Live Assemblages Research Articles

Processes of Benthic Foraminiferal Fossil Assemblage Formation on the Continental Slope

Theoretical analysis of benethic foraminiferal fossil assemblage formation shows that the assemblage eventually preserved in the sediments is an integrated result of species' test production rate, microhabitat behavior, and biogeochemical processes that control the probability of species' test preservation. The biogeochemical processes that influence test preservation in slope sediments are controlled by the flux of organic carbon to the sea-bed and the botton water oxygen concentration. These variables also affect the depth of the biotic habitation zone in the sediments. Therefore, organic carbon flux and bottom water oxygen content should be reflected in benthic foraminiferal fossil assemblages for both ecologic and taphonomic reasons. An integrated study of fossil assemblage generation was conducted on the Gulf of Mexico continental slope using box cores collected along depth transects across the oxygen minimum, and using live and dead assemblage analysis combined with {sup 210}Pb measurements to quantify biotic activity in the sediments and pore water nutrient and metals analysis to quantify biogeochemical processes acting in the sediment habitation zone. The results show that the size of the habitation zone and live standing stock are influenced by organic carbon flux and oxygen supply to the sea-bed. The fossil assemblage is created progressively through themore » upper 10-20 cm of sediment and biologichemically driven test destruction (taphonomic process) is important in determining the assemblage that enters the geologic record.« less

Benthic foraminiferal assemblages from the eastern Weddell Sea between 68 and 73°S: Distribution, ecology and fossilization potential

Surface sediment samples taken with a vented ☐ corer from the eastern Weddell Sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content. The live fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Based on the dead assemblages, potential fossil assemblages were calculated to facilitate the analogy with late Neogene core material. Five distinct live assemblages inhabit the continental margin today. Six dead assemblages and five potential fossil assemblages, respectively, correspond to these biocoenoses. A predominantly calcareous live fauna dominated by Trifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope. Below this, on the upper slope down to 2000 m water depth, the predominantly calcareous Bulimina aculeata assemblage coincides with the core of warm (>0°C) Weddell Deep Water and with fine and more organic-rich sediments. These calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment, which depends largely on the grain size distribution of the sediment and the fluxes of organic matter. Therefore, a still calcareous T. angulosa-dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope, whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated by Martinottiella nodulosa, the characteristic arenaceous fossil remnant of the former predominantly calcareous live B. aculeata fauna. On a continental terrace between 2500 and 3500 m water depth Cribrostomoides subglobosus dominates the live fauna, but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized by Oridorsalis umbonatus and Epistominella exigua comprises the dead assemblage and the potential fossil assemblage, respectively. On the lower continental slope, between the carbonate lysocline (3500 m) and the carbonate compensation depth (4000 m), tests of Nuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna, which is associated with the lower part of the Antarctic Bottom Water (AABW). This corroborates earlier investigations suggesting a relationship between the carbonate-corrosiveness of water masses and the distribution of N. umbonifer. This is important for inferring paleo-routes and estimates of paleo-production rates of AABW during the Neogene.

Taphonomy of the Miocene bone accumulations at Paşalar, Turkey

The bone assemblage from the Miocene deposits at Paşalar, Turkey is described. The fossil bones are concentrated in a single horizon of poorly sorted sands and gravels which were accumulated rapidly. The source of the sedimentary particles was the lower slopes of the hills bordering the site, and the distribution of bones relative to sediment shows that they were transported to the burial site as part of the sedimentary load. Bone size and distribution are correlated with particle size, and orientations and dips of the bones are indicative of turbulent flow during deposition. It is concluded, therefore, that the bones came from the same source area as the mineral constituents of the sediments, namely from the lower hill slopes close to the site. The fossil bones are extremely fragmentary, with isolated teeth being the most common parts preserved, and some of this breakage may be attributed to the effects of transport. The majority of bones are edgerounded, and in most cases the abrasion is superimposed on weathered and broken surfaces, indicating that the bones accumulated and were broken before abrasion. The bone degradation caused by these processes has resulted in the destruction of all but the strongest elements, but it does not appear to have selectively destroyed particular faunal elements. Predator activity is demonstrated for small mammals by the presence on some of their preserved teeth of surface corrosion caused by digestion, and it is inferred for large mammals by age structure anomalies in the samples of certain of the larger sized species. The biases introduced by these processes are considered to be loss of the smallest size groups of small mammals and over-respresentation of rhinocerotids and giraffids, but apart from this the fossil assemblage approximates to a life assemblage accumulated over a short period of time and from a limited geographical area.

