Systematics and biology of some desmodont bivalves from the Australian Permian

The Australian Permian bivalves Megadesmus, Astartila, Pyramtis, and Myonia have previously been grouped in either the Family Pachydomidae (Fischer, 1887) or the Family Edmondiidae (King, 1850). Because of similarities in their musculature (in addition to those in dentition, micro‐ornament, and to some extent, shape) they can now be included in the same family as the deep‐burrowing bivalve Vacunella (previously Chaenomya). Vacunella is closely related to the living genus Pholadomya which has existed at least since the Mesozoic, and the two can be placed in the same family — Pholadomyidae (Fleming, 1828). Because of these similarities between Pholadomya, Vacunella, and the “Pachydomidae” it seems likely that the middle and upper Palaeozoic edmondioid bivalves gave rise to the Mesozoic and Cainozoic pholadomyoids, and that these two groups together constitute a major division of the Class Bivalvia — a division that may have been relatively distinct since Ordovician time.     

Untold muddy tales: Paleoenvironmental dynamics of a “barren” mudrock succession from a shallow Permian epeiric sea

During the late Paleozoic, the intracratonic Paraná Basin, Brazil, in central Gondwanaland, was covered by a huge (>1.600.000 km²), shallow and isolated epeiric sea. Within the Permian succession, oxygen-deficient facies are commonly recorded in the Mesosaurus-bearing Irati Formation (Cisuralian, Artinskian/Kungurian) and the overlaying Serra Alta Formation (Guadalupian, Wordian/Capitanian). Barren, dark-grey mudstones are the main facies preserved in this last unit, which has usually discouraged extensive and detailed stratigraphical and paleontological investigations. However, exhaustive sedimentological, taphonomic and paleontological surveys in those deposits reveal a dynamic and complex depositonal history. Based on sedimentary fabric, autochthonous to parautochthonous occurrences of shelly benthic invertebrates (bivalves) and the presence/absence of concretion-bearing and phosphate-rich layers, we report variations in the oxygen levels of bottom and pore waters, in bathymetry, sedimentation rates, and changes in benthic colonization. Our data indicate that the deposition of this “apparently barren” mudstone-dominated succession was driven by a complex interplay of variations in sedimentation rate and oxygen pulses tied to tectonic and climate changes. Three distinct populations or invertebrate paleocommunities were recorded, which were adapted to (a) normal background low-oxygen (dysoxic) conditions (i.e., minute infaunal suspension-feeding bivalves associated with the trace fossil Planolites), (b) chemically toxic (anoxic/extreme dysoxic) substrates, including gigantic burrowing bivalves (probable chemosymbiotic taxa), and (c) oxic/dysoxic substrates following short-term bottom disruptions.     

Effects of tributyltin on activity and burrowing behavior of the fiddler crab,Uca pugilator

Fiddler crabs,Uca pugilator, collected from the field showed no avoidance to burrowing in 1 μg g^?1 tributyltin (TBT)-contaminated sand held in laboratory trays. Treatment ofU. pugilator with levels of tributyltin as low as 0.5 μg l^?1 for 1–3 wk resulted in an acceleration of the righting reflex in females, indicative of hyperactivity. Crabs of both sexes exhibited a reduction in burrowing activity, as measured by the number of burrows dug at 15 and 60 min after release into laboratory trays containing sand, and by the number of crabs within burrows at those times. The reduction in burrowing activity was not dose-dependent at concentrations of 0.5 to 50 μg l^?1, and was unchanged between one and three weeks of exposure. Since fiddler crabs are dependent on burrows for many aspects of their lives, the reduction in burrowing behavior, should it occur in nature, would have serious consequences for the species.     

