Shell Beds Research Articles

PALEOECOLOGICAL DYNAMICS OF SHALLOW-WATER BIVALVE CARPETS FROM A LOWER JURASSIC LAGOONAL SETTING, NORTHEAST ITALY

Shell beds are products of complex biological, taphonomic, and sedimentological factors. Paleoecological analysis of Pliensbachian shell accumulations from northeast Italy, Southern Alps, records successive phases of colonization and demise, each of which are characterized by specific taphonomic features and sedimentary fabrics. Field studies were augmented by serial sectioning and thin sections revealing microfacies and microtaphofacies, as well as scanning electro-microscopy (SEM) and electron backscatter diffraction (EBSD). The term bivalve carpet is introduced to describe the original geometry of the bivalve accumulation colonizing the substrate. Within a lagoonal depositional setting, small, thin-shelled Isognomonidae-like bivalves lived as endo-byssate, non-siphonate suspension feeders. These bivalves, interpreted as juveniles, constructed bivalve carpets with a primary lateral extension while lacking a distinct vertical growth. Bivalves colonized firm, dysoxic carbonate substrates consisting of a mixture of peloids, bioclasts, and chemolithotrophic iron-oxidizing bacteria. Oxygen-rich bottom currents and microbial activity stabilized the lagoon floor thus allowing for the episodic settlement of fixosessile bivalves and their autochthonous preservation. Short-term anoxic phases interpreted to reflect seasonal decreases in water energy resulted in the mass mortalities of the juvenile bivalves. The abundance of articulated, vertical orientated valve pairs in the shell accumulations indicate that the bivalve carpets were embedded soon after the death of the organisms without prior physical disturbance.

Professor Ron K. Pickerill and the genesis of ichnology in the Antilles (Jamaica and Carriacou)

Antillean ichnology was essentially a blank book when Ron Pickerill of the University of New Brunswick, Fredericton, made his first research visit to Jamaica in February 1990. Ron’s first Jamaican trace-fossil research team worked initially on the Paleogene Richmond Formation, a flysch succession. Subsequent fieldwork examined the diverse sedimentary formations of the Neogene Coastal Group. Ron’s encyclopaedic knowledge of ichnotaxonomy and his enthusiasm for fieldwork led the team in many directions. Investigations were integrated with new studies of the island’s sedimentology and paleontology. For example, the description of the ichnology and sedimentology of the Upper Pliocene Bowden Formation, including the internationally famous Bowden shell bed, was part of a much wider study. The case-hardened rocks of the White Limestone Group discouraged detailed study until Donovan Blissett attacked the diverse ichnobiota of these user-unfriendly rocks for his doctorate under Ron’s supervision at the University of New Brunswick. Carriacou in the Grenadines was the other island to receive detailed examination in terms of its ichnofauna by Ron and his co-workers. The east coast of this small island provides a near-continuous exposure of the deep water succession of the Grand Bay Formation. Deeper-water burrows and borings in allochthonous bioclasts derived from the shallow shelf provide ichnological contrast in this formation.

Exploring Restoration Methods for the Olympia OysterOstrea luridaCarpenter, 1864: Effects of Shell Bed Thickness and Shell Deployment Methods on Shell Cover, Oyster Recruitment, and Oyster Density

Oysters provide habitat, sediment stabilization, and improved water quality, and are important foundation species in many estuarine ecosystems. Worldwide oyster population declines have been dramatic and efforts to restore declining populations and the services they provide are ongoing. Several commonly used oyster restoration techniques were examined to determine which would be the most successful for restoring the Olympia oyster Ostrea lurida in Newport Bay, CA. Replicate (n = 5) 2×2 m shell beds were constructed of two initial shell planting thicknesses (bed thicknesses of 4 versus 12 cm) and two methods of deployment (bagged versus loose shell). Shell cover, oyster spatfall (settlement), oyster recruitment, and adult oyster densities were analyzed over 2 y; 12-cm-thick oyster beds maintained higher shell cover, experienced less sedimentation, and received greater numbers of oyster recruits than 4-cm-thick beds. There was no significant effect of shell deployment method on shell cover, recruitment, or adult density; however, spatfallwas greater on loose shell beds comparedwith bagged shell beds in the final year of the study. Overall, augmenting mudflat habitat with oyster shell significantly increased adult O. lurida oyster density compared with unmanipulated plots and increased oyster density relative to the average density of oysters measured elsewhere in Newport Bay. Collectively, the data suggest that building thicker shell beds might increase the longevity of a constructed shell bed, and therefore, this approach is recommended for future restoration activities in southern California. This study highlights the advantages of augmenting habitat in a manner that provides vertical relief from sedimentation.