A unique bivalve–algae life assemblage from the Bear Gulch Limestone (Upper Mississippian) of central Montana

A single limestone slab is described from the Mississippian Bear Gulch Limestone of central Montana containing more than 100 bivalves, including Caneyella sp. and ?Ptychopteria (Actinopteria) sp., attached in original life positions along an 80-cm-long ?brown alga. The assemblage provides clear evidence of life position and choice of substrate for these pterioids. The bivalves increase in size, and appear to be radially disposed in their attachment sites, along the loosely coiled “kelp-like” alga, indicating little postmortem transport and current activity. The bivalve population is interpreted as living on an algal thallus that was either planktonic or attached to the substrate. Observations and comparisons with modern bivalve—seaweed associations support either view. Evidence for local anoxic conditions and stagnant water help explain the superb preservation and confirm past interpretations of the Bear Gulch Limestone.

Recent Podocopid Ostracoda of the Sedili River and Jason Bay, Southeastern Malay Peninsula

The recent podocopid Ostracoda of the Sedili River, its estuary and the open shelf of Jason Bay, off the southeast coast of the Malay Peninsula, have been studied quantitatively. The distribution of live and dead assemblages is recorded. A total of 101 species were recorded in the study. Of these, 50 were recorded live in the area. In the freshwater system of the Sedili River, only two species were recorded live, in the brackish water of the Sedili Estuary only 4 live species were recorded while 48 live species were recovered from the open shelf. The dead assemblage exhibits a much higher diversity yet lower dominance than the live assemblage and constitutes a total of 98 species from the 3 biotopes. The dead representatives of species which also occur live are more widely distributed than their live counterparts due to postmortem transportation. In this study one new genus is erected, Malaycythereis, n. gen., and the following new species and subspecies are erected: Malaycythereis trachodes, Caudites asiaticus, C. scopulicolusjasonensis, Cytherella incohatus, Hemicytheridea wangi, Keijella gonia, Loxoconcha malayensis, L. triconicula, Neocytheretta murilineata, Paracytheroma ventrosinuosa, Semicytherura contraria, Leptocythere pulchra, and Xestoleberis malaysiana.

Marine molluscs and late quaternary environmental studies with particular reference to the late-glacial period in Northwest Europe: A review

Although it is now commonly thought that temperature is a minor factor in limiting the distribution of marine invertebrates, particularly on a local scale, there is evidence to suggest that some boreal molluscs, particularly those which occur in similar environments on both sides of the North Atlantic, are valuable as oceanic temperature indicators. This applies particularly to planktotrophic species whose northern limits of distribution are apparently controlled by summer surface temperature. The oceanography of the eastern North Atlantic is briefly considered in relation to zoogeographic subdivisions based on temperature and it is concluded that the latter are fairly crude and apply only to shelf seas. Most glaciomarine molluscan faunas are not specific to that environment, an exception being that dominated by the bivalve Portlandia arctica. The role of marine molluscs in palaeoenvironmental research at the local level for the 13-11 ka BP period is illustrated by a site at Inchinnan, Scotland; it is concluded that a shell bed at this locality consists partly of a little modified life assemblage and partly of shells which have been gently reworked under conditions of generally rapid deposition. During the same time interval, the changing composition of molluscan faunas on the NW European seaboard suggests that a weak drift of North Atlantic water replaced polar water at or a little later than 13 ka BP off Scotland and a few hundred years later off northern Norway. Evidence for relatively ‘warm’ intervals early and late in the 13-11 ka BP period has been found particulary on the Scottish coast and (to a lesser extent) more generally. The later ‘warm’ interval may be related to the southward movement of the polar front at the beginning of the Younger Dryas. The value of the bivalve shell itself in the determination of temperature, seasonality and calendar years is briefly discussed.

An account of the techniques using ostracodes in palaeolimnology in Australia

Ostracodes are common in Australian waters, especially in saline lakes where they are particularly diverse. This makes them indispensable “tools” for Australian palaeolimnological studies, especially since many Australian lakes are saline. The following types of subjective information can be obtained from the study of Australian fossil ostracode faunas without knowledge of the ecology or taxonomy of their living representatives: the search for a life assemblage can lead to detection of the presence of water currents and the effects of reworking in addition to the identification of particular facies, such as a lake shoreline or a desiccation phase; rate of sedimentation can be estimated as well as the amount of sediment compaction of drying; information on water oxygenation and acidification can be obtained; and, finally, presence of seasonality affecting lakes can be identified. With additional reference to the ecology of particular species, permanency of lake water can be assessed and so can the variability of rainfall and, even at times, the detection of whether a lake was close to a summer/winter rainfall boundary. Shell ornamentation can inform on water depth. In addition, salinity data can be obtained as well as some information on water chemistry and temperature. The latter from trace elements (Mg, Sr) and stable isotope ( δ 18O) studies of ostracode shells.

