Pliocene marine Bivalvia of Vale do Freixo (Pombal, Portugal): updated taxonomic list and discussion

DOI: 10.1344/GeologicaActa2021.19.13  R.J. Pimentel, P.M. Callapez, P. Legoinha, 2021 CC BY-SA R . J . P i m e n t e l e t a l . G e o l o g i c a A c t a , 1 9 . 1 3 , 1 2 3 ( 2 0 2 1 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 1 . 1 9 . 1 3 Pliocene Bivalvia of Vale do Freixo (Portugal) 2 al., 1993, 2008, 2009; Ramos, 2008). These sucessions form a Pliocene shallow marine unit (Carnide Formation), transgressive on a substrate of Miocene claystones or Lower Jurassic carbonates and evaporites, and often starting with a basal conglomerate with well-rounded clasts and abraded bioclasts of Glycymeris and other rather large bivalves. Locally, the Carnide Formation is very fossiliferous, yielding highly diverse fossil fauna, which includes molluscs and other invertebrates, as well abundant foraminifera, ostracods and other microfossils useful for biostratigraphical studies (e.g. Cachão, 1989, 1990; Cardoso, 1984; Diniz et al., 2016; Pais et al., 2010, 2013; Silva, 2001). Besides its implication for the regional stratigraphy and depositional history of the Cenozoic Mondego Basin (Pais et al., 2012) and the diapiric valleys of Estremadura, this palaeontological record stands out because their paleobiogeographical position in the Pliocene Atlantic realm of southwestern Europe, midway from the Mediterranean and the northwest African margins to the colder waters of Biscay Bay, Channel and British Islands. After the monograph of Dollfus and Cotter (1909), where the taxonomical composition and diversity of the bivalve faunas from the Caldas da Rainha outcrops and other related “classical sites” were first discussed, other works followed. The works of Brébion (1971, 1974), Cox (1936, 1941) and Zbyszewski (1959) focused on the rich assemblages of gastropods from these areas. Cartographical work in the region of Carnide (Pombal) gave Teixeira and Zbyszewski (1951) the opportunity to study several new fossil sites, enlarging considerably the number of Portuguese Pliocene mollusc species. Several decades later, when quarry works discovered the Vale do Freixo outcrop near the southern end of Carnide village, Silva (1993, 2001) described a rich gastropod fauna collected from the well-exposed Pliocene beds. Further works (e.g. Silva et al., 2006, 2010, 2011) enhanced the importance of this fossil site for the Pliocene studies in Portugal, as well as for correlations with other coeval European sites, promoting a better knowledge of the Atlantic mollusc faunas of this interval and their interchanges with the closest biotic realms. However, the bivalve fauna from Vale do Freixo was not studied in detail. Following on from the unpublished MsC dissertation of Pimentel (2018), the aim of this work is to update and discuss the taxonomic list of these bivalves based on the study of many newly collected specimens from the Vale do Freixo fossil site. GEOGRAFICAL AND GEOLOGICAL SETTING The fossil site of Vale do Freixo is located near Carnide, a rural village in the municipality of Pombal, in the Beira Litoral, west Portugal (Fig. 1). The sampled outcrops are located in a disabled quarry, with the coordinates: 39o53’00.35’’N; 8o43’49.45’’W. The site is in a private farm, accessible with the permission of the owner. The Pliocene beds are almost horizontal and extend over both sides of the nearby Carnide Valley, where they follow an alignment of slightly sloped hills cut in siliciclastic beds. Vale do Freixo and other correlative fossiliferous occurrences correspond to scattered lenticular beds exposed along the western side of the valley. The fossiliferous beds overlie unconformably middle Miocene greyish claystones (Amor Formation, Pais et al., 2010, 2012). They locally record the basal parasequence of the succession described as Barracão Group, with marine to alluvial and lacustrine facies (Pais et al., 2010, 2012, 2013). The 1:50,000 Geological Map of Portugal (23-A, Pombal set) reports this unit as “P Marine Pliocene of FIGURE 1. A) Location of the studied area in the Iberian Peninsula. B) Simplified geological sketch of the Cenozoic of the Mondego Basin and related tectono-sedimentary units of the West Portuguese Margin, with emphasis on the Pliocene and Quaternary deposits, and the location of Vale do Freixo and related marine fossil sites. 1= Quaternary; 2= Pliocene (including the Carnide Fm.); 3= Palaeogene and Miocene; 4= Meosozoic series of the Lusitanian Basin; 5= Neogene and Quaternary infill of the Lower Tagus Basin; 6= Variscan basement of the Iberian Massif. Main fossil sites: a= Vale do Freixo; b= Igreja de Carnide; c= Vale da Bouchada; d= Vale Farpado; e= Vale da Cabra; f= Monte Real; g= Mina; h= Senhora da Vitória; i= Famalicão; j= Bom Jesus; k= Salir do Porto; l= Nadadouro; m= Casal do Negreiro; n= Águas Santas.


