Taphofacies analysis of recent shelly cheniers (beach ridges), northeastern baja california,Mexico

Summary This report presents the results of taphofacies analyses of shelly cheniers (mollusk-dominated lag-concentrations) from the tidal flats of northeastern Baja California, Mexico. The three generations of moderm (formed during last 70 years), submodem (younger than 1,500 BP), and subfossil (5,000–2,000 BP) cheniers can be distinguished by their position relative to the shoreline, their topography, and the radiocarbon-age of their shells. The generations differ in the duration and complexity of their taphonomic history. Sixty-one samples from nine localities were collected to test the utility of the taphofacies approach for studying chenier-type shell deposits. The three chenier generations, although all dominated by the bivalve molluskMulinia coloradoensis, differ significantly in their taxonomic composition due to taphonomic and/or biologic factors. The taphofacies analysis included 4,334 specimens ofM. coloradoensis described by nine taphonomic variables. Univariate analysis of those variables indicated that the shells that accumulated in the cheniers are little-affected by biological processes (bioerosion, encrustation), and moderately affected by physical processes (fragmetation, cracking, peeling, edge preservation). Only the luster features of shells (external luster, internal luster, and internal features) vary substantially and consistently with chenier age —a result of subaerial weathering. Multivariate taphofacies analysis discriminates the three generations of cheniers even when the poorly preservable luster variables are excluded from the analysis. This suggests that taphofacies discrimination is possible for fossil cheniers. The shells collected from the chenier surface have substantially poorer preservation than shells from the subsurface, indicating that taphonomic degradation in the chenier plain environment is a surface phenomenon. Chenier plain shelly assemblages are taphonomically distinct from assemblages formed in other marine environments: they have a very low frequency of macroscopically recognizable bioerosion and encrustation. The existence of preservable taphonomic differences between the cheniers that differ in their age (i.e., duration of preburial history), suggests that fossil lag concentrations may be useful in detecting incompleteness gradients along stratigraphic boundaries. A ‘taphonomic clock’—a correlation between a ‘time-sincedeath’ and shell preservation—was found only for luster features, taphonomic attributes that are unlikely to be preserved in the fossil record.     

Taphonomic windows and molluscan preservation

Recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid-outer carbonate ramp settings. If those rare skeletal lagerstätten are representative, then the quality and completeness of the molluscan record are thrown into doubt. Yet database studies suggest that the bivalve fossil record is actually relatively complete. If so, then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution, and which operated on a relatively high frequency, i.e., less than the species duration for bivalves.Storm beds, shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows. Many storm event beds include rich accumulations of shelly benthos. Differences between storm bed faunas and those of the background facies could reflect transportation effects. However, some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents, and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation. Despite Palaeozoic to Cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers, shallow infaunas are commonly concentrated together with epifauna in such deposits.Some low energy, organic-rich mud-dominated settings are associated with preservation of aragonitic molluscs. Infaunal bivalves are a prominent component of shell plasters or pavements in such settings, linked to episodic bottom water anoxia. Decaying algal blooms drew the redox boundary up above the sediment–water interface, and brought populations of infaunal bivalves to the surface where they died. Isolated from the oxic taphonomically active zone, the shells were not dissolved and were buried as thin shell layers. In similar settings, aragonitic shells were preserved as moulds through early pyritisation, or even through preservation of original shell aragonite.In oxic environments, bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution. In some hardgrounds, these delicate moulds were preserved due to synsedimentary cementation, probably using carbonate released by aragonite dissolution. The examples included here come from both intervals of “calcite” and “aragonite” seas, and it is not possible to assess whether the saturation state (with respect to aragonite) of the ambient sea water played a role in the selective removal of aragonitic shells.While taphonomic windows may have captured the diversity of individual groups, it is clear from quantitative data involving skeletal lagerstätten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities.     

