MALACOFAUNA IN RIVER THANATOCOENOSES

Mollusc assemblages in the river thanatocoenoses were analysed at 10 sites distributed along the Mnikowska valley. Individual sites were regularly spaced 100–150 m apart (Fig. 2). Rich and diverse shell material was obtained (46 species; over 3,500 specimens). Seven taxa and 583 specimens represented shade-loving forest snails (F_F ecological group). The most abundant were three species typically found in deciduous or mixed forests: Vertigo pusilla, Aegopinella pura, and Vitrea diaphana. The remaining four taxa were found only as single specimens in a few samples. Shade-loving species typical of open forests and shrubby thickets (ecological group F_B) were represented by 6 taxa and 197 specimens. Three of them, Alinda biplicata, Gonyodiscus rotundatus, and Perforatella incarnata, were present in all samples but usually did not reach high abundance. The remaining taxa were found only as single shells. Shade-loving species typical of high-humidity habitats (ecological group F_H) constituted a significant component of the discussed fauna. Although only four taxa were included here (876 specimens), two of them, Perforatella vicina and Vitrea crystallina, reached very high abundances and were among the most important components of the fauna. Snails typical of the exposed limestone walls (ecological group O_X) appeared very rarely. Only damaged shells of Chondrina arcadica were found (7 specimens). Taxa from open environments were slightly more common (ecological group O_O; 5 taxa, 144 specimens), but except for Vallonia pulchella and Vallonia costata, these were only single shells. Dry-loving mesophilous snails (ecological group M_D) comprised only one species (Cochlicopa lubricella), found as single specimens (4 shells in total). Mesophilous snails of medium-humidity environments (ecological group M_I) were common and abundant (7 taxa, 642 specimens). Euryecological species Cochlicopa lubrica, Vitrea contracta, and Perpolita hammonis, found in almost all samples, were particularly important components. The remaining species appeared less frequently. Mesophilous snails preferring habitats with increased humidity (ecological group M_H) were represented by 4 taxa and 723 specimens. Two of them, Carychium tridentatum and Perforatella bidentata, reached large numbers, constituting an important component of the discussed fauna. The remaining taxa were found only as single shells. Hygrophilous snails (ecological group H; 4 species, 277 specimens) were relatively scarce, except for the common Zonitoides nitidus. The fauna discussed was supplemented by aquatic molluscs (7 species, 177 specimens), primarily Galba truncatula and Euglesa personata. The ecological characteristics of the malacofauna occurring in the river thanatocoenoses are presented in Figure 4.

Fig. 4

Percentage ecological malacological spectra of river (Mno) and slope (Mns) thanatocoenoses

MALACOFAUNA IN SLOPE THANATOCOENOSES

The malacofauna of the slope thanatocoenoses was analysed at 11 sites distributed along the valley slopes at the foot of the rock walls. Four of these represent the southern slope of the gorge, while the remaining seven were located on the northern slope (Fig. 2). The shell material included 45 species of land snails and 3,732 specimens. Shade-loving forest species (ecological group F_F) constituted the most important component of the assemblage (18 species and 1,329 specimens). Noteworthy was the high frequency of taxa typical of deciduous forests: Vertigo pusilla and Vitrea diaphana, which were present in all samples and reached high abundances. Snails living on shaded limestone rocks were also abundant, especially in the northern-exposed sites: Cochlodina orthostoma, Faustina faustina, and Mediterranea depressa. Species of open forests and shrubs (ecological group F_B; 7 taxa, 266 specimens) played a slightly less important role, with Alinda biplicata and Gonyodiscus rotundatus being the most important representatives of this group. Although shade-loving taxa preferring moist habitats (ecological group F_H) (Perforatella vicina and Vitrea crystallina) were found at almost all analysed sites, their numbers were low (128 specimens in total). Taxa inhabiting exposed limestone rocks (ecological group O_X) were a very important component of the assemblage. This group includes two species (Pyramidula pusilla and Chondrina arcadica), represented by 405 specimens. They occur in almost all samples and reach high numbers mainly in southern-facing sites. Snails typical of open grassland biotopes were also common and abundant (4 species and 803 specimens; ecological group O_O), particularly Vallonia pulchella and Vallonia costata. Mesophilous taxa typical of habitats with medium humidity (ecological group M_I) occurred less frequently (9 species, 329 specimens) and constituted a complementary component of the fauna. In this group, Clausilia dubia and Clausilia parvula were most abundant. Mesophilous taxa of moist habitats (ecological group M_H) were represented by the very abundant Carychium tridentatum (464 specimens in total) and a few specimens of Succinella oblonga. Two shells in a single sample of the moisture-loving taxon Vertigo geyeri (ecological group H) were also found in the analysed slope thanatocoenoses. Given their very poor state of preservation and rarity, these shells are likely to have been washed from older sediments. Early Holocene sediments containing Vertigo geyeri were described from cave fillings in the Mnikowska Valley (Alexandrowicz et al. 1992). No aquatic molluscs were detected in the analysed samples. The ecological characteristics of the malacofauna found in the slope thanatocoenoses are presented in Figure 4.

