Sphaeriid Bivalves Research Articles

Freshwater diversity at a biogeographic edge zone: the high-mountain pea-clams of Ethiopia

Mountain ranges are known for their particular level of biodiversity typically including a striking amount of endemism. This has been comparatively well studied for certain mountain ranges worldwide such as the Alps in Europe, the Himalayas in Asia, the Rocky Mountains in North America and the Andes in South America. Apparently, this is not the case for the mountains on the African continent and especially their freshwater fauna. One of Africa’s largest high-elevated regions is the Ethiopian Highlands. The study of freshwater molluscs might potentially allow to provide valuable insights into the interplay of environmental variation and evolutionary change. Here, we present the first genetically based study of Afromontane sphaeriid bivalves in a worldwide phylogenetic framework which allowed us to assess and describe the recent diversity and endemism as well as the fauna’s biogeographic background. Additionally, a new Ethiopian species of Sphaeriidae, Pisidium boessnecki Albrecht & Clewing sp. nov., could be discovered and is formally described here. Our study revealed a total of six sphaeriid species for the restricted study region in the Ethiopian Highlands of which the first two are most likely endemic: Pisidium boessnecki sp. nov., P. ethiopicum, P. cf. kenianum, P. cf. pirothi, P. cf. viridarium, and Pisidium sp. V. Based on the multigene phylogeny generated, we conclude that the observed diversity might have been caused by multiple independent colonization events from Africa. The zoogeographic affinities of all Ethiopian sphaeriids are related to the Afrotropics. The importance of these findings is discussed. http://zoobank.org/urn:lsid:zoobank.org:act:99C2C3A3-533B-4180-BBDD-E4E5A4D28F09

Freshwater biogeography in Wallacea: The case of sphaeriid bivalves in the Malili lake system (Sulawesi, Indonesia)

Ancient lakes are renowned as freshwater hotspots of biodiversity and endemism. However, some of these water bodies are poorly studied with the minute pill clams in the genus Pisidium (family Sphaeriidae) being particularly overlooked. The Malili lake system is located on Sulawesi, which is the largest and possibly biologically most diverse Indonesian island within the biodiversity hotspot region of Wallacea. This lake system consists of 5 interconnected lakes that harbour a unique freshwater fauna including 42 mostly endemic mollusc species described to date. The only bivalves recognized in this lake system have been in the widespread genus Corbicula. We present new insights into the sphaeriid biodiversity of the ancient Malili lakes and provide the first fossil-calibrated phylogeny for this family to discuss the biogeographical history of the Wallacean Pisidium fauna including the dating of speciation events. Both morphological and molecular data suggest that the Malili lake system is inhabited by at least three Pisidium species, two of which are potentially endemic to these lakes. This diversity is probably derived from a single colonization event from the Oriental region. The fossil-calibrated phylogeny further indicates that intra-island diversification occurred well after the island was fully formed. Our study improves understanding of the overall sphaeriid biodiversity in Indonesia as well as the endemic fauna of the ancient lakes.

Thirty years of change in a benthic macroinvertebrate community of southwestern Lake Ontario after invasion by four Ponto-Caspian species

AbstractBeginning in the mid-1980s, the Laurentian Great Lakes underwent successive invasions by Ponto-Caspian species. We quantified major changes in the diversity and relative abundance of pre-invasion benthic macroinvertebrates at the same study site in southwestern Lake Ontario from 1983–2014. The zebra mussel Dreissena polymorpha Pallas arrived at the study site before 1991, the quagga mussel Dreissena rostriformis bugensis Andrusov and the amphipod Echinogammarus ischnus Stebbing arrived before 1999, and the Round Goby Neogobius melanostomus Pallas arrived about 2004. The macroinvertebrate community in 2014 was very different from 3 earlier communities in 1983, 1991, and 1999. In 2014, pulmonate and prosobranch snails and sphaeriid bivalves were absent, D. r. bugensis replaced D. polymorpha, E. ischnus replaced Gammarus fasciatus Say as the dominant amphipod, and a previously diverse community of benthic fish was replaced by abundant N. melanostomus. From 1983 to 1999, the relative abundance of pros...

