Late PleistoceneeRecent marine malacological assemblages of the Colorado River delta (south of Buenos Aires Province): Paleoecology and paleoclimatology

The most recent Quaternary marine transgressions are well represented along the South Atlantic coast. In the Colorado River delta (39 150Se39 550S), south of Buenos Aires Province these deposits are mostly littoral ridges and tidal plains with abundant fossil marine fauna. Seventeen localities were analyzed (five Pleistocene, seven Holocene and five modern ones) representing the Interglacials MIS 9, MIS 5e and MIS 1. A total of 51 species were recorded (29 bivalves and 22 gastropods), together with nine micromolluscs. MIS 1 and the modern coast (37 and 42 species respectively) are the richest ones in species, unlike MIS 5e and MIS 9 (18 and 2 species respectively). In MIS 9, the most abundant species is Pitar rostratus. MIS 5e and MIS 1 have in common the presence and abundance of the gastropod Heleobia australis and the bivalve Tagelus plebeius, as typical fauna of low energy environments, being the bivalve Glycymeris longior and the gastropods Bostrycapulus odites and Buccinanops globulosus common species in both kinds of marine deposits. Approximately 90% of bivalves and 75-71% of gastropods of the marine fauna are recorded from MIS 5e to the present. According to the different descriptive analyses (Bray eCurtis Index and AC) the molluscan fauna was grouped in two and five assemblages respectively, defined by age, type of deposit, and presence and/or abundance of species. Marine species represented in the area of the Colorado River delta vary in abundance among the interglacials but not in faunal composition. One of the possible causes would be the global climatic changes (e.g., rise of sea surface temperature SST) and the heterogeneity of habitats that would have conditioned the development of the different faunal assemblages during the Quaternary. © 2015 Elsevier Ltd and INQUA. All rights reserved. 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112

In the Colorado River delta, south of Buenos Aires Province, marine deposits were studied from a geological and geochronological view (e.g., Alber o et al., 1980;Codignotto and Weiler, 1980;Gonz alez and Weiler, 1983;Weiler, 1984). The interglacials represented are ! MIS 9, MIS 5e and MIS 1 which mostly correspond to paleocliffs, littoral ridges, and tidal plains (Weiler, 1984;Fucks et al., 2012a). The marine fauna associated with these deposits has been briefly mentioned previously (Weiler, 1984(Weiler, , 2000 but it has not been studied, especially the bivalves and gastropods, as "paleoclimatic proxies". The main objective of this paper is to describe the different faunal assemblages of bivalves and gastropods and to study paleoecologic and paleoclimatic issues of the Colorado River delta during the Quaternary, particularly of ! MIS 9, MIS 5e and MIS 1.

Study area and geology
The area of the Colorado River delta extends from Verde Peninsula (39 20 0 S, 62 4 0 W) to Otero Island (39 55 0 S, 62 08 0 W) in southern Buenos Aires Province, Argentina ( Fig. 1 and Table 1). The Colorado River runs from the Andes to the Atlantic Ocean, forming the boundary between Patagonia in the south and the sandy Pampa to the north (Spalletti and Isla, 2003). This river is the most important in the region while others have very little flow or become active in times of flood.
The delta of the Colorado River is a prograding body, composed of large number of active and abandoned channels, such as from N to S: the Colorado Nuevo, Colorado and Colorado Viejo. Deltaic plains of the Colorado River area comprise both the area of recent outlet channels (Colorado Viejo and Colorado Nuevo), and the "old deltas" known today as Caleta Brighman, Verde Peninsula, Verde Bay and Falsa Bay (Isla and B ertola, 2003). Verde Bay is an area in which the main factor to mobilize the material is the tidal current and the currents produced by the waves. In the center of the island there is a sequence of old sandy beach ridges that continue to the outlet of the Colorado Viejo and disappear toward the continent. These beach ridges are bordered by old tidal plains to the north and west.
Islands such as Puerto, Conejos, Word and Ariadna, and sandy banks are distributed within the bay. Among them, Verde Peninsula stands out, a true island during syzygy tides or severe storms (Weiler, 1984).
In the area between the Colorado and Negro rivers, south of Buenos Aires Province (39 30 0 S -41 02 0 S), there are marine deposits assigned to the oldest interglacials because of their geomorphological, altimetric and cementation similarity (Fucks et al., 2012a). Among them, there are deposits assigned to ! MIS 9 which are scarce, thin, and isolated on the continent (Fucks and Schnack, 2011;Fucks et al., 2012a).
According to Gonz alez et al. (1988) the marine deposits of the Sangamon (?; Late Pleistocene) are represented in the area of the Colorado River delta by paleocliffs associated with coast lines no more than 10 m height. The terminal area of the delta is formed by the marine deposits of the MIS 1 ingression, which are beach ridges of intertidal environments (Fucks et al., 2012a). Weiler (1984) described between Verde Bay and Laberinto Point old tidal plains between 5 and 2.5 m height, which are crossed by numerous tidal channels, functional today ( Fig. 2 and Table 2).