DIVERSITY WITHIN THE RIVER AND SLOPE THANATOCOENOSES

Analysis of the constancy and dominance structure of the malacofauna identified in river and slope thanatocoenoses in the Mnikowska Valley allows us to identify species groups that play the most important roles in the communities.

RIVER THANATOCOENOSES

The highest constancy and dominance classes (C = 5; D = 4) are achieved by three taxa: Vitrea crystallina, Cochlicopa lubrica, and Carychium tridentatum. The next three species (Vertigo pusilla, Aegopinella pura, and Perforatella bidentata) achieve classes C = 5 and D = 3 (Fig. 5). These snails are the most important components of this malacofauna. Vitrea crystallina and Carychium tridentatum are taxa that prefer shaded or semi-open habitats with significant humidity. Perforatella bidentata is a shade-loving and moisture-loving snail commonly found on the floodplains of rivers and streams, particularly in alder thickets. Vertigo pusilla and Aegopinella pura are common forest species typically associated with mixed or deciduous forests, and Cochlicopa lubrica is a snail with very high ecological tolerance, often occurring in large numbers in biotopes with varying degrees of shade and humidity (Wiktor 2004, Welter-Schultes 2012, Horsák et al. 2013). It is noteworthy that there is a large group of taxa that occur in almost all samples and achieve the highest constancy classes (C = 5 and C = 4). It includes up to 22 taxa (almost 50% of all identified species). This indicates a high degree of stability in the taxonomic composition of the malacofauna inhabiting the valley floor. On the other hand, only 13 taxa (approximately 25% of the fauna) appeared very rarely (C = 1), usually as single specimens. These taxa are accessory components of the assemblage and can be treated as more or less random admixtures.

Fig. 5

The structure of constancy and dominance of the thanatocoenoses from the Mnikowska Valley

All the six species mentioned above are classified as common and abundant (C>50%, D>5%), with Q index values ranging between 44.40 and 22.82 (Table 1). The next group comprises common but sparse forms (C>50%, D<5%). This includes 19 taxa representing all ecological groups, with Q values ranging from 3.63 to 21.65, forming a complementary assemblage (Table 1). The remaining 20 taxa are rare and sparse species (C<50%, D<5%) with Q values ranging from 3.69 to 0.55 (Table 1), constituting minor and accessory components of the fauna.

Table 1

Structure of the mollusc community of river thanatocoenoses from the Mnikowska Valley: D – dominance, C – constancy, Q – geometric mean of constancy and dominance

Points representing individual samples of the river thanatocoenoses on the triangular diagram are grouped in the fields corresponding to humid and shaded environments (H and F–H) (Fig. 6). Their clustering in proximity indicates uniform ecological characteristics of the malacocenoses in each of the analysed samples and, consequently, little differentiation in the characteristics of natural habitats in the Mnikowska Valley floor.