Freshwater molluscan fauna from the Florissant Formation, Colorado: paleohydrologic reconstruction of a latest Eocene lake

The freshwater molluscan assemblage from the uppermost Eocene Florissant Formation (34.07 ± 0.10 Ma), Colorado, USA, provides a reliable proxy in reconstructing past ecology and environmental characteristics of ancient Lake Florissant. In particular, stable-isotope analyses of aragonitic shell material contribute to our understanding of the paleohydrologic history of this ancient lake. Re-examination of molluscan taxonomy in the middle shale and caprock conglomerate (informal) units produces three sphaeriid bivalves (family Sphaeriidae, genus Sphaerium) and two pulmonate gastropod genera (family Planorbidae, genus Gyraulus; and family Lymnaeidae, genus Lymnaea). The middle-shale assemblage, representing quiet-lake deposition, is dominated by pulmonate gastropods; the shell material in all specimens in this unit has been replaced by silica. The caprock conglomerate assemblage, representing redeposition by a debris flow, is dominated by bivalves; specimens within the caprock conglomerate unit are aragonite, interpreted to be biogenic (original). Carbon and oxygen isotope analyses of the aragonite show a strong covariance (all molluscan data: r = 0.83; sphaeriids: r = 0.76) with consistent grouping (δ13C: 0‰ to −5.5‰, δ18O: +2‰ to −5‰) from all families. This result indicates that ancient Lake Florissant was “closed” and that evaporation had a stronger effect on isotopic values than precipitation, for the duration of the sampled interval. This finding is in agreement with our current understanding of how the sediments of the Florissant Formation were deposited within a lake that formed because of a paleoriver being dammed by debris flows.

Molecular and karyological identification and morphological description of cystocercous cercariae of Phyllodistomum umblae and Phyllodistomum folium (Digenea, Gorgoderidae) developing in European sphaeriid bivalves

Two cystocercous gorgoderid cercariae of the genus Phyllodistomum Braun, 1899, Phyllodistomum umblae (Fabricius, 1780) and Phyllodistomum folium (Olfers, 1816), developing in sphaeriid bivalves from Norway and Lithuania, were studied and compared. Our previous molecular studies revealed very close phylogenetic relation of these two species and proved that cystocercous cercariae of P. folium develop in sporocysts in the gills of sphaeriid bivalves. In the present study morphological descriptions are given for the two cercariae, together with karyological and molecular characteristics. Comparative karyological and sequence analysis using ITS2 and 28S rDNA revealed the conspecificity of cercariae emitted from Pisidium hibernicum and Sphaerium corneum with adult P. umblae from Thymallus thymallus and Coregonus albula. The cercariae of P. folium and P. umblae are very similar morphologically with main differences in the structure of the tail. The two species clearly differ in karyotype structure. The diploid set of P. folium is composed of 18 chromosomes, but 16 chromosomes with one pair of large metacentrics were found in mitotic cells of P. umblae. Interspecific karyotypic difference presumably arose from Robertsonian fusion of two uni-armed chromosomes.

Phyllodistomum kanae sp. nov. (Trematoda: Gorgoderidae), a bladder fluke from the Ezo salamander Hynobius retardatus

The Ezo salamander, Hynobius retardatus, is endemic only to Hokkaido, the northernmost island of Japan. Gravid flukes of the family Gorgoderidae were discovered from the urinary bladder of H. retardatus. The parasites were identified as a new species named Phyllodistomum kanae sp. nov. In the neighboring Honshu island another bladder fluke, Phyllodistomum patellare, has already been found from the Japanese newt. The new species clearly differs from P. patellare in having a spherical ovary and very weakly lobed testes. The discovery of species of Phyllodistomum from urodelan amphibians is very uncommon in Eurasia. A molecular phylogeny based on 28S ribosomal DNA suggests that sphaeriid bivalves may serve as the first intermediate host for the new species.

