New Ludlovian, upper Silurian, graptolite faunas from the Los Espejos Formation, Central Precordillera, San Juan Province, Argentina: correlations and biostratigraphic remarks

FERNANDO E. LOPEZ, OSVALDO A. CONDE, ALEJANDRO R. BRAECKMAN, DIEGO G. SEGURA, JUAN M. DROVANDI, ANDREA J. BUENO, and ULISES ABARCA

Lopez, F.E., Conde, O.A., Braeckman, A.R., Segura, D.G., Drovandi, J.M., Bueno, A.J., and Abarca, U. 2024. New Ludlovian, upper Silurian, graptolite faunas from the Los Espejos Formation, Central Precordillera, San Juan Province, Argentina: correlations and biostratigraphic remarks. Acta Palaeontologica Polonica 69 (3): 351–370.

Graptolites represent one of the most important index fossils for the lower to middle Paleozoic biostratigraphy worldwide. Compared to other regions, graptolite faunas exhibited a marked decrease in diversity in Argentina during the Silurian, what resulted in reduced and low-varied registers and denied biostratigraphic works in its territory. This study introduces new Silurian graptolite faunas from the Los Espejos Formation, Poblete Norte section, Central Precordillera of Argentina. A total of thirteen graptolitic levels were identified, containing Bohemograptus bohemicus, Lobograptus sp., Saetograptus argentinus argentinus, Saetograptus cf. S. varians, Uncinatograptus uncinatus notouncinatus, Uncinatograptus elsae sp. nov., and Uncinatograptus lisandroi sp. nov. The occurrence of Uncinatograptus spp. in association with B. bohemicus could indicate the presence of the lower Gorstian Neodiversograptus nilssoni Biozone. A few meters above, the finding of S. a. argentinus in association with S. cf. S. varians, U. u. notouncinatus, and Lobograptus sp. suggest the presence of the lower Gorstian Lobograptus progenitor Biozone. The upper graptolitic beds yield specimens of S. a. argentinus and B. bohemicus, suggesting late Gorstian to early Ludfordian ages. These new graptolite faunas allow to correlate more precisely with local (Precordillera and Famatina), regional (North Western Argentina and Bolivia) and global equivalent sections. This discovery introduces new graptolite faunas, first-time recorded in South America, proposes for the first time a graptolite biostratigraphy for the Ludlow of Precordillera, and complements so far insufficient knowledge on Ludlovian graptolitic faunas of the continent in a critical time in the history of this fossil group.

Key words: Graptolithina, biostratigraphy, Gorstian Stage, Ludlow Series, Los Espejos Formation, Precordillera, Argentina, South America.

Fernando E. Lopez [felopez@unsj-cuim.edu.ar; ORCID: https://orcid.org/0000-0002-5562-7105 ], Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)-Departamento de Geología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Ignacio de la Roza 590 (O), Rivadavia, San Juan, Argentina.

Osvaldo A. Conde [osvaldoagustin94@gmail.com; ORCID: https://orcid.org/0009-0007-5890-4300 ] and Juan M. Drovandi [drovandijuan@gmail.com; ORCID: https://orcid.org/0009-0004-3022-9444 ], Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)-Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, CIGEOBIO, CONICET, Av. España 400 (N), J5500DNQ, San Juan, San Juan, Argentina.

Alejandro R. Braeckman [geobraeckman@gmail.com; ORDID: https://orcid.org/0000-0003-2580-3873 ], Diego G. Segura [seguradiegogabriel@gmail.com; ORCID: https://orcid.org/0009-0006-3102-0735 ], Andrea J. Bueno [­andreajesusbueno@gmail.com; ORCID: https://orcid.org/0009-0008-4973-5223 ], and Ulises Abarca [likogx89@gmail.com; ORCID: https://orcid.org/0009-0009-7036-9288 ], Departamento de Geología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Ignacio de la Roza 590 (O), Rivadavia, San Juan, Argentina.

Received 8 February 2024, accepted 24 May 2024, published online 28 August 2024.

Copyright © 2024 F.E. Lopez et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

The Silurian System is globally represented by graptolite-rich sections (e.g., Australia, Czech Republic, United Kingdom), where precise biostratigraphic chart have been established (see Zalasiewicz et al. 2009; Loydell 2012; Melchin et al. 2012, 2020; Maletz 2017; Štorch 2023), setting the foundation for paleontological studies in other countries.

In Argentina, Silurian graptolite-rich rocks have been analysed in two different geographical regions. In North Western Argentina, Llandoverian to Ludlovian faunas have been described in the Lipeón Formation (Toro 1995; Maletz et al. 2002; Toro and Maletz 2018). In Precordillera, where this study was conducted, Silurian graptolites were studied in the lower Hirnantian (Ordovician) to Llandoverian Don Braulio Formation and the Wenlockian to Lower Devonian Rinconada Formation of the Eastern Precordillera (Peralta 1986, 1993; Lopez et al. 2023), and from the upper Hirnantian to lower Wenlockian La Chilca Formation and the upper Wenlockian to Lower Devonian Los Espejos Formation of the Central Precordillera (Cuerda 1969; Cuerda et al. 1988; Peralta 1984b; Lopez 2022).

In the aforementioned units, diverse and extensive graptolitic levels have been reported. However, the absence of index taxa and limited taxonomic studies hindered the recognition of most of the biozones identified in other countries. This study introduces new graptolite faunas from the lower Ludlow (Gorstian) found in the middle to upper Los Espejos Formation, Poblete Norte section (Fig. 1). It proposes, for the first time, a graptolite biostratigraphy for the Ludlow of Precordillera, enhancing the limited knowledge of Ludlovian graptolites in Argentina and South America during a crucial period in graptolite history. Additionally, it facilitates more accurate age dating and correlation of the Silurian in Precordillera with other global sections.

Institutional abbreviations.—INGEO, Instituto de Geo­logía, Universidad Nacional de San Juan, San Juan, Argen­tina; UNSJ, Universidad Nacional de San Juan, San Juan, Argen­tina.

Other abbreviations.—2TRD, two thecae repeat distance; FAD, first appearance datum; PNN, Poblete Norte Nuevo; Th, theca(e).

Nomenclatural acts.—This study and the nomenclatural acts it contains have been registered in ZooBank: urn:lsid:zoobank.org:pub:E6FAC9E7-1740-4BDC-9C5A-AEE24B914771

Geological setting

The Los Espejos Formation (Cuerda 1965) crops-out in the Poblete Norte section, 77 km north of San Juan city, Talacasto Range, Central Precordillera of San Juan Province, Argen­tina (Fig. 1). This formation paraconformably overlies the upper Hirnantian to lower Wenlockian La Chilca Formation, and is generally overlain by the Lower Devonian Talacasto Formation. To the south, in the San Juan River area, the Los Espejos Formation passes laterally into the Tambolar For­mation (Heim 1952).

Paleontologically, the Los Espejos Formation possesses one of the most varied and abundant Silurian fossil faunas of Argentina. The assemblage includes brachiopods, graptolites, bivalves, conodonts, tentaculitoids, crinoids, eurypterids, ostracods, cnidarian, and trace fossils, which dated the unit as late Wenlockian to Early Devonian (Cuerda 1969; Brussa and Toro 1989; Benedetto et al. 1992; Gómez et al. 2018, 2021; and studies cited therein).

