Biostratigraphic and paleobiogeographic significance of the Darriwilian microfossils from the top of San Juan Formation in the Los Baños de Talacasto section, Central Precordillera (Argentina)

. The microfossil hosted in the strata of the upper part of the San Juan Formation has been widely studied in several sections to the northward of the Argentinian Central Precordillera. In contrast, the coeval strata at the Los Baños de Talacasto section, in the southern part of the Central Precordillera, have scarce biostratigraphic and sedimentological data. In this work, a conodont association together with single ostracod species are documented for the first time in this section. The record of the Lenodus crassus and L. pseudoplanus zones confirms the Darriwilian age for these beds and accurately correlates them with equivalent strata of the San Juan Formation studied in several sections of the Central and Eastern Precordillera. The microfacies analysis verifies the presence of Nuia síbirica Maslov, 1954, peloids, intraclasts, cyanobacteria, calcareous algae, and a possible microbialite indicating a shallow warm-water subtidal environment, in equatorial to subequatorial climate. This suggests a low latitudes position for Precordillera during the early-middle Darriwilian. The conodont genus Aurilobodus Xiang and Zhang is recognized for the first time from the Precordillera, and the Aurilobodus leptosomatus An specimens are described and illustrated. This genus shows affinities to the warm water in shallow marine environments of North China, Central Asia, South Tibet, western Thailand, Australia, and Newfoundland, suggesting probable ties between Precordillera and these regions. The record of the ostracod Pilla nodospinosa Salas in the study section would agree with the correlation of the top of San Juan Formation with the lower levels of the Las Aguaditas Formation in the Central Precordillera, and also suggests paleobiogeographic links with Eastern Gondwana and Australia regions during the Darriwilian times.

Regarding the ostracods, this group remained scarcely documented from Precordillera until the last decades, in which a series of detailed systematic studies were carried out. Most of the known ostracods are from the Las Aguaditas Formation beds (Darriwilian-Sandbian), where species of the major groups (Palaeocopa, Binodicopa, Leiocopa, and Metacopa) have been described (Schallreuter, 1996;Salas, 2002aSalas, , b, 2003. Both the Floian and Katian faunas are still poorly known, with only isolated reports (Schallreuter, 1995a, b, c;Schallreuter and Hinz-Schallreuter, 1999;Salas, 2007). These studies, in the Ordovician rocks of the Argentine Precordillera, have shown a relatively high diversity of this fossil group in the area.
The study area (Fig. 1) is located in the Central Precordillera, which is considered as typical thinskinned fold and thrust belt with a vergence to the east. All previous studied sections in the Central Precordillera (Mestre, 2012;Mestre and Heredia, 2013a, b;Heredia, 2012), where the top of the San Juan Formation outcrops, are located on the western margin of each thrust fault, by the contrary, there is no data of this formation from the eastern margin. The exploration of new areas and sections from the Central Precordillera is an imperative assignment for increasing the knowledge on the Darriwilian basin paleogeography, as well as the microfossil diversity and distribution.
This work provides an integrated analysis of new data on conodont, ostracods, microfacies analysis, and sedimentary features of the uppermost beds of the San Juan Formation in the Los Baños de Talacasto section with the aim to discuss the biostratigraphic and basin correlation, as well as the paleoenvironmental and paleobiogeographic significance of this information.

Material and Methods
Conodonts and ostracods were collected from 13 samples from limestone beds at 0,5-2 m intervals from the upper part of the San Juan Formation at the Los Baños de Talacasto section (Figs. 1, 2). Initially, 1-2 kg of each sample was dissolved in dilute formic acid with additional material processed if needed, following Stone (1987). The insoluble fraction of each sample was hand-picked for conodonts and ostracods resulting in recovery of ca. 100 identifiable conodont elements and 7 ostracods (the specimens are internal molds with poor preservation) ( Table 1). Conodonts and ostracods are housed in the collection of the Instituto de Geología "Emiliano Aparicio" (INGEO) at the Universidad Nacional de San Juan, under the code-MP. The ostracod specimens were photographed with a SEM microscope in the LAMARX (Laboratorio de Microspopía Electrónica y Análisis de Rayos X) laboratory at the Universidad Nacional de Córdoba, and conodonts with a SEM microscope in the Instituto de Investigaciones Mineras de la Universidad Nacional de San Juan (IIM-UNSJ), Argentina. Eighteen thin and polished sections were made to identify fossils and to analyze the distribution of carbonate components. A petrological investigation of the thin and polished sections was performed using Leica DM2700 microscopes and Lanset binocular microscopes.

