The Paraná Basin is a intracratonic basin located in southernmost Brazil, Uruguay and parts of Paraguay and Argentina. The basin covers a surface area of about 1,700,000 km2, has a present NE-SW elongated shape, and is approximately 1750 km long and 900 km wide. Late Paleozoic rocks along the eastern border of the Paraná Basin in east-central São Paulo state, southeastern Brazil, are represented, from the base up, by the Tubarão Supergroup (Upper Carboniferous to Permian), including the Itararé Group and the Tatuí Formation, and the Passa Dois Group (Permian), consisting of the Irati and Corumbataí Formations (Holz et al., 2010).

The term Irati was initially proposed by White (1906) as “black schisto”, rich in oil, then registered in the states of São Paulo and Paraná. Subsequently the research was extended to the entire Brazilian part of the Paraná Basin (Fig.1), White’s proposal resulted in many stratigraphic researches, until reaching Barbosa & Gomes (1958) who subdivided the Irati into two members: Taquaral (lower) and Assistência (upper). This stratigraphic division was accepted by most of the researchers (Holz et al., 2010; Chahud et al., 2010a, 2010b, 2012).

Hachiro et al. (1993) and Hachiro (1996) proposed upgrade the Irati Formation to Subgroup and the Taquaral and Assistência members to formations. This proposition was not properly formalized, due to mapping problems, so the present researchers rather favor to keep the proposal of Barbosa & Gomes (1958).

Absolute dating on Assistência beds by Santos et al. (2006) and Rocha-Campos et al. (2007) yielded Cisuralian ages, between 279 - 276Ma (Kungurian).

The Taquaral Member occurs irregularly in the Paraná Basin, absent in some places and with thickness close to 30 m in others (Hachiro, 1996; Chahud, 2007, 2011). Dark gray and non-bituminous siltstones and mudstones are the main lithologies (Fig. 2). Three facies are distinguished based on lithologies, fossils and geographic areas.

The first facies is defined by lenticular layers of conglomeratic sandstones, with granules and chert pebbles. Maximum thickness is one meter, normally 10 to 40 cm. These lithosomas are disposed irregularly, without any order of deposition of granulometry. Hachiro (1996) distinguished several facies on granulometric basis. Diversified Chondrichthyes were studied by Chahud & Petri (2008a, 2009a, 2010b, 2010c, 2012a).

The second facies is an almost homogeneous non bituminous silty-shale, generally plane-parallel laminated, no sedimentary structures. The thickness range from 5 to 30 meters. The crustacean Clarkecaris Clarke 1920 is known only in this facies.

A third facies represented by fine carbonate layers can be observed in the thichest stratigraphic sections of the Taquaral Member, in the State of Paraná. The thickness of these carbonates around 30 cm. It is composed of dolomitic micrites, occasionally containing bivalves, located in the upper half of the unit (Hachiro, 1996; Lages, 2004; Matos et al. 2017).

The Taquaral Member has recently been the target of numerous paleontological and sedimentological contributions, with emphasis on the study of facies, vertebrates, arthropods, ichnofossils, palynology and taphonomy (Lages, 2004; Holz et al. 2010; Chahud et al. 2010b; Chahud & Petri, 2010a; 2015b). A synthesis of these contributions is the main goal of this contribution.

The presence of abraded specimens together with teeth, scales and spines with few little breaks or even no kinds of abrasion, are interpreted as deposits laid down under episodic events of storms (Chahud & Petri, 2015b).

The crustaceans of the silty-shale facies are complete or almost complete with scarce examples of little abrasion or broke. The vertebrates are disarticulated, represented mainly by well preserved scales of Sarcopterygii and Palaeonisciformes (Chahud, 2017).

The Taquaral Member bivalves were taphonomically studied by Matos et al. (2017) who noticed a preferential orientation in a unimodal or bimodal arrangement, characterizing shallow water transport to more distant regions through episodic events of storms.

The Irati stratigraphy is heterogeneous, with geographical differences. It is important therefore, to analyze local sections which clarify different paleoenvironments, along the time.

The overall details of the Taquaral basal deposits allow interpretation of a transgressive tract system (Hachiro, 1991, 1996; Assine et al., 2003; Holz et al. 2010).

According to Holz et al. (2010), the Taquaral Member was laid down in a shallow environment with very restricted connection with the open ocean. However Chahud & Petri (2013a, 2013b, 2014) questioned this connection due to the absence of exclusively marine fossils based on comparison with the current Black Sea system, where typical freshwater animals were replaced by a marine fauna after a connection with the Mediterranean Sea, during a few thousand years (Sorokin & Kuprin, 2007).

The silty-shale facies exhibits preserved complete crustaceans and fragile Actinistia scales (Chahud & Petri, 2013a, 2013b), attesting good conditions of preservation. However, no exclusively marine fossil are present, in spite of a long history of Taquaral researches.

