Lower Jurassic calcareous nannofossil taxonomy revisited according to the Neuquén Basin (Argentina) record

. Standard Early Jurassic biostratigraphic studies were performed in the boreal and Tethys realms (western Europe and northern Africa), and biozonations from these areas are the most accurate of the world. Comparatively, investigations in the Paciﬁc realm are scarce, and, in Argentina, they are limited to contributions based on oil-industry subsurface and outcrop reports for the Los Molles Formation. A focused systematic analysis was not previously addressed in the area. The Neuquén Basin in west–central Argentina offers a unique opportunity to study the Early Jurassic calcareous nannofossil history in the south-eastern Paciﬁc Ocean. Calcareous nannofossil assemblages from El Matuasto I section (Los Molles Formation) represent one of the earliest records for the Early Jurassic in the Neuquén Basin and one of the few for the eastern Paciﬁc realm. A detailed systematic analysis allowed the recognition of major bioevents and a comparison with worldwide associations and biostratigraphic schemes. A thorough taxonomic discussion of the Early Jurassic nannofossil species of the Neuquén Basin is presented for the ﬁrst time. Herein, the taxonomic features of coccoliths recorded in the Neuquén Basin are settled. The age of the calcareous nannofossil assemblages recorded in El Matuasto I is early–late Pliensbachian, covering the NJT4a to NJT4c subzones. Similarities between the Neuquén Basin and localities from the proto-Atlantic region suggest an effective connection between the Paciﬁc and Tethyan basins during the Pliensbachian.

Comparatively, investigations in the Pacific realm are scarce Fantasia et al., 2018) and in Argentina are restricted to Los Molles Formation, represented by few general studies (Bown, , 1992Ballent et al., 2000Ballent et al., , 2011Angelozzi et al., 2010; Al-Suwaidi et al., 2010,2016) or contributions based on oil-industry subsurface and outcrop reports (Angelozzi, 1988;Bown and Ellison, 1995 2021). However, a focused systematic analysis was not previously addressed in the area.
In this context, the Neuquén Basin, located in west-central Argentina, offers a unique opportunity to study the Early Jurassic calcareous nannofossil history in the south-eastern Pacific Ocean. The basin yields a Lower Jurassic marine transgression from the palaeo-Pacific and records an extensive sedimentary succession .
Calcareous nannofossil assemblages from Los Molles Formation represent the earliest record for the Early Jurassic in the Neuquén Basin and one of the few for the eastern Pacific realm (Bown, 1992;Fantasia et al., 2018). This contribution aims at characterizing the Pliensbachian calcareous nannofossil assemblages of the south-eastern Palaeo-Pacific region through a detailed study of El Matuasto I section (Neuquén Basin, Argentina). A detailed systematic analysis allowed recognition of major events and a comparison with worldwide associations and biostratigraphic schemes.

Geological setting
The Neuquén Basin is located in west-central Argentina, constituting a series of marine and continental sub-basins that have developed behind the Pacific margin of the South American Plate (Fig. 1a) (Legarreta and Uliana, 1999). Since the beginning of the sedimentary filling in the Late Triassic-Early Jurassic, it has accumulated more than 7000 m of Mesozoic deposits . During the Pliensbachian-Aalenian interval and in the Tithonian, marine sedimentation was widespread in the Neuquén Basin, while it was restricted to a few areas in other time intervals. Until the Early Cretaceous, this basin was part of the south-eastern Pacific Ocean and had a unique record of marine micro-and macrofossils in the world. The detailed stratigraphic characterizations made on well-exposed outcrops resulted in a thorough understanding of the basin evolution (Groeber, 1918;Weaver, 1931;Stipanicic, 1969;Riccardi, 1983; Gulisano et al., 1984;Legarreta and Gulisano, 1989; Riccardi and Gulisano, 1990;Uliana, 1996, 1999;Lanés, 2005;Leanza, 2009;Arregui et al., 2011;Legarreta and Villar, 2012).
The Los Molles Formation (Weaver, 1931) is mainly composed of grey and dark grey mudstones, with fine to medium sandstones interbedded and variable organic content. Sedimentation corresponds to a marine environment with restricted conditions  and represents the earliest Pacific marine transgression in the basin (Legarreta and Uliana, 1996; Legarreta and Villar, 2012). The age of the formation covers the Hettangian to Callovian, considering its total extension in the different areas of the basin (Gulisano and Gutiérrez Pleimling, 1995;Vergani et al., 1995 The El Matuasto I section is located approximately 45 km south of the city of Zapala (Neuquén Province) and 1 km from the Picún Leufú River bridge along the Ruta Nacional 40 (Fig. 1b-c). It is represented by a 33 m thick succession of mudstones with thin fine-medium sandstone intercalations. Sand beds are interpreted as turbidites, where the basin experienced episodes of continental sediment input. Bioturbation and plant debris are common throughout the sequence.

Materials and methods
A set of 26 samples from the El Matuasto I section (Los Molles Formation) was analysed for calcareous nannofossils. The preparation method consists of a slight modification of the technique described by Beaufort et al. (2014). A small amount of powdered rock was diluted with 30 mL of water. The suspension was poured on a coverslip in a Petri dish. The cover slide was weighed before applying the suspension with the study material. Once the sediment was settled (after 4 h), the water was carefully removed to avoid turbulence. The coverslip was then dried to remove the remaining water, weighed again, and then mounted on a microscope slide using Norland 61 optical glue. This method enables us to calculate the absolute abundance (nannofossil per gram of sediment) using the formula described by Menini et al. (2019).
Identification and counting of calcareous nannofossils were performed using a Leica DMP 750 petrographic microscope at 1000× magnification under crossed polars. Photographs were taken with a Leica MC 170 HD camera. For each sample 300 specimens were counted, thus ensuring that the probability of not recovering a rare species is below 5 % (Fatela and Taborda, 2002). In the slides for which a low nannofossil abundance made it difficult to count 300 specimens, the counting stopped at 450 fields of view (FOV; the area of one FOV is 0.0069 mm 2 ).
All nannofossil data were integrated in a distribution chart (Fig. 2). The absolute abundance (as nannofossils per gram of sediment) and preservation index are also indicated. Preservation is a discrete scale based on the general aspect of the specimens (Roth, 1984). G: good -most specimens exhibit little or no secondary alterations and delicate structures such as spines are preserved in most cases. M: moderate -specimens exhibit some degree of overgrowth and/or dissolution (identification of species not compromised). P: poor -the effects of overgrowth and/or dissolution are very intense (identification of species is impaired but possible in some cases).
The systematic palaeontology follows the criteria by de Vargas et al. (2007) for subclass level and up and Young and Bown (1997) and Bown and Young (1997) for levels below subclass.