Geological setting
The Precordillera is placed in the western central region of Argentina and extended in north-south direction through La Rioja, Mendoza, and San Juan provinces (28°-33° S). This morphostructural province was defined by Furque and Cuerda (1979), and then it was divided into three different belts: Western, Central, and Eastern (Ortiz and Zambrano, 1981;Baldis et al., 1982).
The San Juan Formation, mainly developed in Central and Eastern belts of the Precordillera, is composed of fossiliferous limestone, marly limestone and reef, representing a shallow subtidal environment (Keller et al., 1994;Cañas, 1999). The lower boundary is transitional to the La Silla Formation and is indicated by the first nodular wackestone and packstone containing the characteristic open-sea marine fauna (Keller et al., 1994). The San Juan Formation is diachronically overlain by black shales and laminar mudstones of the Los Azules and Las Aguaditas formations, north and south of the Talacasto range (Carrera and Astini, 1998;Mestre, 2010Mestre, , 2014. In the Talacasto range, the San Juan Formation is unconformity overlain by the basal paraconglomerate of the La Chilca Formation. This unit is composed of siliciclastic deposits that represent part of the Ordovician-Silurian glacial event of Gondwana (Peralta, 1990;Astini and Piovano, 1992;Asurmendi et al., 2020).
The fossil contents, in the Los Baños de Talacasto section (Fig. 1), have been carefully studied over the years (Beresi, 1986;Sánchez et al., 1996;Carrera, 1997;Carrera and Ernst, 2010). However, several topics have not been explored in this section, such as conodont biostratigraphy and carbonate microfacies analysis except by Gallardo (2018). Sánchez et al. (1996) conducted a paleoenvironmental and paleoecological analysis based on sponges and brachiopods of the Athiella brachiopod biozone, in the Cerro Viejo, Cerro La Chilca, Talacasto (Baños de Talacasto) and Villicum sections of the San Juan Formation (Fig. 1). These authors recognize a shallow subtidal environment with intermittent high energy conditions associated with a soft substrate in the Talacasto section. On other hands, based on sponge paleoecology studies, Carrera (1997)  are Patellispongia biofacies, recorded in Talacasto, Villicum and Cerro La Chilca section, which represents a middle ramp setting; biofacies of the elongated sponges, that includes several genera such as Archaeoscyphia, Hudsonosponja, and Calycocoelia, recorded in the Cerro Viejo region, developed in the proximal sector of the distal ramp and finally, the roots biofacies that includes roots of elongated sponges in the Las Aguaditas, Las Tunas and Las Chacritas sections ( Fig. 1), representing distal ramp above the storm wave action. Based on the distribution of sponges biofacies Carrera (1997) proposed a shallow water environment for the central and southern region of the last meters of the San Juan Formation (Talacasto, Villicum and Cerro La Chilca) which deepening towards the north (Cerro Viejo) and the west (Las Aguaditas, Las Tunas and Las Chacritas) of the basin.