Disentangling the history of complex multi-phased shell beds based on the analysis of 3D point cloud data

Abstract We present the largest GIS-based data set of a single shell bed, comprising more than 10,280 manually outlined objects. The data are derived from a digital surface model based on high-resolution terrestrial laser scanning (TLS) and orthophotos obtained by photogrammetric survey, with a sampling distance of 1 mm and 0.5 mm, respectively. The shell bed is an event deposit, formed by a tsunami or an exceptional storm in an Early Miocene estuary. Disarticulated shells of the giant oyster Crassostrea gryphoides are the most frequent objects along with venerid, mytilid and solenid bivalves and potamidid gastropods. The contradicting ecological requirements and different grades of preservation of the various taxa mixed in the shell bed, along with a statistical analysis of the correlations of occurrences of the species, reveal an amalgamation of at least two pre- and two post-event phases of settlement under different environmental conditions. Certain areas of the shell bed display seemingly significant but opposing shell orientations. These patterns in coquinas formed by densely spaced elongate shells may result from local alignment of neighboring valves due to occasional events and bioturbation during the years of exposure. Similarly, the patchy occurrence of high ratios of shells in stable convex-up positions may simply be a result of such “maturity” effects. Finally, we document the difficulties in detecting potential tsunami signatures in shallow marine settings even in exceptionally preserved shell beds due to taphonomic bias by post-event processes.

New plesiosaur records from the Lower Cretaceous of the Neuquén Basin, west-central Argentina, with an updated picture of occurrences and facies relationships

In this work new records of elasmosaurid plesiosaurs are described from the Lower Cretaceous Agrio Formation of the Neuquén Basin (west-central Argentina) and their bearing facies and taphonomic modes are analyzed. New records from both members of the upper Valanginian–Hauterivian Agrio Formation are described along with a careful revision of previous records from the Pilmatué Member. These records include the first elasmosaurids reported from the upper Hauterivian of South America. Even though the studied materials comprise solely fragmentary postcranial elements, they suggest the presence of at least two different elasmosaurid taxa. Additionally, the rarity of cervical centra with a ventral notch in the articular faces among pre-Cenomanian elasmosaurids is confirmed. In terms of plesiosaur taphonomy and bearingfacies the studied plesiosaurs from the Agrio Formation were recorded in two different taphonomic modes, 1) preservation of disarticulated but closely associated postcranial elements within dark-grey shales, that probably resulted from bloating, drifting and eventually sinking of carcasses to a muddy and calm seafloor under oxic to suboxic conditions, without further transport or physical reworking; and 2) preservation of isolated plesiosaur bones in shell beds, including mostly weathered and encrusted isolated vertebrae. These taphonomic modes are not randomly distributed in the studied marine succession, but closely follow cycles of changes in the relative sea level and can be linked to fourth-order high frequency cycles. Taphonomic mode 1 includes better preserved specimens deposited in highstand systems tract intervals subjected to higher sedimentation rate while taphonomic mode 2 is related to transgressive systems tract intervals subjected to a reduced sedimentation rate and higher time of exposure of remains on the seafloor, resulting in poorly preserved specimens.