Results
A total of 16 samples of the El Matuasto I section yielded calcareous nannofossils, while 10 were barren. The general preservation of the assemblage is moderate to good, with overall better preservation towards the upper part of the section. Sample richness varies between a minimum of 10 and a maximum of 18 species per sample. Stratigraphic distribution of nannofossils and other features are given in Fig. 2.
For all the recovered species, the entire synonymies provided in the literature, the original papers describing the holotype, and the available literature illustrating a given species were carefully checked. The systematic palaeontology below is thus based upon a careful revisitation of previous literature and presents a synonymy list as complete as possible. For each species, remarks are provided with respect to the original diagnosis, which also make reference to descriptions provided in the literature.
As far as the coccolith morphology is concerned, we refer to Young (1992) for the murolith and placolith description and to  for the protolith and loxolith structure. The protolith rim structure of muroliths comprises a domi- nant distal shield and a proximal shield, both showing a vertical (distal) extension (see Bown, 1987b, text- fig. 6B). The distal shield is composed of elements joined along sutures which are perpendicular to the coccolith base without imbrication. The protolith rim structure is possessed by the genera Crucirhabdus, Mitrolithus, and Parhabdolithus. The loxolith rim structure of muroliths comprises a dominant distal shield and a proximal shield with a vertical (distal) extension (see Bown, 1987b, text- fig. 6A). The distal shield is composed of tall, narrow, steeply inclined and imbricating laths. The loxolith rim is possessed by the genera Crepidolithus and Tubirhabdus. The placolith rim structure possesses elements forming a slightly concave-convex shield developing in the horizontal plane parallel to the cell surface as opposed to the murolith tall upright rims, which were vertically orientated. The radiating placolith rim structure comprises a proximal and distal shield, both unicyclic (Bown, 1987b, text- fig. 7A). The distal shield is composed of blade-like laths lying side by side, with the suture lines between each element orientated radially to the central area of the coccolith. The genera which display this structure include Biscutum, Similiscutum, and Calyculus. The imbricating placolith rim structure consists of a bicyclic distal shield, a unicyclic proximal shield, and a connecting inner wall (Bown, 1987b, text- fig. 7B). The distal shield outer cycle is composed of blade-like laths which are imbricating and joined along sutures with anti-clockwise inclination. The only genus considered here which displays this structure is Lotharingius. Range. Sinemurian-Tithonian (Bown and Cooper, 1998). Occurrence. Given the wide range of C. crassus, this species has been recorded in the entire studied interval. The FO of this species is used by some authors to mark the NJ2-NJ3 zone boundary (Barnard and Hay, 1974;Bown and Cooper, 1998;Fraguas et al., 2015).

Systematic palaeontology
Remarks. The original diagnosis of Discolithus crassus describes an "elliptical slightly elongated, thick coccolith without an elevated rim, exhibiting an undulated longitudinal median line, interrupted in its centre by divergent lateral lines and few punctuations (? perforations)" (Deflandre in Deflandre and Fert, 1954, pp. 115-176). Noël (1965b, p. 88) emended this diagnosis, stating that it is "a typical Crepidolithus". The punctuation reported by Deflandre and Fert (1954) might be the result of poor preservation of the specimen that they illustrate under light microscope (LM). The later description by  p. 16) reports a "broad, high, elliptical rim with a vacant central area often reduced to a lenticular slit. . . The broader the wall the narrower the central area". Actually, scanning electron microscope (SEM) pictures (Bown, 1987b, pl. 1, figs. 6-9, p. 15) show a variable-sized, vacant central area. In fact,  and  observed a certain variability in the central area of C. crassus, which can be slightly open. Suchéras-Marx et al. (2010) reported two differentsized coccolith morphotypes, named "small crassus" with a mean size of 6.5 µm and "large crassus" with a mean size of 8.5 µm. Fraguas and Erba (2010) performed biometric analysis on C. crassus and C. crucifer, and they differentiated the Plate 1. Calcareous nannofossils from El Matuasto I, Los Molles Formation, Neuquén Basin. All pictures from LM under crossed nicols and distal view unless specified. Scale bar: 5 µm. All illustrations adapted after Prins (1969). two species on the basis of biometry, concluding that C. crassus has a smaller size (mean size 6.91 µm) than C. crucifer (mean size 8.96 µm) (see Fig. 3). Despite the small average size of C. crassus reported by Fraguas and Erba (2010), the measured specimens virtually integrate both the small crassus and large crassus of Suchéras-Marx et al. (2010). In fact, the differences between C. crassus and C. crucifer also concern the central-area structures. Range. Pliensbachian (Fraguas and Erba, 2010).
Occurrence. This species has been recorded in the entire studied interval.
Remarks. This species was introduced by Prins (1969) as nomen nudum. Prins (1969) showed a drawing in which a thick cross spanned a narrow central area, and the cross is composed of granular calcite crystals. Rood et al. (1973) provided a SEM picture showing a Crepidolithus in proximal view with a very reduced central area without visible structure, as well as a description stating, "a species of Crepidolithus with a cruciform structure in the central area" (p. 374). According to the latter, Bown (1987b) put C. crucifer in synonymy with C. crassus. However, C. crassus possesses a vacant central area. Thus, the presence of a cruciform structure in the central area of C. crucifer represents a valuable morphological difference between the two species. Moreover, Fraguas and Erba (2010) nicely illustrated by means of SEM and biometry that C. crassus and C. crucifer can be clearly differentiated. They provided an emended diagnosis: "A robust, thick and elliptical species of Crepidolithus with a relatively narrow and large central area filled by a structure consisting of a cross aligned along the major and minor axes of the ellipse that sometimes appears weakly developed" (p. 134). Although they include the presence of a cross aligned to the major and minor axes of the ellipse, in their picture ( fig. 3b) the cross is not visible but a coarse granulation. In their description they stated that the central-area structure often appears as irregular grains. This peculiar morphology is also visible in the SEM picture of C. crucifer showed by Barnard and Hay (1974) (although broken) and in the Medd (1979) C. crassus (see Fig. 3). Other characteristics allowing a differentiation of C. crucifer from C. crassus are a bigger size for C. crucifer (Fraguas and Erba, 2010) and the fact that the elements of the distal shield appear quite large, providing an irregular outline under LM.
Occurrence. Both morphotypes of this species have been recorded in the entire studied interval.
Remarks. This elliptical coccolith has a low distal rim and a large, wide central area filled with small, granular calcite crystals. Bown (1987b) explains that this species shows variable thickness of the distal shield, and, as a consequence, the central-area opening can be more or less developed. This difference is figured in many previous publications (Mattioli et al., . Crepidolithus granulatus is herein presented as two morphotypes described separately below. The rim thickness variation reflects likely a palaeoenvironmental or palaeogeographical control, but further biometric study is necessary to better constrain the differences between the two morphotypes.  1998).

