1. Introduction
The Acoite Formation (Harrington & Leanza, Reference Harrington and Leanza1957), which outcrops in the Cordillera Oriental Argentina, was deposited in an active tectonic setting within the forebulge depozone of the Central Andean Basin, a back-arc foreland basin situated along the western margin of Gondwana at mid to high latitudes, during the late Cambrian (Furongian) to Early Ordovician. The Early Ordovician was characterized by significant terrigenous input, leading to the development of thick siliciclastic successions with subordinate calcareous deposits (Astini, Reference Astini and Benedetto2003; Benedetto, Reference Benedetto and Benedetto2003; de la Puente & Astini, Reference de la Puente and Astini2023).
The graptolite biostratigraphy of the Acoite Formation has been analysed by Toro (Reference Toro1994, Reference Toro1997, Reference Toro1999), Toro & Maletz (Reference Toro and Maletz2007, Reference Toro and Maletz2008), Toro & Vento (Reference Toro and Vento2013), Toro et al. (Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015), Voldman et al. (Reference Voldman, Albanesi, Ortega, Giuliano and Monaldi2016), Toro & Herrera Sánchez (Reference Toro and Herrera Sánchez2019) and Albanesi et al. (Reference Albanesi, Rueda, Ortega and Monaldi2021), among others. Referred authors identified the Tetragraptus phyllograptoides, Paratetragraptus akzharensis, Baltograptus cf. B. deflexus, Didymograptellus bifidus and Azygograptus lapworthi graptolite zones, which enabled the assignment of a Floian to Dapingian (Lower to Middle Ordovician) age to the lithostratigraphic unit.
The purpose of this contribution is to document new biostratigraphic records of graptolites and associated faunas from the middle to upper parts of the Acoite Formation, exposed in a newly studied section of the Cordillera Oriental of Argentina, referred to as the El Moreno section (Monaldi et al. Reference Monaldi, Albanesi, Salfity, Ortega and Moreno Valdez2017; Rueda et al. Reference Rueda, Ortega and Albanesi2024). In addition, previously published records of middle to late Floian graptolites from other localities of northwestern Argentina are reviewed in detail to provide a reappraisal of the regional biozonation for this time interval.
2. Geographical and geological setting
The El Moreno section is located ca. 8 km ESE of the town of El Moreno, along Provincial Route 78 in Jujuy Province, on the western edge of the Cordillera Oriental (Fig. 1a). It lies approximately 7 km to the NE of the classic Angosto del Moreno area (Fig. 1b), where the lower part of the Acoite Formation, assigned to the Tetragraptus phyllograptoides Zone (lower Floian), unconformably overlies the Pupusa Formation (Vaucher et al. Reference Vaucher, Vaccari, Balseiro, Muñoz, Dillinger, Waisfeld and Buatois2020), in which the Anisograptus matanensis Zone (lower Tremadocian) was recognized (Moya et al. Reference Moya, Malanca, Monteros, Albanesi, Ortega, Buatois, Albanesi, Beresi and Peralta2003).
(a) Location map of the study section in the Cordillera Oriental, Jujuy Province, Argentina, highlighting Ordovician rocks in green; Abbreviations: Purm.: Purmamarca, Hum.: Humahuaca, CO: Cordillera Oriental. (b) Geologic map of the El Moreno area (after Coira & Zappettini, Reference Coira and Zappettini2008); 1: Puncoviscana Formation (Precambrian-lower Cambrian), 2: Mesón Group (middle-upper Cambrian), 3: Pupusa and Acoite formations (Ordovician), 4: Salta Group (Cretaceous), 5: Paleogene-Neogene, 6: Quaternary. (c) Satellite image showing the synclinal structure of the El Moreno section and the approximate contact between the Acoite (Lower-Middle Ordovician), Zapla (Upper Ordovician) and Lipeón (Llandovery, Silurian) formations and Lecho and Yacoraite formations of the Salta Group (Campanian-Danian, Cretaceous).
The studied succession represents a partial southward continuation of the Acoite Formation exposed at Chamarra and Los Colorados creeks, in the Los Colorados area (Fig. 1a). In this area, the graptolite biozones of the middle and upper Floian, as covered in this study, were first established for the Cordillera Oriental of northwestern Argentina (Toro, Reference Toro1994, Reference Toro1997).
Monaldi et al. (Reference Monaldi, Albanesi, Salfity, Ortega and Moreno Valdez2017) documented the stratigraphy and structural geology of the studied area. According to these authors, the El Moreno section structurally corresponds to the western flank of a NNE-striking synclinal structure (Fig. 1b, c), extending from San José de Chañi to Los Colorados. In this section, the Acoite Formation exhibits ca. 420 m of stratigraphic thickness. It begins with green pelites interbedded with sporadic sandstone and calcarenite lenses, followed by green pelites containing gray sandstone tempestites and yellowish calcarenites, both with trace fossils. Coquinas are notably abundant towards the middle and upper portion of the section (Figs. 2a and 3). Sedimentary structures such as hummocky cross-stratification and wave-ripples are common in the upper part of the succession (Fig. 2b, c), indicating moderate to high-energy environments characteristic of the shoreface zone.
(a) Panoramic view to the north-northeast of the El Moreno section from provincial route 78. Abbreviation: Z/L: Zapla and Lipeón formations. (b) Hummocky cross-stratification in the middle to upper parts of the Acoite Formation. (c) Wave-ripple structures in the upper strata of the Acoite Formation.
Stratigraphic column of the El Moreno section, showing the locations of fossiliferous samples and the ranges of index conodonts, graptolites and trilobites.
The studied strata correspond to the middle to upper levels of the Acoite Formation. Previous facies analyses of this formation in the Los Colorados area suggest that these levels were deposited in middle to inner shelf environments influenced by a wave-and storm-dominated deltaic system (Astini & Waisfeld, Reference Astini and Waisfeld1993; Astini et al. Reference Astini, Waisfeld, Toro and Benedetto2004). Diamictitic horizons and conglomeratic sandstones of the Zapla Formation (Hirnantian, Upper Ordovician) unconformably overlie, which are succeeded by the Lipeón Formation (Llandovery, Silurian). These units are, in turn, unconformably overlain by continental sediments of the Lecho and Yacoraite formations (Campanian–Danian, Cretaceous) of the Salta Group (Figs. 2a and 3).