Fig. 6

Triangular diagram of river (Mno) and slope (Mns) thanatocoenoses from the Mnikowska Valley

SLOPE THANATOCOENOSES

Five taxa achieved the highest constancy and dominance classes (C = 5; D = 4 or D = 3). These snails are the most important components of the discussed malacofauna. This group includes taxa typical of grassy, open habitats with varying humidity (Vallonia pulchella and Vallonia costata), shade-loving snails typical of deciduous and mixed forests (Vitrea diaphana and Vertigo pusilla), and a mesophilous taxon preferring moist habitats (Carychium tridentatum) (Wiktor 2004, Welter-Schultes 2012, Horsák et al. 2013). Six additional species occurred in all samples but reached lower abundances (C = 5; D = 2) (Fig. 5). This group is dominated by shade-loving taxa and species of open habitats. It should be noted that high constancy classes (C = 4) and especially dominance classes (D = 3) are demonstrated by the petrophilous taxa Pyramidula pusilla, Chondrina arcadica, Clausilia parvula, and Clausilia dubia. As with the river thanatocoenoses described above, a very large group of species appears in almost all samples (C = 5 and C = 4; 17 taxa, almost 40% of the fauna). On the other hand, more than half of the identified species were rare and sparse (C = 1–3, D = 1) (Fig. 5). These values indicate that the internal diversity of slope thanatocoenoses is greater than that of river thanatocoenoses.

Organising the assemblage by Q-index values allowed us to distinguish the taxa that play the greatest role in the malacofauna of slope thanatocoenoses. Seven species are classified as common and abundant (C>50%, D>5%), with Q values ranging from 39.32 to 19.48 (Table 2). The second group comprises widespread but scarce species (C>50%, D<5%), which constitute a complementary component of the assemblage. This group includes 16 taxa, primarily shade-loving species and snails from open environments, particularly rock snails, with Q values ranging from 17.29 to 4.67 (Table 2). The remaining 21 taxa are rare species (C<50%, D<5%), with Q values ranging from 3.69 to 0.55. These are accidental and accessory components of the assemblage (Table 2). In the triangular diagram, points corresponding to the slope thanatocoenoses are grouped into fields F (shaded environments) and F-O (transitional between shaded and open environments) (Fig. 6). Their distribution indicates the existence of two subgroups of samples. Shade-loving species dominate the first, while taxa from open environments dominate the second.

Table 2

Structure of the mollusc community of slope thanatocoenoses from the Mnikowska Valley (for explanations see Table 1)

INTERRELATIONSHIPS BETWEEN THE RIVER AND SLOPE THANATOCOENOSES

The Mnikowska Valley is a narrow gorge bounded by steep rock faces. This, along with its rich malacofauna, provides exceptionally favourable conditions for research on the transport of shell material and the potential for mixing of same-aged thanatocoenoses representing very different types of environments.

At first glance, the analysed thanatocoenoses appear very similar. Both types contained a similar number of species (46 in river thanatocoenoses and 45 in slope thanatocoenoses) and a similar number of specimens (3,570 and 3,732, respectively) at similar sampling densities (10 and 11 samples, respectively). Forty-six species were found in the river thanatocoenoses, of which as many as 20 (44%) did not occur in slope thanatocoenoses. These are primarily humid-loving and aquatic taxa, with a few shade-loving species. A very similar proportion was found in the slope thanatocoenoses, where 21 of the 45 taxa (47%) were absent in the river deposits. This group primarily includes shade-loving taxa and species from open environments, particularly rocky habitats. This simple comparison clearly indicates fundamental differences in taxonomic composition.