Local stratigraphy.—At the Poblete Norte section, the outcrops reveal a stratigraphic sequence ranging from the Lower Ordovician to the upper Carboniferous in the mountain ranges, and Neogene and Quaternary deposits occurring in valleys and river courses (Ramos et al. 2000). The oldest unit is the San Juan Formation of Tremadocian to middle Darriwilian (Ordovician) age (Kobayashi 1937, redefined by Amos 1954). Following this, the Tucunuco Group (Cuerda 1965) is composed of the upper Hirnantian (Ordovician) to Lower Devonian La Chilca and Los Espejos formations (Cuerda 1965). Subsequently, the Gualilán Group (Baldis 1975) consist of the Lower to Upper Devonian Talacasto and Punta Negra formations (Padula et al. 1967; Bracaccini 1950). This is succeeded by the Los Gauchos Formation (lower to upper Carboniferous; Furque 1983; Vergel et al. 2009), the Albarracín Formation (Middle Miocene; Leveratto 1968), and, finally, Quaternary alluvial and colluvial deposits.

In the La Chilca Hill, Cuerda (1969) subdivided the Los Espejos Formation into three members. The Lower Member is characterized by a basal reddish oolitic sandstone, followed by green and purplish red claystones, with interbedded phosphate concretions. The Middle Member presents abundant greenish siltstones, reduced brownish sandstones, and coquinas. Finally, the Upper Member is formed by abun­dant greenish sandstones, coquinas and subordinate silt­stones, with sporadic para-conglomerates at the highest levels. These three members were later recognized in the Poblete Norte section by Lopez (2019).

Structure.—The Poblete Norte section is situated in the Central Precordillera, characterized by a thin-skinned faulted fold belt, exhibiting eastern vergence and detachment level within the Cambrian–Ordovician formations (Ramos et al. 2000).

Specifically, the Poblete Norte section displays a homoclinal and continuous sequence that commences with a faulted zone beneath the San Juan Formation to the east (see Fig. 1). Within the Los Espejos Formation section, the strata exhibit predominantly uniform tilting (ca. 45° W) and azimuth (north-south), with continuous outcropping levels. Conversely, beds located at the east of two minor creeks (pointed as lineaments in Figs. 2 and 3) were observed either vertically or overturned, with slightly turned azimuths (NNW). This variation, coupled with a mismatch of the fossiliferous levels of ca. eleven meters, might suggest the presence of left-lateral strike-slip faults. However, further structural studies are required to validate this hypothesis.

Stratigraphic sections.—The Los Espejos Formation section was traced from the topmost levels of the La Chilca For­mation to the lowermost strata of the Talacasto Formation, encompassing the Los Espejos Formation with a thickness of 138.5 meters. Both formational contacts were identified as paraconformities (Fig. 3A), and within this column, the three members of the unit were recognized.

In an effort to address the structural concerns outlined in the “Structure” section, two detailed columns were examined, one to the north and another to the south of the main column. These detailed segments measured 45.26 meters (North section) and 44.93 meters (South section) in thickness, covering the upper Middle Member and the lower Upper Member of the Los Espejos Formation (Fig. 3B, C). The North section revealed six graptolitic levels (PNN-00N–05N), while the South section exhibited seven levels with graptolites (PNN-02S, 04S, 06S, 06’S, 07, 08’S and 08S; samples 01S, 03S and 05S were barren).

Graptolite faunas from the Los Espejos Formation.—Within the Los Espejos Formation only two genera have been docu­mented, encompassing two species and three subspecies. Specimens of Uncinatograptus uncinatus notouncinatus (Cuerda 1969) have been recognized in the sections of La Chilca Hill, Las Chacritas River, Salto Macho Creek (Talacasto), and del Fuerte Hill (Cuerda 1969; Antonioli 1975; Baldis et al. 1984; Peralta 1984b; Rickards et al. 1996). It is noteworthy that Cuerda (1969) reported undetermined monograptids associated with specimens of U. uncinatus notouncinatus that were tentatively referred to the Pristiograptus dubius group. However, the author did not provide a detailed description or illustration of these findings.

On a different note, several specimens of Saetograptus argentinus argentinus (Cuerda 1969) and Saetograptus argentinus robustus have been described in levels situated a few meters above those yielding Uncinatograptus uncinatus notouncinatus. These findings were recorded in the La Chilca Hill, Salto Macho Creek, Ancha Creek, del Fuerte Hill, and Loma de Los Piojos section (Cuerda 1969; Antonioli 1975; Baldis et al. 1984; Peralta 1984b; Rickards et al. 1996; Maletz et al. 2002; Lopez 2022).

Biostratigraphy of the Los Espejos Formation.—The Los Espejos Formation has been dated based on three fossil groups. First, its brachiopod fauna was subdivided by Benedetto et al. (1992) into four assemblages: the First Association of a Wenlockian to early Ludlovian age, the Second Association of a late Ludlovian to early Pridolian age, the Third Association of a Pridolian age, and a Fourth Association of a Lochkovian (Early Devonian) age. The authors mentioned specimens of the graptolites U. u. notouncinatus and S. argentinus in the lower and middle parts of the Second Association of brachiopods, respectively.

Subsequently, the palynomorph content allowed dating the lower levels as late Wenlockian to early Ludlovian, and the upper levels as Lochkovian (Early Devonian) (García-Muro and Rubinstein 2015).

Finally, the conodont faunas studied in the Los Espejos Formation have allowed to record the Gorstian Kockellella variabilis variabilis Interval Biozone, the Ludfordian Poly­gnathoides siluricus and “Ozarkodina” parasnajdri zones, the Pridolian Ozarkodina eosteinhornensis Interval Biozone and the Lower and Upper O. e. detortus biozones, and the Lochkovian Icriodus herperius Biozone (Albanesi et al. 2006, 2017; Mestre et al. 2017; Gómez et al. 2021). Specimens of S. argentinus were mentioned associated with the K. v. variabilis Interval Biozone (Albanesi et al. 2006).

Until now, no key graptolites have been documented, precluding the biostratigraphic studies. Consequently, the levels hosting U. u. notouncinatus, S. a. argentinus, and S. a. robustus have been dated as Gorstian sensu lato based on phylogenetic relations or the association with other fossils (Cuerda 1969; Benedetto et al. 1992; Maletz et al. 2002; Albanesi et al. 2006; Lopez 2022).

Silurian graptolite faunas in Precordillera and South Ame­rica.—In the Precordillera Geological Province, located in western Argentina, Silurian graptolites have been studied in four stratigraphic units. Llandoverian specimens were documented in the lower Salto Macho Member of the La Chilca Formation (Cuerda et al. 1982, 1988; Lopez et al. 2020; Lopez and Kaufmann 2023), as well as in the “Fangolitas Ocres” and “Ferrífero Superior” members of the Don Braulio Formation (Peralta 1985). Wenlockian graptolites were only mentioned in the upper Salto Macho Member of the La Chilca Formation (Lopez and Kaufmann 2023). Ludlovian specimens have been discovered in the “Middle Psamitic” Member of the Rinconada Formation (Peralta 1984a, 1986) and the Middle Member of the Los Espejos Formation (Cuerda 1965, 1969; Antonioli 1975; Rickards et al. 1996; Maletz et al. 2002; Lopez 2022). Lastly, early Pridolian graptolites have been exclusively described from the upper part of the “Middle Psamitic” Member of the Rinconada Formation (Lopez et al. 2023). As of now, no younger graptolites have been reported in South America.

