Martin Zuschin

Patterns and processes of shell fragmentation in modern and ancient marine environments

Shell fragments are important components of many Recent and fossil marine benthic ecosystems and can provide crucial information on past and present environmental conditions. Interpreting such fragments requires integrated knowledge in various fields and the information potential is therefore rarely optimally utilized. This paper uses the definition of a fragment as being a piece of shell having less than 90% of its original form. It then outlines the potential characteristics, pathways, and fates that shells and their fragments can have. Fragmentation is a key factor shaping the shelly part of death assemblages, but it is difficult to interpret because it can be broadly caused by ecological, biostratinomic or diagenetic processes and also depends on shell strength. Strength, in turn, reflects multifunctionality during ontogeny and depends on a complex set of skeletal and taphonomic factors. Therefore, no particular shell parameter clearly determines strength, but thickness, microstructure type and degree of organic matrix have the strongest influence on pre- and post-mortality strength. Size measurements are usually less important for shell strength than thickness, although ecologically complex size refuges from predation do exist. Similarly, shell shape, sculptural features and specific aperture types (in gastropods) provide various defence strategies rather than increasing strength per se. Key ecological factors of fragmentation include predation due to crushing, peeling, along with mistaken predation, selfinflicted damage during predation and burrowing, and more physical aspects such as impacts by stones. Modern studies must consider damage by benthic commercial fisheries or dredging by scientific vessels. Key biostratinomic factors include transportinduced abrasion (littoral zone of surf-washed beaches), bioerosion and dissolution (mainly sublittoral environments). Diagenetically, fragmentation during compaction mainly occurs when shells are in direct contact with one another or with coarser grains; taphonomic features (e.g., drillholes) have only minor influence. A key step in interpreting fragments is to categorize breakage into repaired versus unrepaired, diagnostic versus nondiagnostic, and severe versus localized damage. Quantifying the above categories can then provide useful information on breakage patterns and underlying processes. Understanding the many characteristics of shells and their fragments is a significant interpretive tool in maximizing the information gain in palaeoecological and taphofacies analyses. D 2003 Elsevier Science B.V. All rights reserved.

Experimental Measurement of Shell Strength and its Taphonomic Interpretation

Abstract Shells were broken experimentally by compressive and compactional loading to determine the best predictors of shell strength among a number of morphologic (length, width, height, weight, thickness) and taphonomic (presence of a drillhole, exposure to seawater, point contacts of shells) features. Compressive strength of fresh (Mercenaria mercenaria, Mytilus edulis) and beach-collected (Anadara ovalis) shells was measured by placing a single valve flat on a surface and compressing it. Compactional strength of beach-collected Mulinia lateralis shells was measured by placing isolated valves and shell layers in fine sand and compacting the sand and shell mixture. The most effective predictor of compressive shell strength of fresh shells was thickness; it was significant for all three species and it easily can be interpreted as an obvious defense against a shell-crushing predator. Size was comparatively less important. The presence of a drillhole reduced shell strength significantly, and strong correlations of weight and shell strength among beach collected Anadara shells may reflect different states of degradation of the original amounts of organic matrix. Supporting this conclusion is the observation that immersion in seawater for seven weeks significantly decreased shell strength of organic-rich shells (Mytilus), but did not affect shell strength of organic-poor shells (Mercenaria). Fracture patterns varied considerably between and within taxa. The breakage pattern of Anadara and Mercenaria shells generally consisted of a set of fractures radiating from the point of loading to the shell margin, including even the thick dorsal margin. The breakage pattern in Mytilus usually consisted of one fracture only, which did not extend from the point of loading but whose path was highly unpredictable. In compaction experiments with beach-collected Mulinia lateralis, the most important factor determining whether or how a shell would break was the contact between shells. Isolated shells, both drilled and undrilled, did not break. Among drilled valves in simulated shell beds, only 26 percent of the fragments > 2 mm fractured through the drillhole.