Taphonomy and taphofacies models of the Pliocene deposits of Vejer de la Frontera (Cádiz, SW Spain)

A quantitative taphonomic analysis of the fossil assemblages, together with a sedimentary study of the Pliocene deposits of the Vejer Basin (Cádiz, Spain), has been carried out. This multidisciplinary analysis has allowed us to establish with great precision the palaeoenvironmental conditions under which these materials were formed. The Pliocene deposits can be divided into three units, named from bottom to top: Unit 1, Unit 2 and Unit 3. Unit 1 is formed of medium-to-coarse-grained sands at the base, corresponding to subtidal bar deposits. At the top, the sands become coarser, and beach and littoral dunes were formed. Within these sedimentological contexts, the fossil remains exhibit a strong degree of taphonomic destruction due to high hydrodynamic energy. Unit 2, consisting of clays, corresponds to deposits formed in a protected coastal bay. A low-energy environment allowed the fossils to maintain a good state of conservation and even thin-shelled bivalves with a delicate ornamentation are preserved. Unit 3 comprises highly bioclastic sands and calcarenites-calcirudites. This unit displays a shallowing upwards trend, passing from deposits generated under shallow subtidal conditions in the lower part to materials deposited on beaches and coastal dunes in the upper part. Under these conditions, the fossil remains show a high degree of taphonomic destruction. In Units 1 and 3, the fossils are mainly accumulated in horizontal beds, with extensive lateral continuity and with erosive bottoms and graded tops. These accumulations of fossils are interpreted as tempestites. As compared with the fossils preserved in surrounding sediments, the taphonomic attributes of these bioclasts suggest a superposition of the background and event processes. During normal conditions, the high energy of the littoral environments produced a strong degree of taphonomic destruction. During storm events, a mixture of taphonomic attributes was produced: high fragmentation and disarticulation and little abrasion and edge rounding of the fragments. This mixture was a consequence of sudden burial after storms, which prevented long periods of exposure of the bioclasts in the taphonomically active zone. Three taphofacies models are proposed, based on taphonomic attributes of fossils contained in the different units: inner-shelf taphofacies, protected shallow-lagoon taphofacies and storm bed taphofacies.     

Upper Pliocene bivalve shell concentrations from the Lower Chelif basin (NW Algeria): Systematics,sedimentologic and taphonomic framework

Sedimentologic and palaeontological investigation of the Upper Pliocene Slama Formation in the Lower Chelif Basin (NW Algeria) led us to collect important bivalve assemblages for taxonomic and taphonomic purposes. A rather comprehensive inventory list of Upper Pliocene bivalves from northwestern Algeria is now available and consists of 30 species, 17 of which are extinct ones. Four principal taphonomic attributes were analysed: bioerosion, encrustation, fragmentation, and abrasion. Physical and biogenic sedimentary structures are used for palaeoenvironmental interpretations. The taphonomic, sedimentologic and ichnological characteristics of most of the deposits suggests they originated from discontinuous processes of winnowing and bypassing of sediments, probably due to the action of storms in shallow waters, mainly in the shoreface depositional environment. The bivalve assemblage is dominated by disarticulated valves and displays significant taphonomic alteration in the shells. Sclerobionts traces in shells particulary affect the oyster shells. Bioerosion traces are predominately those of clionid sponges (Entobia isp.), polychaetes (Maeandropolydora isp. and Caulostrepsis isp.), bivalves (Gastrochaenolites isp.), and of predatory gastropods (Oichnus isp.). Among the sclerobionts, the identified encrusters were juvenile oyster recruits, barnacles, polychaetes (serpulid tubeworms), bryozoans (Microporella sp. and Acanthodesia sp.), and vermetid gastropods (Petaloconchus intortus).     

Sequence stratigraphic significance of sedimentary cycles and shell concentrations in the Upper Jurassic-Lower Cretaceous of Kachchh, western India

