The significance of molluscs as paleoecological indicators of freshwater systems in central-western Argentina

Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 105–113

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

The significance of molluscs as paleoecological indicators of freshwater systems in central-western Argentina Claudio G. De Francesco ⁎, Gabriela S. Hassan CONICET-Centro de Geología de Costas y del Cuaternario, Universidad Nacional de Mar del Plata, CC 722, 7600 Mar del Plata, Argentina

a r t i c l e

i n f o

Article history: Received 4 September 2008 Received in revised form 15 January 2009 Accepted 19 January 2009 Keywords: Freshwater molluscs Distribution patterns Paleoecology Quaternary Mendoza Argentina

a b s t r a c t The objectives of this study were to (a) analyze the distribution pattern of molluscs in freshwater systems from central-western Argentina, (b) investigate the key environmental factors affecting the species distribution, and (c) compare Quaternary and modern mollusc assemblages in order to evaluate the extent and limitations of freshwater molluscs as paleobioindicators. A total of 45 freshwater habitats were sampled for living molluscs. At each selected site, physical and chemical parameters were quantified, and living molluscs collected. Principal Component Analysis (PCA) was used for the ordination of sampling sites based on environmental variables. To explore the relationships between environmental variables and mollusc assemblages, a Canonical Correspondence Analysis (CCA) was performed. Data on fossil mollusc assemblages from three Quaternary alluvial successions outcropping in the area were combined with the modern data set by means of a Detrended Correspondence Analysis (DCA). Eight taxa were identified: Lymnaea viator, Heleobia hatcheri, Heleobia parchappii, Heleobia aff. parchappii, Chilina mendozana, Biomphalaria peregrina, Physa acuta, and a bivalve attributed to the family Sphaeriidae. Except for H. hatcheri and P. acuta, the species were also recorded as fossils in the area. Overall, the occurrence of molluscs in central-western Argentina appeared to be related to calm vegetated shallow water bodies. With the exception of H. parchappii that only occurred in saline waters (8–16 mS cm− 1), the mollusc assemblages were restricted to low salinities (below 1.9 mS cm− 1). Two main groups, indicative of different energetic environmental conditions related to the stability of the flow regime, were recognized: (1) L. viator, P. acuta, and B. peregrina occurred at lentic habitats (shallow lakes) or calm backwaters (pools) within lotic systems such as streams and rivers, (2) H. hatcheri, H. aff. parchappii, C. mendozana, and Sphaeriidae dominated in lotic habitats where a directional water flow exists. These same groups were recognized in the fossil record allowing the reconstruction of habitats that differed in the magnitude of the water flow. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Molluscs are among the most ubiquitous fossils present in Quaternary non-marine sediments, being found in a wide variety of deposits including fluvial, lacustrine, glaciolacustrine and paludal sediments (Miller and Bajc, 1990). They constitute an invaluable source of data for reconstructing past environments, in particular the hydrodynamics of past water bodies. However, the absence of ecological data for many species as well as a general inadequacy in our understanding of the factors controlling their distribution has limited their broad use as paleoecological indicators. This situation is particularly worrying in Argentina because the ecology of freshwater molluscs has been poorly studied and the ecological requirements of species remain in many cases nearly unknown. This lack of modern ⁎ Corresponding author. Tel.: +54 223 4754060; fax: +54 223 4753150. E-mail addresses: [email protected] (C.G. De Francesco), [email protected] (G.S. Hassan). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.01.003

information significantly restricts the quality and extent of paleoenvironmental information that can be gathered from the mollusc assemblages preserved in Quaternary alluvial successions. Recently, paleoecological analysis of Pleistocene and Holocene alluvial successions cropping out in the semiarid region of Mendoza (33–35° S, 69° W) revealed the occurrence of at least six mollusc species. As most of them are common in freshwater habitats from other geographic regions of Argentina (Pampa or Patagonia) the scarce ecological information available was used to reconstruct the past hydrodynamics of these successions. As a result, the Quaternary mollusc assemblages preserved in the basin of Río Tunuyán (northern Mendoza), which were dominated by semi-aquatic snails (Lymnaea viator), suggested the development of damp habitats that were occasionally submerged (De Francesco and Zárate, 2006; De Francesco et al., 2007). On the other hand, the mollusc assemblages preserved in the basin of Río Atuel (southern Mendoza) that showed a higher diversity, with the presence of species not represented in northern Mendoza (e. g., Chilina mendozana), suggested the development of

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C.G. De Francesco, G.S. Hassan / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 105–113

shallow vegetated habitats subject to episodic flooding events although deeper than those represented in the north (Dieguez et al., 2004; De Francesco and Dieguez, 2006). Because of the lack of information on modern species distributions in the area, we cannot predict either the magnitude of the water bodies developed in the past or the mean values of the main environmental parameters that may have characterized them. Some of the species represented as fossils were recently recorded living in Bañado Carilauquen, a tributary of Llancanelo Lake in southern Mendoza (Ciocco and Scheibler, 2008). In this exploratory work, the distribution of molluscs appeared to be related to a gradient of conductivity and hardness. Yet, as Bañado Carilauquen constitutes only an isolated case within the total range of freshwater habitats in Mendoza Province, this study does not provide information on the factors that affect the distribution of molluscs at a regional scale. At present, there is no available information on the distribution of freshwater molluscs in other types of freshwater systems (streams, rivers, shallow lakes, dams, canals) in Mendoza Province. Therefore, an understanding of the factors influencing local distribution of modern species in Mendoza becomes a key issue in assessing the value of freshwater molluscs as paleoecological indicators in the area. The aim of the present study is to assess the paleoecological significance of freshwater molluscs in the semiarid region of Mendoza in order to support paleoenvironmental reconstructions in the area. In particular, the present contribution aims to: (a) analyze in detail the distribution pattern of molluscs in freshwater systems of the area, (b) investigate the key environmental factors affecting the species distributions, and (c) compare Quaternary and modern mollusc assemblages in order to infer the nature of past habitats and, thereby, evaluate the extent and limitations of freshwater molluscs as paleobioindicators.