Crepidolithus granulatus
Occurrence. This species has been recorded in the entire studied interval.
Remarks. Crepidolithus impontus was first introduced by Grün et al. (1974, p. 310), who describe "a large coccolith whose central area is spanned in the proximal side by a bridge made up of two rows of elements parallel to the short axis of the ellipse. A central process is absent". Goy (in Goy et al., 1979, p. 39) emended this diagnosis and proposed "A species of the genus Crepidolithus with a wall composed of calcite laths very inclined and overlapping. The central area is spanned by a bridge parallel to minor axis of the ellipse having in its centre a very small spine". Grün et al. (1974) stated in their remarks that C. impontus resembles to C. cavus sensu Prins, 1969 (nomen nudum). The diagnosis by Goy (in  closely resembles that of C. cavus, which is a species of Crepidolithus with a bridge along the minor axis of the elliptical central area (Rood et al., 1973, p. 375). Accordingly, Bown (1987b) considers C. impontus to be a junior synonym of C. cavus. Eventually, Bown and Cooper (1998) use C. cavus for early Pliensbachian forms with a prominent spine (which according to the description might rather be considered P. liasicus) without figuring it and C. impontus to refer to late Pliensbachian specimens with a wide central area spanned by a delicate bridge and no spine. The specimen of C. cavus drawn by Prins (1969, nomen nudum) figures a murolith with a relatively reduced central area and two prominent insertions of a central structure. The overall features of this specimen may look like a Parhabdolithus liasicus. Thus, C. cavus informally introduced by  was validated by Rood et al. (1973), who show for the holotype an SEM image having a relatively narrow central area and a prominent spine. Also, because the SEM image is very poor, sutural lines of the distal shield are not visible and this specimen resembles a Parhabdolithus. Grün et al. (1974) came to a similar conclusion, stating that the specimen figured by Rood et al. (1973) looks like a distal view of Parhabdolithus marthae. Accordingly, de Kaenel et al. (1996) proposed the new combination Parhabdolithus cavus . Thereby, C. cavus in Prins, 1969 ex Rood et al. (1973) should be considered either P. cavus or a junior synonym of P. liasicus  The confusion between C. cavus (or P. liasicus) and C. impontus is partly due to the loss of the bridge of C. impontus that can be broken in poorly preserved material, making the central area empty; however, the insertions of the bridge are still visible in the wide central area. Also, a certain variability can be observed in the wideness of the central area. (2014) is pro parte considered to be a C. impontus here. In the LM images shown by Fraguas (2014, figs. 3a-e) it is difficult to see the insertions of the bridge because the pictures were not taken at 45 • . In her original diagnosis, Fraguas describes C. cantabriensis as "A medium-sized, normal to narrowly elliptical species of Crepidolithus with an open central area. Its thick proximal shield extends distally to form a collar which appears to be a distal inner cycle. Its bicyclic rim extinction pattern results in a sigmoidal interference figure" (p. 35). However, this diagnosis is invalid because the sigmoidal extinction pattern is the result of the optical discontinuity existing between the proximal and distal shield at 45 • , which is also a typical feature of C. impontus.  1999). According to Angelozzi and Pérez Panera (2016), the LO of this species is a useful event in the Neuquén Basin, and it occurs within the Fanninoceras fannini NAZ, which is considered the time equivalent of the upper part of the Davoei and Margaritatus SAZs (Riccardi, 2008b). In this study, its presence is scarce but continuous, and hence we consider C. pliensbachensis to be an important element to make correlations with other regions. Remarks. A typical Crepidolithus with a thick distal rimwall and a reduced, lenticular central area spanned by a small spine, typically bow-tie-shaped, which is very diagnostic in LM. Occurrence. Crepidolithus timorensis is assigned to the Sinemurian of Timor (Bown, 1992;Bown and Cooper, 1998). Here we found it in the early Pliensbachian within the NJT4b subzone, probably corresponding to the LO of this species, as previously reported by Lozar (1995, northern  Remarks. Kristan-Tollmann (1988a, p. XVIV/86) provided the diagnosis of this species: ". . . broadly elliptical coccolith with high and blocky distal shield. The elements forming the distal shield are elongated and enlarged at their extremity. The central-area size is therefore reduced. The spine is short, ending at or just above the rim. The proximal shield is flat and composed of small elements. In the centre is a weakly developed rhombic structure, made evident by few loose elements (see holotype figs. 3, 5, pl. 2). The elements of the proximal shield are arranged to form two perpendicular furrows, aligned with the major and minor axes of the ellipse. In the case of poorly preserved coccoliths or etched specimens, only the central hole and the cross-shaped furrow can be seen proximally (see plate 2, fig. 1,2,6)". Crepidolithus timorensis is observed in this study as a small coccolith (less than 4.5 µm) with an irregular outline due to the large size of the elements forming the distal shield. The small spine often appears as a small cluster of irregular calcite crystals. Lozar (1995; p. 110) identified a "small" Crepidolithus described as an "elliptical coccolith very similar to C. crassus with a comparable blocky structure, but smaller in size; the elongated central area is closed by a wavy suture". Despite the fact that this description does not match the original diagnosis of Crepidolithus timorensis, the pictures provided correspond to the species because the irregular spine can be seen in one of them. The "small" Crepidolithus mentioned in Cobianchi (1992, p. 104) or C. timorensis pictured by Bown and Cooper (1998) in pl. 4.1, figs. 11-12 should not be confused with the C. crassus "small morphotype" introduced by Suchéras-Marx et al. (2010). Conversely, the pictures shown by Bown and Cooper (1998) pl. 4.9, figs. 13-14 as C. timorensis are C. crassus small morphotypes. In fact, C. timorensis differs from the C. crassus small morphotype due to its smaller size (4.5 µm vs. 6.5 µm on average), but also because of its irregular outline, which is due to the presence of blocky elements composing the distal shield. Conversely, the outline of small crassus is very smooth. Range. Sinemurian-Tithonian (Bown and Cooper, 1998).