Upper Jurassic and Lower Cretaceous siliciclastic shallow water sediments of the Kachchh Basin, western India, form strongly asymmetric coarsening-upward cycles, which are interpreted as recording changes in relative sea level (deepening-shallowing cycles). These cycles correspond to depositional sequences, in which deposits of the lowstand systems tract are not present, the sequence boundary coinciding with the transgressive surface. Shell concentrations are found in the transgressive lags at the base of the transgressive systems tract (TST), in the maximum flooding zone (MFZ), and at or close to the top of the highstand systems tract. They belong to six assemblages, five of them dominated by large bivalves such as Seebachia, Herzogina, Gryphaea, Gervillella, Megacucullaea, Pisotrigonia and Indotrigonia, the sixth by the coral Amphiastraea. Three types of shell concentrations can be distinguished that differ from each other in a number of ecological and taphonomic features, such as species diversity, preservation quality, orientation in cross-section, percentage of disarticulation, and degree of biogenic alteration. Characteristic features of concentrations at the base of the TSTs are moderate time-averaging, sorting, a preferred convex-up orientation, and nearly total disarticulation of shells. They are suggestive of an environment in which reworking and local transport were frequent events. Similar features are shown by concentrations near the tops of the HSTs, except that there shells were largely concentrated in lenses and in pavements rather than in beds as in the transgressive lags. Associated sedimentary structures indicate deposition above fair weather wave base in a high-energy environment. Concentrations occurring in the MFZ, in contrast, are autochthonous and highly time-averaged, having accumulated during times of low rates of sedimentation below storm wave base. This is supported by their high preservation quality (comparatively high percentage of articulated shells, shells of infaunal organisms commonly preserved in life position), biogenic alteration being the most important taphonomic agent. The dominant elements of these shell concentrations, i.e. Seebachia, Megacuccullaea, and Indotrigonia in the Upper Jurassic, and Pisotrigonia in the Lower Cretaceous, are endemic to the Ethiopean faunal province and belong to lineages that rapidly evolved during this time period.     

Taphonomy and compositional fidelity of Quaternary fossil assemblages of terrestrial gastropods from carbonate-rich environments of the Canary Islands

Quaternary aeolian deposits of the Canary Islands contain well‐preserved terrestrial gastropods, providing a suitable setting for assessing the taphonomy and compositional fidelity of their fossil record over ~13 kyr. Nine beds (12, 513 shells) have been analysed in terms of multivariate taphonomic and palaeoecological variables, taxonomic composition, and the stratigraphic and palaeontological context. Shells are affected by carbonate coatings, colour loss and fragmentation. Shell preservation is size‐specific: juveniles are less fragmented and show colour preservation more commonly than adults. In palaeosols, the adult shell density correlates negatively with the proportion of fragmented adults, negatively with the proportion of juveniles, and positively with the proportion of adults with coatings. High bioturbation intensity in palaeosols is associated with low shell fragmentation and high proportion of shells with coatings. These relationships imply that high adult density in palaeosols was driven by an increase in shell production rate (related to a decrease in predation rates on adults and a decrease in juvenile mortality) and a decrease in shell destruction rate (related to an increase in durability enhanced by carbonate precipitation). In dunes, the relationships between taphonomic alteration, shell density and bioturbation are insignificant. However, dune assemblages are characterized by a lower frequency of shells with coatings and higher rates of colour loss, indicating lower shell durability in dunes than in palaeosols. Additionally, non‐random differences in the coating proportion among palaeosols imply substantial temporal variation in the rate of carbonate crust formation, reflecting long‐term changes in bioturbation intensity that covaries positively with shell preservation. Dunes and palaeosols do not differ in species abundances despite differences in the degree of shell alteration, suggesting that both weakly and strongly altered assemblages offer data with a high compositional fidelity. Carbonate‐rich terrestrial deposits originating in arid conditions can enhance the preservation of gastropods and result in fossil assemblages that are suitable for palaeoecological and palaeoenvironmental studies of terrestrial ecosystems.     

Preboreal oscillations inferred from Arctica islandica sclerochronology

An increasingly important source of annually resolved palaeoenvironmental proxy data originates from cross-dated incremental chronologies summarizing the shell growth of several individuals. Here, we have analysed annual increment variations in a collection of radiocarbon-dated shells of ocean quahog (Arctica islandica) from early Holocene prodelta deposits in north Norway. Radiocarbon dating of the shell material showed that the increments were formed contemporaneously during Preboreal times. The biologically youngest shell contains 35 annual increments, whereas the oldest shell shows 169 increments. Time-series of annual increments demonstrated clear age trends with the widest increments during the very early years of bivalve life, followed by a notable decline in increment widths as the bivalves aged. Subsequent to removing these biological trends from the series, a sclerochronological cross-dating was carried out and resolved the temporal alignments of the shell growth increment records relative to each other. The resulting shell growth increment chronology evidences vigorous growth variations. Spectral analysis of the chronology revealed 3.7- and 4.3-year periodicities, indicative of Preboreal environmental oscillations. Periodicities of longer period were not detected. Our results prove the value of radiocarbon-dated shell assemblages to build “floating” geochronologies for periods and regions where dead shells from museum collections or seabed are not obtainable. Increasing constructions of such chronologies enhance the potential of sclerochronological cross-dating of annual shell growth increment chronologies to depict and detail annually-resolved climate variability not only for late Holocene, as previously illustrated, but also for early Holocene times, when large-scale oscillations punctuated the global climate dynamics. Development of longer chronologies with higher sample replication remains an attainable interdisciplinary target.     