2. Study area The province of Mendoza lies within the dominion of the South American Arid Diagonal, a region considered to have been climatically sensitive to the latitudinal shift of the Pacific and Atlantic anticyclonic centres during the late Pleistocene and the Holocene (Abraham de Vázquez et al., 2000). The climate of the region is semiarid with a main annual rainfall of 250 mm in the eastern piedmont (Capitanelli, 2005). Regionally, the study area is cut across by several fluvial systems (Tunuyán, Diamante, and Atuel rivers) with their headwaters located in the high Andes Cordillera (Fig. 1). The river water discharges depend on glacial melting and the mainly winter precipitation of Pacific origin. When reaching the piedmont, these rivers form very extensive and complex alluvial fans. On the margins of these rivers several Quaternary alluvial deposits composed of mollusc shells are continuously exposed along several kilometers (Zárate, 2002). 3. Materials and methods 3.1. Modern dataset 3.1.1. Field sampling In total, 45 sites were selected to represent the maximum heterogeneity of aquatic environments in the area (Fig. 1), that is, the whole range of variation in morphology, substrate, and flow (e.g., streams, rivers, shallow lakes, ponds, dams, canals). As the study aimed to understand the value of molluscs as paleoecological indicators in the area, all the selected sites were located within the same area in which Quaternary molluscs crop out (see De Francesco and Dieguez, 2006; De Francesco et al., 2007). The basins of Río Tunuyán, Río Diamante, Río Atuel, and Río Grande were included. The sites selected represented a wide range of disturbance levels

Fig. 1. Location map of modern (circles) and Quaternary (squares) study sites in central-western Argentina.

C.G. De Francesco, G.S. Hassan / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 105–113

(anthropogenic impact) from almost pristine environments (Andes Cordillera) to streams running across cities or through areas of low to moderate cattle-raising and agriculture (basin of Río Tunuyán). At each site, we quantified current velocity, water temperature, pH, and conductivity. Current velocity was measured using a neutrally buoyant sphere and calculating the time it took the sphere to move 5 m. Temperature, pH, and conductivity were measured with field instruments. Aquatic vegetation cover at each sampling site was estimated visually, and a nominal variable erected for statistical purposes (0 = no vegetation, 1 = low vegetation cover, and 2 = high vegetation cover). One subsurface water sample (1 l) was taken at each site to assess chemical parameters. Water samples were collected in polyethylene bottles and stored in ice until they were transported to the laboratory. A sediment sample (ca. 0.5 kg) was taken for grain size analysis and organic content. We carefully inspected up to 20 m of the shores of every water body investigated, searching for molluscs. Living molluscs were searched for among the submerged vegetation, under stones, or on the substratum. Molluscs were collected both by hand and with the aid of sieves (0.5 mm). In addition, surrounding land was searched for empty shells. Those sites where neither live specimens nor dead shells were present were considered as uninhabited by molluscs. Because of the different substrata sampled (surface, submerged and emergent vegetation), relative abundance of molluscs was estimated by reference to search effort (number of snails caught per hour) following Martín et al. (2001). The collected molluscs were identified back at the laboratory. 3.1.2. Laboratory analyses Water samples were analyzed within fifteen days of collection for − concentrations of nitrate (NO−3), sulfate (SO2− 4 ), chloride (Cl ), fluoride −2 − (F−), phosphate (PO3− 4 ), carbonate (CO3 ), bicarbonate (HCO3), magne2+ 2+ sium (Mg ), calcium (Ca ), and silica (SiO2). Chemical analyses were

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performed using standard methods: chloride following the Mhor method, sulfate by turbidimetry, calcium and magnesium by complexometric titrations with EDTA, potassium by flame spectrometry, silica by the silicomolibdate method, fluoride by the zirconyl chloride method and nitrate by the brucine method (APHA, 1992). Sediment grain size was analyzed with the dry-sieving technique of Folk (1968). It was quantified as the proportion of gravel (N2 mm), coarse sand (N500 μm), medium sand (250–499 μm), fine sand (125– 249 μm), very fine sand (62–124 μm), and mud (silt and clay, b62 μm). Additionally, the organic content of each sample was estimated using the loss-on-ignition method (LOI) for 4 h at 550 °C and water content (humidity) was calculated by drying the sediment for 24 h at ca. 105 °C (Heiri et al., 2001). Molluscs were identified to species (when possible) and their relative abundance calculated. Gastropod identification was based on de Castellanos and Gaillard (1981), de Castellanos and Landoni (1981), Fernández (1981), Gaillard and de Castellanos (1976), and Rumi (1991). 3.2. Application to the fossil record Fossil data were obtained from previous published works. Three different alluvial successions were included: (1) La Bomba section (LB) (De Francesco et al., 2007), (2) Puesto Moya (PM), and (3) Puesto Vicencio (PV) (De Francesco and Dieguez, 2006). The LB succession, which is Pleistocene, is located in the proximity of Río Tunuyán, whereas PM and PV, with Holocene shells, are located in southern Mendoza (Fig. 1). All discrete levels (n = 17) containing mollusc shells were considered. 3.2.1. La Bomba (Pleistocene) This alluvial succession (33°28′ S, 69°03′ W) crops out on the right margin of Arroyo La Estacada, a tributary of Río Tunuyán (in the area of modern sites 1 and 2; Fig. 1). The succession was analyzed in detail by

Fig. 2. Shells of the Quaternary and extant freshwater mollusc taxa identified in central-western Argentina. A: Lymnaea viator; B: Biomphalaria peregrina; C: Chilina mendozana; D: Heleobia parchappii; E: Heleobia aff. parchappii; F: Sphaeriidae; G: Heleobia hatcheri; H: Physa acuta. Scale bars: 1 mm.

