*4.3.2 Interglacial MIS 5e*

In the marine deposits of this interglacial in area D, 44 molluskan species (25 bivalves and 19 gastropods) were recorded. These deposits are represented by littoral ridges and tidal plains along the south of the BAP coast, and by littoral ridges along area D, with scarce content of calcium carbonate, favoring the record of mollusks. In area C the associations of MIS 5e and MIS 1 have respectively 34 and 33 species. This similarity was also found in area D, in which 22 species were recognized for MIS 5e and 23 for MIS 1. Unlike area D, the mollusks of MIS 5e of the northeast of the BAP are characterized by less abundance and diversity of species related to those of the Holocene MIS 1 [32]. This could be due to a less representation of the Interglacial MIS 5e, and because most valves and shells of Pleistocene deposits are dissolved and/or crystallized, preventing the species identification.

Between 50 and 44% of warm water species of bivalves recorded in the Interglacial MIS 5e of area D, are represented in areas A-C, whereas this relationship is only 27% of the species from area D. Warm water mollusks were recognized in the study area, among the most prominent species of this interglacial, in areas A-C, is the bivalve *Crassostrea rhizophorae* (**Figure 7A**). This is a warm lineage species that inhabits currently the Caribbean, Venezuela, Surinam, and Brazil up to Uruguay, but is not recorded in the present Argentine coast. However, it is recorded as fossil in the northeast and south of the BAP in the MIS 5e (e.g., [33, 125]).

Another warm lineage species found in area D, is the bivalve *Anomalocardia brasiliana*, recorded in the area north of the Negro River (**Figure 7B**). This is the most austral record of the species in the Argentine coast. This species is distributed currently from the French Antilles (18°N) up to the coasts of Brazil (33°S), being an infaunal surface species able to support wide salinity ranges (e.g., [126, 127]).

#### **Figure 7.**

*The warm species of Pleistocene. (A)* Crassostrea rhizophorae *(Guilding) (MLP: 34.012, MIS 5e, region B), (B)* Anomalocardia brasiliana *(Gmelin) (CEGH-UNC: 25.609. MIS 5e, region D) and (C) Tegula atra (lesson) (CEGH-UNC: 25.615, MIS 5e, region D).*

As fossil, it was found in Uruguay, both in the MIS 5e of the Nueva Palmira Formation [43] and in the Holocene of the Villa Soriano Formation [128]. In Argentina, this species is reported for marine deposits within the Pampiano Formation of Lomas de Zamora (34°46′S, northeast of BAP; [129]), as well as in the localities of Magdalena, Punta Piedras (BAP) and south of Entre Ríos Province [124]. This species was found in the Pleistocene deposits of Bahía Blanca (south of BAP) together with *Crassostrea rhizophorae* [33].

*Tegula atra* is distributed in intertidal and subtidal shallow rocky substrates, up to 6 m depth (**Figure 7C**) [130]. It is among the most abundant species of the Pleistocene of area D, and is well preserved in deposits of MIS 7 and MIS 5e [5] but it is not recorded in deposits of MIS 1. Whereas in the Pacific coast, *Tegula atra* is recorded in the late Pleistocene and Holocene deposits of the northern and southern coast of Peru (e.g., [131, 132]). In Chile, it is recorded in late Pleistocene deposits of Caleta Coloso (23°45′S/70°28′W), north and south of Antofagasta (23°37' S) [45], as well as in archeological sites such as the late Pleistocene-middle Holocene of Quebrada de Lazareto (south of Chile) [133], middle and late Holocene of the IV Region, Los Vilos (e.g., [134]) and Holocene deposits of the Magellan Straight [135]. *Tegula atra* is currently distributed in the Pacific coasts from Pacasmayo (7°24′S, Peru) up to the Magellan Straight (53°S) [136], but there is no evidence of living specimens in the south Atlantic coasts.

#### *4.3.3 Interglacial MIS 1*

A total of 58 species (31 bivalves and 27 gastropods) was recorded in the marine malacofauna of the Interglacial MIS 1 of northern Patagonia (areas A-D), which differs from the northeast of BAP where Aguirre [89] reported a total of 62 species (25 bivalves and 37 gastropods). Concerning the molluscan composition, in the northeast of BAP, gastropods are more abundant than bivalves both in number of species and of individuals. As a comparison, among the regions studied, area A recorded 51 species (29 bivalves and 22 gastropods), and area B 49 species (25 bivalves and 24 gastropods), being in both regions, bivalves more numerous than gastropods. Whereas in area C, 34 species (17 bivalves and 17 gastropods) were recorded, unlike area D where 42 species (20 bivalves and 22 gastropods) were recorded, being in this latter the number of gastropods slightly higher than bivalves.

The marine deposits of MIS 1 in areas A-C are formed by tidal plains and littoral ridges. Tidal plains yielded mainly *Tagelus plebeius* among bivalves and *Heleobia australis* among gastropods. These species support variable salinity, being recorded in oligohaline–mesohaline–polyhaline associations (salinity between 3 and 30 gr/l), typical of low energy environments, most of them with low diversity indexes. In the littoral ridges, the diversity indexes are mostly higher than those of tidal plains. They yielded among bivalves *Pitar rostratus*, *Amiantis purpurata*, *Ostreola equestris*, and among gastropods *Buccinanops cochlidium* and *Heleobia australis*, typical of high energy environments.

The marine deposits and their malacofauna in the northeast and south (areas A-C) of the BAP are similar in the two types of deposits of the Interglacial MIS 1. The tidal plains are represented in regions A-C and are related to the development of the Colorado River and Bahía Anegada. The most common species are *Heleobia australis*, *Tagelus plebeius* (in life position) and *Corbula patagonica*, being similar to the marine malacofauna of the northeast of the BAP of the Canal 18 Member of the Las Escobas Formation (Holocene, MIS 1) and the estuarine facies of the Mar Chiquita Formation (**Figure 8**).

*Quaternary Marine Mollusk Associations of the Last Interglacials in North Patagonia… DOI: http://dx.doi.org/10.5772/intechopen.99221*

**Figure 8.** *Warm water vs. cold water during the quaternary in all regions (A–D).*