Materials and methods
Seventeen localities were analyzed in the area of the Colorado River delta (five Pleistocene, seven Holocene and five modern ones) representing ! MIS 9, MIS 5e and MIS 1 (Fig. 3). The analysis of each site was made through volumetric samples of 1 dm 3 and in a quadrant 1 m Â 1 m along transects perpendicular to the coast line in modern beaches. These deposits (1 dm 3 ) were separated using three sieves of different mesh size. Each fraction of biogenic content was identified and labeled.
Two indices were used to study the biodiversity of the faunistic assemblages: Shannon index (H 0 ), calculated with the program R version 2.15.0 (package "Biodiversity R"; Kindt and Coe, 2005), and richness of each sample (total number of species) (Pla, 2006). The similarity degree of the sites was analyzed with multivariate analyses (cluster analyses) of program R (vegan package) (Oksanen, 2011) through the BrayeCurtis Index, UPGMA method. This index is used to estimate similarity among species composition in each site. To enhance discrimination among them, we used correspondence analysis (CA), a statistical descriptive method that analyzes the relationships among sites. Both analyses grouped different faunal assemblages.
In order to arrive at paleoenvironmental conclusions focused on paleoecology of the different species, we built tables according to salinity range, life mode, depth, substrate, trophic type and area of distribution. These tables were made on the basis of ecological data of modern representatives of the different species, and following general bibliographic sources (e.g., Bastida et al., 1992;Lasta et al., 1998;Morris and Rosenberg, 2005;Penchaszadeh et al., 2007;Balech and Ehrich, 2008) or specific papers (e.g. Mors an, 1997;Iribarne et al., 1998;Ciocco, 2000;Collin, 2005;Cumplido, 2009). Afterwards, the ecological parameters of gastropods and bivalves of each analyzed area were compared through histograms.
The presence of warm water species and species that today are displaced toward lower latitudes were also used as source of paleoenvironmental information, in this case, as indicators of warmer conditions (e.g., Valentine, 1955Valentine, , 1958Beu, 1974;Aguirre, 1993b;Farinati, 1999, 2000). The illustrated material is housed in the Paleoinvertebrate Collection of the Museo de La Plata, Argentina (MLP-UNLP).

Pleistocene deposits (!MIS 9) in the region of the Colorado River delta
Site 1 is near the locality of Villalonga, at a height of 16 m. It is a beach ridge made of gravel and cemented by calcium carbonate. The age is assigned on the basis of similarity of geomorphological features with other marine deposits south of this area (Fucks et al., 2012a,b;Char o et al., 2013a). It has little fossiliferous content with Pitar rostratus and Corbula patagonica (bivalves), and Buccinanops and Crepidula (gastropods) (Fig. 4).

Pleistocene deposits (MIS 5e) in the region of the Colorado River delta
Site 2 is a beach ridge at a height of 6 m. It is composed of 2 m of brownish silty sands covered by grayish brown clayey silts with scattered clasts, spherical and flattened, and upholstered with calcrete with mollusc remains. The recorded species are Glycymeris longior, Tagelus plebeius (bivalves) and Heleobia australis, Tegula patagonica and Buccinanops globulosus (gastropods).
Sites 3 and 5 form littoral ridges made of sandy sediments at a height of about 6 m. Among the most abundant molluscs are    Glycymeris longior and Buccinanops globulosus Site 4 is a paleochannel 10 m above sea level. This deposit is composed of sandy silt sediments. Among the recorded species are Tagelus plebeius, Corbula patagonica, Pitar rostratus, and Heleobia australis.