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De Francesco et al. (2007). The stratigraphic section, mainly composed of fine sands, interbedded with levels of silty clays and organic matter, records the interval between circa 35 14C ka B.P. and 31 14C ka B.P. and has a total height of 3 m. It consists of 14 discrete levels containing freshwater mollusc shells, all of which were included in the present analysis (LB1–LB14). Shells showed an excellent preservation (Fig. 2). Most specimens were complete, exhibiting neither abrasion nor dissolution, which suggests that they were deposited in the same environment where they lived. For further information on stratigraphy and paleoecology see De Francesco et al. (2007). 3.2.2. Puesto Moya and Puesto Vicencio (Holocene) These two alluvial successions crop out on the right margin of Río Atuel in southern Mendoza, in the proximity of Laguna Llancanelo, in an area of broad meandering sections. The mollusc assemblages have been analyzed in detail by De Francesco and Dieguez (2006). The PM succession is late Holocene whereas the PV succession is early Holocene. Only one level from PM and two levels from PV (PV4 and PV7) contained mollusc shells, and were included in the present analysis. The PM succession (35°15′ S, 69°14′ W) has a total height of 4.67 m and is composed of 8 sedimentary levels, of which only one (81–112 cm depth) contained freshwater mollusc shells. This level corresponds to a hydromorphic paleosol. The PV succession (35°13′ S, 69°06′ W) has a total height of 7.84 m and is composed of 9 sedimentary levels, of which two (PV4 and PV7) contained mollusc shells. PV4 (97–207 cm) is a sedimentary bank composed of clayey sediments, with sand interbedded. PV7 (located at 310–338 cm) is a hydromorphic paleosol, similar to that in PM. Shells from PM and PV exhibited significant shell abrasion (Fig. 2). In addition, most specimens were fragmented, suggesting some degree of post-mortem transportation. Therefore, assemblages from PM and PV probably represent deposits transported by currents that were dropped on the flood plain. For further information on stratigraphy and paleoecology see De Francesco and Dieguez (2006). 3.3. Data analysis Standard product-moment correlation analyses were conducted to identify strongly intercorrelated environmental variables, permitting some of them to be omitted from subsequent statistical analyses. Principal Component Analysis (PCA) was used for ordination of sampling sites based on the reduced set of environmental variables measured. Environmental data were log transformed, as log (x + 1). For each site inhabited by molluscs, the species richness (S), Shannon diversity index (H′), Simpson's diversity index (D), species evenness (E), and equitability (J) were calculated. In order to test for differences between northern and southern sites (Río Tunuyán versus Río Atuel basins), Mann–Whitney tests were performed. All these analyses were carried out with the computer program PAST v 1.81 (Hammer et al., 2008). To explore the relationships between environmental variables and the mollusc assemblages, a Canonical Correspondence Analysis (CCA) was performed. A series of partial CCAs, run with one explanatory variable at a time, was used to separate the total variation in mollusc abundance into components that represent the unique contributions of individual environmental variables, the contribution of covariance between variables and the unexplained variance (Bocard et al., 1992). Statistical significance was assessed by unrestricted Monte Carlo tests (full model) involving 999 permutations at p ≤ 0.001. Fossil mollusc assemblages were combined with the modern data set by means of Detrended Correspondence Analysis (DCA). In order to define groups of major similarities, a Cluster Analysis (CA) based on the Bray–Curtis similarity measure was performed with the program PAST ver. 1.81 (Hammer et al., 2008). All ordinations (except CA) were performed using the program CANOCO version 4.5 (ter Braak and Šmilauer, 1998).

4. Results 4.1. Modern mollusc fauna Only 25 of the 45 sites had live molluscs. In total, 3278 specimens belonging to 8 species (7 gastropods and 1 bivalve) were recorded in the studied sites. The gastropods were Lymnaea viator (Lymnaeidae), Heleobia hatcheri, Heleobia parchappii, Heleobia aff. parchappii (Cochliopidae), Chilina mendozana (Chilinidae), Biomphalaria peregrina (Planorbidae), and Physa acuta (Physidae). The bivalve was attributed to the family Sphaeriidae (Fig. 2). Recently, Ciocco (2008) pointed out the presence of the bivalve Pisidium chiquitanum in Mendoza, which may correspond to the same species recorded here as Sphaeriidae. However, we need to perform additional comparative studies in order to reliably assign these specimens to P. chiquitanum. The three commonest species were H. hatcheri, L. viator and P. acuta. The species richness, Shannon diversity, Simpson's diversity, species equitability, and evenness indices were not significantly different between northern and southern sites (Table 1). Therefore, it can be concluded that the assemblages represented in the different areas are not significantly different, although in many cases the taxonomic composition differed among sites even within the same stream. 4.2. Environmental variables The standard product-moment correlation analysis of environmental data permitted reducing the 25 variables (Table 2) to 13. Hardness, Ca2+, Mg2+, Cl− and SO2− 4 showed a significant correlation − (r N 0.64, p b 0.001) with conductivity. CO2− 3 , and HCO3 were correlated with pH (r N 0.51, p b 0.001). Coarse sand was correlated with gravel and very fine sand (r N 0.62, p b 0.001). Medium sand was correlated with fine sand (r = 0.58, p b 0.001). Mud was correlated with gravel, fine sand, very fine sand, humidity, and LOI (r N 0.61, p b 0.001). Therefore, the reduced matrix included conductivity, pH, temperature, − 3− depth, current velocity, vegetation cover, NO2− 3 , PO4 , F , SiO2, coarse sand, medium sand, and mud. The first two components of the PCA ordination (Fig. 3A) accounted for 63.8% of the variation in the data. The first axis explained 44.1% of the total variation and exhibited a very high positive correlation (r = 0.99) with vegetation cover (Fig. 3B) and, to a lesser extent, with SiO2, medium sand, and mud (r ~ 0.37). In addition, this axis was negatively correlated with conductivity (r = −0.44), current velocity (r = −0.32) and depth (r = −0.20). The second axis, which explained 19.7% of total variation, was positively correlated with mud (r = 0.81), depth (r = 0.44), and NO2− 3 (r = 0.36), and negatively with coarse sand (r = 0.87) and medium sand (r = 0.50). The first axis allowed differentiating sites inhabited by molluscs from uninhabited ones. In fact, the sites inhabited by molluscs were located on the right side of the plot, and characterized as shallow (b1 m) highly vegetated water bodies with low conductivity (b1.9 mS cm− 1) and nil or low current velocity (0–0.7 m s− 1). In addition, these sites displayed finer sediments and were more enriched in SiO2. On the other hand, those sites characterized by nil or low vegetation cover, deeper