The sandy facies, located at the base of the Taquaral Member, is the only evidence of connection with an outside basin. According to Chahud & Petri (2014) the Taquaral waterbody might be connected with the Parnaiba Basin, Pedra de Fogo Formation, northeastern Brazil, during the Permian, for a short time, which might have helped to increase the level of the Taquaral waterbody in the Paraná Basin. The Chondrichthyes Itapyrodus punctatus and Taquaralodus albuquerquei present in both basins, are evidence for this connection (Silva Santos, 1946, 1990; Chahud et al., 2010b; Chahud & Petri, 2010c, 2014). The absence of these fishes in any other basins suggest that they did not migrate from other territories.

Itapyrodus punctatus is reported, in Paraná Basin, only in the Taquaral sandy facies and only in the State of São Paulo and Parnaiba Basin, as seen above. Other species of Itapyrodus, as yet not named, was also reported only in the State of São Paulo, but in the Corumbataí Formation, younger than Irati (Chahud & Petri, 2016).

The Taquaralodus albuquerquei also is known only in the Taquaral sandy facies of the State of São Paulo, without any similar fossils, being later replaced by Xenacanthidae of the genera Xenacanthus and Triodus in more recent units, Corumbataí, Teresina and Rio do Rasto formations (Würdig-Maciel, 1975; Ragonha, 1984; Pauliv et al., 2014; 2017).

The sudden disappearance of the Chondrichthyes in the Taquaral Member after the sandy facies, possibly was a result of decreasing salinity of water body after the sandy facies deposits, isolated forever from the Parnaíba Basin and, eventually, from the open ocean.

After the beds bearing Itapyrodus punctatus and Taquaralodus albuquerquei in the Pedra de Fogo Formation, Parnaíba Basin turned over to a continental paleoenvironment bearing plant fossils and tetrapods (Cisneros et al., 2015; Conceição et al., 2016).

The influence of rivers leading to freshwater environments for the Taquaral Member was first suggested by Mussa et al. (1980) based on the presence of Clarkecaris, which, as Anaspidacea, has been associated with freshwater. However, as discussed earlier, Clarkecaris is a transitional form between the present freshwater Anaspidaceae and the Carboniferous Palaeocaridaceae of marine origin or of variable salinity, so it is not a good indicator.

Although no fossils in Taquaral is exclusively freshwater, on the other hand, no ocean fossils are present. Besides, freshwater flows were recognized by the presence of pollen, spores and algae Botryococcus (Lages, 2004).

Carbonate layers bearing bivalve coquinas (Facies 3, Fig. 2) are solely as yet present in the state of Paraná (Matos et al., 2017). They interfinger with the facies silty shale.

During the carbonate layers deposition, the waterbody keep decreasing, caused by the dry climate, resulting a low energy paleoenvironment and the beginning of Assistência Member, anoxic phase (Matos et al., 2017).

Episodic events, such as storms, have always been common during the deposition of the Irati Formation. According to Chahud & Petri (2015a) and Matos et al. (2017), the storms were responsible for the fish and bivalves deposits.

Many fossils occur in paleoenvironments of variable salinity such as acritarchs, bivalves and fishes. Related species are known in older units, Palermo and Tatuí formations and Itararé Group (Silva Santos, 1946; Fúlfaro et al. 1984; Cazzulo-Klepzig et al. 1989; Marasco et al. 1993; Holz et al. 1998; Chahud & Petri, 2015b; 2015c). Many species may have survived paleoenvironmental changes, reaching younger units due to their adaptability (Chahud & Petri, 2016, Matos et al., 2017). These species were restricted to specific conditions of the basin, allowing survival, in restricted or epicontinental seas, like Baltic, Caspian and Aral (Zenkevich, 1957; Brenner, 2005).

The Taquaral Member, the basal unit of the Irati Formation, is composed mainly of non-bituminous silty-shales, with sandy layers located at the base, near the border of the basin and thin carbonates layers in the upper half, of the thickest stratigraphic sections.

The microfossils include several genera of palynomorphs, spores, pollen, acritarchs and the Botryococcus algae. None were found to be reliable indicators of oceanic, freshwater, or continental environments for their adaptability for changing or ease of transportation.

The lowermost Taquaral sandy-facies, in spite of thin (ten to forty centimeters, seldom 1 meter) is noteworthy for testifying an unique happening a connection between two basins, Paraná and Parnaíba.

Other noteworthy characteristic of the sandy-facies as the great diversity and frequency of fishes and the presence of both Taquaral Member and Pedra de Fogo Formation, of the species Itapyrodus punctatus and Taquaralodus albuquerquei. These species occur only in these two units, absent in any other basin in Brazil and abroad, evidence of the connection between two basins as mentioned above. Several groups of fishes still upholder the lower salinity but others were extinct, like the genus Taquaralodus.

The better preserved fossils are the crustaceans from the silty shale facies. The Clarkecaris crustacean is the most common, endemic, different from all existing Anaspidacea, its evolution in isolation resulted in an unique form, no descendants in the geological record.