Holocene marine deposits in the region of the Colorado River delta
Verde Peninsula is a geomorphological feature generated from the Holocene marine transgression where relict and modern tidal plains, active and stabilized dunes, coastal barriers, and beaches can be identified. Two Holocene sites were analyzed (Sites 6 and 7) the latter with an age of 2.17 ± 0.86 ka 14 C (Alber o et al. 1980). These are tidal plain deposits at a height of 4 m above sea level. Among the most abundant marine molluscs are Amiantis purpurata (bivalve) and Heleobia australis. The southern sector is characterized by the recent formation of a NeS barrier about 3.5 km from the dune line, generating a wide sandy tidal plain, mostly affected by littoral processes during storms and by the wind (Fucks et al., 2012a).
In the vicinity of the mouth of the Colorado Nuevo River, marine sediments are exposed continuously along 26 km of coastline. Sites 8, 9 and 12 are tidal plain deposits at a height of about 5e2 m above sea level. These deposits show 2 m high cliffs, composed of silty clayey sediments, grayish brown, homogeneous, covered in some sectors by eolian sediments. Transitionally overlying are grayish brown sandy sediments, laminated, with a minimum thickness of 0.50 m. These sediments bear a large amount of marine molluscs, mostly Tagelus plebeius, Corbula patagonica, Pitar rostratus, Heleobia australis and Buccinanops globulosus, often articulated and in life position (Fig. 5).
Site 10 is a tidal plain deposit, some 23 km away from the present coast line. It is an exposure at a height of 4e5 m represented by 2.5 m of clayey sand, brown, homogeneous with small clasts and bivalves and gastropods. Among them, the bivalves Amiantis purpurata and Mesodesma mactroides are abundant (Fig. 6).
Site 11 is a 1.40 m thick recent channel deposit at a height of 3 m, with high density of Tagelus plebeius and Heleobia australis. Three levels (A, B and C) were recognized as part of a shallow environment. Level A, is 0.10 m of fine gravel, matrix supported; level B is 0.60 m of clayey fine sand, homogeneous, dark brown with scattered valves and Tagelus plebeius in life position and articulated, covered by a level 0.50 m thick of a green brownish sediment; level C is a pedogenized sand 0.20 m thick. One 14 C date was performed in level B, which yielded an age of 3.69 ± 0.10 ka (LP 2480) (Fig. 7).

Modern beaches in the area of the Colorado River delta
The area between Punta Laberinto and vicinities of the Colorado Viejo River has a wide sandy beach which narrows towards the south, in a stretch of 45 km and a width that ranges from 2 km in Punta Laberinto to 500 m near the Colorado Viejo River (Weiler, 1984). Site 13 is the beach of Verde Peninsula is a sandy beach with a large amount of marine molluscs, most of the valves and shells are fragmented. Among the most abundant molluscs are Adelomelon brasiliana and Zidona dufresnei (gastropods) (Fig. 8).
The most outstanding beach of the area is La Chiquita Beach at Verde Bay, in front of Wood Island (sites 14e16). It is a wide sandy beach characterized by a frontal dune about 4 m high. In the berm near the shoreline, accumulations of valves of Amiantis purpurata and Cyrtopleura lanceolata (bivalves) are common (Fig. 9).
The coast of the east sector of the Colorado River delta is represented by a wide sandy beach (site 17), limited toward the continent by a dune body partially vegetated. Among the most abundant marine molluscs are the bivalves Ostrea puelchana, Pitar rostratus and Amiantis purpurata. In some sectors, as the vicinity of the mouths of the Colorado Chico or Colorado Nuevo rivers, partial erosion phenomena have been observed   (Fig. 10).
The oldest Pleistocene (Site 1) is characterized by the presence of Pitar rostratus and Corbula patagonica and two gastropod genera Crepidula and Buccinanops. The Pleistocene deposits of MIS 5e (Site 2e5) are represented mostly by littoral ridges of high energy, except for Site 4. A total of 18 species was recorded (12 bivalves and 6 gastropods). The abundance of Glycymeris longior, Pitar rostratus, Tegula patagonica, Bostrycapulus odites and Buccinanops globulosus is notable, and also Tagelus plebeius and Heleobia australis were recorded.
The highest values of Shannon Index (H 0 ) are those of the modern sites (1.94e2.72), and the lowest are those of MIS 1 (H 0 1 ¼ 0.10, H 0 3 ¼ 0.58 and H 0 6 ¼ 0.44). The highest values of richness are those of modern sites (S 13 ¼ 30, S 15 ¼ 27) and the lowest ones are those of S1, S3, and S6 with 4. 9 % of the bivalve species of MIS 5e are recorded in MIS 1, and 89.5% of them are still living today (Fig. 15). 75% of the gastropod species of MIS 5e are recorded in MIS 1, and 71.4% of them are still living today (Fig. 16).
In the oldest Pleistocene of ! MIS 9 all the bivalve species are euryhaline, infaunal, of sandy substrate, and filter feeders. Among gastropods, all the species are euryhaline, epifaunal, of sandy or rocky substrates, and filter feeders or carnivorous. In the Pleistocene deposits of MIS 5e most bivalves are euryhaline and polyhaline-euryhaline, infaunal with epifauna species, and cemented. They inhabited sandy substrates, and also rocky substrates. They are filter feeders, but some species are detritivorous and carnivorous. Euryhaline species increase in MIS 1, as well as cemented and detritivorous species increase respect to MIS 5e. Today there is an increase of rocky substrate species.