Table 1 Mean values, standard deviations (SD) and range of the species richness (S), Shannon diversity index (H′), Simpson diversity index (D), evenness (E), and equitability (J) in northern (n = 12) versus southern (n = 13) sites (n. s.: not significant) Site

Richness (S) Shannon (H′) Simpson (D) Evenness (E) Equitability (J)

Northern sites

Southern sites

Range

Mean ± SD

Range

Mann– Whitney (U)

p

Mean ± SD 2.42 ± 1.08 0.40 ± 0.31 0.23 ± 0.20 0.73 ± 0.23 0.40 ± 0.32

1–4 0.00–0.99 0.00–0.58 0.42–1.00 0.00–0.99

2.23 ± 1.09 0.37 ± 0.39 0.22 ± 0.24 0.76 ± 0.19 0.35 ± 0.33

1–4 0.00–0.95 0.00–0.58 0.54–1.00 0.00–0.71

70 60 76 68 70

0.68 (n.s.) 0.34 (n.s.) 0.93 (n.s.) 0.60 (n.s.) 0.68 (n.s.)

Table 2 Values obtained for environmental variables in the 45 sampling sites. Cond: conductivity (mS cm− 1), T: water temperature (°C), Depth (cm), Cu: current velocity (m s− 1), Veg: vegetation cover, Hard: hardness (mg l− 1 of CaCO3), Hum: humidity (%), LOI: organic content (%). Categories of grain size (gravel, CS: coarse sand, MS: medium sand, FS: fine sand, VFS: very fine sand, and mud) are expressed in %. Concentrations of ions are expressed in mg l− 1. Altitude is expressed in meters above sea level Altitude

Cond

pH

T

Depth

Cu

Veg

1—La Estacada 2—Puente El Zampal 3—Puente Roto 4—Arroyo Guajardino 5—Guajardino (nac.) 6—Río Las Tunas 7—Torrecillas 1 8—Río Las Tunas 2 9—Río Las Tunas 3 10—Arroyo medio 1 11—Arroyo medio 2 12—Torrecillas 2 13—Vista Flores 14—Río Tunuyán 15—Arroyo Guiñazu 16—Arroyo Caroca 17—Acequia Claro 18—Arroyo Claro 19—Arroyo Yaucha 20—Presa del Tigre 21—Arroyo Gaby 22—Presa Nihuil 23—Los Molles 24—Río Salado 25—Vertiente Molles 26—Niña Encantada 27—Llancanelo 28—Agua Botada 29—Bañados Grande 30—Río Malargüe 31—Vertiente Malargüe 32—El Chacay 33—Laguna Blanca 34—Arroyo Sosneado 35—Vertiente Sosneado 36—Laguna Sosneado 1 37—Laguna Sosneado 2 38—Vertiente Cañon 39—Río Atuel 40—Salinas Diamante 41—Las Aguaditas 42—Monte Comán 43—Salina La Horqueta 44—Río Desaguadero 45—Arroyo Bebedero

860.1 923.9 936.8 927.3 1080.3 1143.9 1178.5 927.3 908.4 932.9 932.9 943.6 965.4 893.0 868.2 871.5 874.4 874.4 973.9 914.5 863.4 1300.5 1938.1 1932.8 1956.4 2092.9 1336.1 1969.4 1593.4 1538.1 1537.6 1425.4 1637.8 1612.8 1790.3 2112.6 2137.2 934.3 1068.2 1290.5 552.9 523.9 411.1 410.1 402.7

1.15 1.41 1.40 1.01 0.33 0.30 0.17 0.10 0.64 1.69 0.75 0.51 1.21 0.82 0.74 0.81 0.76 1.18 0.21 0.71 0.76 1.16 0.79 2.05 0.62 0.76 8.68 0.59 1.05 0.55 0.72 0.34 3.97 0.79 0.80 0.22 0.22 2.11 1.47 30.00 1.86 1.40 9.83 1.70 16.00

8.16 8.24 8.31 8.05 8.25 8.25 8.10 7.89 8.06 7.91 8.05 8.16 7.87 8.22 8.10 7.88 8.10 7.93 7.90 7.71 8.17 8.04 7.81 7.85 7.47 7.72 7.87 7.92 7.34 8.00 7.93 8.10 8.04 8.11 8.24 9.40 9.00 8.19 7.55 7.55 7.81 8.11 8.26 8.17 7.66

21.2 16.7 25.8 20.7 13.5 22.8 16.7 19.9 18.2 18.8 17.1 15.6 16.0 18.2 16.5 17.0 16.7 18.1 10.1 19.0 19.3 18.3 21.6 14.9 14.5 11.7 19.9 19.9 14.3 16.5 16.0 11.2 15.6 14.3 19.3 22.3 15.9 18.9 19.8 16.1 15.5 18.4 20.8 19.9 20.6

33 34 43 27 6 25 32 125 40 15 150 80 6 200 49 75 15 50 43 20 85 20 26 150 3 55 25 10 15 150 5 28 60 14 7 20 50 10 50 15 50 150 35 150 20

0.30 0.30 0.60 0.55 0.00 1.50 1.00 0.00 1.00 0.00 0.27 0.27 0.10 1.50 0.43 1.00 0.67 0.67 1.35 0.38 0.16 0.00 0.00 1.00 1.00 0.67 0.00 0.75 0.00 0.35 0.10 1.10 0.00 0.17 0.17 0.10 0.00 0.17 0.67 0.00 0.10 0.67 0.00 0.40 0.00

0 0 0 1 2 0 1 2 2 2 2 1 1 0 2 2 2 1 0 2 2 2 2 0 0 2 0 0 2 0 2 2 0 2 2 2 2 0 0 0 2 0 0 0 0