INTRODUCTION
. These sucessions form a Pliocene shallow marine unit (Carnide Formation), transgressive on a substrate of Miocene claystones or Lower Jurassic carbonates and evaporites, and often starting with a basal conglomerate with well-rounded clasts and abraded bioclasts of Glycymeris and other rather large bivalves. Locally, the Carnide Formation is very fossiliferous, yielding highly diverse fossil fauna, which includes molluscs and other invertebrates, as well abundant foraminifera, ostracods and other microfossils useful for biostratigraphical studies (e.g. Cachão, 1989Cachão, , 1990Cardoso, 1984;Diniz et al., 2016;Pais et al., 2010Pais et al., , 2013Silva, 2001).
Besides its implication for the regional stratigraphy and depositional history of the Cenozoic Mondego Basin (Pais et al., 2012) and the diapiric valleys of Estremadura, this palaeontological record stands out because their paleobiogeographical position in the Pliocene Atlantic realm of southwestern Europe, midway from the Mediterranean and the northwest African margins to the colder waters of Biscay Bay, Channel and British Islands.
After the monograph of Dollfus and Cotter (1909), where the taxonomical composition and diversity of the bivalve faunas from the Caldas da Rainha outcrops and other related "classical sites" were first discussed, other works followed. The works of Brébion (1971Brébion ( , 1974, Cox (1936Cox ( , 1941 and Zbyszewski (1959) focused on the rich assemblages of gastropods from these areas.
Cartographical work in the region of Carnide (Pombal) gave Teixeira and Zbyszewski (1951) the opportunity to study several new fossil sites, enlarging considerably the number of Portuguese Pliocene mollusc species. Several decades later, when quarry works discovered the Vale do Freixo outcrop near the southern end of Carnide village, Silva (1993Silva ( , 2001) described a rich gastropod fauna collected from the well-exposed Pliocene beds. Further works (e.g. Silva et al., 2006Silva et al., , 2010Silva et al., , 2011 enhanced the importance of this fossil site for the Pliocene studies in Portugal, as well as for correlations with other coeval European sites, promoting a better knowledge of the Atlantic mollusc faunas of this interval and their interchanges with the closest biotic realms. However, the bivalve fauna from Vale do Freixo was not studied in detail. Following on from the unpublished MsC dissertation of Pimentel (2018), the aim of this work is to update and discuss the taxonomic list of these bivalves based on the study of many newly collected specimens from the Vale do Freixo fossil site.

GEOGRAFICAL AND GEOLOGICAL SETTING
The fossil site of Vale do Freixo is located near Carnide, a rural village in the municipality of Pombal, in the Beira Litoral, west Portugal (Fig. 1). The sampled outcrops are located in a disabled quarry, with the coordinates: 39º53'00.35''N; 8º43'49.45''W. The site is in a private farm, accessible with the permission of the owner.
The Pliocene beds are almost horizontal and extend over both sides of the nearby Carnide Valley, where they follow an alignment of slightly sloped hills cut in siliciclastic beds. Vale do Freixo and other correlative fossiliferous occurrences correspond to scattered lenticular beds exposed along the western side of the valley.
The fossiliferous beds overlie unconformably middle Miocene greyish claystones (Amor Formation, Pais et al., 2010, 2012. They locally record the basal parasequence of the succession described as Barracão Group, with marine to alluvial and lacustrine facies (Pais et al., 2010(Pais et al., , 2012(Pais et al., , 2013. The 1:50,000 Geological Map of Portugal (23-A, Pombal set) reports this unit as "P -Marine Pliocene of  The top of the fossiliferous section of Vale do Freixo (Carnide Formation) is an erosive surface. The section is overlain by the siliciclastics of the Ilha Formation (sensu Carvalho, 1998), which consist of medium to coarsegrained micaceous sandstones with small, well-rounded, gravel clasts of whitish quartzite scattered in the matrix. This rather monotonous sequence of apparently nonfossiliferous beds suggests deposition near an intertidal environment with high energetic oscillation (Carvalho et al., 2005c). Cachão (1990) determined the Pliocene age of the Vale do Freixo fossil assemblages based on calcareous nanofossils, CN12a biozone of Okada and Bukry (1980) and the NN16 unit of Martini (1971). Moreover, Silva (2001) and Silva et al. (2010) suggested that the rich gastropod fauna could be coeval with the Mediterranean Pliocene Molluscan Unit 1 (MPMU1) (Monegatti and Raffi, 2001;Raffi and Monegatti, 1993), and thus essentially somewhat older than 3.0Ma. It has been estimated that the chronostratigraphical range of the fossiliferous interval extens from the top of the Zanclean to the lower half of the Piacenzian (3.70-3.61Ma to 3.0Ma). On the other hand, on the basis of the 87 Sr/ 86 Sr isotopic data obtained from the valves of Palliolum excisum, Silva (2001) placed the lower boundary at 3.52Ma.

