Gastropod Assemblages Research Articles

Vermeij Crushing Analysis: A new old technique for estimating crushing predation in gastropod assemblages

Vermeij Crushing Analysis allows researchers to estimate frequencies of fragmentation due to crushing predation by establishing a taphonomic fragmentation baseline using shells bearing predatory drillholes. We calculated the frequencies of several types of shell damage on groups of drilled and undrilled shells. Shells of the modern species Olivella biplicata, O. baetica, Nassarius perpinguis, and Euspira lewisii were collected at Torrey Pines State Beach, Del Mar, California. The shells were divided into drilled and undrilled groups and examined for multiple types of damage. The frequencies of damage in the drilled groups represent taphonomic damage, as the cause of death of these shells is known to be naticid gastropod predation. In cases where the damage frequencies were higher for the undrilled shells (cause of death unknown, possibly crushing), the differences in fragmentation between the two categories is attributed to crushing predation. The frequencies of damage due to crushing predation are used to estimate a minimum frequency of mortality due to crushing predation in the shell assemblage. Potential confounding factors were also examined, such as potential differences in taphonomic susceptibility and hermit crab habitation between drilled and undrilled shells. This technique bypasses the logistical difficulties of field predation studies and the problems of interpretation of shell repair frequencies, two common methods of assessing crushing predation in shelled prey. Most significantly, Vermeij Crushing Analysis can be performed on both modern and fossil shell assemblages, allowing the study of trends in crushing predation over both space and time.

First Report of a Trilobite in the Carboniferous of Eastern Pontides, NE Turkey

The pre-Mesozoic basement of the Eastern Pontides consists of a Permo-Carboniferous sedimentary sequence including the Çatalçeşme and Hardişi formations. The Çatalçeşme Formation has yielded rich assemblages of fusulinids, corals, gastropods, brachiopods, bryozoans, algae, conodonts and plants. Hitherto trilobites were unknown from this formation; here we report the discovery of a pygidium from a dark grey limestone bed near the top. Trilobites are a minor component of benthic marine communities during the Late Pennsylvanian, and to our knowledge, the discovery of this pygidium constitutes the first report of their occurrence in deposits of this age in Turkey. The morphological characteristics of the specimen, especially the number of axial rings and pleural ribs and the well-defined border (Owens 1983), suggest that it probably belongs to an undescribed species of Ditomopyge Newell, 1931 (Ditomopyginae). It also exhibits a distortion of the posterior part of the axis, and a shortening (right side) or a fusion (left side) of some pleural ribs, which is tentatively interpreted as repaired injuries or the results of a developmental dysfunction.

Escargots through time: an energetic comparison of marine gastropod assemblages before and after the Mesozoic Marine Revolution

The modern structure of marine benthic ecosystems was largely established during the Jurassic and Early Cretaceous (200–100 Ma), a transition that has been termed the Mesozoic Marine Revolution (MMR). Although it has been suggested that the MMR marks an increase in the average energy consumption of marine animal ecosystems, this hypothesis has not been evaluated quantitatively. In this study, we integrate body size and abundance data from the fossil record with physiological data from living representatives to estimate mean per capita metabolic rates of tropical to subtropical assemblages of shallow-marine gastropods—a major component of marine ecosystems throughout the Meso-Cenozoic—both before and after the MMR. We find that mean per capita metabolic rate rose by ∼150% between the Late Triassic and Late Cretaceous and remained relatively stable thereafter. The most important factor governing the increase in metabolic rate was an increase in mean body size. In principle, this size increase could result from secular changes in sampling and taphonomic biases, but these biases are suggested to yield decreases rather than increases in mean size. Considering that post-MMR gastropod diversity is dominated by predators, the net primary production required to supply the energetic needs of the average individual increased by substantially more than 150%. These data support the hypothesis that benthic energy budgets increased during the MMR, possibly in response to rising primary productivity.