Taphonomic signatures,ichnofacies analysis and depositional dynamics of fossil macro-invertebrate assemblages of the San Juan Raya Formation,Zapotitlán Basin,Puebla, Mexico

In this paper, we describe the taphofacies and ichnofacies from Aptian strata of the San Juan Raya Formation in the Santa Ana Teloxtoc area, Puebla, Mexico. A composite stratigraphic section was analysed on a bed-by-bed scale up to a total thickness of 765.5 m. Our results show the presence of 10 taphofacies and 3 ichnofacies. The taphofacies and ichnofacies interpretation, and its correlation with the sedimentary lithofacies, enabled the determination of a palaeoenvironmental model for the study area that corresponds to a shallow marine, open-coast, clastic system with episodic sedimentation as a product of storm events. This system had several variations in sub-environments, from foreshore to offshore. Foreshore and shoreface environments are characterised by taphofacies Tf1, Tf2, Tf3 and Tf10 and PsI and SkI ichnofacies, representing lower faunal diversity moments (with the exception of Tf3 taphofacies). Meanwhile, taphofacies Tf4, Tf5, Tf6, Tf7, Tf8 and Tf9 were representative of shelf environments and are described as moments of medium-to-high faunal diversity (with the exception of Tf5 taphofacies).     

Mid-Neolithic Exploitation of Mollusks in the Guanzhong Basin of Northwestern China: Preliminary Results

Mollusk remains are abundant in archaeological sites in the Guanzhong Basin of Northwestern China, providing good opportunities for investigations into the use of mollusks by prehistoric humans. Here we report on freshwater gastropod and bivalve mollusks covering the time interval from about 5600 to 4500 cal. yrs BP from sites of Mid-Late Neolithic age. They are identified as Cipangopaludina chinensis and Unio douglasiae, both of which are currently food for humans. The shells are well preserved and have no signs of abrasion. They are all freshwater gastropods and bivalves found in pits without water-reworked deposits and have modern representatives which can be observed in rivers, reservoirs, and paddy fields in the studied region. Mollusk shells were frequently recovered in association with mammal bones, lithic artifacts, and pottery. These lines of evidence indicate that the mollusks are the remains of prehistoric meals. The mollusk shells were likely discarded into the pits by prehistoric humans after the flesh was eaten. However, these mollusk remains may not have been staple food since they are not found in large quantities. Mollusk shell tools and ornaments are also observed. Shell tools include shell knives, shell reaphooks and arrowheads, whereas shell ornaments are composed of pendants and loops. All the shell tools and ornaments are made of bivalve mollusks and do not occur in large numbers. The finding of these freshwater mollusk remains supports the view that the middle Holocene climate in the Guanzhong Basin may have been warm and moist, which was probably favorable to freshwater mollusks growing and developing in the region.     

Limpet Shells from the Aterian Level 8 of El Harhoura 2 Cave (Témara,Morocco): Preservation State of Crossed-Foliated Layers