3. Studied material
Fourteen graptolite samples (EMG1–EMG14) were collected from the Acoite Formation, though five of them correspond to fragmentary material (EMG8, EMG10–EMG12, EMG14). The specimens are generally preserved flattened as carbonaceous films in green pelites and, more commonly, in gray sandstones.
This study includes records of trilobites and conodonts, aiming to present the associated fauna. Trilobites were recovered from the upper strata of the section (samples EMT1, EMT2 and EMG9, EMG11–EMG13). Conodonts were recovered from calcarenite and coquina samples (EMC) throughout the entire succession. Only the occurrences of index species are documented in this contribution, whereas the associated conodont species are to be published by Rueda et al. (Reference Rueda, Albanesi and Ortega2025).
Graptolite specimens were illustrated using a conventional optical stereomicroscope (Nikon SMZ 745T) and supplementary line drawings with a camera lucida (Wild M5) at the Museo de Paleontología (UNC). Illustrations of the conodont elements and trilobites were made by polyfocal photomicrographs obtained with a conventional optical stereomicroscope (Nikon SMZ 800) at Centro de Investigaciones Geológicas Aplicadas (CIGEA, UNC). The collections are housed at the Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina, under repository codes CORD-PZ and CORD-MP and Museo de Ciencias Naturales ‘Dr. Saturnino Iglesias’ del Instituto de Geología y Minería, Universidad Nacional de Jujuy, Argentina, under repository code JUY-P.
4. Graptolites and associated faunas
Rueda et al. (Reference Rueda, Ortega, Monaldi and Albanesi2020) briefly reported graptolite tubaria collected from the upper part of the Acoite Formation in the El Moreno section, where Baltograptus deflexus? (Elles & Wood), Baltograptus minutus (Törnquist) and Baltograptus spp., were identified and referred to the Didymograptellus bifidus Zone (upper Floian). Recently, conodonts of the Gothodus andinus and Trapezognathus diprion zones were recovered and associated with graptolites of the Baltograptus jacksoni and Didymograptellus bifidus zones (Rueda et al. Reference Rueda, Ortega and Albanesi2024). An updated taxonomy and biostratigraphy of these fossils have been performed, and additional graptolite material from the same section is studied in the present work.
The new graptolite assemblage described here is dominated by baltograptids, which occur throughout the section (present in all samples except EMG4), whereas specimens of the genera Tetragraptus and Acrograptus are scarce and restricted to the basal levels (samples EMG2, EMG4 and EMG6). This studied material is referred to the Baltograptus jacksoni (middle Floian) and Baltograptus minutus (upper Floian) zones according with the record of the nominal taxa (Fig. 3).
Moreover, well-preserved pyritic internal casts of graptolites were recovered from the sample EMC10 (conodont sample). These casts include proximal ends, stipe fragments and siculae. In some of them, the prosicula and metasicula are distinguishable, and the origin of the first theca is identifiable. Based on their morphology and dimensions, as well as the metasicular origin of th11, these internal casts likely correspond to the genus Baltograptus (Fig. 4).
Pyrite/isoled casts of graptolites recovered from the Floian (Lower Ordovician) Acoite Formation, sample EMC10. (a–c) Siculae with the first two thecae, showing the prosicula and metasicula, in obverse and reverse views. (d, e) Stipes fragments. (f) Proximal end with broken sicula, in obverse and reverse views. (g) Sicula showing prosicula, metasicula and origin of the first theca, in obverse and reverse views. The constrictions of the prosicula, typical of Baltograptus species (Maletz, Reference Maletz2023), are visible in some specimens (a–c, g).
Together with the graptolites, a total of 3281 well-preserved conodont elements were recovered and documented by Rueda et al. (Reference Rueda, Albanesi and Ortega2025). Among several other species, the conodont fauna includes Gothodus andinus (Rao et al.) and Trapezognathus diprion Lindström, which are index species for the eponymous biozones (Fig. 5). In the El Moreno section, the Gothodus andinus Zone correlates with the lower part of the Baltograptus jacksoni Zone, while the Trapezognathus diprion Zone is associated with the upper part of the Baltograptus jacksoni Zone and the lower part of the Baltograptus minutus Zone (Fig. 3). This graptolite-conodont association provides a precise correlation of the Trapezognathus diprion Zone with the Baltograptus jacksoni and Baltograptus minutus zones in the Cordillera Oriental of Argentina.
Floian conodonts from the Acoite Formation, El Moreno section. (a–c) Gothodus andinus (Rao et al.); (a) M element, lateral view, sample EMC8, CORD-MP 83271; (b) Pa element, lateral view, sample EMC8, CORD-MP 83272; (c) Pb element, lateral view, sample EMC2, CORD-MP 83273. (d–f) Trapezognathus diprion (Lindström); (d) M element, lateral view, sample EMC8, CORD-MP 98284; (e) Pa element, antero-lateral view, sample, CORD-MP 98285; (f) Pb element, lateral view, sample EMC8, CORD-MP 98276. Scale bars: 100 µm.
Fragmentary trilobites were also found in the upper strata of the studied section, including cranidia and pygidia of Famatinolithus aff. jujuyensis Benedetto & Malanca, Pliomeridius sulcatus Leanza & Baldis, Ampyx sp. and Colpocoryphe sp. (Fig. 6). This trilobite assemblage suggests the presence of the Famatinolithus Fauna, which is associated with the upper part of the Baltograptus jacksoni Zone and the Baltograptus minutus Zone in the studied section. The Famatinolithus Fauna has been recorded from several sections of the Cordillera Oriental of Argentina (e.g. Waisfeld et al. Reference Waisfeld, Sánchez, Benedetto and Carrera2003; Benedetto et al. Reference Benedetto, Vaccari, Waisfeld, Sánchez, Foglia and Bassett2009; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015), as well as in sections of Bolivia (e.g. Aceñolaza et al. Reference Aceñolaza, Gutiérrez-Marco, Rábano and Díaz Martínez1999) and Peru (Gutiérrez-Marco et al. Reference Gutiérrez-Marco, Carlotto, Cárdenas, Finney, Rábano, Villas, Herrera, Dávila, Carlotto and Chalco2004, Reference Gutiérrez-Marco, Romero, Rábano, Chacaltana, Waisfeld and Vaccari2024). Waisfeld & Astini (Reference Waisfeld, Astini, Albanesi, Beresi and Peralta2003) concluded that this fauna developed in well-oxygenated shallow environments, ranging from the inner shelf to the upper shoreface.