Taxonomic relatedness and spatial structure of a shelf benthic gastropod assemblage

Aim Phylogenetically related species share attributes that lead to common responses to environmental conditions, but which could also produce the exclusion of species by its relatives. These processes could generate the patterns of phylogenetic attraction or repulsion in local communities, where related species would tend to coexist more or less than expected by chance. This paper aims to (1) analyse the phylogenetic structure of a benthic gastropod assemblage in the south-western Atlantic Ocean (SAO); (2) explore the linkages between phylogenetic structure and spatial distribution patterns; (3) compare outcomes driven by the analysis of presence-only data and predictive species distribution models; and (4) explore which aspects of the gained knowledge can be useful to the design of sound conservation and/or management actions. Location Uruguayan shelf and slope in the SAO. Methods Spatial patterns in taxonomical relatedness were assessed using (1) raw presence/absence data (i.e. realized niche approach) and (2) reconstruction of the potential composition of the assemblage from niche modelling (i.e. fundamental niche approach). Null models were used to test hypotheses on assemblage structure. Results Significant departures from the null hypothesis that all species were drawn from the same assemblage were observed irrespectively of the approach, indicating the existence of non-random structures. However, a high proportion of local communities can be thought as random subsets of the regional species pool. This lack of a strong signal of a taxonomic effect could be related to the absence of a linkage between taxonomic distances and ecological similarities. Main conclusions Our results suggest a random assembly of local communities from the regional species pool and/or niche filtering independent of phylogeny as main determinants of local community composition. We also suggest that local assemblages displaying significantly higher (or lower) than expected taxonomic relatedness should be taken into consideration for designing spatially explicit conservation measures.

Effects of structural diversity and identity of patches of habitat on diversity of benthic assemblages

AbstractA fundamental problem in ecology, regardless of habitat or system, is understanding the relationship between habitats and assemblage of organisms. It is commonly accepted that differences in composition and surrounding landscape of habitats affect the diversity of assemblages, although there is not much empirical evidence because of difficulties of manipulating structure in many habitats. These relationships were examined experimentally, using habitats of artificial turfs that are colonized by diverse assemblages of gastropods. Each habitat was made of nine sub‐habitats, which were sampled individually to allow tests of hypotheses about the effect of type of habitat and the influence of other adjacent sub‐habitats on the colonizing assemblage. Turf habitats were deployed for 8 weeks on a rocky shore after which they were collected and the colonizing assemblages of gastropods sampled. Independently of the types of turfs combined to form different habitats, there were more species where there was more than one type of component in a habitat (i.e. structural diversity). The type of habitat (i.e. structural identity) itself had little or no influence on the colonizing assemblage. The number of species colonizing short‐sparse and short‐dense turfs was influenced by which type of habitat was adjacent. Thus, when units of one type (e.g. short‐sparse turf) were added to a patch of habitat of long‐sparse turfs, the number of species in short‐sparse turfs was greater than in patches of the same type. This also increased total number of taxa in the whole patch of habitat. These results show how diversity of gastropods colonizing heterogeneous patches of habitat is influenced not only by the number of types of sub‐habitats, but also by interactions with surrounding sub‐habitats. These findings reiterate the importance of investigating the role of structure of habitats and of their surrounding landscapes across different systems, irrespective of their size or associated assemblages of organisms.

Malacofauna continental holocena y paleoambientes en Villa Valle María (Diamante, Entre Ríos, Argentina)

The molluscan fauna recovered from Holocene continental sediments at Villa Valle María fossiliferous locality (Diamante, Entre Ríos, Argentina), eolian in origin and dated at 2,490 ± 45 14C yrs. B.P. on Bulimulus sp, is reported. It represents the first illustrated record of Quaternary molluscs from loess sequences in Argentina. It is represented exclusively by land snails which belong to five gastropod families (Pupillidae, Succineidae, Ferussaciidae, Orthalicidae and Scolodontidae), 7 genera and 9 species: Gastrocopta nodosaria (d’Orbigny), Succinea meridionalis d’Orbigny, Cecilioides consobrina (d’Orbigny), Bulimulus apodemetes (d’Orbigny), B. bonariensis bonariensis (Rafinesque), Bulimulus sp, Naesiotus pollonerae (Ancey), N. deletangi (Parodiz), Miradiscops brasiliensis (Thiele) and Scolodonta semperi (Doering). The first record of Miradiscops is reported for Entre Ríos province. The gastropod assemblage, typical of habitats with plant cover and leaves and trunks as substrate, is similar to the modern molluscan fauna found today along southern Entre Ríos province (Mesopotamian District in the Subtropical Dominium), an area characterized by a warm-temperate steppe continental scenario. However, the associated record of S. meridionalis, typical of environments with higher humidity contents like ponds, suggests local more humid temporary environmental conditions such as palustrine habitats during the time span considered. Our results provide additional evidence regarding the occurrence of a dominant eolian event which produced the reworking and subsequent accumulation of superficial deflated sediments on the Pampean region. They add to previous models considering a dry period between 3,500 and 1,400 yrs. B.P. over the central Argentine plains.