Facies associations in warm-temperate siliciclastic deposits: insights from early Pleistocene eastern Mediterranean (Rhodes, Greece)

AbstractDiverse, abundant and usually well-preserved communities of skeletal organisms occur in the lower Pleistocene (Gelasian) siliciclastic deposits of the Greek island of Rhodes. Benthic foraminifers, molluscs and bryozoans have been studied in four measured and sampled sections located in the northern part of the island. Among these bottom-dwelling organisms, numerous extant taxa are good environmental indicators and, combined with field observations and sedimentological data, they provide information on the probable conditions in which they developed. The siliciclastic deposits of the Kritika Formation have been divided into 14 different bio- and lithofacies, which have been further grouped into four facies associations corresponding to four different environmental settings: (1) continental to fluviatile; (2) brackish-water (lagoonal/deltaic); (3) infralittoral (0–20 m); and (4) upper circalittoral (depths of 20–40 m, but also down toc.50–60 m). Among the marine facies associations, several characteristic biocoenoses have been recognized: soft-bottoms (fine to coarse sands and gravels); seagrass meadows; biogenic calcareous crusts on drowned beachrock slabs; red algal rhodoliths; and bivalve shell beds. In the studied sections, 13 superimposed genetic sequences have been documented. The repetition of similar facies associations within each sequence suggests: (1) a possibly eustasy-controlled, cyclic sedimentation; (2) a general subsidence of Rhodes during the deposition of the studied facies associations; and (3) a mostly constant range of environmental conditions (i.e. sedimentation rates and temperature) throughout the Gelasian.

The Monotis–Dactylioceras Bed in the Posidonienschiefer Formation (Toarcian, southern Germany): condensed section, tempestite, or tsunami-generated deposit?

The Monotis–Dactylioceras Bed in the Toarcian Posidonienschiefer Formation is a conspicuous shell accumulation, which was used as marble in the late eighteenth century of Bavaria. The shell bed is characterized by left valves of the bivalve Meleagrinella substriata (von Munster) and local accumulations of the ammonite Dactylioceras athleticum (Simpson), and has previously been considered to reflect a regression or a storm event. Indeed, three different interpretations of the bed appear possible: (1) condensed section, (2) tempestite, or (3) tsunami deposit. New observations, specifically swaley cross-stratification and sole marks, point to an event-like formation of the bed. While most of the observations are consistent with the formation by a storm event, the singular occurrence within the 4.4 Ma lasting Lower Toarcian, which spanned about 4.4 Ma, and the inverse grading of the shells point to a possible tsunami origin. The latter hypothesis is supported by contemporaneous seismites in the western Tethyan realm, and micrite injections in strata below the Monotis–Dactylioceras bed, which may reflect quake-induced sediment liquefaction.

Neogene molluscs, shallow marine paleoenvironments, and chronostratigraphy of the Guajira Peninsula, Colombia

The Neogene sedimentary fill of the Cocinetas Basin in northern Colombia preserves a rich record of marine invertebrates and can be analyzed in the context of a high-resolution stratigraphy and excellent chronostratigraphy. Molluscan fossils are highly diverse and often well preserved, offering a window into the rapidly changing paleoenvironments and biogeography of northern South America during parts of the Early to Middle Miocene and latest Pliocene to Pleistocene. Before the evolutionary and biogeographic implications of these fossils can be understood, however, their associated depositional environments and geologic ages must be determined. Here, we present preliminary results from paleoenvironmental, biostratigraphic, and strontium isotope chronostratigraphic analyses of sediments and fossils from the Uitpa, Jimol, Castilletes, and Ware formations found in Cocinetas Basin. The basal unit in the Neogene succession, the Uitpa Formation, comprises mudstones redeposited sandstones and molluscs typical of bathyal to outer shelf environments at its base. It is a shallowing-up sequence and is conformable with the overlying Jimol Formation, which comprises coarse-grained lithic calcarenite, coquina, and mudstone that represent a regressive–transgressive–regressive sequence. This sequence includes foreshore and transition zone through lower inner shelf environments, but generally poorly preserved invertebrate assemblages. The conformably overlying Castilletes Formation contains a varied suite of depositional environments with better-developed shell beds and thicker successions of intervening siltstone. A significant unconformity exists between the Castilletes Formation and the overlying Ware Formation, which represents a deltaic to coastal shoreface deposition environment, rich in shallow marine molluscs from a variety of ecotopes. Biostratigraphic assessment and strontium isotopic results from the Jimol and Castilletes formations indicate that these units contain fossils of latest Early Miocene through Middle Miocene age, while those of the Ware are approximately Late Pliocene in age. These results help to place the shallow marine assemblages of Cocinetas Basin into a wider geologic context that aids our understanding of how these faunas relate to the broader evolutionary and biogeographic history of the southern Caribbean during the Neogene. Additionally, the isotope dating and paleoecology of this fauna help to place co-occurring terrestrial and aquatic vertebrate assemblages into a local paleoenvironmental and chronostratigraphic framework.