Responses of stream benthic macroinvertebrate communities to timber harvesting: Best management practices in a low-gradient watershed in Central Louisiana, USA

AbstractForests are the most dominant land-use type in Louisiana, and timber harvesting is the most economically important of the state’s agricultural commodities. Louisiana has developed voluntary best management practices (BMPs) to minimize negative effects of forest operations on stream water quality, but little is known about how aquatic communities inhabiting low-gradient, headwater streams respond to timber harvesting, and if the current BMPs are effective in protecting community structure and function. In 2005, we initiated a multidisciplinary study in the Flat Creek watershed in central Louisiana to discern the effects of timber harvesting activities, with and without BMP implementation, on benthic aquatic macroinvertebrates. During fall 2007, trees were removed adjacent to two headwater streams in the watershed, and benthic macroinvertebrate samples were subsequently collected at seven stream locations seven times between 2006 and 2009. Our objectives were to describe the community structure of benthic macroinvertebrates in low-order, low-gradient streams, how macroinvertebrate communities responded to timber harvest operations, and whether significant changes in community structure were evident under different levels of BMP implementation. The community structure was predominantly generalist taxa including chironomids, sphaeriid bivalves and asellid isopods. Analyses of variance demonstrated significant increases in bivalve abundance and decreases in the abundance of malacostracan and shredding taxa at most of the sampling sites downstream of BMP-implemented harvest locations in the spring. Additionally, significant increases in bivalves were also found at two sites during late summer. Our results suggest that timber harvesting activities, regardless of BMP implementation, had limited shortterm impacts on resident macroinvertebrates in these lowgradient, subtropical streams. Continued monitoring at the study sites will allow us to better understand the longterm effects of timber harvesting in these stream systems, particularly the resilience of stream biota to harvestingrelated stream conditions.

European Phyllodistomum (Digenea, Gorgoderidae) and phylogenetic affinities of Cercaria duplicata based on rDNA and karyotypes

Genetic markers of some European Gorgoderidae species, including potential adults of Cercaria duplicata, were obtained and used to clarify phylogenetic affinities within the genus Phyllodistomum and to verify conflicting data existing on their life cycles. Molecular data and karyotype, 2n = 18, provide further support for ascription of C. duplicata to the Gorgoderinae. Sequences of C. duplicata form a robustly supported major clade, phylogenetically distinct from other known gorgoderid species in both ITS2‐ and 28S‐based phylogenetic trees. The molecular data revealed no match between C. duplicata and any species of Phyllodistomum, including adults found in the experimental studies. One of them, P. elongatum, showed no differences from type species P. folium. Other, P. angulatum, forms a robustly supported clade, which is closely related to P. macrocotyle clade in all phylogenetic trees. This study supports the concept that only P. macrocotyle is a parasite of Dreissena polymorpha among the phyllodistomes and life cycle described for the type species P. folium by Sinitsin (1905) can apparently be discounted. Previously reported low host specificity of P. folium was justified. Adults of P. folium were detected in eight teleost species from five families and four orders. Cystocercous cercariae of P. folium were recorded in sphaeriid bivalves of the genus Sphaerium and Pisidium. According to our molecular data, P. simile, parasite of bullhead, must be regarded as synonym of P. folium. Phyllodistomum umblae is most closely related to P. folium in the all phylogenetic analyses. Molecular phylogenies support a presumption that Phyllodistomum species with cystocercous cercariae developing in sphaeriid bivalves should be regarded as Phyllodistomum sensu stricto. The results reveal a clear need for reconsideration of the knowledge on gorgoderid life cycles based on experimental studies and re‐evaluation of the validity of some nominal Phyllodistomum species.

Integrative taxonomy identifies a new species of Phyllodistomum (Digenea: Gorgoderidae) from the twospot livebearer, Heterandria bimaculata (Teleostei: Poeciliidae), in Central Veracruz, Mexico

Phyllodistomum inecoli n. sp. is described from the twospot livebearer, Heterandria bimaculata (Teleostei: Poeciliidae), collected in the Río La Antigua basin, Veracruz, Mexico. The new species is described and characterised by using a combination of morphology, scanning electron microscopy, and sequences of nuclear and mitochondrial genes. Diagnostic characters of the new species of Phyllodistomum include a genital pore opening at the level of the caecal bifurcation; oval vitellarium, situated just posterior to the ventral sucker and not extended laterally and anterior extracaecal uterine loops variable in extension (reaching the anterior, median or posterior margin of the ventral sucker). P. inecoli n. sp. most closely resembles P. brevicecum, a species described as a parasite of the central mudminnow, Umbra limi, in other parts of North America; however, the genital pore in P. brevicecum is situated between the caecal bifurcation and the ventral sucker, the ovary is larger, the vitellarium is lobed and extended laterally and the anterior portion of the uterus extends to the posterior margin of the ventral sucker. Comparison of about 1,500–2,200 nucleotides of cox1 and 28S rDNA and ITS1 strongly supports the status of P. inecoli as a new species. Bayesian inference analysis of combined datasets of 28S rDNA and cox1 sequences showed that P. inecoli n. sp. and the other species found in freshwater fishes of Mexico, including the species complex of P. lacustri, are not sister species. Phylogenetic analysis based on 28S rDNA sequences of several gorgoderid taxa revealed the close relationship of P. inecoli n. sp. with several species of Phyllodistomum, Gorgodera and Gorgoderina with cystocercous cercariae developing in sphaeriid bivalves. Dot-plot analysis of ITS1 sequences of P. inecoli n. sp. revealed the presence of eight repetitive elements with different length, which together represent almost half the length of ITS1.