Facies description and environment interpretation
In this work, we logged and described the topmost 17.26 m of the upper part of the San Juan Formation in the Los Baños de Talacasto area. In the study section, the San Juan Formation is composed of fossiliferous nodular wackestone-packstone interbedded with intraclastic grainstone at the base, followed upward by burrowed packstone-wackestone to wackestone-mudstone with increasing fine-grained siliciclastic rocks with chert nodules to the top (Fig. 2). Fossil remains of sponges, brachiopods, bryozoans, pelmatozoan ossicles, and trilobites are abundant at the base of the section and decrease to the top. However, the sponge size increases in the same direction. The microfacies analysis allows recognized four main facies along the section: Bioclastic wackestone-packstone (BT1-BT4 samples): This microfacies is generally light to medium grey with diverse and abundant fossil fauna. The bioclastic and carbonate components consist of remains of brachiopods, pelmatozoan ossicles, gastropods, sponges, trilobites, ostracods, Nuia síbirica (Maslov, 1954) (Fig. 3A), algae, cyanobacteria (Fig. 3A), and pellets. The matrix is micritic, grey in color, locally which is recrystallized and cemented by sparite.
Intrabioclastic grainstone (BT5 sample): This microfacies is composed of poorly sorted grainstone light to medium grey in color. The bioclasts include remains of pelmatozoan ossicles, gastropods, bryozoans, trilobites, brachiopods ( by alternating types of micritic laminae, one with darker dense micritic, and another with lighter micritic and scattered intraclast, giving rise to an alternating simple lamination ( Fig. 3E) (Monty, 1976). The bioturbation is intense along of this facies and there are several colored firmground/ hardground on the beds surface. Bioclastic wackestone-mudstone (BT10-BT13 samples): This microfacies displays a micritic matrix with flasery texture due to the incipient recrystallization of the micrite. There are few ostracod valves, trilobites, echinoderm fragments, and thin brachiopod shells which are embedded in the silty micrite. Locally is observed isolated and badly preserved Girvanella tubes (Fig. 4D). The bioclastic wackestone-mudstone are interbedding with thin levels of greenish shale of 2 to 5 cm of thickness, which increases the thickness to the top, exhibiting a style-nodular to wavy bedding. In the stratigraphically youngest 50 cm of the section is rich in small chert nodules (Fig. 4E).
Based on the presence of fine-grained siliciclastic sediment, intense bioturbation, and the diverse faunal assemblage with robust fossil morphologies, which in some case, they are in live position (especially sponges and brachiopods), we interpret that this facies succession was deposited in a normal shallow subtidal environment below wave action with occasional high-energy episodes (Mamet et al., 1984;Holland, 1993;Mángano and Droser, 2004). The presence of Nuia síbirica, as well as cyanobacteria, calcareous algae, and possible microbialite characterizes a warmwater shallow subtidal environment in the photic zone (Riding and Fan, 2001;Liu et al., 2017;Yu et al., 2019) and supports inferences of an equatorial to a subequatorial climate in a low-latitude region (Vachard et al., 2017).

Conodont biostratigraphy and global correlation
The conodont biostratigraphy studies from the top of the San Juan Formation in the Talacasto range are limited. Lozano and Hünicken (1990) recognized the P. serra Zone for the uppermost beds of the San Juan Formation, but these authors did not offer details of the section sampled. Then, those conodont association was reinterpreting by Lehnert (1995), who proposed the E. suecicus Zone for these levels based on the presence of the conodont Histiodella kristinae Stouge. On the other hand, the L. variabilis Zone was mentioned for the top of San Juan Formation in the Ancha creek (Albanesi et al., 2006).
Despite the low number of conodonts recovered from the carbonate beds of the San Juan Formation at Los Baños de Talacasto section (Table 1), the presence of Darriwilian key conodonts let to identify the following conodont biozones.

Lenodus crassus Zone
The record of the L. crassus Zone in the Los Baños de Talacasto section is based on the occurrence of the eponymous species at the base of the study section of the San Juan Formation, and the upper limit of this biozone is indicated by the first occurrence of L. cf. pseudoplanus (Viira) at 17 m above the base of the section ( Fig. 2; Table 1).
The occurrence of L. crassus in this section is significant for regional and intercontinental correlation because it is used as an index species for the zonal schemes in China, Baltoscandia and Precordillera (Zhang, 1998a, b;Löfgren, 2003;Löfgren and Zhang, 2003;Heredia et al., 2017;Mestre, , 2014Mestre and Heredia, 2013a) (Fig. 6).
The record of L. pseudoplanus Zone in the Los Baños de Talacasto section represents the most FIG. 6. Darriwilian conodont biostratigraphical chart from the Precordillera. The shadow area represents the conodont zones recorded in the study section.
southern record for this conodont biozone in the Central Precordillera. It is also significant for the intercontinental correlation since it is used as an index species for the zonal schemes in China, Baltoscandia, Australia and Precordillera (Zhang, 1998a, b;Löfgren, 2003;Löfgren and Zhang, 2003;Heredia et al., 2017;Mestre, 2012;Heredia, 2012, 2013b;Zhen, 2020) (Fig. 6). The extension of the H. sinuosa range up to the L. pseudoplanus Zone may be related to relative shallow water environment of these stratigraphic levels, owing to the H. sinuosa is characteristic of early Darriwilian carbonate shallow water settings from the Midcontinent (Bauer, 2010). Based on the absence of H. kristinae and the presence of H. sinuosa in the uppermost beds of San Juan Formation in the Los Baños de Talacasto section, we also interpret that in these levels could be registering the base of the L. pseudoplanus Zone. The