Crepidolithus timorensis
Occurrence. Tubirhabdus patulus is reported to be a an extremely long-ranging species, and it is observed throughout the studied interval.
Remarks. This is a narrowly elliptical coccolith with a central-area structure that supports a broad, hollow spine. Although the holotype dimensions state a coccolith smaller than 4 µm , the size range of our studied material goes from 2.75 to 7 µm. Three discrete morphotypes of this species were identified in the studied material based upon size and thickness of the rim. Herein, the "thin" and "thick" morphotypes are partly equivalent to the T. patulus "small" (pl. 4.9, figs. 16-17, p. 71) and "large" (pl. 4.9, figs. 18-19, p. 71) illustrated by Bown and Cooper (1998). A third morphotype is represented by tiny coccoliths clearly displaying a tube-like spine in the reduced central area. Kristan-Tollmann (1988b) introduced two subspecies mainly based on the spine shape and dimensions, namely T. patulus patulus (pl. 3, figs. 2-6; pl. 5, fig. 8) and T. patulus tubaformis (pl. 3, figs. 2-4, 7-8). However, the differences in the spine morphology are difficult to observe with LM.
This species is very abundant in the studied section, constituting 22 % of the total coccolith abundance. Moreover, a shift in the proportions occurs from the base to the top of El Matuasto I between thick and thin T. patulus. Further analysis is necessary to elucidate if these morphotypes respond to preservation or ecological factors.  (1973, p. 373): "A small species of Tubirhabdus with a very broadly open oval to circular central spine". The holotype dimensions state a coccolith smaller than 4 µm; accordingly, in this contribution the pictures provided for the synonymy list comprise specimens between 2.75 and 4.6 µm.
Remarks. Slightly elliptical coccoliths showing a very reduced oval central area infilled by a thick spine. Medd (1979) already stated that the T. patulus proximal view may be confused under the LM with Mitrolithus elegans because in both species the base of a hollow spine is visible. However, the proximal shield architecture is different in the two species. The Tubirhabdus patulus proximal shield is composed of small elements surrounding a broadly open central area, and the foci of the asymmetrical extinction cross are very far away along the major axis of the ellipsis, while the M. elegans proximal shield elements are blocky, making the central opening very reduced, and the foci of the extinction cross are in contact with the base of the hollow spine. Prins (1969) nicely illustrated such features (pl.  Range. Norian-Toarcian (Bown and Cooper, 1998).
Occurrence. This species is a characteristic north-western Europe component (boreal realm;  and tends to be scarce in Tethyan localities during the Early Jurassic Fraguas et al., 2018). In El Matuasto I, this species was recorded consistently from the base to the top of the section (NJT4a-c zones).
Remarks. In the present study C. primulus is widely recognized in side view under the LM and differs from Parhabdolithus liasicus by having a low rim that gives it a "flat" appearance. The proximal shield appears as two tooth-like elements which are very far away each other because of the presence of a widely open central area. The presence or absence of the spine depends on the preservation quality. Small specimens resembling the variety C. primulus nanus described in Prins (1969) were sporadically observed. The recognition of C. primulus in distal view could be difficult because the delicate cross-structure may be easily broken.  fig. 5F. Range. Hettangian-Pliensbachian (Bown and Cooper, 1998;Mattioli and Erba, 1999).
Occurrence. This species is a typical component of Tethys and north-eastern Pacific assemblages (Bown, , 1992Bown and Lord, 1990;Ferreira et al., 2019) and is rarely observed in north-western European associations during the lower Sinemurian to the lower Toarcian (Bown , 1992Mattioli and Erba, 1999). In the El Matuasto I section, we found the first record of common and consistent occurrence of M. elegans in the south-eastern Pacific area in the early Pliensbachian.
Remarks. Mitrolithus species, and especially M. elegans, can be observed in both plan view (proximal or distal) and side view, which is more diagnostic. In this study, Mitrolithus elegans was usually observed in side view, having a prominent spine protruding from the distal shield of the coccolith. Specimens observed in proximal view were also common. Isolated spines (corresponding to the A. dorsetensis of Black, 1969) were rarely observed. The holotype dimensions are 5.8 µm length and 6 µm height (Deflandre in Deflandre and Fert, 1954). Differences between M. elegans and T. patulus have been discussed above.  1999).

Mitrolithus lenticularis
Occurrence. M. lenticularis occurs consistently and abundantly in El Matuasto I from the base of the section, dated as early Pliensbachian (NJT4 biozone). Angelozzi and Pérez Panera (2016) noticed that this species is characteristic of the Pliensbachian-Toarcian boundary assemblages in the Neuquén Basin. Bown (1987b, 1992) and Bown and Cooper (1998) considered M. lenticularis a typical Tethyan species. Its presence in the Neuquén Basin is crucial to establish palaeobiogeographic relationships between the southeastern Pacific and the Tethys realms.
Remarks. Mitrolithus lenticularis, which is usually recognized in side view, differs from M. elegans because of its slightly smaller size (Holotype dimensions are 4.5 µm length, 3.7 µm height;  and because it has a lenticular spine that does not protrude from the rim. Range. Hettangian-Toarcian (Bown and Cooper, 1998). Remarks. This taxon has a high rim and a transverse bar supporting a spine in the central area. Owing to spine dimorphism, an informal separation within the species was recognized by Prins (1969).  proposed two subspecies based on previous descriptions and illustrations by  in Deflandre and Fert, 1954). Parhabdolithus liasicus distinctus has a larger rim and a relatively thick spine compared to P. liasicus liasicus, which is a tiny coccolith with an extremely long and thin spine. Both subspecies are consistently present throughout the El Matuasto I section, even though P. liasicus distinctus abundance is much higher (88 %) than P. liasicus liasicus (12 %).
Remarks. In this latter morphotype, the rim dimensions are very small, while the spine is thin and very long; the spine is often broken. Dimensions given by  are 2.8-3.6 µm length and 1.6-2 µm width. The extinction pattern of the spine in plan view forms a central cross showing a butterfly-like structure aligned with the minor axis of the ellipse. Occurrence. Parhabdolithus robustus was herein recorded for the first time in the south-eastern Pacific, with a consistent and relatively abundant occurrence from the early to late Pliensbachian (NJT4 a to c subzones). The species is common in the Early Jurassic assemblages from Tethys and boreal realms (Bown, , b, 1992Bown and Cooper, 1998), especially abundant in Timor  and rare in the north-eastern Pacific during the Pliensbachian (Bown, 1992). The different relative abundance of this species in the northeastern and south-eastern Pacific would suggest a possible ecological factor. The LO of P. robustus is recorded in the early Pliensbachian within the NJT4a subzone (Bown and Cooper, 1998;Mattioli and Erba, 1999), while in Argentina its presence is observed at least until the NJT4c subzone according to the zonation of Ferreira et al. (2019).