Trilobites from the Acoite Formation in the El Moreno section. (a–d) Famatinolithus aff. jujuyensis Benedetto & Malanca; (a) cranidium, JUY-P-496, sample EMT1; (b) cranidium, JUY-P-480, sample EMT2; (c) cranidium, CORD-PZ 37136, sample EMG12; (d) cranidium, CORD-PZ 37136, sample EMG12. (e) Hypostome, CORD-PZ 37194, sample EMG11. (f, h) Ampyx sp.; (f) pygidium, CORD-PZ 37135, sample EMG12; (h) pygidium, CORD-PZ 37100, sample EMG13. (g) Colpocoryphe sp., cranidium, CORD-PZ 37106, sample EMG13. (i) Pliomeridius sulcatus Leanza & Baldis; pygidium, JUY-P-480, sample EMT2. Scale bars: 2 mm.
5. Graptolite biostratigraphy and correlation
5.a. Baltograptus jacksoni Zone
Toro (Reference Toro1994) introduced the Didymograptus (Corymbograptus) deflexus Zone in northwestern Argentina, its lower boundary is recognized by the first appearance of deflexed Baltograptus species and its upper boundary is defined by the first appearance of pendent forms, corresponding to Baltograptus minutus and Didymograptellus bifidus (J. Hall). The name of this biozone has changed over time, from Didymograptus (Corymbograptus) deflexus (Toro, Reference Toro1994), Baltograptus deflexus (Toro, Reference Toro1997), ‘Baltograptus deflexus’ (Toro & Maletz, Reference Toro and Maletz2007), to Baltograptus cf. B. deflexus (Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015), generating confusion in bioestratigraphic schemes and correlation. This problematic species corresponds to Baltograptus sp. nov. as documented by Toro & Maletz (Reference Toro and Maletz2007). According to Toro & Maletz (Reference Toro and Maletz2008), the specimens of Baltograptus sp. nov. sensu Toro & Maletz (Reference Toro and Maletz2007) from the Argentine Cordillera Oriental are identical to Baltograptus varicosus (Wang), the index species of the eponymous biozone from England (Cooper et al. Reference Cooper, Rushton, Molyneux, Hughesj, Moore and Webbs1995, Reference Cooper, Fortey, Hughes, Molyneux, Moore, Rushton and Stone2004). However, the British specimens previously referred to as B. varicosus were assigned to Baltograptus jacksoni sp. nov by Rushton (Reference Rushton2011), who also included the Argentine material described by Toro & Maletz (Reference Toro and Maletz2007) in the synonymy of his new species. Baltograptus sp. nov. was then recognized as Baltograptus jacksoni by Maletz (Reference Maletz2023), who introduced the eponymous biozone in Scandinavia.
Recently, the Baltograptus jacksoni Zone was introduced by Rueda et al. (Reference Rueda, Ortega and Albanesi2024) in the graptolite bioestratigraphic scheme of the Argentine Cordillera Oriental, and it was considered equivalent to the upper part of the Baltograptus cf. deflexus Zone of previous works (Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015).
In the El Moreno section, the Baltograptus jacksoni Zone is identified by the record of the eponymous species. This biozone extends about 300 m within the middle to upper part of the Acoite Formation, up to the first record of Baltograptus minutus, indicating the beginning of the upper biozone. In this biostratigraphic interval, Acrograptus spp., Tetragraptus reclinatus? (Elles & Wood), Tetragraptus amii? (Elles & Wood), Baltograptus cf. jacksoni Rushton and Baltograptus sp. are also present. Its total extent in the El Moreno area cannot be precisely determined because the basal portion of the unit is covered by Quaternary deposits. In contrast, in other areas of the Cordillera Oriental, the base of the biostratigraphic unit outcrops, such as the Los Colorados area, where the biozone was established (Toro, Reference Toro1994, Reference Toro1997; Toro & Maletz, Reference Toro and Maletz2007), and the Santa Victoria area (Toro & Maletz, Reference Toro and Maletz2007; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015). According to Toro & Maletz (Reference Toro and Maletz2007), in the Los Colorados area, Baltograptus sp. nov. (= Baltograptus jacksoni) appears from the base of the biozone. Toro & Maletz (Reference Toro and Maletz2008) established a correlation of these strata from the Argentine Cordillera Oriental with the Baltograptus vacillans and lower Pseudophyllograptus angustifolius elongatus zones (= Baltograptus sp. cf. B. deflexus Zone sensu Maletz & Ahlberg, Reference Maletz and Ahlberg2011, = Baltograptus jacksoni Zone in Maletz, Reference Maletz2023) of Scandinavia.
Therefore, a regional correlation of the Acoite Formation levels assigned to the Baltograptus jacksoni Zone in the El Moreno area can be established with the levels of the same formation exposed in the Chamarra and Los Colorados creeks in the Los Colorados area (Toro, Reference Toro1994, Reference Toro1997; Toro & Maletz, Reference Toro and Maletz2007), and in the Río La Huerta section in Santa Victoria (Toro & Maletz, Reference Toro and Maletz2007; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015). A partial correlation can be established with the volcano-sedimentary sequences from the Muñayoc section of the Puna (Lo Valvo et al. Reference Lo Valvo, Herrera Sánchez and Toro2020), and with the equivalent levels of the Suri Formation in the Sistema de Famatina (Toro & Brussa, Reference Toro and Brussa1997). Throughout the Central Andean Basin, the upper part of the Baltograptus jacksoni Zone identified in the El Moreno section can be correlated with the upper part of the Expansograptus holmi Zone of Bolivia (Maletz & Egenhoff, Reference Maletz and Egenhoff2003; Egenhoff et al. Reference Egenhoff, Maletz and Erdtmann2004). An intercontinental correlation can be established with sections in Scandinavia where Baltograptus jacksoni has been recorded (Maletz, Reference Maletz2023; Maletz et al. Reference Maletz, Lindskog, Calner and Wallin2023). Similarly, correlations can be made with sections from Great Britain, where the Baltograptus jacksoni Zone was identified by Rushton (Reference Rushton2011). A possible correlation may also be established with southern Morocco where Gutiérrez-Marco & Martin (Reference Gutiérrez-Marco and Martin2016) documented the ?Baltograptus jacksoni Zone (Fig. 7).