Cl− 66.0 112.0 112.0 66.0 13.1 13.1 19.8 59.3 26.3 85.7 39.5 29.6 145.0 102.2 26.3 59.3 52.7 115.4 9.9 85.7 98.9 165.0 115.4 646.4 214.0 26.3 427.4 13.2 58.2 49.4 27.0 10.0 465.0 39.5 26.3 46.1 49.4 178.1 185.0 30000 335.7 234.1 2440.0 442.0 7717.0

SO2− 4

NO−3

PO3− 4

F−

CO2− 3

HCO−3

SiO2

Ca2+

Mg2+

Hard

Hum

LOI

Gravel

CS

MS

FS

VFS

Mud

610.0 795.0 650.0 455.0 123.0 127.0 19.6 376.0 280.0 20.2 286.0 176.0 525.0 393.0 324.0 204.5 202.7 481.0 46.7 315.0 117.0 67.8 68.8 17.8 11.2 392.0 1670.0 174.0 253.2 156.0 124.0 114.0 90.0 256.0 342.0 79.5 58.1 690.0 692.0 51530 680.0 4775.0 4405.0 118.0 2140.0

14.80 7.93 10.00 11.34 6.80 0.00 0.50 0.50 0.50 0.50 5.35 5.35 0.50 5.50 0.00 0.50 3.75 2.46 3.65 0.00 11.70 3.75 4.97 3.23 0.96 12.8 1.41 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11.86 0.00 0.00 1.37 2.51 0.00 0.00 0.00

0.21 0.10 0.12 0.04 0.01 0.00 0.00 0.00 0.00 0.94 0.22 0.00 0.21 1.18 2.54 0.00 1.56 1.89 0.40 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.00 0.00 0.00 0.00 0.08 0.13 0.00 0.60 0.00 0.00 0.00 0.05 0.27 0.30 0.02

0.54 1.69 1.70 1.93 1.45 0.67 1.12 2.89 1.76 2.15 1.24 1.48 0.70 1.03 1.54 2.38 1.51 1.91 0.00 1.62 1.17 1.17 1.55 1.45 0.64 0.70 2.42 2.35 0.76 0.75 1.30 0.40 4.88 0.69 1.45 0.50 0.00 1.61 0.61 1.88 0.52 0.84 0.05 0.00 1.39

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 50.8 50.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

216.3 370.0 216.3 340.0 123.6 69.5 108.1 340.0 231.7 494.4 239.4 216.3 409.4 154.5 301.2 224.0 193.1 278.1 231.7 131.3 168.0 168.0 594.0 92.7 610.0 231.0 139.0 471.0 410.0 285.0 278.0 162.2 84.0 160.0 226.0 58.6 67.0 595.0 243.0 150.8 360.0 234.6 142.0 217.8 150.8

46.4 63.3 34.0 18.5 10.7 10.8 12.6 62.8 48.0 86.5 56.3 42.5 61.4 16.5 48.7 54.6 53.0 52.8 20.0 12.7 11.6 11.0 34.5 9.7 16.5 28.5 5.6 41.3 35.5 42.6 38.0 17.0 21.2 34.4 50.7 43.7 32.6 17.8 22.2 13.2 37.8 36.6 10.0 16.4 72.7

44.0 49.7 38.3 35.6 10.0 16.8 12.0 38.9 34.9 103.5 22.0 22.2 33.8 21.2 23.2 28.2 23.2 28.5 17.2 20.5 35.1 76.3 31.8 62.4 62.4 133.0 90.0 16.0 25.2 27.2 13.3 5.97 43.0 27.2 36.5 17.2 18.0 21.3 33.6 260.0 22.6 34.0 51.8 29.8 345.0

64.3 125.7 114.0 48.7 38.1 34.0 17.7 1.6 68.5 200.0 63.8 79.0 88.0 68.0 43.6 83.8 27.6 50.4 28.1 63.1 58.8 105.6 58.4 52.5 43.6 1.08 89.7 53.2 97.0 42.2 51.5 44.0 356.0 83.2 60.5 24.3 26.2 69.7 172.0 1858.5 200.6 72.6 324.0 158.4 695.0

378.0 648.0 571.0 292.0 184.0 184.0 104.1 606.0 373.0 1095.0 321.0 385.0 451.0 336.0 240.0 420.0 173.0 281.4 160.0 314.0 333.0 630.0 323.0 375.0 338.0 337.0 598.0 262.0 467.3 244.0 248.0 198.0 1546.6 415.0 343.6 144.4 154.4 344.2 800.3 8391.6 892.6 387.6 1481.5 734.5 3764.0

19.98 21.67 20.01 20.80 57.87 8.35 20.00 42.65 40.00 40.97 68.61 27.01 41.78 20.81 37.89 46.22 35.80 35.80 20.74 9.92 24.34 10.55 49.89 22.80 4.67 47.58 34.19 9.92 67.52 21.33 26.66 7.61 19.46 24.49 17.09 56.09 64.74 17.16 10.62 20.38 21.12 27.31 15.94 27.06 32.95

1.90 2.02 1.65 1.19 7.87 1.32 1.70 4.70 5.00 7.99 5.31 2.37 7.81 0.85 4.27 3.53 2.55 2.55 0.88 1.00 3.95 1.15 4.32 1.74 1.21 3.33 2.95 0.65 8.45 2.23 2.23 0.96 0.94 2.38 1.69 5.58 5.84 1.18 1.30 1.37 3.52 1.78 1.04 1.95 5.44

76.24 76.24 76.24 41.51 0.00 74.56 63.35 0.00 0.00 17.95 0.00 0.00 3.79 52.15 7.80 8.05 8.80 8.80 0.00 74.56 0.45 74.56 23.58 0.00 94.81 23.58 0.27 61.60 11.94 63.34 0.00 64.81 22.20 26.36 26.69 0.00 3.24 67.28 44.37 5.27 38.26 0.00 9.24 0.00 0.00