Microbial framework in Upper Triassic (Carnian) patch reefs from Williston Lake, British Columbia, Canada

The Upper Triassic (Carnian) patch reefs from Pardonet Hill, British Columbia, Canada are the only known Upper Triassic reefal buildups from the craton margin of northwestern Pangaea. These buildups occur within the lower Baldonnel Formation and are characterized by a moderately diverse assemblage of corals, spongiomorphs, crinoids, echinoderms, bivalves, gastropods, brachiopods, and microbialites. Despite the evidence that these buildups had a rigid structure (e.g. brecciation and hardgrounds) and relief above the seafloor (visible in outcrop), the exact nature and extent of the framework has previously been undetermined. This study focuses on the microfacies within these patch reefs and highlights the importance of microbialites in the framework of these diminutive buildups (~ 100 m 3 to 500 m 3). The Pardonet Hill patch reefs were mapped in outcrop to determine their structure, geometry, and the importance of large metazoan framework builders. Samples of the reefal horizons and surrounding strata were evaluated utilizing point count analyses of organisms, cements, lithic grains, and microbial textures in thin section. Quantitative analysis of the microfacies in concert with traditional descriptions of reef construction and reef classification allow a more thorough understanding of organism abundance and reef structure. Despite the presence of corals and spongiomorphs throughout the outcrop, they very rarely form a true, intergrown framework, except at the base of Patch Reef B. Instead, cemented microbialites form a rigid framework in the core of some patch reefs. Detailed examination of the microfacies and reef construction demonstrates the importance of non-skeletal framework builders in the structural fabric of these small buildups. Pardonet Hill Patch Reef B is the first true reef with a microbial framework documented from the Upper Triassic of North America. This microbial–coral framework bears a striking similarity to some Triassic and Upper Jurassic coral–microbial reefs from the Tethys, although the depositional environment of the Pardonet Hill patch reefs is likely shallower and more energetic than the Tethyan reefs.

Gastropod evidence against the Early Triassic Lilliput effect

Size reduction in the aftermath of the Permian-Triassic mass extinction event has repeatedly been described for various marine organisms, including gastropods (the Lilliput effect). A Smithian gastropod assemblage from Utah, USA, reveals numerous large-sized specimens of different genera as high as 70 mm, the largest ever reported from the Early Triassic. Other gastropods reported from Serbia and Italy are also as large as 35 mm. Size frequency distributions of the studied assemblages indicate that they were not unusually small when compared with later Mesozoic and modern faunas. The occurrence of large-sized gastropods less than 2 Ma after the Permian-Triassic mass extinction refutes the Lilliput hypothesis in this clade, at least for the last ~75% of the Early Triassic.

Temporal dynamics of gastropod fauna on subtidal sandy sediments of the Ensenada de Baiona (NW Iberian Peninsula)

The temporal variation of the gastropod fauna inhabiting sandy sediments of the Ensenada de Baiona (Galicia, Spain) was studied at three subtidal sites from February 1996 to February 1997 by means of quantitative sampling. A total of 5,463 individuals representing 51 gastropod species and 22 families were found. The family Pyramidellidae was the most diverse in number of species (11 species), followed by Rissoidae and Trochidae (4 species each). The dogwhelk, Nassarius reticulatus, and the rissoid snail, Rissoa parva, were the numerically dominant species at the three studied sites; those and other abundant species showed their greatest densities by the end of summer and the beginning of autumn. In general, univariate measures of the assemblage (number of species, abundance, diversity and evenness) showed variations through time; greater values were recorded between summer and autumn depending on the site. Multivariate analyses done on abundance data showed certain seasonality in the evolution of the assemblage as expected for shallow subtidal sandy sediments at temperate latitudes; those seasonal changes were mostly related to variations in abundance of numerically dominant species. Although the measured sedimentary variables did not show significant correlations with faunal univariate parameters, sediment heterogeneity due to the presence of mats of Zostera marina L. and shells of dead bivalves might explain the differences in composition of the gastropod assemblage among sampling sites.