Biostratigraphic chart of the Floian- lower Dapingian showing the graptolite zones.
5.b. Baltograptus minutus Zone
The Baltograptus minutus Zone is commonly used in biostratigraphic schemes for regions located at mid to high paleolatitudes, such as Scandinavia (Maletz & Ahlberg, Reference Maletz and Ahlberg2011; Maletz, Reference Maletz2023), southern Morocco (Gutiérrez-Marco & Martin, Reference Gutiérrez-Marco and Martin2016), and Bolivia and Peru within the Central Andean Basin (Maletz & Egenhoff, Reference Maletz and Egenhoff2003; Egenhoff et al. Reference Egenhoff, Maletz and Erdtmann2004; Gutiérrez-Marco et al. Reference Gutiérrez-Marco, Maletz, Chacaltana, Obut, Sennikov and Kipriyanova2019). However, in the Cordillera Oriental and the Puna of Argentina, the correlating biozone, the Didymograptellus bifidus Zone, has been used instead, which is associated with schemes from low paleolatitude regions, such as North America (Williams & Stevens, Reference Williams and Stevens1988).
The Didymograptellus bifidus Zone for the Cordillera Oriental of Argentina was introduced by Toro (Reference Toro1994) in the Acoite Formation, Chamarra Creek at Los Colorados area, who defined the base of this biozone by the first appearance of Baltograptus minutus and recorded the presence of Didymograptellus bifidus (Hall) in the middle part of the biozone. Subsequently, this biozone has been identified in various localities of northwestern Argentina, including Los Colorados (Toro, Reference Toro1994, Reference Toro1997), Santa Victoria (Toro & Maletz, Reference Toro and Maletz2007; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015) and the Yavi road sections (Toro et al. Reference Toro, Herrera Sánchez and Lo Valvo2024), all within the Cordillera Oriental, as well as in the Muñayoc section of the Quichagua Range and the Santa Rosa section of the Cochinoca Range, both located in the Argentine Puna (Loss, Reference Loss1949; Martínez et al. Reference Martínez, Brussa, Pérez and Coira1999; Lo Valvo et al. Reference Lo Valvo, Herrera Sánchez and Toro2020; Toro et al. Reference Toro, Herrera Sánchez and Lo Valvo2024). In addition, Toro & Brussa (Reference Toro and Brussa1997) recorded this biozone in the Río Salado section of the Sistema de Famatina, while Bahlburg et al. (Reference Bahlburg, Breitkreuz, Maletz, Moya and Salfity1990) documented it in the Aguada de La Perdíz Formation of the Chilean Puna.
In the Los Colorados area, Didymograptellus bifidus is recorded from the middle part of the eponymous zone, whereas Baltograptus minutus and Baltograptus deflexus appear from its base (Toro, Reference Toro1994, Reference Toro1997; Toro & Maletz, Reference Toro and Maletz2007, Reference Toro and Maletz2008). In the Río La Huerta section of Santa Victoria area, D. bifidus has not been recorded, and the eponymous biozone was identified by the association of B. minutus and B. deflexus (Toro & Maletz, Reference Toro and Maletz2007; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015). The same situation is reported in the Santa Rosa section of the Jujuy Puna, where the Didymograptellus bifidus Zone is recognized by the presence of B. minutus and B. deflexus (Lo Valvo et al. Reference Lo Valvo, Herrera Sánchez and Toro2020).
Considering that previous records show the Baltograptus minutus occurrence preceding Didymograptellus bifidus in sections of northwestern Argentina, and that the latter species has not been even recorded in some of them, the Baltograptus minutus Zone is used here instead of Didymograptellus bifidus Zone. Furthermore, the identification of B. minutus and the absence of D. bifidus in the studied section, together with the widespread use of the Baltograptus minutus Zone in other parts of the Central Andean Basin (Bolivia and Peru), justifies the implementation of the Baltograptus minutus Zone, enabling a more precise regional and intercontinental correlation (Fig. 7).
In the El Moreno section, the Baltograptus minutus Zone is identified by the presence of the eponymous species. Together with Baltograptus minutus, specimens tentatively assigned to Baltograptus deflexus (Elles & Wood) are recorded. In this section, the biozone spans the uppermost 50 m of the succession, and its upper part is truncated by an unconformity that places the Acoite Formation in contact with the Upper Ordovician Zapla Formation.
The upper strata of the Acoite Formation in the El Moreno area can be correlated with equivalent levels of the same formation exposed in Los Colorados, Santa Victoria, and the Yavi road sections (Toro, Reference Toro1994, Reference Toro1997; Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015; Toro et al. Reference Toro, Herrera Sánchez and Lo Valvo2024), as well as with the volcano-sedimentary sequences of the Puna (Martínez et al. Reference Martínez, Brussa, Pérez and Coira1999; Lo Valvo et al. Reference Lo Valvo, Herrera Sánchez and Toro2020; Toro et al. Reference Toro, Herrera Sánchez and Lo Valvo2024), and with the upper levels of the Suri Formation in the Sistema de Famatina (Toro & Brussa, Reference Toro and Brussa1997). Throughout the Central Andean Basin, the Baltograptus minutus Zone identified in the El Moreno section may be correlated with the eponymous zone recognized in the Cieneguillas–Chaupiuno section of Bolivia (Maletz et al. Reference Maletz, Kley and Reinhardt1995; Maletz & Egenhoff, Reference Maletz and Egenhoff2003; Egenhoff et al. Reference Egenhoff, Maletz and Erdtmann2004) and the Libertad and Kimbiri sections of Peru (Gutiérrez-Marco et al. Reference Gutiérrez-Marco, Maletz, Chacaltana, Obut, Sennikov and Kipriyanova2019). An intercontinental correlation can also be proven with sections in Scandinavia (Maletz & Ahlberg, Reference Maletz and Ahlberg2011; Maletz, Reference Maletz2023) and southern Morocco (Gutiérrez-Marco & Martin, Reference Gutiérrez-Marco and Martin2016), where the Baltograptus minutus Zone has been reported (Fig. 7).