Parhabdolithus robustus
Remarks. This coccolith is mainly observed in side view. It possesses a thick, high rim, which gives it a distinctive blocky appearance and a short, broad spine ending at or just above the rim. Occurrence. The consistent presence of this taxon throughout our section and its biostratigraphic reliability allow an accurate age for the sedimentary succession. The FO defines the base of the NJ5 zone (Bown, 1992) and NJ5b (Bown and Cooper, 1998) and NJT4b subzones (Mattioli and Erba, 1999;Ferreira et al., 2019), correlating with the Ibex-Davoei SAZ boundary. Biscutum grande is a species with Tethyan affinities according to Bown ( , 1992. Its abundance in the studied samples provide evidence of biogeographic similarities between the south-eastern Pacific and Tethyan assemblages. Remarks. Biscutum grande is a large, broadly elliptical coccolith composed of a distal shield formed by radial elements and bearing an inner tube cycle. The central area is large, vacant, and sometimes spanned by a thin bar . Under LM, the distal and proximal shields look dark grey, while the inner tube cycle appears as a conspicuously bright rim surrounding the wide central area (Mattioli et al., 2013). The delicate bar is aligned with the minor axis of the elliptic central area, but frequently it is missing; its insertions in the inner tube cycle are, however, seen as two bright lobes. In some cases, the central area can be filled with small calcite crystals (Menini et al., 2019; pl. 2, LAL18) or (very rarely) spanned by a cross (Ferreira et al., 2019; pl. 1, Peniche97). According to de Kaenel and Bergen (1993) Palaeopontosphaera binodosa , is a synonym of Similiscutum finchii; herein we consider P. binodosa to be a synonym of B. grande.
Genus Similiscutum de Kaenel and Bergen, 1993 Type species. Similiscutum cruciulus de Kaenel and Bergen, 1993 Similiscutum cruciulus de Kaenel  Remarks. This group includes S. orbiculus, S. avitum, and S. cruciulus (i.e. small, normal to slightly elliptical coccoliths, with a homogeneously grey unicyclic distal shield and a light grey "collar" surrounding the central area). The three species introduced by de Kaenel and Bergen (1993) are roughly differentiated because S. avitum shows a broadly elliptical coccolith, S. orbiculus has a subcircular outline, and both have a vacant reduced central area, while S. cruciulus shows a subcircular outline with a cross spanning the central area. Nevertheless, Mattioli et al. (2004) proposed a clustering for the Similiscutum cruciulus group based on the absence of diagnostic biometric differences between the species, highlighting the morphological plasticity within the genus Similiscutum.  fig. 9 (3, 4), fig. 13 fig. 12.
Occurrence. The presence of S. finchii in the studied section is observed within the NJT4c subzone. Mattioli et al. (2013) indicate that the FO of this species marks the boundary between the NJ4a and NJ4b subzones within the Margaritatus SAZ. This event matches the record of Angelozzi and Pérez Panera (2016) within the equivalent Fanninoceras fannini NAZ. According to Riccardi (2008b) the F. fannini NAZ correlates with the upper part of the Davoei and most of the Margaritatus SAZ of the western Tethys. In Ferreira et al. (2019), the FO of S. finchii occurs simultaneously with the FO of Lotharingius barozii, and this latter event marks the base of the NJT4c subzone within the Davoei SAZ (late Pliensbachian) in Portugal. Thus, the record of the FO of Similiscutum finchii seems to be quite consistent between Argentina and Portugal.
Remarks. Under the LM, Similiscutum finchii appears as a medium-sized, normal to broadly elliptical coccolith. The distal shield is light grey with an irregular outline. The central area is narrow, elongated, and sub-rectangular in shape. In SEM pictures (like the holotype), the central area is ogiveshaped, elongated, and narrowly elliptical. Biometrics of S. finchii (on average 4.53 µm for the major axis and 3.76 µm for the minor axis;  fall at the small end of the range of sizes reported in the literature  5.4-6.6 µm; Bown, 1987b: 5.8-8.5 µm). Morphologically and biometrically, S. finchii is quite similar to S. novum (average 4.13 µm for the major axis and 3.48 µm for the minor axis; , which is, however, overall smaller in size, less elliptical, and with a less developed central area. In the literature, specimens are figured which are larger, more broadly elliptical, and with a more reduced length of the central area than the S. finchii holotype description. Such specimens are described here as large Similiscutum aff. finchii and were designated as S. giganteum in Ferreira et al. (2019). De Kaenel and Bergen (1993) considered Palaeopontosphaera binodosa Prins, 1969, to be a synonym of S. finchii, but we consider P. binodosa to be a synonym of Biscutum grande. In fact, the drawing in pl. 2, fig. 12 of  shows the presence of a widely open central area spanned by a bridge whose insertions are clearly visible in the inner rim of the coccolith. et al., pl. 1, fig. 10 Ferreira et al., 2019).
Occurrence. This taxon occurs within the NJT4c subzone in El Matuasto I. It was firstly identified in Argentina by  as large specimens of S. finchii. Mailliot (2006)  The occurrence in the studied section would match the previous record in the area  from the late Pliensbachian to the Toarcian.
Remarks. This morphotype of Similiscutum corresponds to a large, broadly elliptical coccolith with a lozenge-like, reduced central area filled by a robust cross. In the literature, it is typically referred to as large Similiscutum/Biscutum finchii de Kaenel and Bergen, 1993;Bown and Cooper, 1998).  unpublished PhD thesis) provided an original diagnosis and proposed Similiscutum giganteum as a new and different species from S. finchii based on biometric significant differences. However, the introduction of a new species in a PhD thesis is invalid because it does not constitute an effective publication (ICBN, Article 30.9; Turland et al., 2018). We agree with the diagnostic description by . Nevertheless, we consider the prompt publication of this species respecting the nomenclature code to be an indispensable and valuable contribution.
Occurrence. The presence of the genus Calyculus is recorded in El Matuasto I since the upper part of the lower Pliensbachian section. This changes the previous record in the Neuquén Basin given by Pérez Panera and Angelozzi (2015) and Angelozzi and Pérez Panera (2016) in the early Toarcian within the Tenuicostatum NAZ. Mattioli (1996) and Mattioli and Erba (1999) reported the FO of this taxon in the late Pliensbachian. Bown and Cooper (1998) identified it sporadically in the ibex SAZ (early Pliensbachian) and continuously from the spinatum SAZ (late Pliensbachian). Afterwards, two morphotypes, namely "small/thin" and "large" Calyculus, were reported from the early and late Pliensbachian, respectively (Matti-oli et al., Ferreira et al., 2019). The earliest record in El Matuasto I section corresponds to the large morphotype.
Remarks. The original diagnosis of the genus Calyculus given by Noël (1972, p. 115) describes it as "Elliptical to subcircular coccoliths made up of subvertical elements placed side by side, enlarged and flattened in their distal region; the central area is slightly conical, deep and closed by a grill". Crux (1987a, p. 53) emended it and states that ". . . Differences in the character of the central grill allow different species to be recognized within the genus Calyculus". Bown (1987b) illustrates as "Calyculus sp. indet." those specimens which lack central structures and therefore cannot be identified at the species level. We follow the same criterion for our material due to the difficult identification of the grid under LM. However, the literature points out two different groups based on the general shape of the coccolith: big, broadly elliptical specimens (i.e. Calyculus cribrum, C. noeliae, C. hommerili, C. serrai, C. derivatus, and C. magnus) and thin, narrowly elliptical coccoliths (i.e. Calyculus depressus and C. absolutus). We recognized two morphotypes of this taxon in distal view. One is characterized by a large, thick, and broadly elliptical rim formed by big, trapezoidal elements individually distinguished, giving an irregular outline and an open central area without visible structure. The other morphotype has a slim, narrowly elliptical rim compared to the size of the central area and lacks central structure. Both are very rarely observed in side view.
Order WATZNAUERIALES Bown, 1987b Family WATZNAUERICEAE Rood et al., 1971 Genus Lotharingius Noël, 1972 emend. Goy in  Type species. Lotharingius barozii Noël, 1972 emend. Goy in ) described the genus Lotharingius as "coccoliths with a rim typical of Lotharingiaceae and the central area with four buttresses aligned with the major and minor axis of the ellipse. . . additional bars can be also present. . . ".
Emended diagnosis (this paper). A placolith-coccolith rim with a bycicilc distal shield. The inner cycle is composed of small elements with radial sutural lines. The outer cycle is composed of elements slightly overlapping and with oblique sutural lines. The central area can be spanned by a cross with additional lateral bars (like L. barozii, L. sigillatus, L. crucicentralis, or L. umbriensis), a prominent transversal bar (like in L. frodoi), a button (like in L. hauffi), or a granular plate (like in L. velatus).
Remarks.  considers the cross-structure and lateral bars spanning the central area of Lotharingius to be useful distinctive features to differentiate this genus from Watznaueria. Mattioli (1996) illustrates the central-area structure variability within the genus and states that a similar cross-structure can be present in the genus Watznaueria. Furthermore, Mattioli (1996) points out the arrangement of the shield as the main difference between these two genera. She provides a clear description, stating that Watznaueria displays distal shield elements possessing more inclined sutural lines than Lotharingius and a prominent concaveconvex coccolith shape; under LM "these features produce an extinction pattern with isogyres displaying right angle bent arms, revealing also the net optical discontinuity between the outer and inner cycles of the distal shield. In the genus Lotharingius the optical discontinuity in distal view is less marked" (p. 402). 2019  According to the holotype SEM images shown by  pl. 11, fig. 3), neither the original diagnosis "as for the genus (see " nor the emended diagnosis (Goy, 1979, p. 43) fit the holotype description: "species of the genus Lotharingius with a massive buttress aligned with mayor and minor axis of the ellipse and a system of dissymmetric radial bars. The coccosphere is slightly ovoidal and possesses 20 coccoliths".
Emended diagnosis (this paper). A placolith-coccolith with a bicyclic distal shield. The inner cycle is composed of small elements with radial sutures. The outer cycle is composed of elements slightly overlapping and with oblique sutural lines. The inner and outer cycles have comparable thickness. The central area is wide and oval and is infilled by buttresses aligned with the major and minor axis of the ellipsis. A few additional lateral bars are visible. In LM images , the coccolith rim is composed of two thin equidimensional cycles surrounding a very wide and elliptical central area spanned by a cross-structure. This delicate structure can be lacking in poorly preserved specimens, but its insertions in the inner wall of the rim remain visible.
Occurrence. In El Matuasto I, the FO of Lotharingius barozii (small specimens) is in the NJT5a subzone of Mattioli and Erba (1999; roughly corresponding to the spinatum SAZ) and defines the base of the NJT4c subzone (within the Davoei AZ) according to Ferreira et al. (2019). In the Neuquén Basin, this event was previously reported within the upper part of the Fanninoceras disciforme NAZ (time equivalent of the Spinatum SAZ; latest Pliensbachian) (Pérez Panera and Angelozzi, 2015; Angelozzi and Pérez Panera, 2016) and for the upper Lias (Toarcian) (Angelozzi, 1988). We consider the earliest presence of Lotharingius barozii in El Matuasto I to match the record from the NJT4c subzone defined by Ferreira et al. (2019). Likewise, this FO approximately correlates with the Davoei-Margaritatus AZ boundary, which defines the transition from the early to late Pliensbachian, representing the earliest occurrence of the Lotharingius genus (Ferreira et al., 2019).
Remarks. The original  and the emended diagnoses  are based upon SEM images, and LM pictures are not available. However, the SEM images shown justify an emended diagnosis. The first paper showing both SEM and LM pictures of Lotharingius barozii is Bown (1987b). We referred to the latter paper for the identification of this species under LM. We recognized two morphotypes within this taxon. These are small specimens showing four tenuous and nearly straight isogyres (pl. 2, fig. 17), similar to the specimens figured by Bown and Cooper (1998, pl . 4.15, fig. 11). The central-area structures described un-der SEM are not present under LM, probably because of preservation issues. The second form shows the typical morphology of a well-developed narrow rim and a wide, subrectangular central area spanned by an axial cross-structure (pl. 2, fig. 18) (Mattioli, 1996;Ferreira et al., 2017); this last morphotype is illustrated by pl. 15, figs. 4-5). After a careful biometric study, Ferreira et al. (2017) recognize the size increment of the species from the Pliensbachian to the late Toarcian. Hence, the small morphotype occurring in the lower part of our section would represent the earliest and smaller forms of this species. Towards the upper part of El Matuasto I, an abrupt change to the typical, larger morphology is observed. According to Mattioli (1996) and Ferreira et al. (2017) Lotharingius barozii is distinguished from the other species of Lotharingius by its overall elliptical shape, equidimensional thickness of inner and outer cycles of the distal shield, and its broadly open, oval central area.  stated that L. barozii is closely related to Bussonius prinsii. Both species possess a similar placolith and central-area structures. The differences between them are that in B. prinsii the outer cycle elements of the distal shield are radially arranged; this feature makes the outer cycle of B. prinsii grey in LM, while both cycles are white in L. barozii. Also, the buttresses spanning the central area of B. prinsii are thicker and more prominent than in L. barozii.