A comparison of living and dead molluscs on coral reef associated hard substrata in the northern Red Sea — implications for the fossil record

Fidelity of death assemblages to live shelly faunas is one of the major palaeontological questions, but quantitative data are scarce and most case studies on this topic have been performed in non-reef sediments. Therefore we studied diVerent types of subtidal reef-associated hard substrata (reef flats, reef slopes, coral carpets, coral patches, rock grounds), each with diVerent coral associations, in order to determine the agreement of assemblages of living and dead shell-bearing molluscs. A total area of 340.5 m2 was investigated and 2846 individuals were counted at 68 sample localities ranging from shallow subtidal to 40 m water depth. Most taxa found dead in the study area were also found live and vice versa; diVerences in this respect can be related to quantitatively unimportant taxa. However, strong diVerences exist in the proportion of living and dead fauna, dominant taxa, and molluscan distribution patterns. The ratio of live to dead molluscs is high. Living molluscs are strongly dominated by taxa with distinct relations to corals, mainly Pedum, Coralliophila and Tridacna, and the encrusting gastropod Dendropoma. Five distinct groups of living molluscs can be diVerentiated and related to specific hard substrata, which are characterized by distinct molluscan life habits. In contrast, the death assemblages are always strongly dominated by encrusting bivalves, mainly Chamoidea and Spondylidae, and cerithiid gastropods in varying dominances. Correspondingly there is no significant correlation of the total abundance of living and dead molluscs and their overall similarity is only 6%. Similarity between living and dead faunas is above 50% at only 12 of the 68 sample locations, and at 17 sample locations significant correlations of living and dead molluscs were recognized. These correlations are mainly based on Chamoidea, which dominate both the living and dead fauna, on rock grounds. Therefore rock grounds are the only bottom type with consistent correlations and similarities of living and dead molluscs. The observed bias is due to the close relationship of molluscan life habits and post-mortem history of shells. Molluscs that live permanently attached to or within living corals (mostly bivalves and encrusting Dendropoma) can easily be overgrown after death by the large amounts of living substrata available. Rapid transport of dead shells into surrounding sediments or into crevices within corals is typical of gastropods that feed on corals. Molluscs that colonize dead surfaces preferentially accumulate on rock grounds. The ecologic information that can be derived from the shells depends on the diVerent post-mortem histories of the faunas.

Molluscan assemblages on coral reefs and associated hard substrata in the northern Red Sea

Abstract Information on spatial variability and distribution patterns of organisms in coral reef environments is necessary to evaluate the increasing anthropogenic disturbance of marine environments (Richmond 1993; Wilkinson 1993; Dayton 1994). Therefore different types of subtidal, reef-associated hard substrata (reef flats, reef slopes, coral carpets, coral patches, rock grounds), each with different coral associations, were investigated to determine the distribution pattern of molluscs and their life habits (feeding strategies and substrate relations). The molluscs were strongly dominated by taxa with distinct relations to corals, and five assemblages were differentiated. The Dendropoma maxima assemblage on reef flats is a discrete entity, strongly dominated by this encrusting and suspension-feeding gastropod. All other assemblages are arranged along a substrate gradient of changing coral associations and potential molluscan habitats. The Coralliophila neritoidea–Barbatia foliata assemblage depends on the presence of Porites and shows a dominance of gastropods feeding on corals and of bivalves associated with living corals. The Chamoidea–Cerithium spp. assemblage on rock grounds is strongly dominated by encrusting bivalves. The Drupella cornus–Pteriidae assemblage occurs on Millepora–Acropora reef slopes and is strongly dominated by bivalves associated with living corals. The Barbatia setigera–Ctenoides annulata assemblage includes a broad variety of taxa, molluscan life habits and bottom types, but occurs mainly on faviid carpets and is transitional among the other three assemblages. A predicted degradation of coral coverage to rock bottoms due to increasing eutrophication and physical damage in the study area (Riegl and Piller 2000) will result in a loss of coral-associated molluscs in favor of bivalve crevice dwellers in dead coral heads and of encrusters on dead hard substrata.

