Differential attrition of molluscan shells in freshwater sediments

1988 ◽  
Vol 25 (1) ◽  
pp. 68-73 ◽  
Author(s):  
Eva Pip
Keyword(s):  
Species Composition ◽  
Energy Conditions ◽  
Small Species ◽  
Freshwater Sediments ◽  
Shell Size ◽  
Attrition Rates ◽  
Delta Marsh ◽  
Fossil Assemblages ◽  
Species Frequencies

Mean seasonal species composition of living molluscan communities was compared with the composition of current dead assemblages in the sediments of three sites located in the Delta Marsh of southern Lake Manitoba. Dead shells were more numerous in vegetated than in bare areas, resulting primarily from the affinities of living molluscs for vegetated areas. Redistribution patterns of empty shells were not significantly different for vegetated and bare areas, as judged from distributions of passively transported land shells in the sediments. Significant differences were observed at all sites between species frequencies in living and corresponding dead assemblages averaged for the season. Proportions of living to dead individuals per unit bottom area indicated higher attrition rates with increasing energy conditions as well as with increasing shell size. Differential attrition may result in overrepresentation of small species in fossil assemblages.

Multiple paleoecological controls on the composition of marine fossil assemblages from the Frasnian (Late Devonian) of Virginia, with a comparison of ordination methods

Paleobiology ◽  
2010 ◽  
Vol 36 (4) ◽  
pp. 573-591 ◽  
Author(s):  
Andrew M. Bush ◽  
Roderic I. Brame
Keyword(s):  
Grain Size ◽  
Species Composition ◽  
Simulated Data ◽  
Ecological Factors ◽  
Detrended Correspondence Analysis ◽  
Late Devonian ◽  
Opportunistic Species ◽  
Disturbance Intensity ◽  
Fossil Assemblages ◽  
Highly Correlated

Ecological ordination can reveal gradients in the species composition of fossil assemblages that can be correlated with paleoenvironmental gradients. Ordinations of simulated data sets suggest that nonmetric multidimensional scaling (NMDS) generally produces less distorted results than detrended correspondence analysis (DCA). We ordinated 113 brachiopod-dominated samples from the Frasnian (Late Devonian) Brallier, Scherr, and lower Foreknobs Formations of southwest Virginia, which represent a range of siliciclastic marine paleoenvironments. A clear environmental signal in the ordination results was obscured by (apparently) opportunistic species that occurred at high abundance in multiple environments; samples dominated by these species aggregated in ordination space regardless of paleoenvironmental provenance. After the opportunist-dominated samples were removed, NMDS revealed a gradient in species composition that was highly correlated with substrate (grain size); a second, orthogonal gradient likely reflects variation in disturbance intensity or frequency within grain-size regimes. Additional environmental or ecological factors, such as oxygenation, may also be related to the gradients. These two gradients, plus the environmental factors that controlled the occurrence of opportunistic species, explain much of the variation in assemblage composition in the fauna. In general, the composition of fossil assemblages is probably influenced by multiple paleoecological and paleoenvironmental factors, but many of these can be decomposed and analyzed.

MOLLUSCAN LIVE-DEAD FIDELITY OF A STORM-DOMINATED SHALLOW-MARINE SETTING AND ITS IMPLICATIONS

Palaios ◽  
2021 ◽  
Vol 36 (2) ◽  
pp. 77-93
Author(s):  
MADHURA BHATTACHERJEE ◽  
DEVAPRIYA CHATTOPADHYAY ◽  
BIDISHA SOM ◽  
AMMU S. SANKAR ◽  
SATYAKI MAZUMDER
Keyword(s):  
Frequency Distribution ◽  
Large Scale ◽  
Regional Distribution ◽  
High Energy ◽  
Shell Size ◽  
Size Frequency Distribution ◽  
Shallow Marine ◽  
Size Frequency ◽  
Optimal Setting ◽  
Fossil Assemblages

ABSTRACT Actualistic studies are important for evaluating the fidelity of fossil assemblages in representing the living community. Poor live-dead (LD) fidelity in molluscan assemblages may result from transport-induced mixing. Large-scale mixing is more common in siliciclastic settings with a narrow shelf, high sedimentation rate, and those that are frequented by episodically high-energy events. Chandipur-on-sea, on the east coast of India has an optimal setting to promote such conditions. By studying the LD fidelity and modeling size-frequency distribution (SFD) of the fauna, we attempted to evaluate the contribution of “out-of-habitat” versus “within-habitat” mixing in developing the molluscan death assemblage. The correlation between the composition of live (LA) and death assemblages (DA) was insufficient; unlike LAs, the DAs do not show environmental partitioning in ordination space. A numerical simulation of the shell size frequency distribution (SFD) for DAs from LAs was compared with the observed SFD of the DAs. The results of this simulation indicate that DAs are not likely to be a product of within-habitat mixing. DAs probably received considerable input via regional transport, facilitated by frequent tropical cyclones affecting the coast of Odisha. Chandipur receives a large proportion of cyclones originating above 15°N, which causes a high degree of lateral transport and shell mixing between 15° to 21°N, explained by the high compositional similarity of species within this latitudinal extent. Our study highlights the significance of out-of-habitat transport in shaping the regional distribution of marine fossil assemblages, especially in storm dominated siliciclastic shallow-marine settings.