The exploitation of mollusks by the first anatomically modern humans is a central question for archaeologists. This paper focuses on level 8 (dated around ∼ 100 ka BP) of El Harhoura 2 Cave, located along the coastline in the Rabat-Témara region (Morocco). The large quantity of Patella sp. shells found in this level highlights questions regarding their origin and preservation. This study presents an estimation of the preservation status of these shells. We focus here on the diagenetic evolution of both the microstructural patterns and organic components of crossed-foliated shell layers, in order to assess the viability of further investigations based on shell layer minor elements, isotopic or biochemical compositions. The results show that the shells seem to be well conserved, with microstructural patterns preserved down to sub-micrometric scales, and that some organic components are still present in situ. But faint taphonomic degradations affecting both mineral and organic components are nonetheless evidenced, such as the disappearance of organic envelopes surrounding crossed-foliated lamellae, combined with a partial recrystallization of the lamellae. Our results provide a solid case-study of the early stages of the diagenetic evolution of crossed-foliated shell layers. Moreover, they highlight the fact that extreme caution must be taken before using fossil shells for palaeoenvironmental or geochronological reconstructions. Without thorough investigation, the alteration patterns illustrated here would easily have gone unnoticed. However, these degradations are liable to bias any proxy based on the elemental, isotopic or biochemical composition of the shells. This study also provides significant data concerning human subsistence behavior: the presence of notches and the good preservation state of limpet shells (no dissolution/recrystallization, no bioerosion and no abrasion/fragmentation aspects) would attest that limpets were gathered alive with tools by Middle Palaeolithic (Aterian) populations in North Africa for consumption.     

Evidence for seasonal variation in δ13C and δ18O profiles of Baculites and implications for growth rate

The life histories of ammonites and the life strategies they employed are difficult to assess without robust modern analogues but placing constraints on ammonite growth rates provides a fundamental first step to understanding this abundant, but poorly understood, fossil group. Here we interpret periodic variations in carbon and oxygen stable isotope profiles from Campanian and Maastrichtian ammonites (Baculites) as seasonally driven and use these records to determine their rate of shell precipitation. Several of these samples are housed in museums and were originally prepared using sealants for display and preservation but testing of these sealants indicated no alteration of the isotopic values of treated carbonate. Diagenetic alteration, as determined by shell microstructure, affected the preservation of isotopic signals, resulting in the loss of seasonal variation in less well‐preserved specimens, and the δ¹³C signal is more robust than δ¹⁸O. The periodicity of isotopic profiles from Baculites shells presented here suggest that these organisms grew at rapid rates (c. 340 mm per year), which may imply an r‐type life strategy in which the animals reach maturity quickly, spawn large quantities of progeny, and die at a young age. Because of the potential mobility of Baculites, reconstructing palaeoenvironmental conditions from these isotopic records is challenging and should be conducted cautiously. Unfortunately, well‐preserved Baculites shells much longer than 350 mm are rarely recovered, which complicates the statistical treatment of potential periodicity in isotopic profiles.     

Opisoma excavatum Boehm,a Lower Jurassic photosymbiotic alatoform‐chambered bivalve

Alatoform bivalves are a polyphyletic group characterized by antero‐posteriorly compressed shells and a ventro‐lateral wing originating from a tight fold of the shell wall. This bizarre shell morphology is interpreted as an adaptation for algal photosymbiosis in heliophilous bivalves. The group contains the living heart cockle Corculum together with four extinct genera ranging in age from the Permian to the Jurassic. The Jurassic alatoform bivalve is Opisoma, which has an aragonitic shell that is divided into two regions, both with different functions: one for stabilization, the other for hosting symbionts. The dorsal part of the shell is massive and played the stabilization role. The ventral part has a very thin and fragile shell that permitted the transmission of light into the internal tissues harbouring photosymbionts. The morphology of this delicate ventral part has thus far remained obscure, due to lack of preservation. Accumulations of Opisoma excavatum Boehm with exquisitely preserved shells containing the fragile winged ventral parts are common within the Pliensbachian shallow‐water, lagoonal carbonate succession of the Rotzo Formation of northern Italy. The wings have internal curved chambers limited by septa parallel to the wing edge. The shell of the ventral part consists of irregular fibrous prismatic and homogeneous structures which progressively infill the chambers. As the chambered wings are analogous structures among alatoform bivalves, they are no longer considered a taxonomic character. According to the observed shell orientation in the field and the consequent organization of the soft parts, Opisoma had an opisthogyrate shell.     