Early Eocene macroflora and associated palynofossils from the Cambay Shale Formation, western India: Phytogeographic and palaeoclimatic implications

A diverse assemblage of plant macrofossils and the associated representative palynofloral elements are documented from the early Eocene subsurface beds of the Cambay Shale Formation exposed in an open cast lignite mine at Vastan Village in the Surat District, western India. The Vastan mine succession is cyclic, each cycle representing a transgressive burial event terminating in the low energy lagoonal conditions. The higher energy cycle begins with sandy lenses having rich biotic remains, followed by mudstones and molluscan shell beds and ends with lignite seams. The dominantly muddy facies and the associated biota demonstrate predominantly low energy near shore or coastal plain depositional setting with conditions varying from dominantly marine (shallow) through brackish to fresh water. The Vastan mine is a well dated fossil locality with a rich and diverse biota of mammals, birds, snakes, lizards, fish, insects, molluscs, foraminifers, dinoflagellates, and plants. The plants comprise leaf and fruit impressions, seeds, fruits, wood fragments, mangrove rooting structures, fungal thalli and spores, pteridophytic spores, and angiosperm pollen grains. Thirteen macrofossil species, including several morphotaxa, are represented by the families Calophyllaceae, Rutaceae, Anacardiaceae, Rubiaceae, Combretaceae, Lythraceae, Sapindaceae, Malvaceae, and Ebenaceae. The palynological assemblage representing fourteen taxa includes the new species, Notothyrites undulatus, Callimothallus semicircularis, and Carallioipollenites integerrimoides. Habitat and distribution of modern taxa comparable with the fossil assemblage from Vastan suggest a terrestrial lowland environment. The macrofossil taxa are indicative of mesophytic, mixed forest growing under tropical to subtropical climate with sufficient humidity. The occurrence of dipterocarp elements along with taxa such as Swintonia, Pterospermum and Diospyros, etc. seems to suggest the presence of a tropical rain forest in the vicinity of Vastan.

New evidences of shell-gravel taxa from Harudi Formation (Middle Eocene), western Kutch

A foraminiferal shell bed (informally referred to as the ‘obtusus bed’) occurring in the upper part of the Harudi Formation (Middle Eocene) in Kutch has yielded evidences of diverse shell-gravel taxa (mostly macroinvertebrates). In this communication, we report new evidences of shell-gravel taxa in the form of clavet and tubular borings of endolithic fauna and epibenthic growth of solitary scleractinian coral and cyclostome bryozoa for the first time. The borings include Gastrochaenolites Leymerie, Trypanites Magdefrau and cf. Trypanites Magdefrau. The coral and the bryozoan are Trochocyathus sp. Edward and Haime and ‘Proboscina’ sp. Audouin respectively. This highly diverse shell-gravel fauna subsisted solely on the larger foraminifera of the ‘obtusus bed’. So far, such an association is unknown from any other part of India.

