolve the bone matrix resulting in branching tunnels, bacteria create microscopical focal destructions with a complex morphology, reorganising the mineral rather than removing it. Different environmental and early post mortem circumstances characterise each type. Bacterial alteration occurs in the early post mortem interval, probably in the first decades after death. A strong link with putrefaction can be observed, indicating that early putrefactive organisms are important contributors to bacterial alteration of bone. Fungal and cyanobacterial alteration occurs when the environment is favourable, i.e., oxygen is present and bone still has sufficient nutrient value. Although microbial alteration causes loss of information in archaeological and palaeontological bone, the study of microbial bioerosion also represents a powerful tool for taphonomic reconstruction.

The use of rocky palaeoshore bioerosion analysis in the study of palaeontological and geological questions is beginning to bear fruit. Five southern Iberian Neogene rocky shores have been analysed and their bioerosion structures have been identified. The observed ichnodiversity is rather low; eleven ichnospecies were identified. These include bioerosion structures produced by polychaete annelids (Caulostrepsis, Maeandropolydora), clionaid sponges (Entobia), echinoids (Circolites), and endolithic bivalves (Gastrochaenolites). The different ichnoassemblages present in Miocene rocky shores in both Portuguese and Spanish sectors correspond to the Entobiaichnofacies. Comparison with the northeastern counterparts of these shores has also been carried out. The study of southern Iberian Miocene rocky shores made it possible to correlate them with the regional tectonic evolution and the main Neogene transgressive events affecting the region.

Although bioerosion is recognized as causing significant destruction of hard substrates, few studies have assessed loss of hard substrates by taphonomic bias against shells with predatory borings (the trace fossil Oichnus). Results of point-load compression experiments suggest preferential loss of Oichnus-bearing shells; however, studies of drilling predation often assume lack of taphonomic bias against drilled shells.This project tested the hypothesis that taphonomic processes preferentially destroyed shells with predatory borings for the Miocene Choptank Formation of Maryland and the Plio-Pleistocene Caloosahatchee Formation of Florida. If bias against drilled shells occurred, drilled shells should be more pristine than undrilled shells because they do not survive exposure, and shells should break through drillholes.

Most measures of taphonomic condition showed no statistically significant difference between drilled and undrilled shells for both formations. In the Choptank, condition of shell sculpture (Boston Cliffs Member) suggested taphonomic bias against drilled shells. However, distinctness of the pallial line (Drumcliff Member) supported the opposite conclusion. In addition, numerous shell fragments contained intact drillholes; breaks passed through drillholes in only 17% of Choptank Oichnusbearing fragments, suggesting that drillholes did not weaken shells significantly. In the Caloosahatchee, taphonomic condition did not vary significantly between drilled and undrilled shells for most of the 24 cases studied. One case, nonpredatory bioerosion in Chione elevata, supported the hypothesis that drillholes cause taphonomic loss of shells. In contrast, visibility of the pallial line and muscle scars and condition of sculpture in Stewartia anodonta indicate that specimens with Oichnus survived more taphonomic damage than undrilled specimens. The predominance of shells with breaks that did not pass through drillholes in the Choptank Formation, and the lack of difference between drilled and undrilled shells for most taphonomic measures in both formations, indicates that drillholes did not weaken shells significantly.

A bibliography of bioerosion-related books and papers has been available on the Internet for several years. The list now includes almost 2200 references. This bibliography is freely available to the public and frequently edited, so it grows in size, coverage, and accuracy. The bibliography shows the diversity of interests in the bioerosion community. References range from the mostly geological to the botanical; from articles concerned with macrobioerosion by bivalves, worms, sponges, and barnacles to numerous papers on microbioerosion by bacteria and fungi. The online nature of this resource means it will grow with the needs of the bioerosion research community.