Grade "NANNOLITHS" Perch-Nielsen 1985b
Family SCHIZOSPHAERELLACEAE Deflandre, 1959 Genus Schizosphaerella  Type species. Schizosphaerella punctulata  Schizosphaerella punctulata  Occurrence. The record of Schizosphaerella punctulata is continuous throughout the studied section, attesting the consistent occurrence of this species in the south-eastern Pacific since the early Pliensbachian.  noticed this taxon from the Neuquén Basin but stated that the locality was undated. Later, Angelozzi (1988) documented the presence of Schizosphaerella in the Toarcian. Bown (1992) considers S. punctulata to be a typical Tethyan component and explains its absence in the Pliensbachian of the Neuquén Basin possibly due to ecological limitations that would be overcome in the Toarcian with the opening of the Hispanic Corridor. Afterwards, Angelozzi and Pérez Panera (2016) reported S. punctulata in the Neuquén Basin since the early Pliensbachian, and this record is confirmed by the present study.
Remarks. This large nannolith is composed of a test of organized calcite crystallites formed by two interlocking subhemispherical valves. The characteristic geometric arrangement of the crystals forming the test of S. punctulata is recognized by its granular appearance under LM. Unfortunately, only isolated, broken pieces were recovered in El Matuasto I. Another species of the genus, namely Schizosphaerella astrea, is reported in the literature; the only difference between the two species concerns the arrangement of the calcite crystal forming the valve (Moshkovitz, 1979). However, this feature is only recognizable under SEM. For this reason, some authors prefer the use of Schizosphaerella spp. when working under LM. Recently, three morphotypes of Schizosphaerella with overlapping size ranges and different palaeoecologies were identified Peti et al., 2021).