PARADOX OF DRILLED DEVIL'S TOENAILS: TAPHONOMIC MIXING OBSCURES CRETACEOUS DRILLING PREDATION IN UTAH OYSTERS

ABSTRACT Pycnodonte newberryi oysters accumulated in voluminous shell beds of Late Cretaceous (late Cenomanian) age in shale deposits of the Western Interior Seaway. Analysis of triplicate grid samples from six shell beds in southern Utah shows that the reclining valves of the oyster are disproportionately represented by the inflated left valve, eight times more often than the smaller, flat right valve, and whole valves are rare. The shells nearly always fragment from the commissure toward the thick umbonal half of the shell. These fragmented shells make up more than half of grid samples, and only 0.5% contain predatory drillholes. By contrast, complete shells are drilled nearly 5% of the time, particularly near the umbo. Fragmentation cannot account for this discrepancy because the preponderance of drillholes occurs in the umbonal part of the shell, the part that is always preserved. To understand this paradox, drilled Pycnodonte were collected from a separate site in Utah and drillhole position on the s...

Palaeoecology, taphonomy, and preservation of a lower Pliocene shell bed (coquina) from a volcanic oceanic island (Santa Maria Island, Azores)

Abstract Massive fossil shell accumulations require particular conditions to be formed and may provide valuable insights into the sedimentary environments favouring such concentrations. Shallow-water shell beds appear to be particularly rare on reefless volcanic oceanic islands on account of narrow, steep and highly-energetic insular shelves where the potential for preservation is limited. The occurrence of an exceptional coquina (Pedra-que-pica) within the Miocene–Pliocene deposits of Santa Maria Island (Azores), therefore provides a rare opportunity to understand the conditions that led to the formation and preservation of a massive shell bed at mid-ocean insular setting. This study provides a detailed analysis regarding a 10–11-m-thick bivalve-dominated fossil assemblage exposed at Pedra-que-pica on Santa Maria Island in the Azores. Integration of taphonomical, palaeoecological and sedimentological observations are used to reconstruct the genesis of the coquina bed and related events, and to discuss why such exceptional sedimentary bodies are so rare on shelves around reefless volcanic oceanic islands. The sequence at Pedra-que-pica demonstrates a complex succession of sedimentary environments in response to the drowning of an existing coastline during a period of rapid sea-level rise. The Pedra-que-pica shell bed incorporates storm-related materials and possible debris falls that originated nearby in a shallow and highly productive carbonate factory. Deposition took place below fair-weather wave base, at around 50 m depth, as inferred from the overlying volcanic succession. The preservation of this coquina was favoured by deposition on a platform laterally protected by a rocky spur, combined with rapid burial by water-settled volcanic tuffs and subsequent volcanic effusive sequences. The recent exhumation of the deposit is the result of island uplift and subsequent erosion.

Experimental treatment of a refinery waste air stream, for BTEX removal, by water scrubbing and biotrickling on a bed of Mitilus edulis shells

The paper presents the results of a two-stage pilot plant for the removal of benzene, toluene, ethylbenzene and xylene (BTEX) from a waste air stream of a refinery wastewater treatment plant (WWTP). The pilot plant consisted of a water scrubber followed by a biotrickling filter (BTF). The exhausted air was drawn from the main works of the WWTP in order to prevent the free migration to the atmosphere of these volatile hazardous contaminants. Concentrations were detected at average values of 12.4 mg Nm−3 for benzene, 11.1 mg Nm−3 for toluene, 2.7 mg Nm−3 for ethylbenzene and 9.5 mg Nm−3 for xylene, with considerable fluctuation mainly for benzene and toluene (peak concentrations of 56.8 and 55.0 mg Nm−3, respectively). The two treatment stages proved to play an effective complementary task: the water scrubber demonstrated the ability to remove the concentration peaks, whereas the BTF was effective as a polishing stage. The overall average removal efficiency achieved was 94.8% while the scrubber and BTF elimination capacity were 37.8 and 15.6 g BTEX d−1 m−3, respectively. This result has led to outlet average concentrations of 1.02, 0.25, 0.32 and 0.26 mg Nm−3 for benzene, toluene, ethylbenzene and xylene, respectively. The paper also compares these final concentrations with toxic and odour threshold concentrations.