A NOVEL CONTEMPORARY FLUVIAL ICHNOCOENOSE: UNIONID BIVALVES AND THE SCOYENIA-MERMIA ICHNOFACIES TRANSITION

Abstract A transitional Scoyenia–Mermia ichnocoenose from the Saint John River, Fredericton, New Brunswick, Canada, is dominated by elements of the Mermia ichnofacies, with traces comparable to Curvolithus, Helminthopsis, Gordia, Spirophycus, and Lockeia. Environmental characteristics are, however, more typical of the Scoyenia ichnofacies, with an emersion event providing conditions favorable to viewing traces preserved in a sand-softground substrate. Observation of in situ trace-making behavior allowed traces to be attributed to their progenitors, which include unionid and sphaeriid bivalves. An omission assemblage of vertebrate tracks also was present, comprising gull, raven, and mink. Oichnus-like borings were observed in some unionid shells. The shallow-tier trace assemblage created in a high-energy river channel may be expected to have a poor preservational potential, with loss of trace definition observed at the water margin during emersion and subsequent deterioration by eolian sediment transport.

The distribution and conservation of threatened Sphaeriidae on British grazing marshland

Freshwater sphaeriid bivalves are poorly known ecologically, particularly uncommon species such as Pisidium pseudosphaerium (UK RDB (Red Data Book) staus = ‘rare’). In the UK, this species occupies grazing marsh where conservation opportunities might be shared with other threatened molluscs. We surveyed sphaeriids including P. pseudosphaerium and snails in 106 drainage ditches in SE England in 1999. P. pseudosphaerium occupied over half the ditches surveyed, at slightly elevated BOD (6 ± 7 mg l−1 sd) but reduced calcium (64 ± 31 mg l−1) and nitrate (0.5 ± 1.2 mg l−1). As part of a sphaeriid assemblage comprising Sphaerium corneum, Musculium lacustre and Pisidium obtusale (=Assemblage 1), P. pseudosphaerium occurred in ditches with floating vegetation and abundant snails of conservation importance, co-occurring significantly with either Valvata macrostoma (RDB ‘vulnerable’) or Anisus vorticulus (RDB: ‘vulnerable’). A more diverse sphaeriid assemblage (=Assemblage 2) included species common in wider, deeper ditches with open water. We suggest that traditional ditch management can support both rare and representative snails and bivalves on grazing marsh. Quasi-traditional and rotational ditch clearance will favour common sphaeriids and pioneer snails during early succession; P. pseudosphaerium, V. macrostoma, A. vorticulus during mid-succession; and the snail Segmentina nitida (RDB: ‘Endangered’) in late successional ditches. In common with threatened snails, P. pseudosphaerium will also benefit from reduced eutrophication. We recommend that P. pseudosphaerium retain RDB status to ensure protection and to emphasise the need for improved ecological information.