Subtropical coral-reef associated sedimentary facies characterized by molluscs (Northern Bay of Safaga, Red Sea, Egypt)

SummaryThe shallow marine subtropical Northern Bay of Safaga is composed of a complex pattern of sedimentary facies that are generally rich in molluscs. Thirteen divertaken bulk-samples from various sites (reef slopes, sand between coral patches, muddy sand, mud, sandy seagrass, muddy seagrass, mangrove channel) at water depths ranging from shallow subtidal to 40m were investigated with regard to their mollusc fauna >1mm, which was separated into fragments and whole individuals.Fragments make up more than 88% of the total mollusc remains of the samples, and their proportions correspond to characteristics of the sedimentary facies. The whole individuals were differentiated into 622 taxa. The most common taxon,Rissoina cerithiiformis, represented more than 5% of the total mollusc content in the samples. The main part of the fauna consists of micromolluscs, including both small adults and juveniles. Based on the results of cluster-, correspondence-, and factor analyses the fauna was grouped into several associations, each characterizing a sedimentary facies: (1) “Rhinoclavis sordidula—Corbula erythraeensis-Pseudominolia nedyma association” characterizes mud. (2) “Microcirce sp.—Leptomyaria sp. association” characterizes muddy sand. (3)”Smaragdia spp.-Perrinia stellata—Anachis exilis—assemblage” characterizes sandy seagrass. (4) “Crenella striatissima—Rastafaria calypso—Cardiates-assemblage” characterizes muddy seagrass. (5) “Glycymeris spp.-Parvicardium sueziensis-Diala spp.-assemblage” characterizes sand between coral patches. (6) “Rissoina spp.-Triphoridae —Ostreoidea-assemblage” characterizes reef slopes. (7) “Potamides conicus—Siphonaria sp. 2—assemblage” characterizes the mangrove.The seagrass fauna is related to those of sand between coral patches and reef slopes with respect to gastropod assemblages, numbers of taxa and diversity indices, and to the muddy sand fauna on the basis of bivalve assemblages and feeding strategies of bivalves. The mangrove assemblage is related to those of sand between coral patches and the reef slope with respect to taxonomic composition and feeding strategies of bivalves, but has a strong relationship to those of the fine-grained sediments when considering diversity indices. Reef slope assemblages are closely related to that of sand between coral patches in all respects, except life habits of bivalves, which distincly separates the reef slope facies from all others.

Effect of hypoxia and anoxia on invertebrate behaviour: ecological perspectives from species to community level

Coastal hypoxia and anoxia have become a global key stressor to marine ecosystems, with almost 500 dead zones recorded worldwide. By triggering cascading effects from the individual organism to the community-and ecosystem level, oxygen depletions threaten marine biodiversity and can alter ecosystem structure and function. By integrating both physiological function and ecological processes, animal behaviour is ideal for assessing the stress state of benthic macrofauna to low dissolved oxygen. The initial response of organisms can serve as an early warning signal, while the successive behavioural reactions of key species indicate hypoxia levels and help assess community degradation. Here we document the behavioural responses of a representative spectrum of benthic macrofauna in the natural setting in the Northern Adriatic Sea (Mediterranean). We experimentally induced small-scale anoxia with a benthic chamber in 24m depth to overcome the difficulties in predicting the onset of hypoxia, which often hinders full documentation in the field. The behavioural reactions were documented with a time-lapse camera. Oxygen depletion elicited significant and repeatable changes in general (visibility, locomotion, body movement and posture, location) and species-specific reactions in virtually all organisms (302 individuals from 32 species and 2 species groups). Most atypical (stress) behaviours were associated with specific oxygen thresholds: arm-tipping in the ophiuroid Ophiothrix quinquemaculata, for example, with the onset of mild hypoxia (< 2mLO(2) L-1), the emergence of polychaetes on the sediment surface with moderate hypoxia (< 1mLO(2) L-1), the emergence of the infaunal sea urchin Schizaster canaliferus on the sediment with severe hypoxia (< 0.5mLO(2) L-1) and heavy body rotations in sea anemones with anoxia. Other species changed their activity patterns, for example the circadian rhythm in the hermit crab Paguristes eremita or the bioherm-associated crab Pisidia longimana. Intra-and interspecific reactions were weakened or changed: decapods ceased defensive and territorial behaviour, and predator-prey interactions and relationships shifted. This nuanced scale of resolution is a useful tool to interpret present benthic community status (behaviour) and past mortalities (community composition, e.g. survival of tolerant species). This information on the sensitivity (onset of stress response), tolerance (mortality, survival), and characteristics (i. e. life habit, functional role) of key species also helps predict potential future changes in benthic structure and ecosystem functioning. This integrated approach can transport complex ecological processes to the public and decision-makers and help define specific monitoring, assessment and conservation plans