The climate changes (mainly cooling) during the Silu­rian and Early Devonian, accompanied by the emergence of plankton predators, marked the gradual decline and subsequent extinction of the graptolites faunas worldwide (see Maletz 2017, 2023; Štorch 2023). This ongoing disappearance is also evident in South America, mainly in the Precordillera, where the most complete record has been found. In this region, the studied Llandoverian assemblages have revealed diverse graptolite faunas, with Coronograptus, Lagarograptus, Metaclimacograptus, Monograptus, Nor­malo­graptus, Pristiograptus, Pseudoplegmatograptus, Sti­mulo­graptus, Talacastograptus, and several other undetermined biserial and uniserial graptolites (Cuerda et al. 1988; Lenz et al. 2003; Lopez and Kaufmann 2023).

Wenlockian associations exhibit a reduced number of genera, only with Monograptus, Pristiograptus, Retiolites, Sti­mulograptus, and undetermined uniserial graptolites (Peralta 1985; Kerlleñevich and Cuerda 1986; Lopez and Kauf­mann 2023). Ludlovian strata, up until now, have shown only two genera, Saetograptus and Uncinatograptus (Cuerda 1969; Antonioli 1975; Peralta 1986; Lopez 2022). Finally, lower Pridolian assemblages are scarce, with only one described so far, consisting of the genera Skalograptus, possibly Enigmagraptus, and undetermined monograptids (Lopez et al. 2023). The pattern outlined above displays a marked decrease in the graptolite taxonomic diversity during the Silurian in Precordillera with at least 9 genera in the Llandovery, 4 genera in the Wenlock, and only 2 genera in the Ludlow and Pridoli series.

A comparable decline in diversity has been observed in other regions of South America as well. In the North Western Argentina and south of Bolivia, Clinoclimacograptus, Para­climacograptus, Spirograptus, and Stimulograptus of Llandoverian age have been documented (Alhfeld and Bra­nisa 1960; Toro 1995; Rubinstein and Toro 2006). Ludlo­vian genera Colonograptus, Monograptus, Neo­di­ver­so­grap­tus, Pristiograptus, and Saetograptus were also described in the same region (Suarez-Soruco 1975; Maletz et al. 2002; Toro and Maletz 2018), with no precise biostratigraphic affiliation for those associations. Finally, in Paraguay, Llando­verian graptolites of the genera ?Demirastrites, Meta­cli­maco­grap­tus, Monograptus, Normalograptus, Para­cli­maco­graptus, and Stimulograptus (Uriz et al. 2008; Tor­tello et al. 2012) were mentioned.

Owing to the last paragraphs, every discovery of upper Silurian graptolite faunas represents a significant paleontological window to a crucial time of the graptolite history. These findings offer insights into their presence and distribution in South America, a continent with scarce graptolite faunal lists, before their final extinction worldwide in the Early Devonian.

Material and methods

The graptolites were collected from brownish sandstones with carbonate cement, greenish massive sandstones, yellowish siltstones, and carbonate coquinas, corresponding to thirteen samples from the Los Espejos Formation, Poblete Norte section, Talacasto Range (PNN-00N–05N; PNN-02S–08S; Figs. 2 and 3). Most of the graptolite specimens are preserved in three dimensions, filled by oxidized pyrite or carbonate-sandstone/siltstone (see Figs. 4–7). In general, the tubaria are parallel to bedding or slightly oblique in siltstones, randomly disposed in coquinas (e.g., PNN-04S), and horizontal and mostly aligned in sandstones (e.g., PNN-02N).

Some of the graptolites from the Los Espejos Formation were partially illustrated and described by Lopez (2019). In the present work, an updated taxonomic revision of these fossils is presented and new specimens from the same area are studied.

The values of the interthecal septum inclination in specimens of Uncinatograptus spp. were measured from the tubarium axis to the top end of the supragenicular wall, positioned as parallel as possible to the thecal development (given the sigmoidal nature of the interthecal septum; see Fig. 6).

The fossil specimens were illustrated using conventional stereomicroscope (Leica S9D) and with a digital camera (Nikon D3400). Material is housed at the Instituto de Geología Dr. Emiliano Aparicio, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, under the repository code INGEO-PI-2016-2248.

Systematic palaeontology

The graptolite fauna collected is dominated by uniserial taxa, aligned (lower levels of the sections) or randomly disposed (upper levels of the sections). Uncinatograptus spp. and Saetograptus argentinus argentinus (Cuerda 1969) are abundant in specific levels, with dozens to hundreds of specimens per sample. Other graptolite taxa, such as Bohemograptus and Lobograptus, are scarce and only present in a few fossiliferous levels. The taxonomic classification of Maletz (2014) and Bates et al. (2023) is followed herein.

Phylum Hemichordata Bateson, 1885

Class Pterobranchia Lankester, 1877

Subclass Graptolithina Bronn, 1849

Order Graptoloidea Lapworth, 1875

Family Monograptidae Lapworth, 1873

Genus Bohemograptus Přibyl, 1967

Type species: Graptolithus bohemicus Barrande, 1850; by original designation; Ludlow of Bohemia (Czech Republic).

Bohemograptus bohemicus (Barrande, 1850)

Figs. 4K, 8A, B.

1850 Graptolithus bohemicus; Barrande 1850: 40, pl. 1: 15–18.

1936 Monograptus bohemicus (Barrande, 1850); Bouček 1936: 3–4, pl. 1: 1–3.

1958 Pristiograptus bohemicus (Barrande, 1850); Urbanek 1958: 77–80, figs. 46, 47, 49.

1967 Bohemograptus bohemicus (Barrande, 1850); Přibyl 1967: 136, pl. 1: 1–6.

1990 Bohemograptus bohemicus bohemicus (Barrande, 1850); Lenz 1990: figs. 3A, B.

1997 Bohemograptus bohemicus bohemicus (Barrande, 1850); Zhang and Lenz 1997: 1236, figs. 6M–S, 7I, 7K–M.

2002 Bohemograptus bohemicus (Barrande, 1850); Nilsson 2002: 16, figs. 8B, 9B, C.

2004 Bohemograptus bohemicus bohemicus (Barrande, 1850); Lenz and Kozłowska-Dawidziuk 2004: 32, pl. 37: 1–5, 7; pl. 44: 1–5.

2012 Bohemograptus bohemicus (Barrande, 1850); Sachanski et al. 2012: pl. 1e.

2014 Bohemograptus bohemicus (Barrande, 1850); Štorch et al. 2014: 1032, fig. 13D.

Material.—Two specimens (INGEO-PI-1865, 1879B) with sicula and firsts thecae, well preserved as carbon film and filled by carbonate siltstone from levels PNN-02N and 07S middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Description.—Specimens moderate to strongly ventrally curved, only with sicula and firsts thecae present. The dorso-ventral width varies between 0.54–0.66 mm at Th1, 0.56–0.73 mm at Th2, 0.74–0.84 mm at Th3, 0.94–1.02 mm at Th4, and 0.94 mm at Th5. Sicula is slightly ventrally curved, 1.81–1.98 mm long, a moderately flared 0.28–0.29 mm wide aperture, and its apex reaches the Th1 aperture level or slightly above. Virgella and dorsal process visible, with 0.34–0.14 mm long and 0.05–0.16 mm long, respectively. Th1 shows a concave ventral wall in profile, whereas Th2–5 present tube-like thecae or slightly undulated ventral walls. Thecae incline 31–36° to the tubarium axis. 2TRD values range from 1.37–1.50 mm proximally, the thecal spacing counts 6 in 5 mm, and the overlap is ½.