LIFE-HISTORY PATTERNS IN THE BROODING FRESHWATER BIVALVE PISIDIUM (SPHAERIIDAE)

Of all freshwater molluscs, the life history of sphaeriid bivalves has been one of the best studied, partly because they brood their young. In Pisidium, one of five genera in the Sphaeriidae, offspring develop and are released synchronously and therefore the reproductive state is easily determined from collections of adults. This study examines life-history trade-offs, morphological constraints on reproductive effort, intraclutch variation and brood production differences in different habitats in the pea clam, Pisidium. My results challenge previous ideas about life-history trade-offs in the pea clams and are as follows: (1) there is a trade-off between offspring number and size, larger embryos being found in smaller clutches when controlling for adult body size; (2) total embryo area is highly correlated with total gill area, but the measured maximum total area of embryos relative to gill area is rarely achieved; (3) intraclutch variance itself varies non-normally across Pisidium and does not appear to relate strongly to other measured variables; (4) creeks, ponds, springs and marshes appear to have higher relative per-clutch brood production than lakes in the best-sampled taxon, P. casertanum. Relative brood production is lower in high alpine lakes (above 3350 m), perhaps due to low dissolved organic material and ions.

The Physiological Ecology of the Zebra Mussel,Dreissena polymorpha, in North America and Europe

Synopsis. The zebra mussel, Dreissena polymorpha (Pallas), was intro? duced into North America in 1986. Initial North American (N.A.) studies suggested that physiological responses varied between N.A. and European populations. However, literature review indicates agreement on most as? pects of physiological adaptation including: respiratory responses; hypoxia/anoxia tolerance; salinity limits; emersion tolerance; freezing resis? tance; environmental pH limits; calcium limits; starvation responses; and bioenergetic partitioning. The main differences among N.A. and European mussels appear to be elevated upper thermal limits and temperatures for optimum growth among N.A. populations. N.A. zebra mussels probably originated from the northern shore of the Black Sea in the warmest por? tion of the mussel's European range. However, most European physio? logical data come from northern Europe where populations may be adapt? ed to colder temperatures. Alternatively, N.A. research suggests that mus? sels may have a capacity for seasonal temperature acclimatization such that responses recorded in warmer N.A. waters may be different from those recorded in northern Europe even after short-term laboratory accli? mation. Studies of genetic variation and physiological response among European and N.A. D. polymorpha populations are required to elucidate the basis for physiological differentiation. Recently evolved D. polymor? pha has poor resistance adaptations compared to unionacean and sphaeriid bivalves with longer freshwater fossil histories. Poor resistance adapta? tions make it less suited for stable habitats, instead, its high fecundities, early maturity, and rapid growth are adaptations to unstable habitats where extensive resistance adaptations are of little value.

Preliminary study to compare body residues and sublethal energetic responses in benthic invertebrates exposed to sediment‐bound 2,4,5‐trichlorophenol

AbstractRelationships between concentration of 2,4,5‐trichlorophenol (TCP) in sediment, body residues of the chemical, and sublethal biological effects in three benthic invertebrates were studied. Uncontaminated lake sediment was spiked with four concentrations (23–85 μg/g dry sediment) of TCP. Chironomid larvae (Chironomus riparius), oligochaete worms (Lumbriculus variegatus), and sphaeriid bivalves (Sphaerium corneum) were exposed to the sediment. The effect of chlorophenol on the rate of heat dissipation of animals was monitored by direct microcalorimetry. It appeared that both the behavior of the animals and their body residues explained the energetic response. Valve closure behavior of S. corneum reduced the accumulation of toxicant (<0.3 μmol/g) but was observed as a complex energetic response. Heat dissipation of L. variegatus was at the same level in control animals and those with high body residues of TCP (>1.5 μmol/g). Regardless of the amount of TCP accumulated to C. riparius (0.1–0.6 μmol/g), the rate of heat dissipation was almost two times higher than that of the control animals, probably reflecting uncoupling of oxidative phosphorylation, which is the primary mode of toxic action of chlorophenols. However, when a threshold concentration was exceeded there was no concentration‐response dependence until acute toxicity appears.

Physiological energetics of a midge, Chironomus riparius Meigen (Insecta, Diptera): normoxic heat output over the whole life cycle and response of larva to hypoxia and anoxia.