2.77 2.77 2.77 5.31 0.00 15.08 9.51 0.76 0.76 8.23 0.93 0.00 1.84 0.29 10.23 5.21 6.91 6.91 0.57 15.08 3.74 15.08 19.35 0.59 3.18 19.35 5.05 26.34 13.47 9.51 1.61 8.08 8.31 13.68 11.64 1.04 7.12 27.56 40.82 7.28 17.07 0.00 16.30 0.00 1.68

1.19 1.19 1.19 11.42 25.55 8.88 7.72 1.60 1.60 18.93 13.48 0.05 6.88 2.13 20.14 14.61 13.27 13.27 15.70 8.88 7.56 8.88 15.14 16.21 1.05 15.14 8.45 8.14 26.31 7.73 16.60 6.85 34.65 19.22 12.69 10.99 14.88 3.68 11.74 6.86 18.17 1.82 7.54 0.05 32.62

2.97 2.97 2.97 18.51 25.57 1.33 9.88 12.75 12.75 17.84 24.31 2.16 19.93 17.04 26.36 19.73 25.46 25.46 50.96 1.33 35.08 1.33 13.62 57.20 0.69 13.62 21.52 3.30 26.58 9.88 26.89 10.13 34.39 24.04 26.69 28.30 22.90 0.71 2.21 18.78 14.77 28.27 34.95 2.36 21.54

6.23 6.23 6.23 11.76 24.02 0.13 5.12 37.85 37.85 16.19 23.09 26.40 19.30 18.34 16.54 18.43 19.72 19.72 24.47 0.13 30.52 0.13 10.15 14.21 0.18 10.15 23.08 0.41 12.49 5.12 26.12 4.29 0.22 9.41 13.14 34.54 25.12 0.31 0.39 41.04 5.67 49.10 22.54 29.79 23.04

10.59 10.59 10.59 11.47 24.85 0.02 4.42 47.02 47.02 20.85 38.19 71.39 48.25 10.05 18.92 33.96 25.83 25.83 8.29 0.02 22.64 0.02 18.16 11.78 0.08 18.16 41.63 0.21 9.20 4.42 28.78 5.83 0.21 7.29 7.14 25.13 26.74 0.46 0.48 20.77 6.06 20.81 9.42 67.80 21.12

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Site

109

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Fig. 3. (A) Principal components analysis (PCA) biplot for sites and environmental variables. Sites inhabited by molluscs are indicated by white circles, uninhabited sites by black circles (B) Correlations of environmental variables with the first component of the PCA.

(N1.5 m), or displaying higher conductivity (N4 mS cm− 1) and current velocity (N1 m s− 1) were uninhabited by molluscs and located on the left side of the PCA plot. The unique sites with high conductivity values (8–16 mS cm− 1) that contained live snails were represented by Laguna Llancanelo (27) and Arroyo Bebedero (45). Both sites contained only specimens of the mud snail Heleobia parchappii, not represented at any other site. As these two sites represent outliers from the modern data set (because they represent brackish water conditions) they were discarded from subsequent statistical analyses. Although site 25 displayed low conductivity, it was located on the left side of the plot mainly because of the absence of vegetation and the exclusively dominance of hard substrata (boulders).

sediments. In addition, they appeared to be related to higher temperatures (13.5–22.3 °C) and higher depths (6–125 cm). On the other hand, Heleobia hatcheri, Chilina mendozana, Heleobia aff. parchappii and the sphaeriids dominated in less vegetated lotic water bodies with coarser sediments and lower pH (7.3–8.2). In addition, these species appeared to be more tolerant to slightly higher conductivity (0.5–1.2 mS cm− 1), and lower temperatures (14.3–19 °C).

4.3. Relationships between mollusc assemblages and environmental variables The variables of the reduced matrix obtained from standard product-moment correlation analyses were used as representatives of the environmental data to relate to the abundance of molluscs in the CCA (Fig. 4). The 13 variables explained 78% of variation in the data. The unrestricted Monte Carlo test for all canonical axes was significant (F = 2.46; p b 0.001). Partial CCAs showed that the total explained variance was mainly composed of pH (18.2%), NO−3 2 (12.3%), conductivity (11.8%), temperature (10.7%), F− (9.8%), mud (6.9%), coarse sand (6.9%), depth (6.7%), vegetation cover (6.6%), SiO2 (6.4%), and current speed (4.8%). The first two axes of the CCA accounted for 43.7% of variation in the abundance of molluscs and 55.9% of the species– environment relationship (Table 3). The species–environment correlations of CCA axis 1 (r = 0.96) and axis 2 (r = 0.95) were high and indicated a strong relationship of mollusc abundance to environmental variables. The CC1× CC2 plot showed that the species Lymnaea viator, Physa acuta, and Biomphalaria peregrina dominated in highly vegetated sites −1 characterized by high pH (7.7–9.4) and NO2− 3 (0.5–12.8 mg l ), on fine

Fig. 4. Canonical correspondence analysis (CCA) ordination plot showing the relationship between sampling sites (white circles), species (black squares) and environmental variables (arrows). For variable abbreviations see Table 1.

C.G. De Francesco, G.S. Hassan / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 105–113 Table 3 Summary statistics for the first two axes of CCA, with the 13 selected environmental factors CCA axes Eigenvalues (λ) Species–environment correlations Cumulative % variance – Of species abundance – Of species–environment relationship Total inertia Sum of all canonical eigenvalues

Axis 1

Axis 2

0.766 0.959

0.697 0.954

22.9 29.3

43.7 55.9 3.35 2.61

4.4. Application to the fossil record In total, 4831 fossil shells were combined with the modern data set. According to the cluster analysis and DCA, 7 groups were recognized, of which only 3 included both living and fossil samples (Fig. 5). Results showed that fossil level PV7 was similar to modern sites 9, 16 and 22.