Role of Pretreatments in the Thermal Runaway of Hazelnut Shell Pyrolysis

This study examines the effects of various pretreatments (size reduction, pelletization, hot water washing, torrefaction) on the pyrolysis of packed beds of hazelnut shells, exposed to moderate heating along the lateral surface, aimed at identifying the controlling or predominant mechanisms for the high reaction exothermicity leading to thermal runaway. As long as the particles preserve their chief structural properties (roughly or finely crushed shells), the process dynamics are not altered by size variation. Instead exothermic effects are highly reduced or almost disappear for milled shells, even in the pelletized form, depending on the actual powder size. Hence, secondary intraparticle reactions, owing to a peculiar scarcely porous microstructure, play a paramount role in the process exothermicity and thermal runaway. The displacement of the beginning of the reaction process at higher temperature, following sample washing, is enhanced by deepening the pretreatment. In this way sample conversion takes place in the presence of large spatial gradients, which hinder the occurrence of thermal runaway. Finally torrefaction, by partial or complete conversion of the more thermally labile chemical components, also eliminates the conditions that trigger and partly sustain the thermal runaway.

Behavior of a Woody and a Loose Biomass in a Downdraft Gasifier

Even though wood is the preferred feedstock for fixed bed gasifiers to generate producer gas, its limited availability and increasing cost have necessitated the use of bioresidues. Bioresidues may be used in either loose form or briquetted form. For small gasifiers, the use of bioresidues in loose form is more economical. But certain drawbacks like increased pressure drop, choking, and channeling of biomass bed may occur in the reactor. In this work: (a) characteristics like densities, voidage, specific surface area, etc., of beds of loose groundnut shells and wood pieces were determined experimentally; (b) pressure and temperature distribution in the gasifier during their gasification were measured; and (c) all these parameters were compared between loose groundnut shells and wood pieces. For loose groundnut shells: (i) the bed pressure drop was determined to be 266% of that for wood pieces; and (ii) the upward propagation velocity of oxidation zone in the gasifier at unsteady state was determined to be 56% of that for wood pieces. From the tests, certain design and operating parameters for gasification of loose groundnut shells have been known.

Real-time echosounder based acoustic seabed segmentation with two first echo parameters

Seabeds with different physical and biological properties (for example, mud, sand, gravel, rock, seagrass, shell beds) generally produce differently shaped echoes in response to echosounder pings. After allowance for propagation losses and for artefacts caused by sampling at equal time intervals, instead of equi-angular spacing, geometrical (i.e. shape based) estimates of the similarity of echo envelopes may be used to geographically segment the seabed into areas with similar acoustic responses. Post-processing of entire echoes by direct clustering provides a statistically and geometrically optimal method of segmentation which in itself does not need ground truthing, since it is based on the actual seabed acoustic response. However, real-time processing allows sampling strategies to be adapted in response to findings during survey. A real-time segmentation method is presented which uses only two simple echo parameters. Classes formed by direct clustering of actual echoes map to unique areas of the 2-parameter space in a very simple and regular fashion, fully validating its use as a segmentation scheme. Groundtruth (seabed samples or video) must be taken if descriptions (labels) are to be assigned to the 2-parameter segmentations (the segmentation becomes a classification). However, the relation to direct clustering results means that in principle groundtruth is not needed to validate the segmentations. In contrast it has never been shown that echo features used by existing real-time schemes contain sufficient information to adequately characterize seabeds.