Systematic Paleontology
Conodonts obtained from the studied samples were mainly described and illustrated in previous publications (Lehnert, 1995;Albanesi, 1998;Mestre and Heredia, 2013;Mestre, 2014). Hence, only one species is included in the following description in this report, since this represents the first mention for the Precordillera. A single species of ostracod was recovered from the study section, however, it represents the first ostracod record from the top of the San Juan Formation, for this reason, a brief description of it is also included.

Remarks:
The present record represented the first mention of the genus Aurilobodus from Darriwilian strata of the Precordillera. Xiang and Zhang (in An et al., 1983) defined the genus Aurilobodus as having a bimembrate apparatus including symmetrical and asymmetrical elements. Nevertheless, An et al. (1983) described six species of Aurilobodus from the Darriwilian from North China, which are composed of four morphotypes: geniculate M, symmetrical Sa, asymmetrical Sb, and strongly asymmetrical Sc elements . Lehnert (1995) proposed the genus Aurilobodus as a probably junior synonym of the genus Juanognathus, but the arguments to support this statement are not clear. However, Zhen et al. (2020) proposed both genera as valid, Juanognathus which typically dominated the Floian offshore faunas of the Open-Sea Realm, and Aurilobodus, which inhabited mainly from the Darriwilian shallow marine setting. The species Juanognathus serpaglii Stouge, 1984, was reported from the Table Head Formation (Newfoundland) by Stouge (1984), in the present contribution is considered as a junior synonym of the species A. leptosomatus, following the proposal of Lehnert (1995) and Zhen et al. (2020).  (Salas, 2002a) in the Precordillera Argentina. The studied material has all the diagnostic features of P. nodospinosa, species, nodes separated from each other, both protruding beyond the dorsal margin, with the posterior one spine like. These features differentiate this species from the remaining species of the genus, where the nodes are rounded and the sulcus narrow. The more similar species is P. reedi (Wolfart, 2001a), from Darriwilian-early Sandbian western Thailand, by its broad sulcus and its smaller posterior node, however, in P. nodospinosa the posterior node is spine-like. Moreover, in this species the valves show a well-developed and ridge-like pseudovelum extended from the anterior cardinal angle to the postero-ventral sector of valve, while the remaining species show a rounded and wide pseudovelum. Some specimens of the recorded

Regional correlations
The presence of the index taxon L. crassus was first recorded in the Precordillera from the uppermost level of the San Juan Formation in Del Aluvión section (Fig. 1) (Mestre, 2010;Mestre and Heredia, 2013a) and subsequently, it was found at numerous localities from Central and Eastern Precordillera Heredia, 2013a, b, 2020a, b;Mestre, 2014;Heredia et al., 2017). This biozone is also recorded from the uppermost level of the San Juan Formation in the Los Amarillitos section, Las Aguaditas section and several sections in the Villicum range (Fig. 1), where the San Juan Formation is overlain by the Los Azules or Las Aguaditas formations Heredia, 2013a, 2020a, b;Mestre, 2014) (Fig. 8). uppermost strata of the San Juan Formation and lower levels of the Los Azules and Las Aguaditas formations (Heredia, 2012;Mestre, 2012;Heredia, 2013a, b, 2020b) (Fig. 8).
Regarding the presence of the ostracod P. nodospinosa in the Los Baños de Talacasto section, it represents the first record of the genus in the top of the San Juan Formation. The previous record of the species corresponds to the lower-middle levels of the Las Aguaditas Formation (sensu , assigned to the L. pseudoplanus and L. suecicus zones indicating a Darriwilian age (Heredia, 2012;Feltes et al., 2016;Heredia, 2013b, 2020a, b). Although the ostracods are not good biostratigraphic markers, the presence of this species in Los Baños de Talacasto section would agree to the age and correlation documented by conodonts.