Patches of barnacles and ascidians in soft bottoms: Associated motile fauna in relation to the surrounding assemblage

Epibenthic patches dominated by barnacles Balanus crenatus Bruguiere and solitary ascidians Styela spp., Bolthenia echinata (L.) and Molgula spp. in the White Sea shallow subtidal develop on bivalve shells and small stones surrounded with muddy sand. The space between barnacles and ascidians is filled with muddy sediment inhabited by motile taxa. We hypothesized that (i) epibenthic patches and unstructured sediment would attract different motile fauna and (ii) motile fauna of the patches would be affected by local abundances of epibenthic foundation species. Most dominant motile species demonstrated a significant difference in abundance between the two microhabitats. In contrast to the fauna of the sediment, species composition observed in aggregations of barnacles and ascidians was stable across different locations. In the field experiment initially clear bivalve shells after 5 years of exposure developed barnacle clusters with motile fauna similar to that observed in natural aggregations. Amphipods, isopods and bivalves, capitellid polychaets, Cirratulus cirratus (Müller) and Pholoe minuta Fabricius (Polychaeta) dominated in the sediment inside epibenthic patches. The proportion of capitellids, known to be sensitive to organic enrichment, was much higher within the patches than outside. The abundances of motile taxa found in aggregations were mostly determined by the number of barnacles of different size and of their empty shells, the biomass of ascidians, and the effect of location. Different dominant species demonstrated sensitivity to different parameters.Physical structure of the habitat, provided by barnacles and ascidians, as well as their biodeposition activity are regarded as the main factors structuring the motile fauna in the community studied. The spatial pattern observed seems to imply a range of pattern-generating biogenic processes, similar to those previously revealed in patches of filter-feeding bivalves, tube-building worms and seagrass.     

Geoducks (Panopea abrupta) as isotopic bioarchives of seasonality in the climate of British Columbia

Geochemical analyses of geoduck shells (Panopea abrupta) have yielded new high-resolution isotope records of individual growth increments. The geoducks, which were collected from Parry Passage at Langara Island, British Columbia, are the most northern specimens that have yet been analyzed for stable oxygen and carbon isotopes. The isotope records represent the entire shell growth during the calendar years 1918, 1987, and 1991. The analyses showed that values of δ¹⁸O_shell and δ¹³C_shell vary within their growth increments. Temperature derived from δ¹⁸O_shell is consistent with modern instrumental data of sea-surface temperature. High-resolution δ¹⁸O records from individual growth increments substantiate the assumption that they are formed annually. The instrumental data indicate that the growth season was from May through October/November. The sea temperature became gradually warmer during the beginning of the growth season, and decreased steeply during autumn. Similar subannual trends can be recognized in shells growing almost a century ago. Correlation between δ¹⁸O_shell and δ¹³C_shell is negative for 1917–1919 and 1986–1988, whereas the correlation is positive for 1990–1992. Differences in correlation may be related to the composition of available food particles, i.e., algal blooms and plant debris. The food composition is partially affected by the Pacific Decadal Oscillation (PDO). These findings have major implications for the understanding of the interpretation of carbon isotopes in marine bivalves. Because of their spatial and temporal distribution, the geoduck shells are highly suitable for oceanographic reconstructions using stable-isotope analysis and sclerochronology. Furthermore, the large shell size and thickness and their deeply buried life position suggest a high preservation potential in ancient deposits. It is well known that the genus Panopea has a long-range fossil record.     

The isotopic biosignatures of photo‐ vs. thiotrophic bivalves: are they preserved in fossil shells?

Symbiont‐bearing and non‐symbiotic marine bivalves were used as model organisms to establish biosignatures for the detection of distinctive symbioses in ancient bivalves. For this purpose, the isotopic composition of lipids (δ¹³C) and bulk organic shell matrix (δ¹³C, δ³⁴S, δ¹⁵N) from shells of several thiotrophic, phototrophic, or non‐symbiotic bivalves were compared (phototrophic: Fragum fragum, Fragum unedo, Tridacna maxima; thiotrophic: Codakia tigerina, Fimbria fimbriata, Anodontia sp.; non‐symbiotic: Tapes dorsatus, Vasticardium vertebratum, Scutarcopagia sp.). ?¹³C values of bulk organic shell matrices, most likely representing mainly original shell protein/chitin biomass, were depleted in thio‐ and phototrophic bivalves compared to non‐symbiotic bivalves. As the bulk organic shell matrix also showed a major depletion of δ¹⁵N (down to –2.2 ‰) for thiotrophic bivalves, combined δ¹³C and δ¹⁵N values are useful to differentiate between thio‐, phototrophic, and non‐symbiotic lifestyles. However, the use of these isotopic signatures for the study of ancient bivalves is limited by the preservation of the bulk organic shell matrix in fossils. Substantial alteration was clearly shown by detailed microscopic analyses of fossil (late Pleistocene) T. maxima and Trachycardium lacunosum shell, demonstrating a severe loss of quantity and quality of bulk organic shell matrix with time. Likewise, the composition and δ¹³C‐values of lipids from empty shells indicated that a large part of these compounds derived from prokaryotic decomposers. The use of lipids from ancient shells for the reconstruction of the bivalve's life style therefore appears to be restricted.     