6. Systematic paleontology
Order Graptoloidea Lapworth (in Hopkinson & Lapworth, Reference Hopkinson and Lapworth1875)
Suborder Sinograptina Mu, Reference Mu1957
Family Kinnegraptidae Mu, Reference Mu1974
Subfamily Sigmagraptinae Cooper & Fortey, Reference Cooper and Fortey1982
Genus Acrograptus Tzaj, Reference Tzaj1969
Type species. Didymograptus affinis Nicholson, Reference Nicholson1869
Acrograptus spp.
Floian acrograptids and tetragraptids from the El Moreno section, Cordillera Oriental, Argentina. (a, f) Tetragraptus amii? Elles & Wood; (a) JUY-P-541, sample EMG2; (f) JUY-P-508A, sample EMG4. (b, c, j) Tetragraptus reclinatus? Elles & Wood; (b) JUY-P-539, sample EMG2; (c) JUY-P-548, sample EMG2; (j) JUY-P-551, sample EMG2. (d, e, g–i) Acrograptus spp.; (d) CORD-PZ 37219, sample EMG6; (e) JUY-P-515, sample EMG4; (g) JUY-P-517, sample EMG4; (h) CORD-PZ 37222, sample EMG6; (i) CORD-PZ 37217, sample EMG6. Scale bars: 1 mm.
Camera-lucida drawings of the Floian graptolites from the El Moreno section, Cordillera Oriental of Argentina. (a, b) Tetragraptus reclinatus? Elles & Wood; (a) JUY-P-548, sample EMG2; (b) JUY-P-551, sample EMG2. (c–f) Acrograptus spp., (c) CORD-PZ 37217, sample EMG6; (d) JUY-P-517, sample EMG4; (e) CORD-PZ 37222, sample EMG6; (f) CORD-PZ 37219, sample EMG6. Scale bars: 1 mm.
Material. Scarce and incomplete specimens from samples EMG4 and EMG6, Baltograptus jacksoni Zone. CORD-PZ 37217, 37219, JUY-P-515, 517, 521.
Description and remarks. Juvenile specimens with two slender stipes and poorly preserved. The tubaria have different shapes and dimensions, and may correspond to two different species. One of the specimens (Figs. 8g and 9e) has the stipes arranged sub-horizontally and an artus-type proximal development. The sicula is obliquely oriented, reaching a length of ca. 1 mm and an aperture width of 0.25 mm. The free ventral side of the sicula is 0.36 mm long. A short nema, less than 0.08 mm, is visible. The stipes have a width of 0.38 mm at the apertures of the th11 and appear to originate at nearly the same level, giving a symmetrical appearance to the proximal end. There are about 9.7 thecae in 10 mm (2TRD is 2.08 mm proximally). The morphology and dimensions of the recovered specimen are similar to those of Acrograptus sp. cf. A. gracilis (Törnquist) described and illustrated by Maletz (Reference Maletz2023) from the Baltograptus vacillans Zone at Diabasbrottet, Sweden (2023, fig. 13h, j, k; sicula is 1–1.1 mm long, measuring less than 0.2 mm across the aperture, and the stipes are 0.3 mm wide), but the proximal development is of artus type.
The other specimens (Figs. 8d, h, i and 9c, d, f) have declined stipes, ranging from 0.26 to 0.36 mm in width. The stipes diverges from the sicula at different levels, giving the proximal end of the tubarium an asymmetrical appearance. Despite the material is incomplete, two specimens that appear have an artus-type proximal development. The sicula is about 1 mm long, with an apertural width of 0.2 mm, and a free ventral side of 0.35 to 0.4 mm. The thecae are long and slender, reaching up to 2 mm in length and 0.19 to 0.26 mm in width. These specimens are similar to Acrograptus(?) artus described and illustrated by Maletz (Reference Maletz2022), but differ of those in having a smaller sicula (sicula is about 1.1–1.2 mm long sensu Maletz).
Suborder Dichograptina Lapworth, Reference Lapworth1873
Family Phyllograptidae Lapworth, Reference Lapworth1873
Genus Tetragraptus Salter, Reference Salter1863
Type species. Graptolithus bryonoides Hall, Reference Hall1858
Tetragraptus reclinatus? (Elles & Wood, Reference Elles and Wood1902)
? 1902 Tetragraptus reclinatus sp. nov. Elles & Wood (Reference Elles and Wood1902, p. 67, pl. VI, fig. 5a–e).
Material. Abundant fragmentary specimens occur in the sample EMG2, Baltograptus jacksoni Zone. JUY-P-537, 539, 548, 551, 560.
Description. Juvenile tubaria with symmetrically arranged stipes, growing first horizontally and becoming reclined. The initial width of the stipes is 0.8–0.88 mm at the aperture of th11, and the maximum width is about 1.32 mm at th4. The sicula has a length of 1.56–1.64 mm and an apertural width of 0.3–0.48 mm. It is straight for most of its length, with the distal portion gently curved towards the dorsal side. A short rutellum is visible at the aperture of the sicula. The free ventral side of the sicula is about 0.3–0.38 mm long. A short nema of 0.25 mm is present (Fig. 8b). The proximal end is preserved in some specimens, showing isograptid proximal development and high origin of th11 ca. 0.36 mm below the apex of the sicula. The thecae are straight with concave apertures measuring 0.52–0.58 mm. There are about 14–15 thecae in 10 mm proximally (2TRD is 1.12 mm at th3).