Application of Ostracoda to sea‐level studies

Ostracoda are probably better indicators of past sea level than has hitherto been realized. This is so because very characteristic low diversity, high productivity faunas occur in the upper reaches of the estuarine environment. Moreover, a fair amount of knowledge exists concerning their ecology. Details are given on the faunas that are present in each estuarine facies, and on how death assemblages can be distinguished from life assemblages. Sea level can be estimated to an accuracy of 50 cm to 100 cm in some NW European microtidal areas, but further research is required on macrotidal coasts to ascertain whether they can be used to pinpoint sea level here. Examples of where Ostracoda have proved of value in sea‐level studies are given for North Denmark, The Netherlands, East Ireland, and the Gulf of Gascony.

Distribution and ecology of benthic foraminifera in McMurdo sound, antarctica

Foraminifera from modern sediments in McMurdo Sound have been studied to provide a basis for interpreting fossil assemblages there. The sound is 50 km across, more than 900 m deep, and ice-covered for at least nine months of the year. The salinity and temperature of the bottom water are nearly constant (33.96–34.99%, and −1.95°C − −1.4°C). Sea floor sediment is mainly fine sand and mud with a little ice-rafted gravel. Nearly 40,000 specimens of foraminifers (2334 living and 36,875 dead) were identified from 28 samples collected by gravity corer and grab from sites varying in depth between 79 and 856 m. Three present day assemblages can be recognised: 1. (1) Shallow open water assemblage (SWA) from 110 to 560 m, found in shallower areas of the open sound, dominated by calcareous species; 2. (2) Deep open water assemblage (DWA) from 620 to 856 m, found in deeper areas of central McMurdo Sound; dominated by agglutinated species; and 3. (3) Harbour assemblages (HA) from 79 to 796 m in the enclosed basins and harbours of the area, dominated by agglutinates species. The composition of the assemblages is controlled chiefly by the calcium carbonate compensaion depth (CCD). Calcareous species are abundant and varied (84 calcareous species) in the SWA above 620 m, but are almost absent from the DWA. The dominance of agglutinated foraminifera in the HA indicates a shallower CCD (about 230 m) in restricted coastal settings. Comparison of McMurdo Sound assemblages to those from other areas of the Antarctic, the Arctic and the deep-ocean indicates that there are grpss similarities in the over-all faunas, but actual species in commin range from 6 to 41% of the McMurdo Sound total assemblage (124 species). Death assemblages have a similar species diversity to corresponding life assemblages from 620 (the CCD) to 420 m, where death assembalges are representative of them, except for open water assemblages from 620 (the CCD) to 420 m, where death assemblages are depleted in calcareous taxa. The diversity of the agglutinated component of each assemblage remains nearly constant in all habitats and at all water depths.

The shape of the Upper Carboniferous non-marine bivalve Anthraconaia in relation to the organic carbon content of the host sediment

ABSTRACTAnthraconaia lived on clay grade sediments. In highly variable life assemblages of the A.prolifera group of the E German Wettin Shales (Stephanian C) organic carbon percentage of the host sediment correlates with wu/m and A/L ratios of the shell (where wu is the maximum width ventral to the line of maximum growth, m, L the maximum length measured parallel to the line of the hinge, and A the length anterior to the umbo). Decrease in the organic carbon of the sediment is associated with decrease in umbonal development, elongation of the shell along the m axis, and straightening and reflection of the ventral margin; the latter becomes subparallel to the dorsum or to the m axis with a concomitant decrease in size. These results confirm and amplify earlier work on Anthraconaia in the Appalachian coalfields. The same morphological trends in relation to organic carbon characterise the Anthraconaia modiolaris group of Britain, both within a single succession in upper Westphalian A sediments in Yorkshire, and also in three shell bands of Lower Westphalian B age in S Wales, Yorkshire and central Scotland. In the last named, above the Musselband Coal, statistical formulae (of Leitch 1940) for the species A.salteri are shown to define neither the type assemblage nor its stratigraphical position. The shell-shape/organic carbon relationship has been broadly confirmed on other horizons of lower Westphalian B age in the Scottish and Pennine coalfields. Far from embarrassing the stratigrapher, the relationship, which reflects ultimately trophic level of deltaic palaeoenvironments, contributes to our understanding of non-marine bivalve faunas, especially of the apparent regional distribution of ‘species’ of Anthraconaia.