Influence of Size-sorting on Diversity Estimates from Tempestitic Shell Beds in the Middle Miocene of Austria

Abstract Paleontological data frequently are extracted from genetically and stratigraphically complex shell beds. It is therefore important to recognize taphonomic biases that can lead to major errors in paleoecological interpretations (e.g., on ancient local biodiversity). The strong influence of transport-related shell-size sorting on diversity estimates from single samples was studied in a transect of the middle Miocene Grund Formation (Lower Austria), which contains allochthonous, psammitic event beds with channel structures, sharp erosional bases, and graded bedding. These event beds are interpreted as proximal tempestites, and contain densely packed, polytaxic molluscan assemblages. The faunal composition and taphonomic features of shells indicate that transport occurred from wave- or current-agitated nearshore habitats into a pelitic, inner-shelf environment. The different skeletal concentrations contain a highly diverse molluscan fauna with 130 species identified from more than 4200 individuals. Although the quantitatively most-important species are the same in standardized samples from five different shell beds, species richness differs significantly among the three samples from the base of the transect and the two samples from its top. Diversity depends on size-sorting and therefore reflects the transport history of the individual tempestites, not the species richness of the original paleocommunity. Poorly sorted samples (indicating relatively minor transport) approximate the diversity of single samples of that environment better than well-sorted samples (which indicate stronger transport). Diversities of shelly assemblages from parautochthonous and allochthonous assemblages cannot be compared directly. Even comparisons among tempestites are problematic because transport intensity governs diversity. The intensity of any taphonomic process, however, is difficult to predict without detailed investigations. The use of samples from taphonomically complex shell beds for diversity comparisons can bias results, especially on the fine-scale level of local diversity patterns. Studies at such fine scales of resolution should consider the taphonomic framework of assemblages, which is necessary to recognize the dominant taphonomic factors and their intensities.

Fidelity of molluscan life and death assemblages on sublittoral hard substrata around granitic islands of the Seychelles

This study quantified the degree of coincidence between living and dead molluscan faunas in a shallow-water coral reef environment in the Indian Ocean. The results were compared with those from a similar life:death study in the northern Red Sea, and with those published for reef corals and soft substrata molluscs. The proportions of quantitatively important taxa are robust to sampling intensity, but fidelity indices and rank-order correlations are strongly influenced by quantitatively unimportant taxa. Distinct differences between life and death assemblages were recognized, which are due to distinct biases in the death assemblage. Bivalves that lived in close contact with living corals are preferentially overgrown after death and should provide considerable temporal and ecological information in a potential fossil record, as they will be preserved within a rapidly growing reef framework. Some gastropod taxa are preferentially transported into surrounding soft substrata postmortem. Here they will be affected by time-averaging and taphonomic disintegration typically occurring in sediments resulting in the associated loss of much temporal information. Most gastropod shells, however, are inhabited by hermit crabs postmortem, which may strongly alter the fossil gastropod community structure. The results are similar to a comparable study in the northern Red Sea, with a major exception being the strong dominance of hermit crab-inhabited gastropod shells in the death assemblage of the Seychelles. Comparison of life:death assemblages between hard and soft substrata, in keeping with the northern Red Sea study, showed the strong dominance of dead shells in the soft substrata with the converse on the hard substrata. This results from different accumulation conditions for dead shells in soft substrata. Fidelity indices are well suited to demonstrate that sedimentary death assemblages are typically remarkably robust reflections of local community composition but they do not record the strong biases in death assemblages of coral reef associated hard substrata molluscs and are therefore unsuitable for comparisons of life and death assemblages in reef environments.