Remarks. The dimensions of the sicula and the width of the stipes in the specimens from El Moreno are consistent with those of Tetragraptus reclinatus described from Floian strata of North America and Europe by Elles & Wood (Reference Elles and Wood1902), Monsen (Reference Monsen1937), Cooper & Fortey (Reference Cooper and Fortey1982), and Williams & Stevens (Reference Williams and Stevens1988), among others (sicula length of 1.5–1.8 mm, free ventral side of 0.2–0.3 mm, and stipes width of about 0.75 mm proximally, widening to a maximum of 2.2 mm in mature specimens). These authors describe a slightly smaller thecal spacing (11–13 thecae in 10 mm) than that of our specimens. However, Koren et al. (Reference Koren, Gorshenina and Tolmacheva2004) described and illustrated Tetragraptus reclinatus from the Dapingian-Darriwilian of St. Petersburg, Russia, with a thecal spacing of 14–16 thecae in 10 mm in the proximal portion (2TRD is 1.1–1.3 mm proximally). The lack of mature specimens and the poor preservation of the material make an accurate taxonomic assignment difficult.
Tetragraptus amii? Elles & Wood, Reference Elles and Wood1902
(Fig. 8a, f)
? 1902 Tetragraptus amii sp. nov. Elles & Wood (Reference Elles and Wood1902, p. 60, pl. V, fig. 4a–c).
Material. Two specimens occur in the samples EMG2 and EMG4, Baltograptus jacksoni Zone. JUY-P-508.
Description. One specimen has a typical tetragraptid shape with four horizontal stipes (Fig. 8a), measuring about 3.8 mm in length and 0.88 mm in maximum width. The funicle is 1.6 mm long and 0.36 mm wide. The other specimen, preserved in lateral view, has two subhorizontal stipes (Fig. 8f). The proximal end is poorly preserved, hindering measurement of the sicula. The stipes appear to diverge from the sicula at the same level, giving a symmetrical appearance to the proximal end of the tubarium. They are 5.36 mm long and 1–1.1 mm wide proximally, reaching a maximum width of 1.72–1.84 mm at th4, and then their width seems to decrease. The thecae have an inclination of approximately 60° in the proximal portion and 35–40° distally. A very short rutellum at each thecal aperture is observed. There are 13 thecae in 10 mm (2TRD is 1.48 mm at th5).
Remarks. The presence of a strong tubarium with horizontal to subhorizontal stipes resembles that of Tetragraptus amii Elles & Wood. The width of the stipes at the level of the third or fourth theca is consistent with that of specimens from southern Sweden (1.2–1.3 mm sensu Maletz, Reference Maletz2023), but the 2TRD is slightly lower than that of the Swedish material (1.6–1.7 mm sensu Maletz, Reference Maletz2023). The scarcity and poor preservation of the specimens preclude a reliable diagnosis.
Family Didymograptidae Mu, Reference Mu1950
Genus Baltograptus Maletz, Reference Maletz, Chen, Erdtmann and Ni1994
Type species. Didymograptus vacillans Tullberg, Reference Tullberg1880
Baltograptus minutus (Törnquist, Reference Törnquist1879)
Floian baltograptids from the El Moreno section, Cordillera Oriental, Argentina. (?a, b, g, h, ?m) Baltograptus cf. jacksoni Rushton; (a) JUY-P-486A, sample EMG9; (b) JUY-P-563, sample EMG1; (g) CORD-PZ 36806, sample EMG5; (h) JUY-P-488A, sample EMG9; (m) JUY-P-488A, sample EMG9. (c, l) Baltograptus sp.; (c) CORD-PZ 36806, sample EMG5; (l) CORD-PZ 36806, sample EMG5. (d, i, k, n) Baltograptus jacksoni Rushton; (d) JUY-P-552, sample EMG2; (i) JUY-P-505, sample EMG7; (k) JUY-P-550, sample EMG2; (n) CORD-PZ 36806, sample EMG5. (e, j) Baltograptus minutus (Törnquist); (e) CORD-PZ 37187, sample EMG13; (j) JUY-P-484B, sample EMG9. (f) Baltograptus deflexus? (Elles &Wood), CORD-PZ 37184A, sample EMG13. Scale bars: 1 mm.
Camera-lucida drawings of the Floian graptolites from the El Moreno section, Cordillera Oriental of Argentina. (a–c, j) Baltograptus jacksoni Rushton; (a) JUY-P-550, sample EMG2; (b) CORD-PZ 36806, sample EMG5; (c) JUY-P-505, sample EMG7; (j) JUY-P-552, sample EMG2. (d–f, ?g) Baltograptus cf. jacksoni Rushton; (d) JUY-P-488A, sample EMG9; (f) CORD-PZ 36806, sample EMG5; (g) JUY-P-486A, sample EMG9. (h, i) Baltograptus sp.; (h) CORD-PZ 36806, sample EMG5; (i) CORD-PZ 36806, sample EMG5. (k) Baltograptus deflexus? (Elles & Wood), sample EMG13, (k1) CORD-PZ 37184A; (k2) CORD-PZ 37184B. (l, m) Baltograptus minutus (Törnquist), (l) JUY-P-484B, sample EMG9; (m) CORD-PZ 37187, sample EMG13. Scale bar: 1 mm.
1879 Didymograptus minutus sp. nov. Törnquist (Reference Törnquist1879, p. 447, 2 figs).
1994 Didymograptus (Didymograptellus) minutus (Törnquist); Toro (Reference Toro1994, pl. 2, figs. 6, 12).
1994 Baltograptus minutus (Törnquist); Maletz (Reference Maletz, Chen, Erdtmann and Ni1994, p. 36, fig. 6g, h).
1997 Baltograptus minutus (Törnquist); Toro (Reference Toro1997, pl. III, fig. 10).
2004 Baltograptus minutus (Törnquist); Egenhoff et al. (Reference Egenhoff, Maletz and Erdtmann2004, fig. 5e).
2011 Baltograptus minutus (Törnquist), Maletz & Ahlberg (Reference Maletz and Ahlberg2011, fig. 5b).
2013 Baltograptus minutus (Törnquist); Maletz & Slovacek (Reference Maletz and Slovacek2013, p. 1113, figs. 1e, 3a, 5c, 6, 7).