Modeling Bedload Transport of Juvenile Bivalves: Predicted Changes in Distribution and Scale of Postlarval Dispersal

The potential consequences of bedload transport of postlarvae for patterns of distribution of marine invertebrates were explored by developing a bedload transport model for juvenile bivalves in a small estuary in New Jersey, USA. A simple numerical model of tidal current hydrodynamics was developed based on field measurements of shear stresses near the bottom. Burrowing behavior of bivalves was incorporated into the model of bedload transport by using estimates of entrainment rates of Gemma gemma and Mya arenaria in a laboratory flume, and jump lengths of the bivalves were estimated by methods previously developed for noncohesive particles. Based on the flood domination and strong gradient of shear stresses in the Navesink estuary, our model predicted that juvenile bivalves would accumulate in the center of the estuary, traveling up to several kilometers over 30 days. Field distributions of juvenile bivalves were consistent with the model predictions for other species of bivalves but not for G. gemma, for which field distributions of both <500- and >500-μm individuals were concentrated in the eastern end of the estuary. Differences between the bedload model and G. gemma distributions suggest that spatial variation in burrowing behavior or biological interactions are playing an important role in maintaining distribution patterns of this species in spite of high levels of bedload transport. This modeling approach is applicable to other juvenile benthic invertebrates that disperse as bedload and is a useful model against which to compare field observations of rates of transport and patterns of distribution and abundance.     

Taphonomy and palaeoecology of the gastropod fauna from a Late Cretaceous rocky shore, Sweden

A gastropod fauna comprising 17 species, each represented by a limited number of specimens, is described from a Late Cretaceous, late early Campanian rocky shore at Ivö Klack, southern Sweden. The gastropod fauna is associated with the most diverse ancient rocky shore fauna ever found. However, the low gastropod species diversity compared to the faunas of modern rocky shores is ascribed to taphonomic factors, notably dissolution of the aragonitic shells, but the predominance of epifaunal herbivores is indicative of a guild structure similar to that found on modern rocky shores. The presence of drill holes assigned to the ichnospecies Oichnus simplex suggests the former presence of muricid gastropods which have not been found as body fossils. A single drill hole is referred to Oichnus paraboloides and was probably made by a naticid gastropod. The infaunal mode of life of naticids makes preservation of such drill holes difficult, since the majority of infaunal prey such as burrowing bivalves has aragonitic shells which are not preserved. The relatively high number of species in comparison to many other Late Cretaceous rocky shore faunas, offers an opportunity to compare gastropod guild structure at Ivö Klack with modern counterparts, even though taphonomic processes such as mechanical destruction and aragonite dissolution have played an important role in the fossil gastropod assemblage.     

Vertical distribution of infauna in sediments of a subestuary of central Chesapeake Bay

The vertical distribution of infauna was quantified in eight strata from 0–35 cm in sand and mud sediments of a lower mesohaline subestuary of Chesapeake Bay. Large numbers of small polychaetes, amphipods, and clams occurred in the upper 5 cm of both sediment types, whereas large clams (Macoma balthica in mud andMya arenaria in sand) extended down to 30 cm and comprised most of the biomass in their respective sediment types. There was extensive overlap of the species inhabiting both sediment types. Vertical stratification within and among species apparently reflected constraints on burrowing depth related to body size rather than resource partitioning among competitors. The maximal sediment penetration of 35 cm, which was exhibited byHeteromastus filiformis, was considerably less than the maximal penetration for deep burrowing species in some marine infaunal communities. Several species which burrowed deeper than 5 cm exhibited significant temporal shifts in their vertical distribution.     