Remarks.—Uncommon component of the Los Espejos For­mation graptolite fauna. The specimens collected in the Los Espejos Formations resemble Bohemograptus bohemicus as described and illustrated by Berry (1964), Urbanek (1970), Lenz (1984), Nilsson (2002), Koren’ and Sujarkova (2004), Lenz and Kozłowska-Dawidziuk (2004), and Wilkinson (2021). This species differs from its derivative species Bohe­mograptus tenuis (Bouček, 1936) in its greater width and thecal overlap (Lenz and Kozłowska-Dawidziuk 2004).

A possible B. bohemicus was mentioned but no illustrated by Suarez-Soruco (1975) in lower Ludlovian levels. This contribution presents the first description and illustration of this taxon in South America.

Stratigraphic and geographic range.—Level PNN-02N and 07S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province of Argentina.

Genus Lobograptus Urbanek, 1958

Type species: Monograptus scanicus Tullberg, 1883; Ludlow of the Scania (Sweden).

Lobograptus sp.

Fig. 7D, 8F.

Material.—One specimen with mould and countermould (INGEO-PI-53A and B) well preserved in relief filled by carbonate siltstone. Level PNN-04S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Description.—Straight 9.82 mm long fragment, with proximal end not recovered. The dorso-ventral width increases from 0.60 mm to 0.71 mm. Thecae inclined 10–18° to the tubarium axis. Thecae show ventral bends in its ventral wall or are slightly geniculate, giving a weakly monoclimacid-­shape in profile. The thecal aperture shape is straight to sigmoidal, with orthogonal to obtuse angles to the tubarium axis. Thecal spacing counts 4 in 5 mm, the 2TRD varies 2.70–3.06 mm, and the thecal overlap is ¼ to two-fifths.

Remarks.—Very rare component of the Los Espejos For­ma­tion graptolite fauna. The specimen exhibits morphological similarities with the material described and illustrated by Palmer (1971), Lenz (1990), Zhang and Lenz (1997), Koren’ and Sujarkova (2004), Lenz and Kozłowska-Dawidziuk (2004), Pashko (2020), and Wilkinson (2021) as Lobograptus sp. In particular, the studied specimen shares features with L. progenitor in thecal shape, middle to distal dorso-ventral width, thecal inclination, shape, spacing, and overlap. On the other hand, the absence of essential diagnostic characteristics (proximal end and tubarium curvature shown by Palmer 1971), does not allow for a specific determination.

The specimen of Lobograptus described herein represents the first record of this genus in South America.

Genus Saetograptus Přibyl, 1942

Type species: Graptolithus chimaera Barrande, 1850; original description Přibyl 1942; Neodiversograptus nilssoni Zone, Gorstian, Ludlow of Bohemia (Czech Republic).

Saetograptus argentinus argentinus (Cuerda, 1969)

Fig. 7A, B, 8E, I.

1969 Monograptus argentinus; Cuerda 1969: 231–234, pl. 1: a–d; pl. 2: 4–6.

1971 Monograptus argentinus Cuerda, 1969; Cuerda 1971: 399, pl. 5: 1–11, pl. 31: 4–6.

1996 Saetograptus (?Colonograptus) argentinus (Cuerda, 1969); Rickards et al. 1996: 120, figs. 7e–h, 11h.

2002 Saetograptus argentinus argentinus (Cuerda, 1969); Maletz et al. 2002: 334–336, text-fig. 2g, i–m, pl. 1: 2, 4.

2019 Saetograptus argentinus? (Cuerda, 1969); Lopez 2019: 77, figs. 21h–i, 22b.

2022 Saetograptus argentinus argentinus (Cuerda, 1969); Lopez 2022: 28–30, figs. 3.1–3, 3.5–8, 4.1–3.

Material.—Several juvenile and mature tubaria (INGEO- PI-2138–2146, 2178, 2190, 2195–2197, 2202–2203, 2206, 2208A–B, 2214–2217, 2219, 2221–2227, 2247) well preserved and filled by carbonate siltstone from levels PNN-04N, 05N, 04S, 07S, 08’S, and 08S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Description.—The material found in the Los Espejos For­mation possesses both juvenile and mature specimens with an absent proximal end, and a few complete tubaria preserved in relief, filled by carbonate or oxidized pyrite. Straight tubaria, with a maximum length of 26.4 mm, having thirty-one thecae and a broken proximal end. The proximal dorso-ventral width without spines is 0.82–1.11 mm at Th1, 0.93–1.39 mm at Th2, 1.07–1.44 mm at Th3, 1.11–1.36 mm at Th5, and 2.07–2.51 mm distally. Thecae possess ventrally directed lateral apertural spines up to Th3–6, with a rapid distalward transition to pristiograptid thecae with even apertures. The sicula is 2.05–2.26 mm long, and 0.25–0.46 mm wide at the aperture which possesses 0.37–0.44 mm long virgella and 0.14–0.51 mm antivirgellar spine. Sicular apex reaches up to the level of the aperture of Th2–3. The 2TRD is 1.17–1.21 mm at Th2, and 1.79–1.97 mm distally. Thecal spacing equals 5–8 thecae in 5 mm, and the thecal overlap is one half proximally to three-fifths distally. The thecal inclination decreases from 50° proximally to 36–30° distally.

Remarks.— These measures agree with those reported by Cuerda (1965), Cuerda (1969), Rickards et al. (1996), Maletz et al. (2002) and Lopez (2022) for S. a. argentinus. This subspecies can be differentiated from S. a. robustus mainly by its narrower tubarium (2.51 mm vs 3.2–3.8 mm) and much lower thecal inclination distally (30° vs 55°).

Stratigraphic and geographic range.—Levels PNN-04N, 05N, 04S, 07S, 08’S, and 08S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province of Argentina.

Saetograptus cf. varians (Wood, 1900)

Fig. 7C, 8C, D.

Material.—Two immature specimens with sicula and firsts 7–8 thecae (INGEO-PI-2187, 2198A, B) well preserved in relief filled by carbonate siltstone from level PNN-04S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Description.—Straight tubarium of a maximum recorded length of 7.67 mm (INGEO-PI-2198A), with a gently ventrally curved proximal end. The dorso-ventral width is 0.65–0.89 mm at Th1, 0.78–0.93 mm at Th2, 0.92–0.93 mm at Th3, 0.83–1.05 mm at Th4, 0.89–1.13 mm at Th5, and 0.99 mm at Th8. The sicula is slightly ventrally curved, 1.55 mm long. Sicular aperture is 0.25–0.34 mm wide. The sicular apex reaches up to the level of Th2 aperture. A 0.38 mm long virgella and 0.09–0.15 mm long antivirgellar spine are present. Thecae are biform, with Th1 to Th6–7 displaying spine-like lateral apertural lappets. Subsequent thecae gained simple pristiograptid appearance. Thecae incline at 37–39° to the tubarium axis. 2TRD values range from 1.39 mm at Th2 to 1.55 mm at Th5. Thecae number 6.5–7 in 5 mm, and overlap for three-fifths to ¾ their length.

Remarks.—The studied specimens from the Los Espejos Formation are similar to S. varians in its proximal width, 2TRD, thecal spacing, etc. (Hutt 1969; Lenz and Kozłowska-Dawidziuk 2004; Wilkinson 2021). On the other hand, the presence of spine-like thecal lappets in theca Th4–7, of antivirgellar spine, and lower dorso-ventral width (cf. Wilkinson 2021), suggest that the Argentinian material belongs to a different taxon, for which a reliable specific determination requires a further material.