Taphonomic Feedback in Miocene Assemblages: Testing the Role of Dead Hardparts in Benthic Communities

The accumulation of dead hardparts can directly influence the structure and dynamics of benthic communities by changing the physical characteristics of the sea floor. Biotic changes driven by such liveldead interactions have recently been termed taphonomic feedback (TF) to stress a) the role of post-mortem processes in the availability of hardparts, and b) that not only does the life assemblage influence the death assemblage, but the accumulation of a death assemblage affects the living one. This paper presents a quantitative test for the operation of taphonomicfeedback in the fossil record. Assemblages from Miocene strata of Maryland exhibit a statistically significant correlation between the abundance of shell-gravel dwellers (including epifauna and infauna) and sediment shelliness consistent with faunal changes predicted by TF. Alternative explanations for the correlation, such as differential preservation potentials of soft-bottom and shell-gravel taxa, faunal response to changing water energy, and livellive interactions, can be rejected. The operation of TF has several implications for analysis of the fossil record: a) as a driving mechanism for faunal change in benthic communities; b) as an approach to reconstructing patterns in the accumulation (and non-accumulation) of hardparts and sediment; and c) as an indication of the apparently prolonged post-mortem persistence of skeletal material in some settings and the biologically and taphonomically complex origins of many densely fossiliferous deposits.

Palaeoecology of Quaternary Shorelines and Sea-Level Change in Mallorca

descriptive, and merely attempted to fit the shorelines into the classic Tyrrhenian framework of Deperet (1918). This placed the shorelines in the last interglacial period, and related their formation to periods of high eustatic sea levels. The interglacial origin of the shorelines was supported by the presence of the thermophilous gastropod Strombus latus {-bubonious), which is today found living in West Africa. A Tyrrhenian I 'transgressive complex' was recognized by Butzer and Cuerda (1962) at heights between +4 m and +34 m; two stages of the Tyrrhenian II were identified with shorelines at +10.5 m, +12.5 m and +2 m to 9 m; and Tyrrhenian III deposits were found at +0.5 m to +2.8 m. The fauna associated with the shorelines is similar to that reported from many other areas of the Mediterranean, with a rich Strombus fauna characterizing the Tyrrhenian II shoreline. Following this initial stratigraphic analysis, 15 Thorium/Uranium (Th/U) dates were obtained for shell material in the shorelines (Stearns and Thurber, 1965). The dates fell into three basic age ranges which tended to support the threefold division of the Tyrrhenian, but the spread of dates also indicated that the sequence of marine and terrestrial deposits was far more complex than was originally thought. Six major marine transgressions were eventually recognized in the Mallorcan sequence (Table I). Each marine hemicycle was identified by a beach overlain by a palaeosol, and the terrestrial hemicycle was equated with a silt/aeolianite complex followed by a pedocal. A useful review of the Mallorcan literature is presented in the Quaternary Research Association field guide for the island (Crabtree et al, 1978). The guide also contains location maps and sections of all the key sites. Most of the sites were visited in the present study, and their molluscan fauna was sampled. Methods Samples of the fossil molluscan fauna were collected from each site over a set time period of one hour. This method provides comparability with abundance data obtained for modern live and dead molluscan assemblages in the Mediterranean (Richards, 1982; in press). Thus each site could be matched with a modern analogue with a known relationship to sea level, allowing the position of the contemporaneous sea level to be established to within ?0.5 m or better.

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MOllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.