A new record and a new ichnospecies of Arachnostega from the Middle Miocene of Egypt

The ichnogenus Arachnostega Bertling, 1992 is recorded for the first time from a bioeroded, coral-rich, Middle Miocene limestone bed in the middle Siwa Escarpment Member (Marmarica Formation) at Siwa Oasis, northern Western Desert of Egypt. These burrowing traces are preserved on the surface of a few internal moulds of bivalves and gastropods. Until now, the ichnogenus Arachnostega included a single ichnospecies, A. gastrochaenae Bertling, 1992. In the material studied herein, two ichnospecies are identified, described, illustrated and compared with other, similar traces. One of these is a new ichnospecies of Arachnostega; this is here named A. siwaensis ichnosp. nov. It is distinguished from A. gastrochaenae in the possession of tunnels that usually branch in a V-pattern; intervals of ramification are longest in the main branch and decrease in second- and third-order branches. Arachnostega traces were produced in a warm, low-energy, shallow-marine (<10?m water depth) environment. Polychaetes are the most likely producers of marine Arachnostega, which are commonly occurred in the same studied bed. This new occurrence is significant in extending the known stratigraphical and geographical ranges of Arachnostega into the Middle Miocene of Egypt.     

Association of cobia,Rachycentron canadum,with cownose ray,Rhinoptera bonasus

Field and laboratory observations on an association of cobia,Rachycentron canadum with cownose ray,Rhinoptera bonasus are reported. Cobia main-tained a position in close proximity to the back of the rays. The cobia would move away from the rays to take food presented. This opportunistic and probably facultative feeding on rejected food scraps or displaced benthos may be the first step in the evolution of commensal attachment as has been shown for the remoras.     

Palaeoecology of well-preserved coral communities in a siliciclastic environment from the Late Pleistocene (MIS 7), Kish Island, Persian Gulf (Iran): the development of low-relief reef frameworks (biostromes) in increasingly restricted environments

Major changes in community structure and depositional relief of high-latitude coral communities in the southern Persian Gulf between marine isotope stage (MIS) 7 and the present day suggest that the area has become increasingly restricted. Corals and bivalves from outcrops on Kish Island, Iran, were identified in order to interpret the Late Pleistocene palaeoenvironmental setting. U/Th disequilibrium dating was used to constrain the ages of the stratigraphic units. During MIS 7, two coral-bearing sequences were deposited on what is now Kish Island. The lower sequence is dated as MIS 7.5 and changes laterally from an assemblage dominated by Cyphastrea sp. and Platygyra daedalea in the west to one characterized by branching Montipora in the east. By contrast, the upper sequence, dated as MIS 7.1, transitions from an assemblage dominated by platy Montipora in the west to a diverse assemblage of Platygyra and other faviids in the east. The assemblages of both sequences are within a marl matrix and bounded by thin lithified mollusc-rich layers. Corals and bivalves indicate that the sequences were deposited on gentle slopes in sheltered environments less than 20 m deep. The MIS 7 deposits may be classified as coral carpets or biostromes that developed a low-relief framework. During MIS 5, coral communities were no longer framework building and are now limited to an Acropora-rich layer of coral rubble that covers large parts of the island, and two small incipient reefs with sparse faviids. Similarities between the MIS 5 and modern nearshore coral communities suggest that the environmental conditions during MIS 5 were comparable to those of today. The late Pleistocene coral carpets and non-framework coral communities of the southern Persian Gulf may serve as models for coral biostromes in the fossil record, which formed under restricted environmental conditions such as elevated terrigenous input, high turbidity, and strong seasonal changes in temperature and/or salinity.     

Taphonomy of Early Cretaceous freshwater bivalve concentrations from the Sihetun area,western Liaoning,NE China

The Early Cretaceous Yixian Formation at Sihetun, Beipiao, western Liaoning, northeastern China, is well-known for yielding diverse and excellently preserved fossils of the Jehol Biota. The lower unit of Yixian Formation, dominated by lacustrine deposits, is rich in concentrations of two freshwater bivalves: Sphaerium anderssoni and Arguniella ventricosa. These bivalve concentrations can be divided into three types that comprise either paucispecific A. ventricosa or S. anderssoni, or both bivalves in similar amounts. The lithological, biotic, and taphonomic features of 12 bivalve concentrations are recorded, and the taphonomic signatures (such as shell articulation, size-frequency distribution, and orientation) are analyzed. Autochthonous as opposed to allochthonous bivalve concentrations are discriminated. A very short time-averaging effect is recognized in some concentrations, which was probably caused by seasonal or episodic water-level fluctuations and hypoxia. Three factors operated on the bivalve concentrations before final burial: in-situ reworking, transport, and time-averaging. Although time-averaging of the death assemblages was limited to several years, it is in this way that several generations of the two bivalve species could become preserved together. Reworking of the bivalve concentrations was most likely caused by storm action.     