Density compensation suggests interspecific competition is weak among terrestrial snails in tabonuco forest of Puerto Rico

In the Luquillo Experimental Forest (LEF) of Puerto Rico, composition of terrestrial gastropod assemblages exhibits little temporal variation, even in response to intense hurricanes. One might therefore expect snail assemblages to be equilibrial and structured by interspecific interactions such as competition. If competition is strong and persistent throughout an assemblage, then it should manifest as density compensation (a negative correlation in population density between competitors) and be most pronounced for species pairs that are morphologically similar, as they should compete most strongly for resources. We assessed the degree to which terrestrial snails in the LEF exhibit density compensation, from both spatial and temporal perspectives. In general, density compensation was weak, especially from a spatial perspective. Moreover, the degree of density compensation was not significantly associated with dissimilarity in body size or shell shape. These results suggest that external factors such as predation or disturbance probably maintain population densities at low enough levels that the effects of interspecific competition do not become intense or persistent enough to mold patterns of assemblage structure for terrestrial snails.

Palaeoecological significance of turritelline dominated gastropod assemblage from the infratrappean beds of the Rajahmundry area, Andhra Pradesh

Palaeoecological significance of turritelline dominated gastropod assemblage from the infratrappean beds of the Rajahmundry area, Andhra Pradesh

A fossil Potadoma (Gastropoda: Pachychilidae) from Pleistocene central Kalahari fluvio-lacustrine sediments

A shell of the gastropod Potadoma was discovered in fluvio-lacustrine sediments of the Kalahari (Botswana). A late Pleistocene age of the sediments of ca. 46,000 cal. years BP was established by radiocarbon dating. It is the first record of the freshwater genus Potadoma from southern Africa. Modern counterparts as well as other Pleistocene species of Potadoma had been reported elsewhere from (palaeo-) habitats located at least 2,000 km further in the north, e.g., in the Congo Basin. So far it can be only speculated about possible causes for such a disjunct distribution. The discovery of Potadoma and five more fossil gastropods from the same sediments was used to outline palaeolimnological features. Considering the geomorphological setting, the assemblage of fossil gastropods indicates that the nowadays mainly dry Boteti River was permanently flowing through the western Makgadikgadi Pans ca. 46,000 cal. years BP. The existence of such a riverine palaeoenvironment in the pans demonstrates that the so-called Lake Palaeo-Makgadikgadi was comparatively small during that time, not spanning the area of investigation.

Oligocene and Early Miocene gastropods from Kutch (NW India) document an early biogeographic switch from Western Tethys to Indo-Pacific

Shallow marine gastropod assemblages from Chattian, Aquitanian and Burdigalian sections in the Indian Kutch Basin are described. They provide insight into the composition and biogeographic relations of the gastropod assemblages at this junction between the Western Tethys and Proto-Indo-Pacific Ocean. For the first time, an improved biostratigraphy allows a clear separation of the assemblages, especially for the hitherto undifferentiated Early Miocene faunas. Throughout the Oligocene, about one-third of the species are also frequently found in the Western Tethys, documenting a passable Tethyan Seaway for nearshore molluscs. A considerable provincialism is evident as well. The expected turnover during the Early Miocene, due to the closing of the Tethyan Seaway, is reflected in the Miocene assemblages. Surprisingly, however, the cut appears very early, i.e. already during the Aquitanian, when the West–East interrelation drops to zero despite the passage having been open during this interval. In contrast, the Burdigalian assemblages witness a minor re-appearance of Western Tethys taxa, suggesting the re-establishment of rather ineffective migration pathways prior to the final closure of the Tethyan Seaway. Cerithium bermotiense and Lyria (Indolyria) maniyaraensis are introduced as new species.