MATERIALS AND METHODS
Sampling was made bed by bed, following the stratigraphic section. Most specimens were collected individually, which was complemented by a volumetric bulk sample for each bed. Fieldwork also included taphonomic observations. Because large specimens occurred in unconsolidated sediments, they were easily extracted in the outcrop.
In the laboratory, the mechanical preparation and cleaning were often difficult due to the extreme fragility of many fossil shells, such as those of Tellinidae, Lucinidae and Mactridae. For these shells, the use of water to remove the sediment matrix was avoided, and a less aggressive meticulous dry cleaning was adopted.
Bulk-samples collected for volumetric purposes were washed through a column of sieves with meshes from 2mm to 0.125mm. Macro and microfossils from these fractions were picked up using a binocular and classified into taxonomic groups. The bivalve specimens were classified into genus and species.
A bulk collection of c.1,400 specimens was obtained. Several specimens were partially covered by matrix, and others consolidated with application of a film of transparent glue dissolved in acetone.
For the commonest species, we selected the bestpreserved specimens. For the remaining species, the criterion was to use all specimens, even if incomplete or badly preserved. All specimens were numbered and catalogued with the references RP-VF2-B001 to RP-VF2-B061, RP-VF3-B001 to RP-VF3-B738, and RP-VF4-B001 to RP-VF4-B385. The collection is housed in the Earth Sciences Department of the University of Coimbra.
Cardita scalaris j.c. de sowerby, 1825 is here placed in genus Scalaricardita sacco, 1899 and for the Carditidae we followed the systematic scheme suggested by Pérez (2019).
Cardium hians brocchi, 1814 in Procardium ter poorten and la perna, 2017. The mention of this species in the Pliocene beds of Alfeite and S. Joanes (Zbyszewski, 1943), and in the Carnide area (Teixeira and Zbyszewsky, 1951;Zbyszewski, 1959) seem to correspond to occurrences of P. diluvianum for the Portuguese Neogene.
In line with the propositions of Fisher-Piette and Métivier (1971) and Canapa et al. (2003), that the genus Paphia röding, 1798 should be restraint to the Indo-Pacific Realm, we considered Tapes megerle von mühlfeld, 1811 as the most adequate genus for Venus vetula de basterot, 1825.
The richness and abundance of the bivalves are similar to those of the gastropods. The molluscan faunas also contain scaphopods and polyplacophora (Dell'Angelo and Silva, 2003).
They occur with a diverse fossil content of macrofaunal elements, including small scleractinian corals, branched and incrusting bryozoans, a terebratulid brachiopod, serpulid worms, balanoids (Ferreira et al., 2019) and decapod crustaceans, spatangoid and epifaunal echinoids, and small fishes (Nolf and Silva, 1997). The microfauna is also very abundant, including benthic and planktonic foraminifera, ostracods and dinoflagellates (Vieira et al., 2006). These assemblages contain many elements that suggest a location in the subtropical Mean Monthly Sea Surface Temperature (MMSST), in a position close to the seashore and slightly protected from the direct influence of the open ocean (e.g. Silva and Landau, 2009).
The 178 species of bivalve molluscs from the Redonian (Pliocene) of the Loire Basin, northwest of France, cited by Lauriat-Rage (1981), despite the strategic geographical position on the edge of the Pliocene French-Iberian Bioprovince, it is not easy for us to establish a general comparison given their heterochronic character from the upper Miocene to the Pleistocene. Only the Redonian sites in the Nantes area and Vendée, Loire-Atlantique, would be of early Pliocene age (e.g. Brault et al., 2004;van Dingenen et. al., 2015) and could be correlative in some extent to the Vale do Freixo assemblage. In addition, Landau et al. (2020) considered the provincialism of the Zanclean (Pliocene) gastropods, including those of the Portuguese fossil site, enough to create a distinct Ligerian molluscan subprovince. Other Britany Redonian localities would be of Miocene age (e.g. Brébion, 1964;Dollfus, 1900Dollfus, , 1904Mercier et al., 2000;Monegatti and Raffi, 2010;Néraudeau et al., 2002Néraudeau et al., , 2003. According to ter Poorten and la Perna (2017), the Early Pliocene to Early Pleistocene large cardiid Procardium diluvianum occurred from the Atlantic Guadalquivir Basin to the eastern Mediterranean. This range area probably extended southwards, along the west coast of Africa. We include Vale do Freixo and the Mondego Basin in the Atlantic margin of West-Central Portugal as the northernmost known area of distribution for this genus, during the Pliocene.

CONCLUSIONS
The Vale do Freixo fossil site, in the West Portuguese Margin of Iberia, yeilded the most diverse and wellpreserved marine faunal assemblages of Pliocene age reported in the area. The Mondego Basin should correspond to the northernmost Piacenzian occurrence area for thermophilic bivalve molluscs such as Cardilia michelottii and Procardium diluvianum. Further studies on cardiliidae specimens from Huelva and Vale do Freixo would be needed to verify that a distinct Pliocene cardiliid species of Atlantic distribution does exist, althrough close to the Mediterranean Cardilia michelottii. It would be also convenient to revise the taxonomic status of the Spaniorinus specimens from the Iberian Pliocene. The results of the present work on the Pliocene bivalves of Portugal emphasize the importance of the Mondego Basin and the Estremadura mountain ranges, in the biogeographic knowledge of the Atlantic marine faunas of Europe, and their relations with the Mediterranean and North-African warmer ones.