Human harvesting impacts on managed areas: ecological effects of socially-compatible shellfish reserves

We examined how human harvesting impacts on managed areas affect the abundance and size distribution of the edible mangrove shellfish Anadara granosa and Polymesoda spp. in the Roviana Lagoon, Solomon Islands. We tested two hypotheses: (1) in areas permanently and temporally closed to human exploitation, abundance and size distribution of these shellfish species is significantly greater than in sites open to exploitation and (2) moderate human disturbance of shell beds, particularly of Polymesoda spp., increases their abundance. Firstly, we studied perceptions of environmental states and processes coupled to foraging and management interventions to assess sociocultural influences on harvesting practices and ascertain the types of management regime that people would consider in a context where poaching and interloping are common practices. Secondly, we compared shellfish abundance and shell size from areas that were permanently protected, temporally reserved for communal harvest, and permanently open for exploitation. Thirdly, drawing from women’s local knowledge, we measured the abundance of Polymesoda spp. in relation to mud compactness in quadrats across the three management regimes. Results showed that both species were significantly more abundant in permanent and temporally closed sites than in open sites. In the mud compactness study, however, while shell abundance was greater in moderately compacted quadrats, there was no statistical relationship between mud compactness and shell abundance within or across the three management regimes. Results suggest that even under the strong impacts of poaching, temporally closed areas have more clams than open areas and are as effective as areas that are permanently closed nominally. The results also suggest that human harvesting regimes can influence the effectiveness of local management decisions and thus are important when designing community-based conservation programs in the Solomon Islands and other Pacific Islands.

The Upper Pliocene Bowden shell beds, southeast Jamaica

The Neogene fossil record of the Antilles is particularly rich and can satisfy the interests of most collectors, from foraminifers to vertebrates. The molluscs are among the commonest and most diverse groups, being found in both terrestrial and marine deposits. The Pliocene Bowden shell beds of southeast Jamaica, best known for its hundreds of species of molluscs, also yields diverse members of other groups, the total exceeding 800 species from a mere 5 m of stratigraphical section.

<em>Octopus tetricus</em> (Mollusca: Cephalopoda) as an ecosystem engineer

Summary: The Sydney octopus (Octopus tetricus) occurs in unusual numbers on a shell bed of its prey remains that have accumulated as an extended midden where additional octopuses excavate dens. Here, O tetricus are ecosystem engineers, organisms that modulate availability of resources to other species and to their own species by causing physical state changes in materials. A community of invertebrate grazers and scavengers has developed on the shell bed. Fishes are attracted to the shell bed in numbers significantly greater than in nearby habitats. Large predators, including wobbegong sharks, were attracted to and fed on concentrations of fish, inhibiting the activities of the original engineers, the octopuses. Positive feedbacks included the accumulation of shell debris, increasing shelter availability for additional octopuses and aggregating fish. Negative feedbacks included reductions of nearby prey size and availability, aggression among octopuses, and predator limitation to octopus activity that would otherwise maintain the shell bed.

The origin of echinoid shell beds in siliciclastic shelf environments: three examples from the Miocene of Sardinia, Italy

Three mass accumulations of sea urchins from the Miocene of Sardinia show a number of taphonomic features which set them apart from previously described echinoid assemblages from the Cenozoic in which they represent: (1) monotypic assemblages; (2) include very well-preserved remains of either regular or spatangoid echinoids; and (3) originate in deeper water environments. These accumulations are compared using a detailed sedimentological and taphonomic analysis including preservational fabrics, taphonomic signatures, size frequency distributions, density of occurrences and preferred orientations. The possible role of gregarious behaviour contributing to mass occurrences and the specific sedimentary events leading to the excellent preservation are discussed. The interpreted depositional environment of all three deposits is that of a storm-dominated, siliciclastic shelf environment. A phymosomatid assemblage represents rapid burial through obrution of a highly dense, freshly dead community. A Brissopsis-dominated spatangoid assemblage represents a mixed accumulation of parautochthonous and transported skeletons. The third assemblage consisting of regular echinoid spines and rare tests represents a composite tempestite. Differences in the depositional environments are related to their position along onshore–offshore gradient with the first two beds originated in a deeper setting than that of the spine accumulation. This study shows that the preservation of assemblages containing complete regular echinoids and spatangoids is higher in deeper water settings than in shallow water environments.