Leaf beds in the Early Miocene lacustrine deposits of the Geumgwangdong Formation,Korea: Occurrence,plant–insect interaction records,taphonomy and palaeoenvironmental implications

Occurrence, preservation and evidence of plant–insect interactions in the leaf deposits of the early Miocene Geumgwangdong Formation are described, and the taphonomy and palaeoenvironmental implications are interpreted. The Geumgwangdong leaf beds are tuffaceous lacustrine deposits consisting mainly of thin-bedded sediments, including shale. Deposition was dominated through the vertical aggradation of fine-grained sediments by suspension sedimentation, rather than by the fluvial deposition of coarse-grained sediments. The majority of the fossil leaves were preserved by compression rather than merely preserved as impressions. The Geumgwangdong leaf deposits can be characterised as a wind-transported, dysaerobic, deep-water, lake taphofacies with some influence of water-transport. The fossil flora of the Geumgwandong Formation consists of 64 taxa belonging to 27 families and 43 genera. The dominant taxa were Metasequoia, Fagus, Betula, Quercus, Acer, Zelkova, and Leguminosae, which is comparable to the Early Miocene cool–temperate Aniai-type Flora of north-east Japan. The similarity between the lake settings of the Geumgwangdong Formation and the Aniai Coal-bearing Formation might have been a factor in the development of similar plant–fossil assemblages. Although traces of insect damage in the fossil leaves of the Geumgwangdong Formation were commonly observed, more than 90% of the damaged leaves showed a low level of diversity and degree of insect damage (< 10%), which is indicative of a cool–temperate palaeoclimatic condition. Despite latitudinal differences and geographic separation, the development of similar floras in the Geumgwangdong Formation of south-east Korea and the Aniai Coal-bearing Formation of north-east Japan during the early Miocene could have been influenced by the incipient NW–SE sea-floor spreading that resulted in the opening of the East Sea (Sea of Japan), which might have enabled floral migration between the two regions. This study provides useful data for understanding not only the taphonomy and palaeoenviroments of the leaf deposits, but also the spatial development of the flora resulting from palaeogeographic changes driven by tectonic movement during the early Miocene in Far East Asia.     

The interplay between event and background sedimentation and the origin of fossil‐rich carbonate concretions: a case study in Permian rocks of the Paraná Basin,Brazil

The Permian Serra Alta Formation was generated under transgressive conditions within a large, calm epeiric sea. A monotonous succession of ‘barren’, massive mudstones deposited under oxygen‐deficient conditions (mainly below storm wave base) is the main lithofacies of this unit. Fossils are generally rare and diluted in the matrix, but certain intervals contain shell‐rich concentrations with well‐preserved, closed articulated bivalves, mixed with shells and comminuted debris with variable quality of preservation, all encased in carbonate concretions. Two main scenarios may account for the origin of these bivalve‐rich concretions (i.e. unique events in sea‐water chemistry or unique burial‐starvation couplets). Sedimentological and taphonomic information indicates that the final deposition of the original shell‐rich mudstone intervals was probably tied to episodic influx of fine‐grained sediments in distal settings. Moderate bioturbation is also recorded suggesting low rates of sedimentation prior to early diagenesis. Hence, the fossil concentrations in concretions were formed due to the interplay of event and background sedimentation. These are internally simple concentrations with complex depositional histories. The concretion‐bearing beds are not randomly distributed in the Serra Alta Formation. Rather, they are found in the sparsely fossiliferous offshore deposits of the basal to intermediate portions of the unit. Thus, the concretionary mudstone beds and associated deposits are preserved in particular intervals and can be tracked for kilometres. This indicates that the conditions essential for concretion development existed only at particular stratigraphical intervals. Finally, our study strongly corroborates the idea that concretions are critical sources of sedimentological, taphonomic and stratigraphical information.     