Paleoecological remarks of the conodont association
The Darriwilian conodont fauna from the Precordillera is characterized by high diversity and abundance perhaps as response to the GOBE (Gobal Ordovician Biodiversification Event) rise (Stigal et al., 2019). Regarding the paleoenvironmental preference of the Histiodella species, they show a wide distribution on all marine environments, from shallow to deep water and from carbonate to siliciclastic settings, having pelagic behavior (Ethington and Clark, 1981;Stouge, 1984;Zhang, 1998a;Löfgren, 2004;Tolmacheva, 2014;Jing et al., 2016).
P. horridus and P. macrodentatus are the most abundant species in the Darriwilian conodont fauna in the Precordillera, representing about the 50% of the population in the L. crassus and L. pseudoplanus zones, especially in western sections from the Central Precordillera, such as Las Chacritas river and Cerro La Chilca sections (Mestre, 2010;Serra et al., 2017b;Mestre and Heredia, 2020b). In other regions of the world, the genera Periodon and Paroistodus characterized deep and open-sea biotopes, occupying upper to lower slope environments (Stouge, 1984;Zhang, 1998a;Wu et al., 2014;among others). However, low number of specimens recovered of P. horridus and P. macrodentatus from the top of San Juan Formation in the Los Baños de Talacasto section and the presence of the A. leptosomatus, may be related to shallow environment recognized for those beds (Table 1). Nevertheless, the regional study for recognizing the distribution of these shallow water settings and its conodont faunas in the Central and Eastern Precordillera should be developed in the future for corroborating this hypothesis.