THE STRATIGRAPHIC AND SEDIMENTOLOGIC FRAMEWORK OF FINE-SCALE FAUNAL REPLACEMENTS IN THE MIDDLE MIOCENE OF THE VIENNA BASIN (AUSTRIA)

Abstract Fine-scale paleocommunity dynamics were studied in a short (∼16 m) section in the Middle Miocene (Badenian Stage) of the Central Paratethys, which consists of siliciclastic, pelitic, and sandy-to-gravely shallow-water deposits. Two basal, coarsening- and shallowing-upward parasequences of a late highstand systems tract are separated by a third-order sequence boundary from the deepening-upward basal part of a transgressive systems tract at the top of the section. Benthic faunas in this succession are primarily autochthonous and storm-influenced, level-bottom assemblages, but a distinct oyster-vermetid boundstone occurs near the base of the transgressive systems tract. Additionally, three tempestitic shell beds were included, which were found out of sequence in an associated basinal setting; their faunal content relates them closely to the fine-grained deepest parts of the transgressive systems tract. Ordination of species and samples using detrended correspondence analysis and analysis of similarity suggest that two basic benthic assemblages can be distinguished. The oyster-vermetid boundstone is tied to a unique set of environmental conditions and indicates a major environmental change at the sequence boundary. The faunal assemblage in the boundstone shows a weak gradient into the pelitic (deeper and quiet-water) level-bottom assemblage, which in turn is characterized by strong overlaps with the fauna of sandy (shallower and more agitated) habitats. Therefore it is concluded that the benthic assemblages in the studied section belong to the same basic metacommunity, which was not seriously affected by the strong facies changes at the sequence boundary and at the flooding surfaces. Moreover, the species in the studied benthic assemblages reacted to changes in the environment by habitat tracking.

Fossil evidence for chemoautotrophic bacterial symbiosis in the thyasirid bivalve thyasira michelottii from the middle Miocene (Badenium) of Austria

Many modern thyasirid bivalves are known to engage with chemoautotrophic bacteria in symbiosis, which can be inferred from the specialized feeding and digestive systems of these bivalves, as well as from stable carbon isotope ratios of their soft tissues. Fossil evidence for this life habit consists largely of facies criteria and bio‐geochemical markers. Further evidence for chemosymbiosis is found in the characteristic burrow‐systems of these bivalves, by which sulfide is obtained to sustain the symbiotic bacteria. These burrows were previously known only from observations made in aquaria. Such a burrow is described here for the first time in a fossil thyasirid bivalve. Thyasira michelottii (R. Hörnes, 1875) burrowed in sandy sediment in an anterior‐up position, approximately 5–10 cm below the surface, to which it was connected by an inhalant tube. It also produced a prominent posterio‐ventral tunnel, extending up to 300 mm into the sediment. Like modern thyasirids, this animal lacked an exhalant tube. By comparison with the ecology of modern thyasirid bivalves, we suppose that oxygen was obtained by active ventilation through the inhalant tube and that the prominent posterio‐ventral tunnel reflects search by the probing, vermiform foot for short‐lived pockets of sulfidic material in an otherwise low‐sulfide environment. The monospecific occurrence of Thyasira and the paleogeographic setting indicate dysaerobic, warm water conditions. A short distance to the coast and the occurrence of proximal tempestites suggest an inner shelf setting, prone to occasional disturbance by storms.