The rate of metabolism of laboratory reared Chironomus riparius was monitored by direct calorimetry over the entire life cycle from egg to adult stage. The metabolic response of the fourth instar larva to decreasing oxygen concentrations and anoxia was also measured. Normoxic measurements were carried out at 20°C and the hypoxic-anoxic experiments at 10°C. In larvae with body sizes ranging from 0.0028 to 0.645 mg ash-free dry mass (afdm), the rate of heat dissipation was related to body mass by a power function, with a mass exponent of 0.71±0.02 corresponding to an exponent of -0.29 for the relationship between mass-specific metabolic rate and body mass. However, the allometric equations applicable to larvae would not predict the metabolic rates of eggs, pupae and adults. Single egg batches used in the experiments consisted of 354±90 eggs, the individual egg with a mass of 0.99±0.01 μg (mean±SD). The mass-specific rate of heat dissipation of the egg (13.7±1.8 μ W mg-1 afdm) was considerably lower than that of the first and second instar larvae (44-53 μ W mg-1) but equal to that of fourth instar larvae (13.1±3.9 μ W mg-1). Heat dissipation by a pupa shortly before adult emergence was high (14.8±1.8 μ W mg-1), probably due to high metabolism during metamorphosis. Emergence of the adult in the calorimeter was indicated by a short but intense burst of heat. The newly emerged imago had a ca. 20-35% higher metabolic rate than the pupa. In response to reduced O2 partial pressure the fourth instar larva of C. riparius displayed metabolic regulation. In continuously declining oxygen partial pressure, the fourth instar larva maintained its aerobic energy metabolism (4.2 μ W mg-1) with only a small decrease down to 0.8 kPa, corresponding to an oxygen concentration of 0.42 mg O2l-1 H2O. Below this critical oxygen concentration (Pc), the rate of heat dissipation decreased rapidly down to the anoxic level which was only 14-17% of the normoxic level. The high relative reduction of metabolic rate under anoxia gives a wrong impression of short-term tolerance of C. riparius to anoxia. The absolute energetic costs of C. riparius associated with anaerobic energy metabolism (0.64±0.11 μ W mg-1) are almost 6 times higher than those of more anoxia tolerant invertebrates such as sphaeriid bivalves.

Depositional Environments in an Alluvial-Lacustrine System: Molluscan Paleoecology and Lithofacies Relations in Upper Part of Tongue River Member of Fort Union Formation, Powder River Basin, Wyoming: ABSTRACT

The upper part of the Tongue River Member of the Fort Union Formation (Paleocene) in the northern Powder River basin, Wyoming, contains assemblages of excellently preserved nonmarine mollusks which occur in laterally continuous outcrops of diverse lithologic sequences and sedimentary structures. These attributes offer a unique opportunity for interdisciplinary interpretation of depositional environments based on molluscan paleoecology and lithofacies relations. Taphonomic histories of mollusk assemblages as reflected by molluscan biofabric (size, End_Page 478------------------------------ shape, orientation, and distribution of faunal elements), and the taxonomic composition, diversity, and abundance representation of species in assemblages are of particular importance in interpretations of paleoecology and depositional environments. Three facies are recognized vertically within an alluvial-lacustrine system. The interfluvial lake and lake splay facies is characterized by sequences of coarsening-upward detritus, abundant continuous limestone beds, and few beds of discontinuous coal and continuous carbonaceous shale. Limestones, contain two lacustrine mollusk assemblages: a locally reworked assemblage dominated by the bivalve Plesielliptio (two species), and the gastropods Viviparus, Lioplacodes (three species), and Clenchiella; and a quiet-water assemblage dominated by sphaeriid bivalves. This facies reflects development of numerous lakes bordered by well-drained backswamps in interfluvial flood basins. Lakes were repeated infilled by carbonate mud, splay detritus, and backswamp deposits. The interfluvial crevasse splay-crevasse channel facies is characterized by sequences of coarsening-upward detritus and few discontinuous limestone beds, separated vertically by thick, continuous coal and carbonaceous shale beds. This facies includes small crevasse channel sandstones which scour into splay sandstones. Biofabric of lacustrine mollusk assemblages, which are identical in composition (but with dwarfed species of Plesielliptio) to locally reworked lacustrine assemblages of the interfluvial lake and lake splay facies, reflects deterioration of lakes through active infilling by crevasses. The fluvial channel and interchannel facies is typified by thick channel sandstones laterally separated by sequences of coarsening-upward detritus, overbank sediments, and rare limestones. This facies includes thick, continuous coal and carbonaceous shale beds. Muddy-substrate flood basin lakes are characterized by an untransported lacustrine mollusk assemblage differing from those of the interfluvial lake and lake splay facies in both composition and relative abundance of species. Transported mollusk assemblages dominated by the gastropod Hydrobia eulimoides occur in crevasse sandstones. Relative to northerly flowing major channels, proximal and distal parts of splay deposits can be differentiated by their mollusk assemblage composition. The fluvial channel and interchannel facies re lects deposition on flood basins that were formerly occupied by lake, crevasse, and backswamp deposits. Thick, continuous coal beds reflect the spread of backswamps from interchannel depressions to abandoned major channel ridges. Cessation of fluvial channel and interchannel sedimentation was marked by widespread lacustrine deposition. The vertical sequence of facies just described also reflects the areal distribution of deposits of major channels that pass laterally into overbank and crevasse splay channel areas, which in turn merge into large and small lakes. Alluvial-lacustrine deposition in the upper part of the Tongue River Member is similar to that of anastomosed reach of the Saskatchewan River, Canada, as reported in a 1980 study by Smith and Putnam. End_of_Article - Last_Page 479------------