Specimens of S. cf. S. varians and S. varians were mentioned by Branisa (1969) and Suarez-Soruco (1975) in Bolivian outcrops in association with S. argentinus. The material of the Los Espejos Formation means the first mention and description of this species in Argentina.

Genus Uncinatograptus Tsegelnyuk, 1976

Type species: Monograptus uncinatus Tullberg, 1883; 12a, lectotype, selected by Přibyl 1948: 35 (Tullberg 1883: pl. 1: 25; ?LO collection, specimen not identified); 12b–c, proximal end in lateral (3b) and ventral (3c) views; from the Ludlow of Bohemia (Czech Republic).

Uncinatograptus elsae sp. nov.

Figs. 4D–F, 5F–J, 6C.

Zoobank LSID: urn:lsid:zoobank.org:act:B90B4DF2-6ECB-4D16-8B 64-0ED5DBC1FF78

Etymology: In honour of Elsa Nelly Borcia (1957–2020), mother of FEL, a woman with an exceptional affection and patience, who instilled to FEL the love for the knowledge, and to whom is greatly thankful.

Type material: Holotype: INGEO-PI-2115, almost mature relief preserved tubarium. Paratypes: INGEO-PI-1870, well preserved fragmented tubarium; INGEO-PI-2042, moderate preserved specimen; INGEO-PI-2035, proximal specimen; INGEO-PI-2137A and B, specimen with preserved hoods. All from the type locality and horizon.

Type locality: Poblete Norte Creek, Talacasto, 77 km NNW from San Juan city. Central Precordillera, San Juan, Argentina.

Type horizon: PNN-02N, North detailed section, Middle Member of the Los Espejos Formation, Poblete Norte section, Talacasto, lower Gorstian, lower Ludlow, Silurian.

Material.—13 proximal ends or complete specimens in relief (INGEO-PI-2023, 2035, 2042, 2048A, B, 2066A, B, 2069, 2082, 2087, 2088, 2091, 2111A, B, 2115, 2132A, B, 2137A, B, 2162) filled by carbonate or oxidized pyrite from levels PNN-02N and 02S, all from the type locality and horizon.

Diagnosis.—Straight, robust, and rapidly widening Uncinato­graptus reaching a dorso-ventral width of more than 1 mm (without hood) at Th3–4, decreasing to 0.8 mm at Th10 and then continuously increasing distally, giving a half spindle-shape to the tubarium. Hoods rarely preserved. Thecal number 5.5–7 in 5 mm, the 2TRD is 1.45 mm proximally and 1.86 mm distally, and interthecal septum inclination is 48–56° proximally and 40–33° distally.

Description.—Straight tubarium of a maximum length of 9.21 mm (INGEO-PI-2115), with slightly to moderately ventrally curved proximal end. The dorso-ventral width varies 0.64–0.80 mm (0.70 mm mean value) at Th1, 0.72–0.89 mm (0.79 mm) at Th2, 0.83–1.05 mm (0.91 mm) at Th3, 0.89–1.21 mm (1.05 mm) at Th5, 0.82–1.18 mm (1.03 mm) at Th7, 0.74–0.90 mm (0.80 mm) at Th10, and 0.97–1.06 mm (0.98 mm) distally. Sicula is moderate to strongly ventrally curved, with 1.91–2.44 mm long, with 0.25–0.39 mm wide aperture, and the apex generally attaining the level of the Th2 aperture. Ventrally-directed, up to 0.68 mm long virgella and 0.05–0.16 mm long dorsal apertural process are usually seen. The distance between the sicular and Th1 apertures ranges 1.12–1.65 mm (1.32 mm in average). Thecae are of Uncinatograptus-type, with dorsal hoods and ventrally facing apertures. Due to taphonomy these hoods are usually not visible, giving the aspect of a pseudomonoclimacid-like thecae or apertural spines. The supragenicular ventral walls are slightly oblique proximally, to parallel medially and distally to the tubarium axis. The interthecal septum inclines 48–56° proximally, to 40–33° distally. 2TRD values range from 1.45 mm proximally to 1.86 mm distally, and the thecal spacing counts 5.5–7 in 5 mm. Thecal overlap for about half their length.

Remarks.—Common species of the lower graptolitic levels of the Los Espejos Formation (PNN-02N and 02S) preserved in different astogenetic stages. Specimens of Uncinatograptus elsae sp. nov. can be differentiated from other Uncinatograptus species by its dorso-ventral width, which initiates in near values (0.64–0.80 mm) and rapidly increases and exceeds 1 mm from Th3 (1.05 mm maximum) to Th7 (1.18 mm maximum); subsequent dorso-ventral width values decrease to less than 1 mm, to finally stabilize in values similar to those in Uncinatograptus uncinatus notouncinatus distally (0.97–1.06 mm). The increase-decrease width results in a half spindle-shape of mature tubaria (from Th1–10) or a conic-shape in juvenile specimens, that helps to distinguish this species from the parallel-shaped Uncinatograptus lisandroi sp. nov., and the steadily widening of U. u. uncinatus and U. u. notouncinatus (with maximum distal values of 1.56 mm and 1.02 mm, respectively). In addition, proximal interthecal septum tilting can be helpful to differentiate the respective species, where U. elsae sp. nov. shows higher values (48–56°) than U. u. notouncinatus (40°) and U. lisandroi sp. nov. (32–40°), and distal values, with U. elsae sp. nov. showing slightly higher angles than U. u. uncinatus (25–20°). Moreover, sicula length is similar in all the species, showing only slightly higher values in sicular aperture width in U. elsae sp. nov. than U. u. uncinatus (0.22–0.26 mm; Urbanek 1958). These differences are summarized in Fig. 6. On the other hand, if the specimen shows only the firsts pair of thecae, its features will be similar between species denying a reliable taxonomic determination. It is important to mention that similar tubaria shape and sizes to U. elsae sp. nov. are illustrated by Garrat (1978: figs. 6A, B), classified as Monograptus aff. uncinatus.

Stratigraphic and geographic range.—Levels PNN-02N and 02S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Uncinatograptus lisandroi sp. nov.

Figs. 4G–J, 5A–E, 6A.

Zoobank LSID: urn:lsid:zoobank.org:act:09E7BF7D-D620-457B-B6 BA-824DDEEAB8CA

Etymology: In honour of Lisandro Lopez Gordillo (born 2022), son of FEL, a lovely little boy that gives love and kindness to everyone, and who nowadays inspires FEL to continue his research.

Type material: Holotype: INGEO-PI-2134A, mature well preserved tubarium. Paratypes: INGEO-PI-2134B, countermould of holotype; INGEO-PI-2040, 2062, 2092, mature specimens with moderate preservation; INGEO-PI-2046A, 2048A, juvenile specimens. All from the type locality and horizon.

Type locality: Poblete Norte Creek, Talacasto, 77 km NNW from San Juan city. Central Precordillera, San Juan, Argentina.

Type horizon: PNN-02N, North detailed section, Middle Member of the Los Espejos Formation, Poblete Norte section, Talacasto, lower Gorstian, lower Ludlow, Silurian.

Material.—13 proximal ends or complete specimens in relief (INGEO-PI-2023, 2029, 2040, 2042, 2044, 2046A–C, 2048A, B, 2083, 2092, 2093, 2108A–C, 2126, 2133A, B, 2134A, B, 2137A, B) filled by carbonate or oxidized pyrite. Levels PNN-02N, 06S, and 06’S, all from the type locality and horizon.