scholarly journals Vertical distribution of planktonic foraminifera in the Subtropical South Atlantic: depth hierarchy of controlling factors

2019 ◽  
Author(s):  
Douglas Lessa ◽  
Raphaël Morard ◽  
Lukas Jonkers ◽  
Igor M. Venancio ◽  
Runa Reuter ◽  
...  
Keyword(s):  
Environmental Factors ◽  
Species Composition ◽  
Vertical Distribution ◽  
Planktonic Foraminifera ◽  
Standing Stock ◽  
South Atlantic ◽  
Subsurface Water ◽  
Sst Variability ◽  
Fossil Assemblages ◽  
Total Fauna

Abstract. Temperature appears to be the best predictor of species composition of planktonic foraminifera communities, making it possible to use their fossil assemblages to reconstruct sea surface temperature (SST) variation in the past. However, the role of other environmental factors potentially modulating the spatial and vertical distribution of planktonic foraminifera species is poorly understood. This is especially relevant for environmental factors affecting the subsurface habitat. If such factors play a role, changes in the abundance of deeper dwelling species may not solely reflect SST variation. In order to constrain the effect of subsurface parameters on species composition, we here characterize the vertical distribution of living planktonic foraminifera community across the subtropical South Atlantic Ocean, where SST variability is small but the subsurface water mass structure changes dramatically. Four planktonic foraminifera communities could be identified across the top 700 m of the E–W transect. Gyre and Agulhas Leakage faunas were predominantly composed of Globigerinoides ruber, Globigerinoides tenellus, Trilobatus sacculifer, Globoturborotalita rubescens, Globigerinella calida, Tenuitella iota and Globigerinita glutinata, and only differed in terms of relative abundances (community composition). Upwelling fauna was dominated by Neogloboquadrina pachyderma, Neogloboquadrina incompta, Globorotalia crassaformis and Globorotalia inflata. Thermocline fauna was dominated by Tenuitella fleisheri, Globorotalia truncatulinoides and Globorotalia scitula in the western side, and by G. scitula in the eastern side of the basin. The largest part of the standing stock was consistently found in the surface layer, but SST was not the main predictor of species composition, neither for the total fauna at each station nor in analyses considering each depth layer separately. Instead, we identified a consistent vertical pattern in parameters controlling species composition at different depths, in which the parameters appear to reflect different aspects of the pelagic habitat. Whereas productivity appears to dominate in the mixed layer (0–60 m), physical-chemical parameters are important at depth immediately below (60–100 m), followed by parameters related to the degradation of organic matter (100–300 m), and parameters describing the dissolved oxygen availability (> 300 m). These results indicate that the seemingly straightforward relationship between assemblage composition and SST in sedimentary assemblages reflects vertically and seasonally integrated processes that are only indirectly linked to SST. This also implies that fossil assemblages of planktonic foraminifera should also contain a signature of subsurface processes, which could be used for paleoceanographic reconstructions.

Conidae and Terebridae (Gastropoda: Neogastropoda) from the Plio-Pleistocene of the Philippines

Zootaxa ◽  
2017 ◽  
Vol 4221 (5) ◽  
pp. 545 ◽  
Author(s):  
ENATE A. HELWERDA
Keyword(s):  
Species Composition ◽  
Deep Water ◽  
Fossil Record ◽  
The Philippines ◽  
Fossil Assemblage ◽  
Fossil Assemblages ◽  
Eleven Species ◽  
Lack Of Knowledge

Six species of Conidae and seven species of Terebridae are reported from the Plio-Pleistocene “Cabarruyan” fauna of Pangasinan, the Philippines. Eleven species are identified; these species all occur in the Recent Indo-Pacific fauna and seven of these are previously known from the fossil record as well. The species composition of this fauna shows little overlap with other fossil assemblages, except with the Fijian fossil assemblage. This is attributed to a lack of knowledge on Indo-pacific fossil faunas as well as to the relatively deep water setting (200–300 m) of this fauna. More research is needed to determine why the Fijian assemblage is relatively similar. 