Old fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake Malawi

Studies on environmental changes provide important insights into modes of speciation, into the (adaptive) reoccupation of ecological niches and into species turnover. Against this background, we here examine the history of the gastropod genus Lanistes in the African Rift Lake Malawi, guided by four general evolutionary scenarios, and compare it with patterns reported from other endemic Malawian rift taxa. Based on an integrated approach using a mitochondrial DNA phylogeny and a trait-specific molecular clock in combination with insights from the fossil record and palaeoenvironmental data, we demonstrate that the accumulation of extant molecular diversity in the endemic group did not start before approximately 600,000 years ago from a single lineage. Fossils of the genus from the Malawi Rift, however, are over one million years older. We argue that severe drops in the lake level of Lake Malawi in the Pleistocene offer a potential explanation for this pattern. Our results also challenge previously established phylogenetic relationships within the genus by revealing parallel evolution and providing evidence that the endemic Lanistes species are not restricted to the lake proper but are present throughout the Malawi Rift.

Old fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake Malawi

You have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Schultheiß Roland, Van Bocxlaer Bert, Wilke Thomas and Albrecht Christian 2009Old fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake MalawiProc. R. Soc. B.2764083http://doi.org/10.1098/rspb.2009.1443SectionYou have accessCorrectionsOld fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake Malawi Roland Schultheiß Roland Schultheiß Google Scholar Find this author on PubMed Search for more papers by this author , Bert Van Bocxlaer Bert Van Bocxlaer Google Scholar Find this author on PubMed Search for more papers by this author , Thomas Wilke Thomas Wilke Google Scholar Find this author on PubMed Search for more papers by this author and Christian Albrecht Christian Albrecht Google Scholar Find this author on PubMed Search for more papers by this author Roland Schultheiß Roland Schultheiß Google Scholar Find this author on PubMed Search for more papers by this author , Bert Van Bocxlaer Bert Van Bocxlaer Google Scholar Find this author on PubMed Search for more papers by this author , Thomas Wilke Thomas Wilke Google Scholar Find this author on PubMed Search for more papers by this author and Christian Albrecht Christian Albrecht Google Scholar Find this author on PubMed Search for more papers by this author Published:13 May 2009https://doi.org/10.1098/rspb.2009.1443This article corrects the followingResearch ArticleOld fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake Malawihttps://doi.org/10.1098/rspb.2009.0467 Roland Schultheiß, Bert Van Bocxlaer, Thomas Wilke and Christian Albrecht volume 276issue 1668Proceedings of the Royal Society B: Biological Sciences13 May 2009Figure 1 was presented incorrectly. It is presented correctly here asFigure 1. Evolutionary scenarios relative to the ecological stability of a lacustrine system. The stability is indicated by the width of the grey surface underlying the schematic species trees (broad representing stable; narrow, unstable conditions). See text for details.Download figureOpen in new tabDownload PowerPointIn §2a, the first line of the first paragraph should read: Three endemic species of Lanistes (Ampullariidae) are described from within the lake (figure 2): Lanistes solidus Smith 1877, Lanistes nasutus Mandahl-Barth 1972 and Lanistes nyassanus Dohrn 1865.In §2c, the third sentence of the first paragraph should read: COI and LSU rRNA sequences of Pila ovata (Olivier 1804), Pila conica (Gray 1828) and of seven Lanistes specimens were obtained from NCBI GenBank (see table 1 in the electronic supplementary material). Previous ArticleNext Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsRelated articlesOld fossils–young species: evolutionary history of an endemic gastropod assemblage in Lake Malawi13 May 2009Proceedings of the Royal Society B: Biological Sciences This Issue22 November 2009Volume 276Issue 1675 Article InformationDOI:https://doi.org/10.1098/rspb.2009.1443Published by:Royal SocietyOnline ISSN:1471-2954History: Published online13/05/2009Published in print22/11/2009 License:© 2009 The Royal Society Citations and impact Subjectsevolutiontaxonomy and systematics Large datasets are available through Proceedings B's partnership with Dryad

Recolonization of a burned Mediterranean area by terrestrial gastropods

Wildfires are common natural perturbations in Mediterranean ecosystems. Their frequency and extent have changed in recent decades to become one of the main ecological problems for wildlife. The response of fauna to wildfires depends greatly on the life histories and biological traits of each species. Terrestrial gastropods have limited mobility, and their presence is restricted by the vegetal and abiotic characteristics of habitats. For this reason, they are expected to have a low ability to recolonize burned areas. We have explored their survivorship and recolonization patterns according to the cryptic-refuge and fire-edge models in a Mediterranean protected area affected by a large fire in August 2003. The low number of species recorded at burned sites demonstrates the negative effects of a wildfire on the richness of gastropod assemblages 4 years after the perturbation. However, the total number of living individuals did not vary between burned and unburned areas, suggesting an after-fire shift in dominant species from woodland to open-space species. Forest species with wide European distributions dominated in unburned sites, whereas open-space species and xerophytic Mediterranean species were present at burned sites. These differences were evident even at the burned sites closest to the unburned forest, suggesting low recolonization rates from the fire edge. By contrast, the abundance of xerophilous species as well as isolated records of mesophilous species in the burned areas suggests the survival of small populations and further recuperation after fire following the cryptic-refuge model.