Discussion
Calcareous nannofossil biostratigraphic studies and improvements of "standard" biozonations for the Early Jurassic are mostly based on sections from the Tethys realm, which from a palaeogeographic point of view represents a very small region. Previous works dealing with the Los Molles Formation (Neuquén Basin, Argentina) only presented a general characterization of the nannofossil assemblages and a broad correlation of nannofossil events with the boreal and Tethys realms (Angelozzi, 1988;Angelozzi andPérez Panera, 2013, 2016;Pérez Panera and Angelozzi, 2015), without a thorough systematic approach. Nevertheless, the Neuquén Basin offers a unique opportunity to compare the evolutionary history of this group between the south-eastern Pacific Ocean and the classic localities situated in the Northern Hemisphere and may contribute to achieving a more comprehensive global biostratigraphic scheme for the Early Jurassic. However, as demonstrated in this contribution, detailed systematic studies on the Neuquén Basin calcareous nannofossil record need to be carried out before detailed correlations can be established.

Taxonomy
When comparing the nannofossil record of the Los Molles Formation in Argentina with the Tethys nannofossil record, some inconsistencies appeared in the literature concerning some murolith-coccoliths. A careful and extensive examination of the original diagnoses was thus undertaken, along with a revision of the known literature for evaluating the synonymies. Thus, it appeared that Crepidolithus crucifer has been cited inconsistently in the literature due to its informal introduction made by Prins (1969) as nomen nudum (Fig. 3). Because Prins (1969) did not provide a diagnosis, the taxon was not formally recognized until the contribution of Rood et al. (1973). Unfortunately, the SEM image presented then for the holotype is a proximal view of the coccolith, which hinders a proper assessment of the species based on central-area  (1969), , Barnard and Hay (1974), and Fraguas and Erba (2010).
structures that are more clearly visible in distal view. Based on the proximal view features of the holotype, Bown (1987b) established C. crucifer as a junior synonym of C. crassus. Yet, diverse and sometimes unclear usage of these two names to refer to the same species has been found in the literature for more than 20 years. Fraguas and Erba (2010) differentiated the two taxa by means of biometry and emended the diagnosis of C. crucifer. According to this latest description, the SEM picture provided by Barnard and Hay (1974) can actually be considered the first illustration of the species.
Crepidolithus cavus was introduced as nomen nudum by Prins (1969) and eventually validated by Rood et al. (1973). Later, Grün et al. (1974) introduced the new species Crepidolithus impontus and Goy (in ) emended its diagnosis. The C. impontus imaged by Grün et al. (1974) resembled C. cavus by Prins (1969), and the emendation of C. impontus by Goy (1979) better fits with C. cavus as described in . Considering these is-sues,  tried to clarify the taxonomy of both species, setting C. impontus as a junior synonym of C. cavus. Nonetheless, the confusion in the literature persisted through time and no consensus about the taxonomic status of these names was reached. In particular, the species with a bridge were assigned to C. impontus by some authors, while C. cavus was considered for specimens with an empty central area. However, Menini et al. (2019) lumped C. cavus-C. impontus because the bridge which characterizes C. impontus can be broken, although its insertions are generally still visible. After a careful revision, it appears that the holotype of the C. cavus SEM image shown by  possesses a relatively narrow central area and a prominent spine. Also, because the SEM image is very poor, sutural lines of the distal shield are not visible, and this specimen resembles a Parhabdolithus. A similar interpretation was inferred by Grün et al. (1974) and de Kaenel et al. (1996). Thereby, C. cavus in Prins, 1969 ex Rood et al. (1973) should be considered either P. cavus or a junior synonym of P. liasicus . We support Crepidolithus impontus as the correct and valid name because the wide central area may or may not display a central structure according to the preservation quality of the material, but its insertions are still visible. Even if a certain variability in the rim thickness and wideness of central area is observed, the specimens shown in the literature possessing a reduced, narrowly elliptical central area spanned by a spine more likely apply to Parhabdolithus liasicus. Recently, Fraguas (2014) described the new species Crepidolithus cantabriensis; the diagnosis is, however, incorrect because the cited bicyclic rim extinction pattern does not apply to the genus or the murolith coccoliths. Furthermore, the specimens shown in Fraguas (2014) were taken at 45 • under LM crossed polars. Various muroliths display a bicycliclike extinction pattern at 45 • . Accordingly, we assigned some of the Crepidolithus cantabriensis figured in Fraguas (2014) to C. impontus and some others to C. crassus depending on the size of the central area. In fact, Bown (1987b) described a certain variability in the size of the central area of C. crassus, which can be vacant, slightly open, or narrower, depending on the size of the distal shield elements. Bown and Cooper (1998) figured out different-sized morphotypes of Tubirhabdus patulus, even though they did not consider these to be subspecies. We recognized three different morphotypes for this species, which are differentiated by a variable central-area shape, besides by their size. Accordingly, we name them "tiny" for the very small, "thick" for those specimens with a thick wall and a very reduced central area, and "thin" for large morphotypes with a large central area in which the hollow tube is well outlined. Bown (1987) and Bown and Cooper (1998) also described thin and thick Crepidolithus granulatus without specifying if these are subspecies or eco-phenotypes. Accordingly, we differentiate these two morphotypes that likely correspond to more or less heavily calcified coccoliths.
A wide morphological variability is described for Early Jurassic murolith-coccoliths, which makes systematic analysis and palaeontology difficult. As biometric studies (e.g. Fraguas and Erba, 2010; Suchéras-Marx et al., 2010) are still scarce, it is difficult to assess if such morphological differences are related to the existence of pseudo-cryptic species, or if they are ecologically driven changes in geometry or calcification. A better understanding of the morphological variability is needed for improving biostratigraphy and also for accurately describing evolutionary patterns.
A quite large morphological variability also occurs within the placolith-coccolith group. Although some biometric studies have been performed on placoliths of Biscutaceae and Lotharingius (e.g. Fraguas and Young, 2011;Ferreira et al., 2017), the systematic palaeontology is not yet perfectly established and some morphological species should be revisited, especially within the Calyculus and Similiscutum groups. Also, we found an inconsistency within the original diagnosis of the Lotharingius genus, which comprises several species and dominated the nannofossil assemblages in Toarcian and Aalenian rocks. We propose an emended diagnosis for the genus, which better reflects the morphological characters of the figured holotype, as well as for the type species L. barozii.