Impacts of tree invasion on floristic composition of subtropical grasslands on the Bunya Mountains, Australia

2006 ◽  
Vol 54 (3) ◽  
pp. 261 ◽  
Author(s):  
Don William Butler ◽  
Russell James Fairfax ◽  
Roderick John Fensham
Keyword(s):  
Species Composition ◽  
Plant Species ◽  
Vascular Plant ◽  
Floristic Composition ◽  
Grass Species ◽  
Grassland Species ◽  
Tree Invasion ◽  
Eucalypt Forest ◽  
Eastern Australia ◽  
Species Frequencies

The effect of ongoing forest invasion on floristic composition of subtropical montane grasslands was considered by examining vascular plant species frequencies across 13 adjoining areas of grassland, invaded grassland (eucalypt forest <50 years old) and eucalypt forest (>50 years old) on the Bunya Mountains in subtropical eastern Australia. Tree invasion of grasslands over the last 50 years has had substantial facilitative or antagonistic net impacts on populations of many plant species. Increases in species frequency, indicating net facilitation, generally appear to occur earlier in forest development than do decreases in frequency, indicative of net antagonism. Although more than 20% of the flora showed substantial association with either grassland or forest, the dominant ground-stratum species in each habitat were quite similar and very few grassland species were not recorded in grassy forests. Forb species composition appears to change more rapidly after tree invasion than grass species composition. Relatively few forbs preferred forest to grassland, whereas shrubs, trees or lianes were substantially more frequent in forests. Replacement of grasslands by grassy forests would reduce landscape diversity and impact on other values in the Bunya Mountains. However, this study suggests that most, if not all, vascular plants that currently occur on the grasslands will persist in the area provided the grassy character of the eucalypt forests is maintained.

scholarly journals Distribution of planktonic foraminifera in the subtropical South Atlantic: depth hierarchy of controlling factors

2020 ◽  
Vol 17 (16) ◽  
pp. 4313-4342
Author(s):  
Douglas Lessa ◽  
Raphaël Morard ◽  
Lukas Jonkers ◽  
Igor M. Venancio ◽  
Runa Reuter ◽  
...  
Keyword(s):  
Environmental Factors ◽  
Species Composition ◽  
Vertical Distribution ◽  
Planktonic Foraminifera ◽  
Standing Stock ◽  
South Atlantic ◽  
Subsurface Water ◽  
Assemblage Composition ◽  
Sst Variability ◽  
Fossil Assemblages

Abstract. Temperature appears to be the best predictor of species composition of planktonic foraminifera communities, making it possible to use their fossil assemblages to reconstruct sea surface temperature (SST) variation in the past. However, the role of other environmental factors potentially modulating the spatial and vertical distribution of planktonic foraminifera species is poorly understood. This is especially relevant for environmental factors affecting the subsurface habitat. If such factors play a role, changes in the abundance of subsurface-dwelling species may not solely reflect SST variation. In order to constrain the effect of subsurface parameters on species composition, we here characterize the vertical distribution of living planktonic foraminifera community across an east–west transect through the subtropical South Atlantic Ocean, where SST variability was small, but the subsurface water mass structure changed dramatically. Four planktonic foraminifera communities could be identified across the top 700 m of the transect. Gyre and Agulhas Leakage surface faunas were predominantly composed of Globigerinoides ruber, Globigerinoides tenellus, Trilobatus sacculifer, Globoturborotalita rubescens, Globigerinella calida, Tenuitella iota, and Globigerinita glutinata, and these only differed in terms of relative abundances (community composition). Upwelling fauna was dominated by Neogloboquadrina pachyderma, Neogloboquadrina incompta, Globorotalia crassaformis, and Globorotalia inflata. Thermocline fauna was dominated by Tenuitella fleisheri, Globorotalia truncatulinoides, and Globorotalia scitula in the west and by G. scitula only in the east. The largest part of the standing stock was consistently found in the surface layer, but SST was not the main predictor of species composition either for the depth-integrated fauna across the stations or at individual depth layers. Instead, we identified a pattern of vertical stacking of different parameters controlling species composition, reflecting different aspects of the pelagic habitat. Whereas productivity appears to dominate in the mixed layer (0–60 m), physical properties (temperature, salinity) become important at intermediate depths and in the subsurface, a complex combination of factors including oxygen concentration is required to explain the assemblage composition. These results indicate that the seemingly straightforward relationship between assemblage composition and SST in sedimentary assemblages reflects vertically and seasonally integrated processes that are only indirectly linked to SST. It also implies that fossil assemblages of planktonic foraminifera should also contain a signature of subsurface processes, which could be used for paleoceanographic reconstructions.

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