Stratigraphic significance of Middle Triassic radiolarians from the central part of Kotel’nyi Island (New Siberian Islands)

New data on Middle Triassic radiolarians from the central part of Kotel’nyi Island are discussed. The Triassic section is represented by clays and mudstones with diverse macro- and microfossils, which confirm the presence of lower Anisian, upper Anisian, and upper Ladinian sediments on the island. The lower Anisian Substage 10–15 m thick is composed of clays with ammonoid species Karangatites evolutus. The upper Anisian Substage 20 m thick consists of clays with phosphate concretions and bivalves Daonella sp. cf. D. moussoni Merian, D. americana Smith, ammonoid form Indigirophyllites sp. ex gr. I. spetsbergensis (Oeberg), and radiolarian assemblage with Glomeropyle boreale Bragin. The upper Ladinian Substage 25–30 m thick is composed of clays with phosphate concretions and bivalves including Daonella frami Kittl., ammonoid species Indigirophyllites sp. ex gr. I. oimekonensis Popow, and radiolarian assemblage with Muelleritortis firma (Gorican). The middle Anisian and lower Ladinian substages are missing, which may be explained by the incompleteness of the section. The Middle Triassic section of Kotel’nyi Island is first stratified on the basis of radiolarians, which are supplemented by the data on cephalopods and bivalves. Glomeropyle saccum Bragin, sp. nov. is described.     

Bioerosion and encrustation rates in recent mollusk death assemblages on a supratidal siliciclastic setting,Playa Norte,Veracruz State,Mexico

This study presents both qualitative and quantitative data regarding marine mollusk (gastropods and bivalves) shell bioerosion and encrustation based on death assemblages obtained from a recent supratidal environment in Playa Norte, Veracruz State. The objectives of this study were to assess the nature of bioerosion and encrustation processes and to investigate the role of these taphonomic features contributing to the deterioration of natural shell accumulations within a tropical siliciclastic tidal environment. The assemblage comprises 31 species: 13 gastropods and 18 bivalves. The bioerosion and encrustation degrees were low to moderate for both types. The most abundant traces were predatory gastropod structures (Oichnus paraboloides and O. simplex), whereas sponge borings (Entobia isp.), polychaete dwellings (Caulostrepsis taeniola), and echinoid raspings (Gnatichnus isp.) were less frequent. The encrusting organisms include polychaete serpulids, bryozoans, and rare foraminifers (Homotrema rubrum). Because of the low bioerosion and encrustation degrees occurring in this area, accumulation is expected to predominate over biotic destruction. As deposition conditions (richly fossiliferous carbonate sandstone beds) were similar to those prevailing in the Tuxpan Formation during the Miocene (Langhian), it is suggested that this study provides an equivalent reference to interpret mollusk fossil assemblages located in this site.     