Diagnosis.—Slightly to moderately dorsally curved Unci­nato­graptus, with generally slender tubarium. Sicula and first theca gently to moderately ventrally curved. Dorso-ventral width of 0.52 mm proximally, 0.71 mm medially, and 0.51 mm distally, giving a parallel-sided shape to the tubarium. Hoods rarely preserved. Thecae number 5–8 in 5 mm, 2TRD is 1.38 mm proximally and 1.74 mm distally, interthecal septum inclination decreases from 32–40° proximally to 28° distally.

Description.—Specimens with slightly to moderately proximal ventral curvature, gently to moderately dorsally curved in the middle and distal part. The maximum length is 10.99 mm (INGEO-PI-2092). The dorso-ventral width varies from 0.49–0.69 mm (0.52 mm mean value) at Th1, 0.52–0.78 mm (0.62 mm) at Th2, 0.49–0.78 mm (0.64 mm) at Th3, 0.60–0.79 mm (0.71 mm) at Th5, 0.63–0.78 mm (0.73 mm) at Th7, 0.61–0.62 mm (0.61 mm) at Th10, and 0.45–0.70 mm (0.51 mm) distally. Sicula is moderately to strongly ventrally curved, 1.89–2.21 mm long, and its apex reaches up to the level of the Th2 aperture. Sicular aperture is 0.23–0.40 mm wide. Downwardly to slightly ventrally-directed virgella 0.14–0.75 mm long, commonly having 0.05–0.14 mm long dorsal tongue. The distance between the sicular and the Th1 apertures varies from 1.18–1.65 mm (1.28 mm mean value). Thecae of uncinatograptid-type finished with hoods and extroverted ventrally facing apertures. Due to taphonomic reasons these hoods are usually not visible or seen in cross section, giving the aspect a pseudomonoclimacid appearance or apertural spines. The supragenicular ventral walls are parallel or inclined slightly inwards to the tubarium axis. Interthecae septum inclines at 32–40° proximally, decreasing to 28° distally. The 2TRD values range from 1.38 mm proximally to 1.74 mm distally. Thecae number 5–8 in 5 mm, and overlap for mostly half their length.

Remarks.—Common components of the lower and middle graptolitic levels of the Los Espejos Formation in different astogenetic stages. Specimens of U. lisandroi sp. nov. can be differentiated from other Uncinatograptus species by its dorso-ventral width, which starts 0.49–0.69 mm at Th1, and from Th2 maintains constant values measured in the respective specimens (minimums of 0.45–0.52 mm to maximums of 0.62–0.79 mm). This width development differs from the constant growth of U. u. uncinatus and U. u. notouncinatus, and the half spindle-shape of U. elsae sp. nov. Furthermore, proximal interthecal septum inclinations are smaller than those showed by U. u. uncinatus (54°), and distal interthecal septum inclination and distal 2TRD show lower values than U. u. notouncinatus (40° and 1.97 mm) and U. elsae sp. nov. (48–56° and 1.86 mm). These minute differences are summarized in Fig. 6. On the other hand, if the tubarium presents only proximal features or belongs to a juvenile astogenetic stage, a taxonomic classification at species level will be imposible.

Stratigraphic and geographic range.—Levels PNN-02N, 06S, and 06’S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province, Argentina.

Uncinatograptus uncinatus notouncinatus (Cuerda, 1969)

Figs. 4A–C, 6B.

1969 Monograptus uncinatus notouncinatus; Cuerda 1969: 228–231, pl. 1e–h; pl. 2: 1–3.

1971 Monograptus uncinatus notouncinatus Cuerda, 1969; Cuerda 1971: 396–399, pl. 5: 12–23; pl. 31: 1–3.

1975 Monograptus uncinatus notouncinatus Cuerda, 1969; Antonioli 1975: 212–216, pl. 1a–k; pl. 2: 1–2.

1996 Monograptus uncinatus notouncinatus Cuerda, 1969; Rickards et al. 1996: 114–115, figs. 6j–l, 11c–g.

2019 Monograptus uncinatus var. notounciantus Cuerda, 1969; Lopez 2019: 75–77, figs. 21a–g, j, 22a.

Material.—Several juvenile and mature tubaria (INGEO-PI- 1863, 1868, 2016A, B, 2023, 2025, 2032, 2044, 2062, 2067, 2074, 2077, 2093, 2101, 2103A, B–2104A, B, 2109A–C, 2115, 2123, 2126–2127A, B, 2134A, B, 2152, 2156, 2173A, B, 2193, 2197, 2209, 2229A, B, 2235, 2238–2246) well preserved as carbon films or filled by carbonate siltstone, from levels PNN-00N–03N, 02S, 04S, 06S, and 06’S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province of Argentina.

Description.—The specimens are abundant in the Los Espe­jos Formation and represented by both juvenile and mature tubaria. Generally, they are preserved in relief, filled by carbonate or oxidized pyrite, as carbon films or negative casts. The analysed material exhibits a straight to slightly dorsally curved tubarium, with a ventrally curved proximal end. The longest specimen with 14 thecae attained the length of 14.5 mm. The dorso-ventral width, thecal spacing, and 2TRD measures are summarized in the Fig. 6. Thecae are extroverted throughout the tubarium, although most have lost the hood as a result of preservation and simulate a hood-like aperture or the presence of spines. The supragenicular walls are parallel or inclined with obtuse angles to the tubarium axis. The sicula is 1.75–2.32 mm long, presents an aperture of 0.28–0.36 mm long, its apex reaches the Th2 aperture, and possesses from two to seven sicular annuli. The thecal overlap is one half. The interthecal septum tilting varies from 40° proximally to 30–27° distally.

Remarks.—These features agree with those given by Cuerda (1969), Antonioli (1975), and Rickards et al. (1996) for U. u. notouncinatus. It is important noting that the dorso-ventral width continuously increase from the proximal (0.62 mm) to the distal area (0.99 mm), in contrast to U. elsae sp. nov. and U. lisandroi sp. nov. (Fig. 6).

Stratigraphic and geographic range.—Levels PNN-00N–03N, 02S, 04S, 06S, and 06’S, middle to upper Los Espejos Formation, lower Gorstian, lower Ludlow, upper Silurian. Poblete Norte section, Talacasto area, San Juan Province of Argentina.

Results

Graptolite faunas.—The graptolite faunas reported in this contribution represent the most diverse associations found in the upper Silurian of South America until now. In addition to the three subspecies previously described, Uncinatograptus uncinatus notouncinatus, Saetograptus argentinus argentinus, and Saetograptus argentinus robustus, five new taxa have been identified.

At the North section (Fig. 3B), the first appearance of U. u. notouncinatus corresponds to the PNN-00N and 01N samples, located in the Middle Member of the Los Espejos Formation (Fig. 3B, indicated by the sky-blue area). Upwards, and 15 m above the first lineament, the identified association is made up of U. u. notouncinatus, U. elsae sp. nov., U. lisandroi sp. nov., Bohemograptus bohemicus, and an unidentified uniserial taxon with pristiograptid thecae (level PNN-02N; Figs. 4, 5). Subsequently, one meter higher, an association of U. uncinatus, U. lisandroi sp. nov., and several plant remains was discovered (PNN-03N). Still 8.20 m upwards, the first appearance of S. a. argentinus was recorded in the sample PNN-04N (Fig. 3B, indicated by the blue area). Finally, 7.90 m above the last lineament, the level PNN-05N was found, showing only a few specimens of S. a. argentinus.