2016 Baltograptus minutus (Törnquist); Gutiérrez-Marco & Martin (Reference Gutiérrez-Marco and Martin2016, fig. 4n).
2023 Baltograptus minutus (Törnquist); Maletz et al. (Reference Maletz, Lindskog, Calner and Wallin2023, fig. 9i).
2024 Baltograptus minutus (Törnquist); Toro et al. (Reference Toro, Herrera Sánchez and Lo Valvo2024, fig. 5.4).
Material. Four specimens occur in the samples EMG9 and EMG13, Baltograptus minutus Zone. CORD-PZ 37185, 37187, JUY-P-482A, 484.
Description. Slender tubaria with two pendent stipes and artus-type proximal development. The stipes are 0.45–0.54 mm wide at the aperture of th11 and 0.6–0.76 mm at th61. The sicula has a length of 1–1.28 mm. The sicular aperture is 0.24–0.32 mm wide and the free ventral side is 0.32–0.36 mm long. The thecae have straight margins and are inclinated at 20–25°. There are about 17 thecae in 10 mm proximally (2TRD is 1.12–1.4 mm at th21).
Remarks. The shape and dimensions, as well as the artus-type proximal development of the specimens from the El Moreno section, agree with those described by Maletz & Slovacek (Reference Maletz and Slovacek2013) for Baltograptus minutus (Törnquist).
Geographic and stratigraphic distribution: Baltograptus minutus is restricted to the upper Floian and is widely distributed in the eponymous biozone of Baltoscandia (Törnquist, Reference Törnquist1879; Monsen, Reference Monsen1937; Maletz & Slovacek, Reference Maletz and Slovacek2013; Maletz & Ahlberg, Reference Maletz and Ahlberg2011, Reference Maletz and Ahlberg2018; Maletz et al. Reference Maletz, Lindskog, Calner and Wallin2023). This species is also recorded in the Baltograptus minutus Zone of Morocco (Gutiérrez-Marco & Martin, Reference Gutiérrez-Marco and Martin2016), Bolivia (Maletz et al. Reference Maletz, Kley and Reinhardt1995; Egenhoff et al. Reference Egenhoff, Maletz and Erdtmann2004) and Peru (Gutiérrez-Marco et al. Reference Gutiérrez-Marco, Maletz, Chacaltana, Obut, Sennikov and Kipriyanova2019). In the Argentine Cordillera Oriental, B. minutus is restricted to the Didymograptellus bifidus Zone, as recognized in the Los Colorados and Chamarra sections (Toro, Reference Toro1994; Toro & Maletz, Reference Toro and Maletz2008), the La Huerta section in the Santa Victoria area (Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015) and the El Moreno section (Rueda et al. Reference Rueda, Ortega, Monaldi and Albanesi2020; Rueda et al. Reference Rueda, Ortega and Albanesi2024). This species is also found in the Cerro Tafna, Muñayoc and Santa Rosa sections of the Puna region (Martínez et al. Reference Martínez, Brussa, Pérez and Coira1999; Lo Valvo et al. Reference Lo Valvo, Herrera Sánchez and Toro2020; Toro et al. Reference Toro, Herrera Sánchez and Lo Valvo2024).
Baltograptus deflexus? (Elles & Wood, Reference Elles and Wood1901)
? 1901 Didymograptus deflexus sp. nov. Elles & Wood (Reference Elles and Wood1901, p. 35, pl. II, fig. 12a–c).
Material. Two specimens from sample EMG13, Baltograptus minutus Zone. CORD-PZ 37116, 37184.
Description. Juvenile tubaria with two pendent stipes and artus-type proximal development. The stipes have a width of 0.54–0.64 mm at the aperture of th11. The sicula is long and slender, with a length of 1.44–1.52 mm and an apertural width of 0.24–0.28 mm. The ventral free side of the sicula is 0.3–0.4 mm long. A short nema of ca. 0.32 mm in length, is present. The thecae are inclined at about 30° from the dorsal side of the stipe.
Remarks. According to Maletz & Slovacek (Reference Maletz and Slovacek2013), the proximal end of Baltograptus deflexus is virtually identical to that of Baltograptus minutus and it would be impossible to differentiate early astogenetic stages of both species. Nevertheless, the studied specimens have a longer and slightly thinner sicula compared to Baltograptus minutus, which has a sicula length of 1–1.28 mm. Following Rushton (Reference Rushton2000, Reference Rushton2011), Baltograptus deflexus has a sicula length of 1.4–1.6 mm. Although the dimensions of the specimens from El Moreno agree with those of Baltograptus deflexus, due to the scarcity and poor conservation of the collected material, the specimens are assigned to this specie with doubt.
On the other hand, our specimens can be distinguished from Baltograptus extremus Maletz & Slovacek by having a shorter sicula (1.7–2.3 mm in B. extremus sensu Maletz & Slovacek, Reference Maletz and Slovacek2013).
Baltograptus jacksoni Rushton, Reference Rushton2011
(Figs. 10d, i, k, n and 11a–c, j)
1994 Baltograptus cf. deflexus (Elles & Wood); Maletz (Reference Maletz, Chen, Erdtmann and Ni1994, fig. 6f, stated wrongly as fig. 6g in figure caption.
2007 Baltograptus sp. nov.; Toro & Maletz (Reference Toro and Maletz2007, figs. 4e, 5a–d).
2011 Baltograptus jacksoni sp. nov. Rushton (Reference Rushton2011, p. 323, figs. 4–7, 8d, ?8e).
? 2016 Baltograptus cf. jacksoni Rushton; Gutiérrez-Marco & Martin (Reference Gutiérrez-Marco and Martin2016, fig. 4o).
2023 Baltograptus jacksoni Rushton; Maletz (Reference Maletz2023, p. 106, figs. 50a–g, 51l–n, cum syn.).
Material. About ten specimens from samples EMG2, EMG5 and EMG7, Baltograptus jacksoni Zone. CORD-PZ 36806, JUY-P-505, 537, 538, 550, 552.