Biostratigraphy
The calcareous nannofossil assemblage and the species morphology from the studied El Matuasto I section, Los Molles Formation, closely resemble the ones documented in the western Tethys (Fig. 4). The biozonation scheme of Ferreira et al. (2019) was the most suitable for application to the El Matuasto I section. Yet, the Neuquén Basin records several marker species, such as Biscutum grande, Lotharingius barozii, Similiscutum finchii, and the S. cruciulus group, allowing good correlation with biostratigraphic schemes established for western Europe. We recorded the Similiscutum cruciulus group from the base of the studied section. Bown and Cooper (1998) already considered the FO of the placolithcoccoliths (including Similiscutum spp.) to be the major evolutionary event to set the base of the NJT4 zone. Thereby, Ferreira et al. (2019) used the FO of the Similiscutum cruciulus group to define the base of the NJT4 zone.
The NJT4a subzone (Mattioli and Erba, 1999 emend. Ferreira et al., 2019) is defined as the interval between the FO of Similiscutum cruciulus and the FO of Biscutum grande, and it corresponds to the NJ4a subzone of Bown and Cooper (1998) and the NJT4a subzone of Mattioli and Erba (1999). We identified this subzone because of the presence of S. cruciulus group from the base of the section until the identification of the ?FO of B. grande. Within this subzone, Ferreira et al. (2019) reported the LO of Crepidolithus timorensis, but we observed the sporadic presence of this species after the FO of B. grande. According to the literature, Crepidolithus timorensis has a range restricted to the Sinemurian (Bown and Cooper, 1998) or the lower Pliensbachian (Kristan-Tollman, 1988b), with its LO lying within the NJT4a subzone (Ferreira et al., 2019). The Neuquén Basin record is slightly different because this taxon last occurs within the NJT4b subzone. If not due to reworking (turbidite levels are recorded in the section), this new record constitutes a difference with respect to the Tethyan bioevents, and further analysis in other sections of the Neuquén Basin will help to corroborate the new datum.
The two subsequent NJT4b and NJT4c subzones were easily recognized in the El Matuasto I section. They were defined by Ferreira et al. (2019) as follows: the NJT4b subzone spans the FO of Biscutum grande to the FO of Lotharingius barozii, and the NJT4c subzone ranges from the FO of Lotharingius barozii to the LO of Parhabdolithus robustus. Both partly correspond to the NJ4b subzone (Bown and Cooper, 1998) and to the NJT4b subzone (Mattioli and Erba, 1999). Two morphotypes of Lotharingius barozii occurred, namely earlier forms with a very thin rim and larger specimens whose features are more consistent with the holotype description. For biostratigraphic purposes, the FO of Lotharingius barozii was considered since the occurrence of the earliest forms. This represents the earliest occurrence of the Lotharingius genus according to Ferreira et al. (2019).
The FO of Similiscutum finchii happens synchronously with the FO of Lotharingius barozii classic morphotype within the NJT4c subzone. Within this subzone, the LO of Crepidolithus pliensbachensis also occurs. These events perfectly fit the Ferreira et al. (2019) findings in the Lusitanian Basin, Portugal.
Parhabdolithus robustus co-occurs with Crepidolithus impontus in the upper part of the El Matuasto I section. Such a coexistence is not observed in the north-eastern Pacific (Bown, 1992) or in the boreal (Bown and Cooper, 1998) and Tethys realms (Mattioli and Erba, 1999;Ferreira et al., 2019). Hence, if not related to resedimentation, this cooccurrence may constitute another important difference between the Northern and Southern Hemisphere settings.

Palaeobiogeography
Some species previously considered typical representatives of the Tethys realm were reported for the first time in the Neuquén Basin and in the south-eastern Pacific, such as Crepidolithus timorensis, Mitrolithus elegans, Parhabdolithus robustus, Similiscutum avitum, S. cruciulus, and S. orbiculus. In particular, Crepidolithus timorensis is a distinctive component of the assemblages from Timor (southwestern Tethys), and its presence in the proto-Atlantic region (Portugal, France) and now in the south-eastern Pacific provides new insights on the species distribution and range. Similarly, M. elegans has long been considered a typical component of Tethys assemblages . The similarity observed between the Neuquén Basin and the western Tethyan assemblages is remarkable, especially within the Lusitanian Basin, Portugal. The assemblage composition, therefore, may suggest the existence of a connection between the Pacific and Tethys oceans since the Pliensbachian (Fig. 1b).

Conclusions
Although the calcareous nannofossil assemblages from the south-eastern Pacific Ocean has been already described in a few papers (e.g. Al-Suwaidi et al., 2010 Angelozzi and Pérez Panera, 2013; Fantasia et al., 2018), a thorough taxonomic discussion of the Early Jurassic nannofossil species of the Neuquén Basin is presented for the first time. This contribution settles the taxonomic features of coccoliths recorded in the Neuquén Basin. After the seminal work of , very little detailed literature exists on coccolith taxonomy. Our review of the existing literature, including very old papers (and species diagnosis in French or German) and studies published in journals that are difficult to access, revealed some taxonomic inaccuracies, which were propagated in the subsequent literature.
The age of the calcareous nannofossil assemblages recorded in the Los Molles Formation is early Pliensbachian and could be well constrained by applying the Ferreira et al. (2019) zonation conceived for the Lusitanian Basin covering the NJT4a to NJT4c subzones. Considering the limited information on the Lower Jurassic Andean successions, the studied calcareous nannofossils are a valuable contribution to the biostratigraphy of the region. New biostratigraphic data are now available for the Southern Hemisphere, and the global biostratigraphic value of calcareous nannofossils is established.
Many similarities were found between the Neuquén Basin and far locations, particularly within the Lusitanian Basin, Portugal, situated in the proto-Atlantic region. These similarities suggest that a connection between the Neuquén and the Lusitanian basins was established through the Jurassic Hispanic Corridor by the Pliensbachian. ments and suggestions to improve the paper, Mateo Gutierrez for helping with the figures, and Gladys Angelozzi for the careful reading of the paper.