New gadiform fishes (Teleostei,Gadiformes) from the Miocene of Algeria

As part of the completion of studies on the Miocene fishes of the Chelif Basin (north-western Algeria), this paper represents a contribution to the knowledge of the Messinian gadiform diversity of this western Mediterranean, semi-enclosed, Neogene basin. A new genus and species of the family Macrouridae is erected (Razelainia paradoxa n. gen. et sp.), two specimens are tentatively referred to already existing taxa (Gadiculus cf. jonas; Merluccius cf. merluccius), and a species formerly assigned to the gadid genus Brosme is transferred to the genus Gaidropsarus (Gaidropsarus murdjadjensis). The macrourid Razelainia paradoxa n. gen. et sp. is characterized by an unusual combination of: plesiomorphic gadiform features, such as low vertebral number (presumed), well-developed caudal-fin rays, presence of a single continuous dorsal fin originating just posterior to the neurocranium, anal-fin rays slightly longer than dorsal-fin rays; and derived, typically macrourid features, such as the presence of spinoid scales and the anterior anal-fin pterygiophores extending forward over the abdominal wall. A paleoecological analysis reveals that the Messinian gadiform assemblage of the Chelif Basin had a subtropical/warm temperate affinity, with a marked north-eastern Atlantic-Mediterranean biogeographic character.     

A general aspect in the genesis of nodular limestones documented by the Upper Cretaceous limestones of Jordan

Upper Cretaceous nodular limestones in Jordan were originated by burrowing organisms of the types Thalassinoides, Callianassa or Ophiomorpha. Although burrows are difficult to recognize in the pure lutitic limestones of northern Jordan, in the south they are filled by early diagenetic stochiometric dolomite or chert. One may conclude that some nodular limestones hitherto described, are products of burrowing organisms and not of purely chemical processes.     

The occurrence of the anemone Peachia parasitica as a symbiont in the scyphozoan Cyanea capillata in the lower Chesapeake Bay

Juveniles of the burrowing anemone Peachia parasitica, living on the scyphozoan Cyanea capillata, were obtained from Virginia, thus extending the southern limit for this species. Anemones were easily maintained in the laboratory; one lived for over four years. The feeding behavior is described.     

Reduction of sea star predation by the burrowing response of the hard clam Mercenaria mercenaria (Mollusca: Bivalvia)

The hard clam Mercenaria mercenaria burrows deeper into the sediment when the predaceous sea star, Asterias forbesi is present. The supposition that this increase in burial depth represents an escape response designed to reduce predation was tested experimentally by regulating clam burial depth through manipulation of the amount of sediment available for burial. Mercenaria maintained at zero depth were eaten by Asterias at greater rates than those held at ordinary burial depths (2.5–3.0 cm). These clams in turn were eaten at greater rates than those maintained at escape depths (4.0–4.5 cm). The results unambiguously establish an anti-predator function for the burrowing response, as well as underscoring the protective function of the fossorial habit. They are not confounded by behavioral predator food preferences, inherent differences between prey species, or debilitating side effects of preventing prey from escaping. Mechanisms by which the burrowing response may reduce predation are discussed and observations on the unreported clam-digging behavior of Asterias forbesi are presented.     

Epifauna on ammonites from the Lower Jurassic of the Severn Basin,southern England,and their palaeoenvironmental and taphonomic significance

Ammonites with bivalves or worm tubes attached are relatively rare among the abundant specimens in the Lower Jurassic (Sinemurian) mudstones at Bishop's Cleeve, Gloucestershire, UK, but provide evidence for ammonite taphonomy, environmental conditions and biological interactions between ammonites and epifauna. Epifauna attached only to one side or within the body chambers of ammonites usually indicate post-mortem attachment. Epifauna on both sides of, or overgrown by, ammonites attached in vivo. One large example of Oxynoticeras has at least 51 encrusting bivalves attached exclusively to one side indicating it formed a ‘benthic island’. Four, presumably annual, bivalve cohorts are recognized, with variation in preferred orientation in each cohort suggesting that current direction varied. Both the ammonite and its epifauna were buried by a single sedimentation event. Other informative examples include a Cheltonia that overgrew an epifaunal bivalve, and an example of Bifericeras which bears two clusters of four worm tubes, one of which apparently attached in vivo, the other post-mortem. Other examples bear too few specimens to be certain of the timing of attachment, but most probably attached post-mortem.     