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These sites represent streams dominated by coarse sediments (mainly boulders), moderate conductivity (0.87 ± 0.26 mS cm− 1), and mean depths of 45 cm. They were dominated by Heleobia aff. parchappii and H. hatcheri. On the other hand, fossil level PV4 (located above PV7) was similar to modern sites 26, 34 and 20. These sites represent shallower vegetated streams (mean depth= 29 cm) characterized by moderate water velocity (0.41 ± 0.25 m s− 1) and lower conductivity (0.75 ± 0.04 mS cm− 1). They were dominated by H. hatcheri, C. mendozana and Sphaeriidae. Most levels from the LB section (LB1–LB3, LB7–LB9; LB10, LB12, LB13) and PM were similar to modern site 36 (Laguna El Sosneado). This freshwater vegetated shallow lake is located in the proximities of the High Cordillera (Fig. 1) and, therefore, is an environment with very low anthropogenic impact. Snails (mostly L. viator) were found exclusively on the submerged vegetation present in a very shallow area (20 cm deep) where a small stream flows into the lake. The remaining levels from LB (LB4–LB6, LB11, LB14) did not have a modern analogue in the present model. These levels were dominated by B. peregrina, a species that did not dominate in any modern site.

Fig. 5. (A) Detrended correspondence analysis (DCA) ordination plot of modern (white circles) and Quaternary (black circles) mollusc samples. (B) Cluster analysis (CA) of modern and Quaternary mollusc samples based on Bray–Curtis similarity measures. Groups defined by CA at similarities N 0.5 are encircled in DCA. PV: Puesto Vicencio (Holocene), PM: Puesto Moya (Holocene), LB: La Bomba (Pleistocene).

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5. Discussion and conclusions The PCA clearly discriminated between sites inhabited by molluscs and uninhabited sites. The former was characterized by high vegetation cover, low conductivity and current speed as well as predominance of fine sediments (mainly mud). Depth, substrate, flow, and vegetation have been considered important in explaining snail distributions (see Dillon, 2000 and references therein). Pulmonate snails are primarily adapted to calmer water, with few species inhabiting lotic environments, their occurrence in rivers restricted to calm backwaters and eddies (Dillon, 2000). Therefore, molluscs in the semiarid region of central-western Argentina occur primarily in calm, vegetated, shallow water bodies. With the exception of a few species that can tolerate high conductivity such as Heleobia parchappii (see De Francesco and Isla, 2004), assemblages are also confined to low salinities (below 1.9 mS cm− 1). Heleobia parchappii was the only mollusc species recorded in saline waters (8–16 mS cm− 1). In similar saline habitats in central-western Argentina, Doering (1884) reported Heleobia occidentalis. Based on their similar shell morphology, Gaillard and de Castellanos (1976) suggested that H. occidentalis could be a geographical variety of H. parchappii adapted to saline waters. De Francesco (2007) suggested that they could be synonyms. Ciocco and Scheibler (2008) found no conchological differences between the two species. We did not find any significant morphological or anatomical (penial structure) difference between them concluding that they may be synonyms. In the Pampean region, H. parchappii occurs both in fresh and saline water. However, when the species occurs in saline waters it is the only mollusc species present (De Francesco and Isla, 2004; Peretti, 2005) while in fresh water it is usually accompanied by other mollusc species. Ciocco and Scheibler (2008) found that along a gradient of conductivity in the Bañado Carilauquen, H. parchappii coexisted with other species only in the headwaters and middle reaches at low salinities (0.9–1.6 mS cm− 1) but was the only species found in the extremely saline waters (8.0–19.5 mS cm− 1) of the outlet into the lake. Therefore, although we cannot morphologically distinguish the populations of H. parchappii in fresh water from those inhabiting saline waters, the composition of the assemblage may be indicative of these two sorts of habitats, which has important implications for the value of freshwater mollusc assemblages as indicators of past continental salinities. Although the shell morphology of Heleobia aff. parchappii is different from typical H. parchappii, the anatomy of the penial complex is similar. We suggest that this species may be a variety of H. parchappii. It is known that Heleobia displays a significant phenotypic plasticity in shell morphology (De Francesco, 2007). Another possibility is that this taxon is an undescribed species, ecologically similar to H. parchappii. A third possibility is that this taxon corresponds to Heleobia kuesteri as recently suggested by Masi and Ciocco (2008). Heleobia kuesteri is a problematic species that was originally described for Mendoza, of which there is no updated taxonomic or ecological information. In fact, it has been considered as species inquirenda (Cazzaniga, 1981) because of the difficulties for assessing its taxonomic identity. Although we cannot be sure of the taxonomic identity of this taxon with the scattered observations presented here, the presence of this same morphology as fossil in the area (it was published as Heleobia sp. or Heleobia cf. H. australis by De Francesco and Dieguez, 2006) allows their utility as a reliable bioindicator with important consequences for the reconstruction of past environments. In other words, despite its taxonomic identity the ecological characteristics of this taxon are still an informative indicator of the original depositional environment. The CCA clearly indicated a strong relationship between mollusc taxa and environmental variables. Two main groups can be recognized: (Group 1) the species L. viator, P. acuta, and B. peregrina dominated in highly vegetated sites characterized by high pH and NO2− 3 , and lower current velocity, on fine sediments, and (Group 2) the two Heleobia