Warm water species
Concerning the amount of warm water bivalve and gastropod species represented in each interglacial, from ! MIS 9 to MIS 5e, 50% of the species are warm water ones. From MIS 1 to today, they increased by 10% (Fig. 18). 4.6. Malacological assemblages 4.6.1. Cluster analysis According to the BrayeCurtis index, the marine malacofauna of the area of the Colorado River delta is divided into two major groups. Group A corresponds to the malacological association mostly of modern sites, and Group B to Pleistocene and Holocene sites. Group A is divided into two subgroups: A1) malacological association of beach sites with abundance of Crepidula and presence of bivalves such as Mesodesma mactroides, Cyrtopleura lanceolata and Solen tehuelchus; and A2) malacological associations mostly of beaches except for three sites (1, 7, and 8) with abundance of Pitar rostratus and Amiantis purpurata. Sites 1, 7, and 10 correspond to beach ridges with abundant malacological remains, similar to those found in modern beaches. Sites 7 and 10 (MIS 1) resemble each other by the abundance of Amiantis purpurata.
Group B is subdivided into three subgroups: B1) malacological association of MIS 5e represented by littoral ridges; B2) malacological association of MIS 1 represented by tidal plains with abundant Heleobia australis; B3) malacological associations of two sites of MIS 5e (sites 2 and 4) and two sites of MIS 1 (sites 9 and 12). Sites 4 and 12 are very similar, because of the presence and abundance of Tagelus plebeius, Pitar rostratus, Corbula patagonica and Heleobia australis (Fig. 19).