Affinities of the recorded microfauna and its paleobiogeographic implications
The genus Aurilobodus has affinities to the warm water fauna of the North China Platform (An et al., 1983;Wang et al., 2014) and it inhabited the shallow waters on the shelf with normal temperatures and salinities of the Australasian Superprovince (Agematsu et al., 2006(Agematsu et al., , 2008Agematsu and Sashida, 2009;Zhen and Percival, 2017;Kuhn and Barnes, 2005;Yu et al., 2019). The A. leptosomatus occurrence in the Precordillera indicates a probable paleobiogeographic tie between North China, South Tibet, Central Asia, western Thailand, Australia, Newfoundland and the Precordillera during the early-middle Darriwilian (Fig. 9).
The previous studies on Ordovician conodont paleobiogeography interpreted a temperate domain for the Ordovician from the Precordillera (Zhen and Percival, 2004) or suggested that the Precordillera was closer links to North America (Marathon basin, Newfoundland, and the Antelope Valley) (Albanesi and Bergström, 2010). On the other hand, the Darriwilian conodont biostratigraphy chart is based on Baltoscandian and South-Central China biostratigraphy schemes evidencing ties with those areas (Heredia andMestre, 2011, 2013;Heredia et al., 2017) (Fig. 4).
The probable paleobiogeographic links between North China, South Tibet, Central Asia, western Thailand, Australia, Newfoundland, and the Precordillera during the Darriwilian, show the necessity to re-evaluate the significance of the spectrum of the conodont biofacies, from shallow to deep water, in the conodont provincialism or paleobiogeography affinities. As first noted by Sweet et al. (1959) and Sweet and Bergström (1962), the correlation of shallow-and deep-water facies is difficult by the conodont provincialism in the Ordovician. In several regions were defined different conodont biostratigraphic chart for the platform and the slope environment, e.g., North and South China Jing et al., 2017) or British Columbia (Pyle and Barnes, 2002), showing the strong biofacial control on the provincialism.
Respect to the ostracods, the genus Pilla Schallreuter and Siveter, 1988 has so far six known species from Australia, western Thailand (Sibumasu), southwestern China and northern India, in addition to the two of Precordillera. The recorded species P. nodospinosa, was defined for material from other localities of the Precordillera Argentina, so their only previous record corresponds to the Darriwilian of the lower-middle levels of the Las Aguaditas Formation (sensu meanwhile, P. austramericana Schallreuter, 1996 comes from the middle-upper levels of this formation (lower Sandbian). The oldest species of the genus is P. latolobata Jones and Schallreuter from Floian-Dapingian levels of the Amadeus Basin (Jones and Schallreuter, 1990). P. piformis Schallreuter and Siveter (type species of the genus), also from Australia, comes from Sandbian-Katian levels of New South Wales (Schallreuter and Siveter, 1988). Moreover, P. reedi (Wolfart, 2001a) from western Thailand in Sibumasu, is also recorded during the Darriwilian, in younger levels than of the Precordillera. The species is known from late Darriwilian to early Sandbian Thong Pha Phum Silt-Mudstone Formation (Wolfart, 2001a, b). Recently, P. reedi was also recorded in the late Darriwilian to early Sandbian Shihtzupu Formation of South China (Zhang, 2020). Finally, the youngest species of the genus, P. pinensis Schallreuter, occurs in the Pin Formation (late Katian), northern India, northwestern Himalaya (Schallreuter et al., 2008). The paleogeographical distribution of the genus was discussed by Zhang (2020) who suggest that the temporal and spatial distribution of Pilla might have originated in low-latitude areas and then migrated to peri-Gondwana terranes during the Middle Ordovician.
In this context, the presence of the genus Pilla suggests, at least since the Darriwilian, a paleobiogeographic relationship between the Precordillera Argentina with Eastern Gondwana and Peri-Gonwana terranes. This paleobiogeographic affinities are also suggested by other genera of ostracods like Eodominina Schallreuter and Velapezoides Mcgill recorded both in Precordillera (Salas, 2002a, b) and Australia (Schallreuter and Siveter, 1988); while with South China, in addition to Pilla, genera like Aechmina and Longiscula are also common (Salas, 2003(Salas, , 2007Yichi Zhang, 2020). This connection between the Precordillera and Australia begins during the Darriwilian with the record of the Pillinae (Salas, 2002a), even though the remaining fauna exhibit a high percentage of endemic genera, and a mixture of genera with several affinities, Baltic and peri-Gondwanan (Salas, 2007).
Other groups like trilobites show similar behaviour, with genera that suggest biogeographic relationships FIG. 9. Paleogeographical reconstruction for the Middle Ordovician (Torsvik and Cocks, 2013) showing geographical distribution of the Aurilobodus leptosomatus (triangle) and ostracod genus Pilla with several localities around Gondwana since the Darriwilian, such as North and South China, Australia, Tasmania, Himalaya, Turkey and Armorica (Edgecombe et al., 1999;Waisfeld et al., 2001). The same occurs with the rostroconch Tolmachovia, that also suggest affinities with Australia and Tasmania since Darriwilian (Sánchez, 1998). Regarding rhynchonelliform brachiopods, they display a significant percentage of genera with Baltic and Celtic affinities (Benedetto, 2003). Nevertheless, during the Darriwilian the Precordillera is thought to have been situated relatively close to the Gondwana margin but not too far from Laurentia (Benedetto, 2004). Moreover, based on Darriwilian Baltic signature of a number of linguliform and rhynchonelliform brachiopods from the Precordillera, Lavié and Benedetto (2016) infer that it was located in low-temperate latitudes like to Baltica. On the other hand, the acrotretids and other lingulid microbrachiopods from the San Juan Formation in the La Chilca section and La Brecha creek section display major similarities with those inhabiting low-latitude paleocontinents (Holmer et al., 2016;Lavié et al., 2021). Accordingly, the paleobiogeographic affinities shown by the Darriwilian conodont assemblage in the Los Baños de Talacasto section are in agreement with those observed in ostracods and other fauna.
The discovery of the shallower water marine environment at the Baños de Talacasto section and the conodont association retrieved from these beds provided substantial information on the biofacial control on the paleobiogeography affinities in the Ordovician conodonts from the Precordillera and denoting the strong control of the biofacies on the provincialism. In a similar way the analysis of Ordovician ostracod palaeobiogeography has also shown that geography and environment appear to be an important control on the distribution of the fauna. Thus, the composition of ostracods, at least at an upper taxonomic group level (suborder and family), displays an important palaeobiogeographic component, which can be observed since the Middle to Late Ordovician (e.g., Vannier et al., 1989;Williams et al., 2003;Salas, 2011).
In the light of this new data, is evident the necessity of future studies for understanding the distribution of the shallow water carbonate environment and the microfossil faunas that inhabit these biotopes in the Precordillera basin, as well as the paleogeographic location of the Precordillera and surface currents configuration in the Iapetus during the Middle Ordovician.