Analysis of the dendrogram of similarities and correspondences also clearly indicates differences between the analysed types of thanatocoenoses (Fig. 7). Based on these, three types of fauna can be distinguished. The first represents the river thanatocoenoses (Fig. 7A, B). The very high similarity between the samples, ranging between 0.85 and 0.95, is noteworthy. This indicates very similar characteristics of the mollusc assemblage, resulting from similar taxonomic composition and species frequency. This indicates uniform environmental conditions on the valley floor. Correspondence analysis reveals a cluster of projection points corresponding to moisture-loving and shade/partly shade faunas. The second fauna type includes seven samples representing slope thanatocoenoses (Mns-A) (Fig. 7). It exhibits internal diversity but forms a well-defined cluster. In the correspondence analysis diagram, the points corresponding to this fauna are located in an area associated with relatively dry and shaded environments. It occurs on the northern slopes of the valley and features limestone rocks covered with dense deciduous forest. The last type of fauna includes four samples representing slope thanatocoenoses (Mns-B) (Fig. 7A, B). It is internally diverse but forms a single cluster. In the diagram illustrating the results of correspondence analysis, the points representing the above-mentioned samples form a separate cluster associated with an assemblage with a high proportion of open-country species. It was found on the southern slopes of the valley and inhabits open, dry limestone walls.

Fig. 7

Cluster and correspondence analysis of river (Mno) and slope (Mns) thanatocoenoses from the Mnikowska Valley Mno, Mns-A, Mns-B. Molluscan assemblages described in text

TRANSPORT OF SHELL MATERIAL WITHIN THE VALLEY

The transport of shell material accumulated in various types of thanatocoenoses is of significant importance for paleoecological and paleoenvironmental interpretations. The impact and role of transport are difficult to assess in taphocoenoses (accumulations of subfossil shells within deposited sediments). In the case of thanatocoenoses, it is possible to observe these processes almost directly, and consequently determine the degree of change in the composition and ecological structure of the assemblages. The possibility of such an assessment, extrapolated to analyses of subfossil mollusc assemblages, is fundamental to the accuracy of environmental reconstructions in Quaternary sediments (Briggs et al. 1990, Keen 1990, Schreve et al. 2007, Alexandrowicz & Skoczylas 2017, White et al. 2013, 2017, Alexandrowicz 2020).

TRANSPORT OF SHELL MATERIAL ACROSS A VALLEY

For considerations of shell thanatocoenoses and the interrelationships between shell material accumulated in the immediate vicinity of the riverbed and that accumulated on the valley slopes, the characteristics of the floodplain and valley floor, particularly their width and the nature of the vegetation covering them, are fundamental. The presence of permanent or periodic watercourses dissecting the valley slopes is also significant. Flood sediments, along with mollusc shells, are deposited within the floodplain and may lead to the mixing of river and slope thanatocoenoses (i.e., mixocoenoses). In large river valleys, the width of the floodplain can significantly exceed several hundred metres. In smaller stream valleys, especially in upland and mountainous areas, the width of the zone in question usually does not exceed 200 metres, and in source areas it is only a few metres wide. In the Mnikowska Valley, the floodplain extends along the riverbed throughout the analysed section. Its width ranges from 60 to 120 m (Figs 2, 3). The narrow valley width causes the accumulation zones of the river and slope thanatocoenoses to be located close to each other (20–50 m apart). They are separated by a flat valley floor covered with grass and a narrow strip of moisture-loving shrubs stretching along the streambed. The valley slopes are steep, rocky in many places, with exposed limestone walls, and covered with mixed forests. The ability to compare the taxonomic composition and ecological structure of the river and slope thanatocoenoses allows for an assessment of the scale of shell material transport across the valley. The analysed thanatocoenoses differ in both taxonomic composition and ecological structure. This is particularly evident in the case of rock species that are common and abundant in slope thanatocoenoses, found only as single, usually damaged shells in river sediments. This observation leads to the conclusion that mixing of these two types of assemblages occurs on a very limited scale, despite their proximity. The most important factors limiting the transport of shell material are the flat valley floor, the absence of permanent or periodic watercourses, and the presence of vegetation. The first two of these factors prevent the gravitational downslope movement of shells (either through slumping or water transport). The only practically available means of transport remains surface wash associated with heavy rainfall or rapid melting of the snow cover. In this case, dense vegetation, primarily grasses, becomes an effective barrier. Studies of the taxonomic composition and ecological structure of thanatocoenoses in large river valleys with wide, flat floodplains indicate a very slight admixture of shell material from the valley slopes. At the same time, the spatial differentiation of thanatocoenoses identified on the valley floor closely reflects local habitat conditions. These observations indicate a limited role for shell material transport within floodplains (Alexandrowicz 2000a, Čejka et al. 2008, Ilg et al. 2009, 2012, Jurkiewicz-Karnkowska 2009, 2019, Myšák & Horsák 2011, Juřičková et al. 2013, Horáčková et al. 2015). Similar transport constraints are also generated in the valleys of smaller rivers and streams, provided the floodplain is flat (Alexandrowicz 1997, 2002, 2021, Myšák & Horsák 2011, Juřičková et al. 2013, Horáčková et al. 2015, Podroužková et al. 2021, 2025). In mountainous areas, river valleys often have a V-shaped configuration. Floodplains are usually very narrow or absent. Consequently, there is no barrier hindering or preventing the movement of shell material from the slopes directly into the river channels. In the thanatocoenoses described in such zones, the share of shells originating from the slopes is usually large and often dominant (Alexandrowicz 2000b, 2002, 2021, Podroužková et al. 2021, 2025). In this respect, the Mnikowska Valley exhibits significant similarities to both lowland and upland rivers.