Correspondence analysis
According to the Correspondence analysis (CA), five malacological associations are distinguished. A) malacological association of MIS 5e (sites 3 and 5) which correspond to littoral ridges with presence of Glycymeris longior. B) malacological association of two bivalves Pitar rostratus and Amiantis purpurata with presence of the bivalves Corbula patagonica, Lyonsia alvarezii and Diplodonta vilardeboana, and the gastropods Crepidula argentina, Urosalpinx rushi, Tegula patagonica, Bostrycapulus odites, Buccinanops globulosus, and Buccinanops cochlidium. C) malacological associations of littoral ridges of sands and clasts with presence of the bivalves Ostrea, Ostreola equestris, Brachidontes rodriguezii and Mactra guidoi and the gastropod Buccinanops uruguayensis. D) malacological association of MIS 1 (sites 6, 8 and 11) corresponding to tidal plains with abundance of Heleobia australis. Site 8 is characterized by the particular presence of two micromolluscs: Parvanachis isabellei and Turbonilla argentina. E) malacological association that corresponds to modern sandy beaches (sites 15 and 16) close to each other composed of Mesodesma mactroides, Cyrtopleura lanceolata, Barnea lamellosa and Raeta plicatella (bivalves). Site 15 is outstanding by the presence of the bivalve Atrina seminuda and the gastropod Epitonium striatum (Fig. 20).
The Malvinas Current is a branch of the Circumpolar Current and is divided into two branches: the western branch of cold water and low salinity, due to the contribution of inland water, flows northward along the continental shelf of Argentina until the edge of the continental shelf of Uruguay in front of the Río de la Plata. The eastern branch surrounds the Malvinas Islands and also flows northward, but contouring the edge of the continental shelf, running from south to north the South Atlantic from an approximate latitude of 55 S and up to the area between 39 and 36 S (e.g., Barr e et al., 2006;Balech and Ehrich, 2008).
The contact of the two currents generates a large region of dynamic characteristics, both spatial and temporal, and mixtures of subtropical with sub-Antarctic waters. This area of confluence, called the Transition zone, is located between 30 and 46 S, from which the currents flow in opposite directions (Piola and Matano, 2001).
The area of the delta of the Colorado River (39 S), is located in the transition zone of the warm Brazil Current and the cold Malvinas Current, but when compared to the coastal area of Uruguay (33 52 0 O) this latter is influenced by the Brazil Current (Scarabino, 1977). The Uruguayan Pleistocene deposit (Nueva Palmira Formation, 31e34 ka;  in Nueva Palmira (33 52 0 S), are related to the study area in 44.4% of the molluscan fauna. The species are mostly polyhaline-euhaline and euhaline, and of rocky and sandy substrate. This Pleistocene deposit is characterized by the presence of two warm-water species (Anomalocardia brasiliana and Nioche subrostrata), currently displaced towards lower latitudes, not found in the delta area. The Holocene deposits (Villa Soriano Formation, 6.8e1.8 ka;  are richer in species than in the study area (65 vs. 37 species respectively) and have a similarity of 54.05% with the Holocene marine malacofauna present in the delta area. Most species of the study area are euhaline, infaunal, and of sandy substrates. The higher percentage in species similarity between the two areas and the existence of warm water species in Villa Soriano Formation (Anomalocardia brasiliana, Marshallora nigrocincta, Nioche subrostrata, Bulla striata and Miralda sp.) currently displaced further north may be due to warmer waters and the displacement of the saline front of the estuary of the Río de la Plata during the Holocene .
In southern Gualeguaychú (southern Entre Ríos province, 33 1 0 S; 58 31 0 W), the Pleistocene deposits (26.6e33 ka, minimum age; Guida and Gonz alez, 1984) are considered by geographical vicinity to be similar in age to the Uruguayan Pleistocene deposits (Martínez and del Río, 2005). The Pleistocene faunal assemblages in southern Entre Ríos are characterized by low species richness (5 species), 11.1% agreeing with the study area, with the presence of Tagelus plebeius and Heleobia australis in both areas. The Holocene deposits (5.6e6 ka; Guida and Gonz alez, 1984), are limited to three species in common (8.1% similarity). The difference between the two areas is the existence of the estuarine species Erodona mactroides, indicating brackish waters during the Holocene in Uruguay.
In the area of Bahía Blanca (38 43 0 S, south Buenos Aires Province), Pleistocene deposits (35.5e25 ka, minimum age; Gonz alez, 1984), are characterized by the presence of two warm water species (Crassostrea rhizophorae and Anomalocardia brasiliana) (Chaar and Farinati, 1988), exclusive to the Pleistocene (Chaar et al., 1992), and only one of them is represented in the MIS 5e of the delta area. The Holocene deposits (7.4e1.4, MIS 1;Spagnuolo, 2005), correlated with the ridges of the Las Escobas Formation (Schnack et al., 2005), coincide 91.1% with the malacofauna of the study area (Farinati, 1985). In the Holocene deposits of Bahía Blanca, Erodona mactroides, Tagelus plebeius and Heleobia australis are dominant, the two latter being present and abundant in the Colorado River delta, indicating a lower salinity gradient.

Southern Buenos Aires Province
In the Colorado River delta, Interglacial ! MIS 9 is represented by species that continue in MIS 5e and MIS 1. This is also seen in the south of the study area, where at Anegada Bay (39 S), Char o et al. (2014) report marine deposits at 18 m height of Interglacial ! MIS 9 (Fucks et al., 2012b), with the most represented marine fauna being Glycymeris longior, Zidona dufresnei and Buccinanops cochlidium, recorded in this area today. In the area of San Blas Bay (40 S) the same fauna of Anegada Bay is recorded (Char o et al., 2013a(Char o et al., , 2014 with Pitar rostratus and Olivancillaria urceus. In the northeast of the Buenos Aires Province (35 e36 S), deposits of this interglacial have not been preserved (Aguirre and Fucks, 2004). None of the studied deposits yielded warm water marine fauna which are displaced currently to lower latitudes (Char o et al., 2014).
During MIS 5e, changes of the geographic distribution of warm water marine molluscs are recorded (e.g., Muhs et al., 2002;Zazo et al., 2003Zazo et al., , 2010Aguirre et al., 2005). Some authors (e.g., Aguirre, 1993; Q4 explain the high SST during MIS 5e by the southward displacement of the warm Brazil current. In the area of the Colorado River delta, MIS 5e and MIS 1 have in common the presence and abundance of Heleobia australis and Tagelus plebeius, typical species of low energy environments, and the record of Glycymeris longior, Bostrycapulus odites and Buccinanops globulosus in both deposits. Table 5 Ecological requirements and distribution of bivalves:  Table 6 Ecological requirements and distribution of gastropods:  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64 In general, MIS 1 in Buenos Aires Province is represented by two types of deposits: tidal plains and littoral ridges. Tidal plains of the south of the Province of Buenos Aires are related to the development of the Colorado River and with the coeval geomorphological development related to island and tidal plains formation that affected the Colorado River, Anegada Bay and northern San Blas Bay. In the study area, as well as in Bahía Blanca area , Anegada Bay (Char o et al., 2013b) and north of San Blas Bay (Char o et al., 2013a) the best represented bivalve in low energy marine deposits is Tagelus plebeius. The modern malacofauna of the study area records a higher richness index of species in MIS 1 respect to MIS 5e and ! MIS 9.