At the South section (Fig. 3A), the first appearance of U. u. notouncinatus was recorded in the sample PNN-02S, along with U. elsae sp. nov. and fragments of undetermined retiolitid lists. Higher, the sample PNN-04S, located 15.60 m above, exhibited the first appearance of S. a. argen­tinus, associated with S. cf. S. varians, U. u. noto­uncinatus, and Lobograptus sp. (Figs. 7, 8). This sample represents the first record of S. a. argentinus and U. u. notouncinatus in the same stratigraphic level. The samples PNN-06S and 06’S, positioned 7 and 8 m higher in the section, yield specimens of U. u. notouncinatus and U. lisandroi sp. nov. Subsequent, the sample PNN-07S is 4.60 m upwards, and yields S. a. argentinus. Finally, at 9.90 m (PNN-08’S) and 11.10 m (PNN-08S) of the section, specimens of S. a. argentinus and B. bohe­micus were collected.

The FAD of S. a. argentinus is recorded in both detai­led columns and reliably correlated (PNN-04N and 04S), whereas the FAD of U. u. notouncinatus cannot be reliably correlated due to the presence of possible strike-slip faults in the North section, which might represent repetitions or omissions in the stratigraphic column. In the latter case, a tentative correlation is proposed between the levels PNN-00N and 02S.

Associated fossils.—In the North section, graptolites were found associated with brachiopods Australina jachalensis, Amosina sp., and Castellaroina sp. (levels PNN-00N–04N). Specimens of Clarkeia sp. were observed in levels above the PNN-05N. Most of the levels presented tentaculitoids and ostracods, and a few gastropods, like Loxonema sp. and Bellerophontidae gen. et sp. indet., were present in the levels PNN-00N and 01N. Eurypterid plates, classified as Eurypteracea by Brussa and Toro (1989), together with spicules, crinoids, and scolecodonts were collected in the PNN-02N (Figs. 9, 10A–G). Concretions with conularid cnidarians were found in the level PNN-00N. It is worth mentioning that undetermined carbonaceous wisps were collected in the levels PNN-02N, 03N, and 05N, first-time registered in the Los Espejos Formation. Finally, in the interbedded strata, traces as Chondrites isp., Conostichnus isp., and Planolites isp., were observed.

In the South section, as well as the North section, the bra­chiopods Australina jachalensis, Amosina sp., and Cas­tel­laroina sp. were present in most of the levels, together with Har­ringtonina australis (PNN-02S) and Clarkeia sp. (PNN-04S and 08S). Specimens of Nuculites sp. and undetermined bivalves were collected in the levels PNN-02S, 04S, and 08S. Eurypterid plates were found in the levels PNN-02S, 04S, and 08’S. Most of the sampled levels showed tenta­culitoids and ostracods, in some cases associated with spicules (PNN-06S), scolecodonts (PNN-02S) (Fig. 10A–G), favositid cnidarians (PNN-02S), trilobites (PNN-06’S and 08S), and conodonts in bedding planes (PNN-02S and 04S) (Fig. 10H, I). In the levels PNN-02S and 04S, a few carbonaceous wisps were collected. Lastly, a similar ichnofossil association were observed in both detailed columns.

Local, regional, and global correlation.—The list of lower Gorstian graptolites in the middle–upper Los Espejos Formation allows to establish a regional correlation with other studied section of Precordillera, as del Fuerte Hill, Loma de Los Piojos section, La Chilca Hill, Las Chacritas River, and Ancha and Salto Macho creeks for the Los Espejos Formation (Cuerda 1969; Antonioli 1975; Baldis et al. 1984; Peralta 1984b; Rickards et al. 1996; Maletz et al. 2002; Albanesi et al. 2006; Gómez et al. 2018; Lopez 2022), the Villicum Range and the La Rinconada sections for the Rinconada Formation (Peralta 1984a, 1986), the San Juan River section for the Tambolar Formation (Peralta and León 1993; Mestre 2009), and the Famatina Geological Province for the Villacorta Formation (Césari et al. 2020).

A regional correlation is proposed with graptolitic levels of the upper Lipeón Formation (Argentina) and with the middle–upper levels of the Kirusillas Formation (Bolivia) (Martínez et al. 1971; Brockmann et al. 1972; Suarez-Soruco 1975; Maletz et al. 2002; Edwards et al. 2009; Aris et al. 2011; Toro and Maletz 2018).

Finally, a global correlation could be proposed with contemporaneous sections from Albania (Maletz et al. 1998; Pashko 2020), Australia (Garrat 1978; Garrat and Rickards 1984; Rickards and Sandford 1998; Rickards 2000; Talent et al. 2003), Canada (Lenz 1984, 1990, 2011; Lenz and Kozłowska-Dawidziuk 2004), China (Chen 1984; Hou and Shu 1986; Lenz et al. 1996; Rong et al. 2003), Czech Republic (Kozłowska-Dawidziuk et al. 2001; Manda et al. 2012; Štorch et al. 2015), England (Berry 1964; Rickards 1967; Cocks et al. 2003), Kyrghistan (Koren’ and Sujarkova 2004), Lithuania and Latvia (Radzevičius and Paškevičius 2005), Poland (Urbanek 1958, 1960, 1966; Podhalańska 2019), Sweden (Nilsson 2002; Baarli et al. 2003), Romania (Rickards and Iordan 1975), Russia (Koren’ 1973; Koren’ and Sujarkova 2004; Baarli et al. 2003), USA (Berry and Satterfield 1972; Saltzman 2001; Cramer et al. 2006), and Wales (Watkins and Berry 1977).

Discussion

Biostratigraphic considerations.—Although biostratigraphic data have been obtained from other fossil groups, such as brachiopods, conodonts and palynomorphs (see Biostratigraphy of the Los Espejos Formation section), and a Gorstian age was estimated for host rocks, the described graptolite taxa could not provide a reliable age information. This issue is caused by the endemic nature of the species previously described, so far only known from South America, and by the absence of index graptolites of the Ludlow. Generally speaking, the two genera, Saetograptus and Uncinatograptus, appear in the fossil record in the lower Gorstian Neodiversograptus nilssoni Biozone, and disappear in the upper Ludfordian S. leintwardiniensis Biozone and in the Lower Devonian U. yukonensis Biozone, respectively (Bates et al. 2023). By comparison with other taxa worldwide, the species found in the Los Espejos Formations are consistent with a supposed Gorstian age (Cuerda 1969; Maletz et al. 2002), with the FAD of U. u. notouncinatus in levels slightly below those with the FAD of S. a. argentinus (Cuerda 1969; Benedetto et al. 1992; Rickards et al. 1996; this study).

This contribution introduces three new graptolites, generally cosmopolitan taxa, first time recorded and described in Argentina and South America. Bohemograptus bohemicus is a typical representative of the lower Gorstian (Go1) N. nilssoni and Lobograptus progenitor biozones, or the base of the lower Ludfordian (Lu1) Saetograptus leintwardinensis Biozone (Fig. 11), then replaced by B. tenuis (Urbanek 1970; Rickards and Wright 1999; Kozłowska-Dawidziuk et al. 2001; Nilsson 2002; Koren’ and Sujarkova 2004; Lenz and Kozłowska-Dawidziuk 2004; Rickards 2012; Štorch et al. 2014; Pashko 2020).