TRANSPORT OF SHELL MATERIAL ALONG A VALLEY

The extent to which mollusc shells are transported along a valley is more difficult to estimate than lateral transport. Empty shells are usually highly buoyant (they are filled with air or gases produced during the decomposition of a dead mollusc’s body). The transportability of shells depends on many factors, two of which play a decisive role (Cummins 1994, Alexandrowicz 1999, Alexandrowicz & Alexandrowicz 2011). First, the shell’s morphology (e.g., shape, size, wall thickness) determines its mechanical resistance. This factor is largely responsible for the selective destruction of shells during transport (Pip 1988, Kidwell & Flessa 1996, Chaves-Campos et al. 2012, Kidwell 2013, Rumm et al. 2018). Another important element is the nature of the valley. Gentle slopes and great width characterise lowland river valleys. As a result, even during periods of flooding, fine-grained material is transported, and the flow is calm, often nearly laminar. As a result, mechanical destruction of shells occurs on a small scale and they can be transported over considerable distances. At the same time, the rising and falling of flood waters occur relatively slowly, which allows for the segregation of shell material within the floodplain (Alexandrowicz 2000a, Pišút & Čejka 2002, Čejka et al. 2008, Ilg et al. 2009, 2012, Cernohorsky et al. 2010, Jurkiewicz-Karnkowska 2009, 2019, Myšák & Horsák 2011, Juřičková et al. 2013, Horáčková et al. 2015, Čiliak et al. 2015). Narrow valleys with steep gradients often characterise mountain and upland streams, and during periods of flooding, they generate turbulent, high-energy flows that transport coarse-grained material. Consequently, numerous impacts rapidly destroy shell material, significantly impeding its transport over longer distances. Observations conducted in this context indicate that the movement of shells occurs over distances ranging from several hundred metres (Alexandrowicz 1997, 2021) to a maximum of several kilometres (Podroužková et al. 2021, 2025). Typically, the rapid subsidence of the flood wave prevents the sorting of shell material within the floodplain. The character of the valley banks and elements directly adjacent to the channel is also crucial, particularly the characteristics of the vegetation covering these areas (Alexandrowicz 1997, 2000a, 2000b, 2002, 2021, Chaves-Campos et al. 2012, Myšák & Horsák 2011). Within the Mnikowska Valley study area, the Sanka River has a relatively low gradient and primarily transports fine-grained material. Its low, riverbed-adjacent terraces are composed primarily of fine-grained sands. The characteristics and nature of the vegetation covering the valley floor remain almost constant throughout the entire analysed section. This is the main reason for the observed low variability in the taxonomic composition and ecological structure of the river thanatocoenoses. In this respect, the Mnikowska Valley displays significant similarities to lowland rivers.