Response of snails and slugs to fragmentation of lowland forests in NW Germany

Habitat fragmentation is a major cause for species loss, but its effect on invertebrates with low active dispersal power, like terrestrial gastropods, has rarely been studied. Such species can not cross a hostile habitat matrix, for which the predictions of island theory, such as positive relations between species richness and patch size, should apply. In order to test this prediction, we studied gastropod species diversity by assessing gastropod assemblage characteristics from 35 sites in 19 fragments of deciduous old-growth forests in the Lower Rhine Embayment, Germany. Assemblages differed between larger (≥700 ha) and smaller forests ( 1,000 ha, i.e., well above the size of most fragments. In conclusion, the prediction is valid only for forest species. The response to fragmentation is species specific and seems to depend on habitat specialization and macroclimatic conditions.

Gastropod Assemblages as Indicators of Sediment Metal Contamination in Mangroves of Dumai, Sumatra, Indonesia

Impacts of heavy metal concentrations in sediments on the gastropod community structure were assessed in the coastal waters of Dumai, Sumatra, Indonesia. The objective of this study was to relate the levels of heavy metal pollution with the changes of gastropod community structure in the study area. Concentrations of Cd, Cu, Pb, Zn, Ni, and Fe in surface sediments collected from five sampling stations were analyzed and correlated with the abundance, species richness, and diversity of gastropod populations. Sediments at stations with more anthropogenic activities accumulated higher concentrations of heavy metals and consequently displayed lower abundances, lower species richness and diversity; while stations with lower heavy metal concentrations in sediments showed the opposite pattern in gastropod community metrics. The above findings are complemented by significant negative correlations p < 0.05–0.01) between concentrations of Cd, Cu, Pb, Zn, and Ni in the sediments and gastropod abundance and species richness. Among the metals analyzed, Cu and Zn had strongest negative correlations with the gastropod diversity which suggests that these metals may be the most detrimental to gastropod populations in the mangrove area of Dumai coastal waters.

Does developmental mode influence distribution patterns of megabenthic gastropods from the Uruguayan shelf?

We analyzed the effect of development mode (direct vs. pelagic development) on the spatial structure of a megabenthic gastropod assemblage at a regional scale. The research was carried out along the bathymetric, saline and thermal gradients generated by the interaction between the shelf topography, the freshwater discharge of Rio de la Plata and the presence of oceanic water masses. Direct developers dominated in terms of number of species in the entire area and when the inner and outer shelves were considered separately. Species with pelagic development were not observed to be ecologically successful (i.e. have higher abundances) on the inner shelf, which suggests that high environmental variability may not necessarily favour this strategy. No patterns were found in the relative abundance of species in each developmental mode between the inner (i.e. < 50 m) and outer shelves. However, richness patterns of the two groups were differentially affected by environmental conditions. Species richness for direct developers was affected by mean annual temperature, bathymetry and longitude while pelagic developers showed evident trends in relation to temperature range and latitude. Further studies are necessary to derive general predictions concerning the relative advantages of each developmental type in relation to these gradients in ecological time scales and local or regional spatial scales.

The current state of study of the gastropods of the Mandrikovka Beds (Upper Eocene of Ukraine), with the description of a new species of Conorbis

Gastropod assemblages from the Mandrikovka Beds of two main localities (village of Mandrikovka and Chapli granite quarry) are compared. The Chapli Assemblage is much more diverse in the number of species, which are mostly small-sized, collected by washing in sieves; the use of this technique is the main reason for the apparently higher diversity of this assemblage. However, some large-sized species occur in one locality and are absent from the other. In particular, the new species Conorbis veselovi sp. nov. is only recorded in the Chapli quarry; it is here described and evidence for its assignment to the genus Conorbis is presented.