Stratal,sedimentary and faunal relationships in the Coniacian 3rd-order sequence of the Iberian Basin,Spain

The Coniacian 3rd-order sequence in the Iberian Basin is represented by a carbonate ramp-like open platform. The biofacies is mainly dominated by nekto-benthic (such as ammonites) and benthic organisms (such as bivalves, mainly rudists) with scarce solitary corals (hermatypics are absent), showing major differences among the Transgressive System Tract (TST) and Highstand Normal Regression (HNR). During the TST, platform environments were dominated by Pycnodonte, other oysters and molluscs (with only subordinate rudists) and ammonites, which were represented by ornamented platycones (Tissotioides and Prionocycloceras), and by smooth oxycones (Tissotia and Hemitissotia). During the HNR, shallow water depositional areas were occupied by rudist-dominated associations. Storm- and wind-induced currents and waves acting on these associations produced large amounts of loose bioclastic debris that covered outer platform areas. This facies belt graded landwards into protected, lower-energy settings (inner platform, lagoon and littoral environments). Rudist biostromes were preserved in seaward areas of these protected shallow environments of overall moderate to low hydrodynamic gradient, which was punctuated by storms. In this environment and landwards, large areas of marly substrate favoured the presence of gastropods, other bivalves, echinoderms, benthic foraminifera and solitary corals. Because of the input of siliciclastics and, probably, the lack of nutrients in suspension, the establishment of rudist communities was difficult in more landward areas of the lagoon and in tidal environments. This heterozoan carbonate factory was thus controlled by warm-water conditions and high energy levels, which were responsible for high-nutrient contents in suspension.     

Recognizing cryptic environmental changes by using paleoecology and taphonomy of Pleistocene bivalve assemblages in the Oga Peninsula,northern Japan

Multivariate analyses applied to Pleistocene bivalve assemblages from the Oga Peninsula (northern Japan) discriminate three distinct assemblages. The assemblages and their taphonomy were used to recognize environmental settings and changes. The Astarte–Cyclocardia–Glycymeris assemblage indicates shelf environment (below the storm wave base) where gravels and shells are transported from shallower settings. Supply of the exotic coarse sediment probably enabled epifaunal bivalves to inhabit the sea floor. The Glycymeris assemblage is characterized by dominance of G. yessoensis and represents current-swept shoreface environment (above the storm wave base). The Moerella assemblage is characterized by bivalves inhabiting bay to open-marine conditions and diverse deposit-feeders, indicating a moderately land-locked environment, such as an open bay or a bay mouth. Fine-grained substrata rich in organic matters in the bay were probably suitable for the deposit-feeders. Ordination also shows the assemblages along two environmental gradients, a bathymetrical one and the other related to open-marine and bay conditions. The environmental changes are explained mainly by glacio-eustatic sea-level changes and alternation of coastal geomorphology caused by local crustal movements. This study also suggests that fossil assemblages can be a powerful tool to reconstruct environments and depositional dynamics even in intensely bioturbated sedimentary facies.     

The heterodont bivalve Maghrebella forgemoli (Coquand, 1862) from Cenomanian of Batna,northeastern Algeria: Palaeobiogeography,biostratigraphy and palaeoenvironment

In the northern Aurès Range near Batna, Algeria, the Cenomanian Smail Marls Formation consists of fossiliferous deposits rich in diverse benthic macrofauna, mostly dominated by bivalves, among them the carditid Maghrebella forgemoli (Coquand, 1862). Almost exclusively Cenomanian, the abundance zone of Maghrebella forgemoli is in the higher levels of Lower Cenomanian, extending from the Sharpeiceras schlueteri zone to the Mantelliceras dixoni zone and corresponds to a limited circalittoral interval with relatively low energy, soft substrata, and relatively cold temperatures. The distribution of Maghrebella forgemoli in Mountains of Batna evidences bathymetric variation controlled by the tectonic activity affecting the ante-Triassic and the Upper Cretaceous substrata and generating a system of tilted blocks, at the beginning of the uppermost Albian-Cenomanian and Turonian, in the eastern Atlasic domain that extends to central Tunisia.