Southern Patagonia
The malacological assemblages of southern Argentinean Patagonia have currently higher percentage of typical cold-water species, belonging to the Magellan Malacological Province, which is related since post-Miocene times to the cold Malvinas Current (del Rio, 2004). The Magellan Malacological Province extends along the Atlantic Ocean south of 42e55 S, comprising the Fuegian archipelago, Malvinas Islands and the Burdwood Bank, extending into the Pacific Ocean to southern Chile (44 S; Bastida et al., 2007).
During the Holocene, the proportion of bivalves and gastropods of the Colorado River delta is higher than in Península Valdez (42 31 0 S, Province of Chubut) (37 species vs. 28 species) with 13.5% similarity with the study area in two sites: Isla de los P ajaros and Punta Pardelas (Pastorino, 1989). In Isla de Los P ajaros (42 25 0 S) there are two Holocene ridges (Terrace VI or Comodoro Rivadavia terrace, F. San Miguel) (Feruglio, 1950;Haller, 1982) with four species in common with the study area (Mytilus edulis platensis, Notochoclis isabelleana, Buccinanops globulosus and Olivella tehuelcha; Pastorino, 1989). Most species are euryhaline, of sandy substrate and carnivores, and belong to the Magellan Malacological Province (Pastorino, 2000). In Punta Pardelas (42 36 0 S), the Holocene marine malacofauna (Terrace VI or Comodoro Rivadavia Terrace; Pastorino, 1989Pastorino, , 2000 coincides in three species with the study area (Notochoclis isabelleana, Odontocymbiola magallanica and Ostrea puelchana). Both sites have in common the presence of Aulacomya atra, Ameghynomia antiqua, and Hiatella arctica, among the most important, which are not recorded in the area of the Colorado River delta.
The proportion of Quaternary bivalves and gastropods is higher in the area of the Colorado River delta than in the area of Bahía Vera e Camarones (44.2 Se45 S) (51 species vs. 41 species). In the Pleistocene deposits 135e92 ka (Terrace V of Feruglio, MIS 5e?a?c?;Rostami et al., 2000) 22.2% of the marine malacofauna of the study area are shared with the latter area. All species are euryhaline, mostly epifaunal, both of rocky and sandy substrates, and filter feeders. Most of these species persisted since the Pleistocene, except for Pitar rostratus, Brachidontes rodriguezii and Tegula patagonica which are today displaced farther north, and Tegula atra which is extinct in the Atlantic Ocean (Aguirre et al., 2011;Char o et al., 2014). Feruglio (1950) recorded the bivalves Corbula patagonica, Diplodonta vilardeboana and Mactra cf. patagonica (Aguirre et al., 2007) in Pleistocene deposits of the area of Vera Bay e Camarones (Terrace III, 400 ka, MIS 11), which are today displaced to lower latitudes, and inhabiting the area of the delta, suggesting warmer waters in MIS 11 than today (Ortlieb et al., 1996). Holocene deposits (Terrace VI of Feruglio, 8e2.5 ka, MIS 1; Codignotto et al., 1988Codignotto et al., , 1993Schellmann and Radtke, 2000;Rostami et al., 2000) have 18.9% similarity, with all the species epifaunal, mostly euryhaline, of rocky substrate and filter feeders.