Evaluation de l’état diagénétique de tests de Mollusques du Pléistocène de la Mer Caspienne

A microstructural examination of some molluscan shells (bivalves and gastropods) collected in Pleistocene levels of the Kraynovka core (north-western coast of the Caspian Sea) shows that the shells are altered: exfoliated shell layers, abundant superficial cavities, disappearance of the organic matrix … Although the microstructures are apparent, chemical analyses show high Si, Al, K and Fe contents. These enrichments lack after a light etching of the shells. Very thin infiltrations and/or secondary deposits are present on the shells, without any structural or mineralogical changes in the hard parts. Shells are aragonitic. Alterations in these shells can be detected only by microstructural observations related to localized chemical analysis.     

Ordovician trimerellacean brachiopod shell beds

The large, thick-shelled, inarticulate brachiopod Eodinobolus forms many conspicuous deposits of shells in the Upper Ordovician limestones of central western New South Wales. Both in situ and reworked shell beds arc preserved at recurrent intervals through the successions, in similar facies of both transgressivc and regressive phases of deposition. In situ shell beds arc best developed in transgressivc sequences, with up to four generations of shells exhibited in the individual in situ beds. These monotypic and very low diversity shell beds are interpreted as having formed in marginal marine, quiet water conditions: (1) on the fringes of an offshore island (in part the Molong High of the Tasman Orogen), with the island still providing a fairly continuous supply of terrigenous material: and, (2) after submergence of the island, on the resulting terrigenous-free, major offshore Bahamas-like platform. This may imply that the shell beds developed in different salinity regimes. Possibly Eodinobolus was capable of tolerating a wider than normal range of salinity, from slightly brackish through normal marine, even to marginally hypersaline. However, in both settings, Eodinobolus, in its role as the dominant member of the respective pioneer community, colonized similar substrates in the low energy mud zone. This appears to suggest depositional environments most directly analogous to those of Palaeozoic virgianid pentamerides, and perhaps also comparable with some modern marginal marine oyster and mussel-bed occurrences. ?Ordovician, Brachiopoda, Eodinobolus, palaeoecology, facies, shell beds. New South Wales.     

Shell survival and time-averaging in nearshore and shelf environments: estimates from the radiocarbon literature

Radiocarbon dates provide a means for estimating the time a shell may persist in active sedimentary environments and the actual temporal extent of time-averaging in marine deposits. Information compiled from the published literature on the radiocarbon age of marine shells gave information on a total of 734 radiocarbon dates from 276 localities from nearshore (< 10 m depth) and shelf (> 10 m depth) habitats. The median age of 128 nearshore shells is 2,465 years; that of 158 shells from the shelf is 8,870 years. The distribution of shell ages in both nearshore and shelf environments is strongly skewed: most dates are in the 0–3,000 range, and the number of shells in older age-classes falls off rapidly. The maximum age of a shell in an active sedimentary environment is a measure of time-averaging, because it estimates the amount of time represented in the deposit. The median duration of time-averaging in 63 nearshore deposits is 1,250 years; the median duration of time-averaging in 129 shelf deposits is 9,190 years. Radiocarbon-dated shells from fossil deposits confirm our estimate of time-averaging in nearshore environments: the median difference between maximum and minimum ages in 49 inactive beach ridges is 1,390 years; the median difference in other, predominantly nearshore deposits, is 830 years. Greater shell survival and longer durations of time-averaging in shelf settings may result from lower rates of sedimentation, lower rates of taphonomic destruction, greater rates of bioturbation, the history of post-glacial sea level, sample bias, or some combination of these factors. Our results may estimate the actual magnitude of time-averaging of shelly deposits forming under conditions of low net sediment accumulation. Such extensive time-averaging may confound attempts at detailed paleoecological and paleoenvironmental reconstruction. □ Time-averaging, benthic assemblages, taphonomy, radiocarbon, paleoecology.