species (H. hatcheri and H. aff. parchappii) together with C. mendozana, and the sphaeriids dominated in less vegetated lotic water bodies with higher current velocity and coarser sediments, as well as lower pH. In addition, these species appeared to be more tolerant to slightly higher conductivity. Even though the waters of the Bañado Carilauquen are clear and variations in pH scarcely significant, Ciocco and Scheibler (2008) also observed a predominance of L. viator and B. peregrina at higher pH, and in turbid waters, whereas C. mendozana and H. hatcheri dominated in more clear and oxygenated waters, with high HCO−3. These results are in agreement with previous studies that suggested the occurrence of L. viator and B. peregrina in lentic bodies (shallow lakes, ponds) of areas containing submerged aquatic vegetation (de Castellanos and Landoni, 1981; Rumi, 1991) as well as the presence of H. hatcheri and C. mendozana in rivers and streams (Gaillard and de Castellanos, 1976; de Castellanos and Gaillard, 1981). Thus, from a paleoecological standpoint, we conclude that these two assemblages are indicative of different energetic environments related to the stability of the flow regime. The assemblage dominated by L. viator, P. acuta and B. peregrina represents lentic habitats (shallow lakes) or calm backwaters (pools) within lotic systems such as streams and rivers; the assemblage dominated by H. hatcheri, H. aff. parchappii, C. mendozana, and Sphaeriidae indicates lotic habitats where a directional water flow (of variable velocity) exists. The two main groups observed in the modern dataset were recognized in the fossil record. According to the DCA, the mollusc assemblages preserved in Puesto Vicencio (PV7 and PV4) are included in the group 2 mentioned above. Consequently, they represent shallow vegetated streams characterized by moderate current velocity and low conductivity. According to the clusters observed, a subtle difference between both levels can be inferred. This is due to the occurrence of some L. viator specimens in PV4, which may be consequence of taphonomic alteration. According to the poor preservation of these shells (in most cases, the weathered shells simply disintegrate when touched), it is probable that they represent a mixed assemblage, including specimens that would have been deposited after some transport by currents. Kotzian and Simões (2006) found that small scale lateral transport of shells does occur in fluvial systems. The occurrence of L. viator here suggests shallow water bodies probably connected to the main lotic system. This mixed assemblage probably represents a taphocoenosis deposited during flooding events. Moreover, the whole PV succession may reflect the variations in water flow that occurred in the floodplain during the Holocene in Río Atuel. An important conclusion is that the molluscs preserved in this sort of alluvial succession represent mixed assemblages that suffered some degree of taphonomic alteration. Because of the natural dynamics of the fluvial environment, it is expected that molluscs usually be subject to transport and redeposition. The LB and PM successions represent lentic habitats with nil or very low water velocity (Group 1). These habitats would have been vegetated shallow lakes within the floodplain and not connected to the fluvial system. The excellent preservation of the shells from LB supports this interpretation (see De Francesco et al., 2007). As L. viator inhabits the submerged vegetation in very shallow areas where small streams flow into the lake, shells may be deposited in situ and, thus, represent parautochthonous assemblages. Some LB levels (LB4–LB6, LB11, LB14), dominated by B. peregrina, did not have a modern analogue in the present model. However, B. peregrina is widely distributed in shallow lakes from La Pampa and Buenos Aires Province (Rumi, 1991; Peretti, 2005), regions characterized by higher humidity than the region studied here. Thus, these LB levels probably indicate relatively wetter conditions, which agrees with the previous interpretation, based on the whole assemblage together with the isotopic evidence, that the studied section records a mild interval corresponding to an interstadial (Marine isotope stage 3) of the last glacial cycle (De Francesco et al., 2007). Although the results obtained here indicated that mollusc species are useful for inferring past habitats in the semiarid region of

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Mendoza, the presence of species not recorded as fossils in the area (H. hatcheri and P. acuta) introduces a complication in using the modern assemblage to infer paleoenvironments. P. acuta (= P. cubensis) is an introduced species widely distributed in Argentina (Núñez and Pelichotti, 2003). This species has a superior reproductive capacity compared to other pulmonate snails, as well as ability to migrate upstream and to quickly recolonize a water-body (see de Cock and Wolmarans, 2007 and references therein). Consequently, it may have a negative impact on indigenous freshwater molluscs in particular and on the biodiversity of freshwater habitats in general. One important caveat regarding paleoenvironmental interpretations is the lack of knowledge of the ecological role played in the past by other modern coexisting species such as L. viator and B. peregrina. It is likely that, in the absence of P. acuta, these species would have occupied a different range of habitats or, eventually, experienced an increase in their distribution. Similarly, the absence of H. hatcheri in the past suggests that the species colonized Mendoza in recent times (in fact, the species was not recorded as fossil in Mendoza even in younger alluvial deposits of up to ca. 500 years B.P.; De Francesco and Zárate, 2006), so the habitats they occupy today would have been absent in the past or occupied by a different species, which cannot be ascertained. Even though the total biodiversity has suffered changes in recent times, many of them probably as a consequence of human activities, the good news is that mollusc assemblages still constitute reliable bioindicators of past environmental conditions and can be used with confidence. Acknowledgements Financial support for this study was provided by Agencia Nacional de Promoción Científica y Tecnológica (PICT 16-20706). We thank Adolfo Gil, Gustavo Neme, Miguel Giardina, and Alejandra Guerci (Museo de Historia Natural de San Rafael) for support during field work. We acknowledge Gustavo Bernava for chemical analyses, and Mauricio Quiroz for providing data on altitude. Robert Cowie and Néstor Ciocco improved the manuscript with constructive reviews. The authors are members of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). References Abraham de Vázquez, E.M., Garleff, K., Liebricht, H., Regairaz, A.C., Schäbitz, F., Squeo, F.A., Stingl, H., Veit, H., Villagrán, C., 2000. Geomorphology and paleoecology of the Arid Diagonal in southern South America. In: Miller, H., Hervé, F. (Eds.), Geology, Geomorphology and Soil Science. Sonderheft Zeitschrift fur Angewandte Geologie, Sonderheft, pp. 55–61. APHA, 1992. Standard Methods for the Examination of Water and Wastewater. APHA, Washington, DC. Bocard, D., Legendre, P., Drapeau, P., 1992. Partialling out the spatial component of ecological variation. Ecology 73, 1045–1055. Capitanelli, R.G., 2005. Climatología de Mendoza. Editorial de la Facultad de Filosofía y Letras de la Universidad Nacional de Cuyo, Mendoza, Argentina. Cazzaniga, N.J., 1981. Notas sobre hidróbidos argentinos. III (Gastropoda Rissoidea) Strobeliella, un nuevo género de la Patagonia. Neotrópica 27, 3–10. Ciocco, N.F., 2008. Primer hallazgo de Pisidium chiquitanum Ituarte, 2001 (Bivalvia, Sphaeriidae) en el centro-oeste de Argentina. 4° Congreso Argentino de Limnología, San Carlos de Bariloche, Abstracts, p. 90. Ciocco, N.F., Scheibler, E.E., 2008. Malacofauna of the littoral benthos of a saline lake in southern Mendoza, Argentina. Fund. Appl. Limnol. 172, 87–98.

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