Significance of Limestone-Shale, Rock-Stratigraphic Contacts--The Connecting Links Between Areas of Contemporaneous Carbonate and Terrigenous Detritus Sedimentation: ABSTRACT

In undisturbed depositional sequences one rarely, if ever, observes lateral change directly from mud-supported carbonate Synthetic seismograms after normal moveout correction for a hypothetical 20-m (66 ft) high impedance sand reservoir in shale. Ratio of compressional velocity to shear velocity in this model is 1.8 for shale, 1.57 for brine sand, and 1.5 for gas sand. Variation in reflectivity with offset in this model is a first order effect over normal recording range. End_Page 417------------------------------ into grain-supported siltstones, sandstones, or conglomerates. Shale or claystone always intervene. Such bounding and internal contacts are the most conspicuous and most informative--yet most neglected--aspects of the limestone-shale record. Significant progress in understanding carbonate to terrigenous detritus facies changes can come from closer attention to these contacts. A majority of Mid-Continent Pennsylvanian limestone-shale contacts are of regional extent and commonly are represented by upward gradations from shale to limestone. Shale units in carbonate sections commonly range from fractions of inches to tens of feet in thickness. Genesis of thin (2 ft ot .6 m or less) shale breaks and argillaceous partings has been neglected. This is a serious oversight since such breaks are the connecting links between contemporaneous-land- and shallow-inland-sea-derived sediment. The following conclusions result from the study of numerous limestone-shale contacts in outcrops and conventional cores in the period 1957 to 1981. (1) The history of Mid-Continent Pennsylvanian sedimentation, including river mouth shifts and numerous floods, is recorded in limestone-shale contacts. (2) The day-to-day regime was one of minor terrigenous sediment bypassing laterally adjacent embayments in which carbonate sediment accumulated. (3) Day-to-day deltaic progradation was minor with carbonate sedimentation curtailed only along the narrow junction between deltas and adjacent embayments. (4) During and immediately after floods, major deltaic progradation spread relatively thin and widespread deltaic packages which stifled carbonate sedimentation over several hundred square-mi e (minimum) adjacent areas. (5) Thin shale or claystone breaks to featheredge argillaceous partings in limestone record flood-deposited prodeltaic increments. (6) Shale or claystone interbeds (more than 2 ft or .6 m thick) in limestone as well as shale- or claystone-sandstone units tens to a few hundreds of feet thick and laterally equivalent to largely limestone are predominantly the composite record of many floods and many times of small-scale mass movements. (7) Within the thicker terrigenous detritus units, individual flood or individual mass movement record is extremely difficult to define. (8) The lateral movement of life assemblages was slower than the influx of terrigenous clay in flood-generated plumes and organic communities were buried before they could vacate the area. (9) Te rigenous detritus sedimentation rates exceeded carbonate sedimentation rates. (10) The writer is unaware of a convincing modern analog supportive of all aspects of these conclusions. Possibly, the problem is that day-to-day tides reach most present-day shorelines whereas that may not have been so in the Paleozoic epieric sea setting. End_of_Article - Last_Page 418------------

Vertebrate fossil-bearing paleosol units (Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.): Implications for taphonomy, biostratigraphy, and assemblage analysis

In the lower 300 m of the fluvial Willwood Formation of northwest Wyoming, most large concentrations of vertebrate fossils consist of disarticulated and broken skeletal remains that occur in widespread, tubular, thin (2 cm-1 m) greenish- and bluish-gray mudstones that are low in percentage of free iron, aluminum, and manganese, relatively high in percentages of organic carbon, and often show evidence of clay eluviation. These units lie atop sesquioxide-rich, sometimes clay-illuviated variegated unit (red, yellow, purple, and mottled variants) and are relicts of the A horizons of podzolic spodosols (aquods). The wide distribution of bone fragments, the numerical proportions of bone elements, weathering of the fragments, and biogenic evidence indicate the vertebrate remains accumulated gradually as litter on the surface of the soils and became incorporated into the A horizon. The bones were disarticulated, broken, and otherwise disturbed by the action of scavengers prior to burial. Occurrence of abundant vertebrate remains in discrete, readily identifiable, and widespread paleosol units makes these units valuable biostratigraphic markers (zonules, faunules). Evidence of geologically short-term lag accumulation of the bones is important to assemblage analysis; the paleosol concentrations result from attritional mortalities that more closely reflect composition of the life assemblage than do remains concentrated by fluvial transport or predators.

A soft substrate community with edrioasteroids, from the Verulam Formation (Middle Ordovician) at Gamebridge, Ontario

The edrioasteroids Isorophusella incondita (Raymond) and Cryptogoleus chapmani (Raymond) occur in profusion on one horizon in the lower member of the Verulam Formation (Middle Ordovician) near Gamebridge, Ontario. The associated fauna, including brachiopods, trilobites, and gastropods, suggests a smothered life assemblage from soft bottom shallow marine platform conditions. The edrioasteroids settled on overturned shells of Rafinesquina alternata and on local hard surfaces provided by previously formed internal moulds of molluscs.