Application of Nonmarine Mollusca to Paleoenvironmental Interpretation and Correlation of Paleogene Rocks: ABSTRACT

Nonmarine mollusks, the most consistently occurring element of Mesozoic and Cenozoic nonmarine macrofaunas, are abundant and locally dominant in many preserved terrestrial and freshwater paleocommunities. Maximum interpretive potential of mollusk assemblages is derived from detailed analysis of several biologic and physical factors: (1) taxonomy based on modern malacologic and paleontologic concepts that include differentiation of genetic and nongenetic morphologic variability; (2) biostratonomy (the history of the faunal assemblage from death to final burial); (3) community structure; (4) time-space variability of assemblages relative to a detailed lithostratigraphic framework; and (5) rock types, fabric, and structures of the enclosing sedimentary rock. Collectively, hese factors indicate whether the faunal assemblage was preserved in the original environment in which it lived. Failure to gather and/or interpret adequately these data has promoted the widely held misconception that mollusks are of little value in the interpretation of depositional environments, biostratigraphy, correlation, and age determination of nonmarine rocks. Two examples of the interpretive value of Paleocene and Eocene nonmarine mollusks are (1) depositional environments and regional paleoenvironmental reconstruction of part of the Green River and Wasatch Formations, southwestern Wyoming and northwestern Colorado; End_Page 828------------------------------ and (2) biostratigraphy and correlation of part of the Tongue River Member of the Fort Union Formation, northern Powder River basin, Wyoming and Montana. The Green River and Wasatch Formations are complexly intertonguing lacustrine and fluvial units that were deposited during early and middle Eocene time in Western Interior basins of Wyoming, Colorado, and Utah. The structure of nonmarine molluscan associations within these strata delineates littoral and sublittoral lacustrine, pond, fluvial, and terrestrial habitats. Littoral and sublittoral lacustrine habitats are characterized by a low-diversity association of prosobranch gastropods and unionid bivalves. Rank and relative abundance of taxa differ in these habitats. Ponds are dominated by a diverse association of aquatic pulmonate gastropods with sphaeriid bivalves. Lowland and flood-plain habitats are characterized by a locally diverse association of terrestrial pulmonate gastropods and a fluvial association dominated by unionid bivalves with prosobranch and aquatic pulmonate gastropods. Comparison with structurally similar molluscan associations from modern habitats, paleosynecology of fossil taxa, and lithostratigraphic data provide bases for paleoenvironmental interpretation. Analysis of paleogeographic and stratigraphic distribution of these Eocene molluscan associations, relative to a detailed lithostratigraphic framework, permits regional paleoenvironmental reconstruction within the Green River and Wasatch Formations. In the northern Powder River basin, the Tongue River Member of the Fort Union Formation contains diverse, commonly excellently preserved assemblages of Paleocene nonmarine mollusks. Detailed study of the distribution of gastropods and bivalves in the stratigraphic interval from below the Wall coal bed to above the Arvada coal bed clearly indicates the value of mollusks in correlation of sedimentary sequences between the Wall, Anderson, Smith, Roland, and Arvada coal beds. Clinal morphologic variation in shell form and sculpture through time within a lineage of viviparid gastropods provides an additional method for correlation within part of the stratigraphic interval. These studies clearly indicate the value of mollusks in the interpretation of depositional environments, biostratigraphy, and correlation of Paleogene nonmarine rocks. End_of_Article - Last_Page 829------------