Darriwilian Nuia record
The genus Nuia was recognized in the Early Ordovician carbonate beds of the San Juan Formation in several sections from Eastern and Central Precordillera (Beresi, 1986;Cañas, 1999;Keller, 1999) and the siliciclastic Volcancito Formation in Famatina (Astini, 2001). This microfossil was considering as algae, cyanobacteria, or microproblematic organism that was common and widespread in the Late Cambrian-Middle Ordovician shallow water deposits (Riding and Fan, 2001). Recently, Vachard et al. (2017) proposed that the Nuia is probable a rivulariacean cyanobacteria exclusively from the Early Ordovician, dismissing the Late Cambrian and Middle Ordovician records, also these authors defined the Nuia paleogeographic province that characterizes a tropical to subtropical low-latitude settings. This paleogeographic province would include the Laurentia, Siberia, Tarim, Kazakhstan, North China, South China, and Precordillera during the Early Ordovician.
The paleolatitudinal position of Precordillera was probably tropical to subtropical (low-latitude) until the latest Dapingian-earliest Darriwilian, due to the presence of the Zondarella-dominated reefs in the upper part of the San Juan Formation at the Eastern Precordillera (Lehnert and Keller, 1993;Mestre et al., 2020). However, the youngest strata over this reef facies most likely were deposited on a carbonate and mixed platform in middle latitude as result of the collision of the Precordillera (Cuyania terrane) against to the western margin of Gondwana (Astini et al., 1995;Thomas and Astini, 1996;Thomas et al., 2000).
The finding of Nuia in the studied strata would allow to place the Precordillera in a low-latitude position during the L. crassus Zone, extending up to this time interval the tropical to subtropical condition for the south-western margin of Gondwana, as is proposed by Torvisk and Cocks (2013Cocks ( , 2017 and Cocks and Torsvik (2020). On the other hand, we confirmed the record of Nuia up to the Middle Ordovician, as have already been verified by Guildbault et al. (1976), Riding and Fan (2001) and Shen and Neuweiler (2016) in Canada and Tarim basin (North China).

Conclusion
A new conodont and ostracod fauna, as well as carbonate component across the upper part of the San Juan Formation, have been analyzed based on material from the Los Baños the Talacasto stratigraphic section. The main conclusions are as follows: • Darriwilian conodont and ostracod association is documented for the first time from the top of San Juan Formation at Los Baños de Talacasto section. • The L. crassus and L. pseudoplanus zones are recorded, providing a substantial improvement in the knowledge on the distribution of these biozones in the Central Precordillera. • A shallow warm-water subtidal environment in equatorial to subequatorial climate in low latitudes was recognized in the last meters of the San Juan Formation in this section. • The A. leptosomatus occurrence in the Precordillera Darriwilian strata shows a probable paleobiogeographic link with equivalent beds from North China, South Tibet, Central Asia, western Thailand, Newfoundland, and Australia during the Darriwilian. • The presence of P. nodospinosa is recorded for the first time in San Juan Formation, its record would be in agreement with the biostratigraphic and paleobiogeographic analysis based on conodont associations.