Later, specimens of the genus Lobograptus have been found from the middle Homerian (Ho2) Colonograptus praedeubeli Zone to the lower Ludfordian (Lu1) S. leintwardinensis Biozone. On the other hand, the thecal morphology of the studied fragment resembles to L. progenitor or slightly younger species (from the L. progenitor or L. scanicus biozones), based on what a Gorstian age sensu lato can be estimated (Fig. 11) (Bates et al. 2023; and references therein).

Lastly, Saetograptus varians have been found as a scarce integrant only of the early Gorstian L. progenitor Zone in sections of Canada and Czech Republic (Fig. 11) (Lenz 1988, 1990; Lenz and Kozłowska-Dawidziuk 2004; Štorch et al. 2015).

In accordance with the aforementioned points, the FAD of U. u. notouncinatus in the levels PNN-00N and 02S might correspond to the lower Gorstian (Go1) N. nilssoni Zone, supported by the presence of B. bohemicus in immediately overlying levels (PNN-02N). On the other hand, the FAD of S. a. argentinus in the levels PNN-04N and 04S might suggest assignment to the lower Gorstian (Go1) L. progenitor Zone, in accordance with the presence of S. cf. S. varians and Lobograptus sp. The latter stratigraphic assignment is complacent with S. argentinus reported from the K. v. variabilis Interval Biozone by Albanesi et al. (2006). Finally, samples PNN-05N, 08’S, and 08S with S. a. argentinus and B. bohemicus, could correspond to the upper Gorstian or lower Ludfordian due to the ranges of the mentioned species, associated with brachiopods of the Ludlovian genus Clarkeia (Benedetto et al. 1992, 1996). These biostratigraphic considerations are the first proposed for the Ludlovian graptolite faunas of the Precordillera unit of Argentina.

On the other hand, the new species of Uncinatograptus described herein (i.e., U. elsae sp. nov. and U. lisandroi sp. nov.) were found in the lower to middle detailed sections, for which a Gorstian age sensu lato is assigned.

Paleoenvironmental and paleoecological remarks.—Grap­tolite fauna distribution has shown significant variations depending on the paleolatitude and the depth of the water column. The graptolite presence and taxonomic abundance is directly related with the water temperature and the basin location of the deposits (Boucot and Chen 2009; Goldman et al. 2013; Bates et al. 2023; and references therein), i.e., the shallower and colder the water, the poorer the taxonomic assemblage. According to the latter, Goldman et al. (2013: fig. 26.1, and references therein) subdivided a sedimentary basin into three vertical areas with regards on the graptolite fauna: endemic (shallow waters), pandemic (deeper waters) and transition zone; and two horizontal areas or biotopes: epipelagic (0–250 m deep) and mesopelagic (250–1000 m deep) (Fig. 12). Furthermore, Boucot and Chen (2009: fig. 9) delineated a water depth subdivision into four segments (1–4), and related them with the graptolite diversity.

At the local scale, the Los Espejos Formation has been associated to a transgressive-regressive marine cycle in a foreland basin, evolving from a muddy platform without wave influence, later to an inner platform with storm influence, and finally to a transitional platform dominated by storms (Astini and Piovano 1992; Astini and Maretto 1996). This basin evolution modelled a sedimentary succession characterized by heterolithic levels, which alternate shales, siltstones, sandstones, and abundant coquinas, and were deposited in progressively shallower waters with moderate to high energy (Astini and Maretto 1996). Moreover, the paleogeographic reconstruction proposed by Torsvik and Cocks (2013) and Scotese (2021) indicates that the Precordillera was located in middle to high latitudes for the Ludlow–Pridoli, ranging from 45° S to 60° S, suggesting the presence of cold waters within its basin.

The graptolite assemblage of the Los Espejos Forma­tion, presented herein, includes endemic taxa as Saeto­graptus argentinus argentinus, Uncinatograptus uncinatus notoun­cinatus, Uncinatograptus elsae sp. nov., and Uncinatograptus lisandroi sp. nov. Further­more, this study adds new pandemic specimens first time recorded in South America as Bohemograptus bohemicus, Saetograptus cf. S. varians, and Lobograptus sp., together with retiolitid lists and the previous possible Pristiograptus dubius-like tubarium recorded by Cuerda (1969). The first group is the most abundant in the graptolitic levels, reaching amounts of up to hundred specimens per sample. Conversely, representatives of the second group are extremely uncommon, with only one or a few specimens in all the sampled levels. This abundance contrast, in conjunction with the shallow water and moderate-high energy paleoenvironment of the Los Espejos Formation, and with a middle-high paleolatitude location of Precordillera during the Ludlow, would have generated a low-varied graptolite assemblage, high abundance of endemic and epipelagic graptolites, and sporadic representation of pandemic specimens in the basin. According to this, an in-shore position in the proposal of Goldman et al. (2013) or Depth 1 in Boucot and Chen (2009) can be estimated for the Los Espejos Formation levels studied in the Poblete Norte section, which is consistent with the paleoenvironmental features described for the unit (Fig. 12).

Conclusions

New graptolite faunas from the Los Espejos Formation, Poblete Norte section, Talacasto area of Central Precordillera, are presented herein, enabling more precise biostratigraphic knowledge on the Silurian of Argentina.

Two detailed section were studied and thirteen graptolitic levels were sampled. These strata presented classical examples of the Silurian succession of Precordillera, Uncinatograptus uncinatus notouncinatus and Saetograp­tus argentinus argentinus, associated with three worldwide taxa as Bohemograptus bohemicus, Saetograptus cf. S. varians, and Lobograptus sp., and two new species as Uncinatograptus elsae and Uncinatograptus lisandroi.

The occurrence of U. u. notouncinatus in association with B. bohemicus could indicate the presence of the lower Gorstian Neodiversograptus nilssoni Biozone in the lower graptolitic levels. Few meters above, the existence of S. a. argentinus with S. cf. S. varians, U. u. notouncinatus, and Lobograptus sp. might correspond to the lower Gorstian Lobograptus progenitor Biozone. The last graptolitic levels, which present specimens of S. a. argentinus and B. bohe­micus could indicate upper Gorstian to lower Ludfordian ages. The new species of Uncinatograptus described herein, U. elsae and U. lisandroi, were found in the lower to middle detailed sections, for which a Gorstian age sensu lato is assigned. This proposal represents the first Ludlovian graptolite biostratigraphy in Precordillera, Argentina.

The discovery of these new graptolite taxa in the Los Espejos Formation enables a more precise local correlation with other sections from Precordillera and Famatina, as well as regional correlations with the Lipeón (Argentina) and Kirusillas (Bolivia) formations. Additionally, it contributes to global correlation efforts with several equivalent sections worldwide.

This study introduces new graptolite faunas from the Ludlow of Precordillera, first-time recorded in Argentina and South America, enriching insufficient knowledge on Ludlow graptolite fauna of this subcontinent in a critical time in the graptolite history.

Acknowledgements

The authors are grateful to Michael Melchin (Department of Earth Sciences, St. Francis Xavier University, Antigonish, Canada) and Petr Štorch (Institute of Geology of the Czech Academy of Sciences, Prague, Czech Republic) for their revisions and suggestions, which significantly enhance the original manuscript; to the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) for its continuous support in the graptolite studies; and to the Instituto y Museo de Ciencias Naturales (IMCN) and Universidad Nacional de San Juan for their instrumental facility and workplace.

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Acta Palaeontol. Pol. 69 (3): 351–370, 2024

https://doi.org/10.4202/app.01139.2024