Microstratigraphic sampling and the limits of paleontologic resolution

Rates of modern sedimentation in various depositional settings have been compiled and adjusted for compaction for the purpose of estimating the spans of time represented in thin stratigraphic samples. Owing to discontinuous or low rates of sedimentation, it is either impossible or impractical to recover a continuous series of discrete life assemblages of fossil populations by collecting microstratigraphic samples from continuously fossiliferous intervals. Assemblages that are less time-averaged may be recovered from discontinuous records formed in depositional environments with higher rates of intermittent sedimentation, but these are isolated in time and cannot be used to observe the dynamics of local faunal history. Patterns of fossil distribution may appear similar to patterns of living populations explained by neontologic processes, but the longer time scale of even the best fossil sequences suggests the action of uniquely paleontologic processes that may be driven by environmental changes.

Foraminiferal Life and Residue Assemblages from Cretaceous Slope Deposits

Distinctive foraminiferal assemblages and dissolution patterns are associated with Cretaceous clastic deep-water sediments along the eastern North Pacific continental margin. These sediments were deposited in bathyal, low-oxygen chemically reducing environments, as is evidenced by their foraminiferal composition, lithology, sedimentary constituents, organic-carbon content, and associated organisms. In species composition and morphology, the foraminiferal assemblages closely resemble modern faunas from low-oxygen environments on the continental slope and in deep-water basins along the eastern North Pacific Ocean. The Cretaceous life assemblage is characterized by species of Praebulimina , the subfamily Chilostomellinae, and several agglutinated and nodosariid genera, among others. In southern California this assemblage is restricted to the laminated mudstone facies of the Upper Cretaceous Point Loma Formation, which exhibits limited bioturbation and contains large amounts of organic detritus and pyrite. By comparison with modern depositional environments, bottom conditions probably were oxygen deficient but not anaerobic. Differential preservation of the foraminiferal fauna in the Point Loma Formation resulted from diagenic dissolution in the chemically reducing sediments. The subsequent residue assemblage is enriched in resistant forms such as thick-walled, compact calcareous and agglutinated benthic species; it is impoverished in planktonic and less resistant benthic species. Selected species are ranked according to their susceptibility to dissolution by noting their successive stage of preservation. These data illustrate the complex interactions between environmental and diagenetic processes that strongly influence interpretations of paleontologic age and environment of deposition.

Palaeoecology of post-glacial beaches in East Lothian

Synopsis Molluscan assemblages, not previously described in detail, from raised beaches in the Aberlady and Gosford Bay areas indicate that several facies are present: rocky shoreline deposits formed at high-water mark; sandy shore facies from a little lower in the shore zone; a sandspit and various intertidal and estuarine deposits. In addition there are fossil life assemblages of Ostrea edulis and Cerastoderma edule. The raised beaches are correlated on the evidence of their depositional facies combined with their present height O.D., from which have been determined the relative heights of high-water marks at the times of their formation. The older life assemblages and the highest shoreline (8–9 m O.D.) have thus been grouped together and by means of C14 dating found to be about 5500 years old, corresponding with the age of the main post-glacial marine transgression in the Firth of Forth area.

Gypidulid Brachiopods: Their Life Position and Paleoenvironment: ABSTRACT

Life assemblages of 2 species of gypidulid brachiopods, preserved in their original positions of growth, have been collected from Upper Silurian and Lower Devonian limestones of the Appalachian basin. The 2 assemblages inhabited the same environmental locus at different times in a transgressing carbonate sea. The earlier G. prognostica is in the Keyser Formation in Pennsylvania, Maryland, Virginia, and West Virginia, and the later G. coeymanensis is found in the Coeymans Formation of New York state. The gypidulids, in both cases, lived clustered in a beak-down position on poorly sorted skeletal sand substrates in association with a diverse faunal assemblage of brachiopods, bryozoans, crinoids, and trilobites. The specimens appear to have no pedical openings, indicating lack of pedical development in mature forms. The inner prismatic shell layer in the pedical umbo is many times thicker than the total shell elsewhere on the organism. The resulting weighting of the pedical umbo and lateral contact with other individuals in the cluster promoted an upright, posterior down orientation. Gypidulids are restricted to shallow (near, but above wave base) shelf environments. Onshore skeletal barrier sands and skeletal lagoonal muds, as well as offshore fossiliferous muds, are barren of gypidulids. The gypidulid environmental position occurs only in transgressive stratigraphic sequences. End_of_Article - Last_Page 637------------