Description. Tubaria at various astogenetic stages, with deflexed stipes. The stipes have a maximum length of 8 mm and a variable width, ranging from 0.88 to 0.92 mm at the aperture of the th11 to 1.12–1.2 mm distally. Although most of the specimens are incomplete or preserved in obverse view, a few appear to have an isograptid proximal development (Figs. 10i and 11b). The sicula is 1.56–1.76 mm long and 0.36–0.44 mm wide at the aperture. The ventral free side of the sicula measures 0.4–0.6 mm in length. The origin of th11 is about 0.68 mm below the apex of the sicula. The thecal apertures are concave. The thecae are inclined at about 25–40°. There are 12 thecae in 10 mm proximally (2TRD is 1.6–1.78 mm at th22) and about 11 thecae in 10 mm distally (2TRD is ca. 1.8 mm at th62).
Remarks. The morphology and dimensions of the El Moreno specimens agree with those described for Baltograptus jacksoni by Rushton (Reference Rushton2011) from the Lower Ordovician Skiddaw Group in England, as well as with the descriptions by Maletz (Reference Maletz2023) for specimens from Scandinavia. Specimens documented as Baltograptus sp. nov. by Toro & Maletz (Reference Toro and Maletz2007) from the Cordillera Oriental of Argentina were referred to Baltograptus jacksoni by Rushton (Reference Rushton2011) and Maletz (Reference Maletz2023).
Geographic and stratigraphic distribution. This species is widely distributed in Scandinavia and Britain and is the index species of the eponymous biozone (Rushton, Reference Rushton2011; Maletz, Reference Maletz2023). In the Anti-Atlas region of Morocco, Gutiérrez-Marco & Martin (Reference Gutiérrez-Marco and Martin2016) collected deflexed specimens, which they compared with Baltograptus jacksoni (unfortunately, these were not described), and tentatively identified the ?Baltograptus jacksoni Zone. The presence of Baltograptus jacksoni and the eponymous biozone in the Cordillera Oriental of Argentina were considered by Rushton (Reference Rushton2011), Maletz (Reference Maletz2023), Gutiérrez-Marco & Martin (Reference Gutiérrez-Marco and Martin2016) and Gutiérrez-Marco et al. (Reference Gutiérrez-Marco, Maletz, Chacaltana, Obut, Sennikov and Kipriyanova2019); and were recently identified in the middle part of the Acoite Formation at El Moreno section by Rueda et al. (Reference Rueda, Ortega and Albanesi2024).
Baltograptus cf. jacksoni Rushton
(Figs. 10?a, b, g, h, ?m and 11d–f, ?g)
cf. 2011 Baltograptus jacksoni sp. nov. Rushton (Reference Rushton2011, p. 323, figs. 4–7, 8d, ?8e).
Material. Scarce juvenile specimens from samples EMG1, EMG5 and EMG9, Baltograptus jacksoni and Baltograptus minutus zones. CORD-PZ 36806, JUY-P-486, 488, 563.
Description. Tubaria with deflexed stipes reaching a maximum length of 7.3 mm in mature specimens, widening from 0.68–0.76 mm at the aperture of th11 to 0.96 mm at th8. The sicula is 1.36–1.52 mm long, with an apertural width of ca. 0.4 mm. The free ventral side of the sicula is 0.4–0.46 mm long. The first theca originates ca. 0.56 mm below the sicular apex. In some specimens (Fig. 11e, f), an isograptid proximal development is observed. The thecal inclination is 25–35° and the thecal overlap is about 1/2 of their length. There are 14 thecae in 10 mm proximally (2TRD is 1.38 mm at th21, and 1.4 mm at th31), and 15 thecae in 10 mm distally (2TRD is 1.2 mm at th7).
Two specimens (Figs. 10a, m and 11g) show a broad initial divergence angle of the stipes (145º) and a stipe width of 0.8 mm in th71. Because of these characteristics, they are doubtfully assigned to this species.
Remarks. Baltograptus cf. jacksoni differs from Baltograptus jacksoni in the dimensions of the tubarium. Baltograptus jacksoni has slightly more robust stipes, widening from 0.8–0.9 mm at the aperture of th1 to 1.4–1.5 mm distally, and a longer sicula, 1.7 mm (Rushton, Reference Rushton2011), as well as greater thecal spacing, 2TRD is 1.7 mm at th1–th3 and 1.6–1.7 mm distally (Maletz, Reference Maletz2023). The width of the stipes and the isograptid-type proximal development of Baltograptus cf. jacksoni are consistent with those of specimens described as Baltograptus deflexus (= Baltograptus cf. B. deflexus sensu Toro et al. Reference Toro, Meroi Arcerito, Muñoz, Waisfeld and de la Puente2015) by Toro (Reference Toro1999) from the Santa Victoria area. The dimensions of Baltograptus cf. jacksoni agree with those documented for Baltograptus cf. B. deflexus by Vento (B. Vento, unpub. Ph.D. thesis, Universidad Nacional de Cuyo, 2013) from the Aguilar Range in the Cordillera Oriental, and by Lo Valvo (GA. Lo Valvo, unpub. Bachelor thesis, Universidad de Buenos Aires, 2019) from Muñayoc in the Puna. These forms could correspond to Baltograptus jacksoni, and their differences may be due to intraspecific variation or to poor preservation of the material. Nevertheless, in this contribution, we prefer to retain this form as Baltograptus cf. jacksoni, pending an exhaustive review and even statistical analysis, encompassing all material from Argentina.
Baltograptus sp.
Material. Two specimens occur in the sample EMG5, Baltograptus jacksoni Zone. CORD-PZ 36806.
Description. Robust tubaria consisting of two deflexed stipes, with a proximal width of 0.88–0.96 mm at the aperture of th11 and 1.04–1.08 mm at the aperture of th12, slightly increasing from 1.16 mm at the th4–th5 to 1.2 mm at the th72. The stipes diverge from the sicula at an initial angle of about 130°, then decrease to 90–95° and diverge distally to 110°. The sicula has a length of 1.32 to 1.4 mm and an aperture width of 0.46–0.54 mm with a short rutellum. The supradorsal part of the sicula is 0.6–0.8 mm long, and the free ventral side is 0.4–0.46 mm. Due to poor specimen preservation, the origin of the first theca is unknown. One of the specimens (Figs. 10l and 11h) appears to have an isograptid proximal development. The thecae are simple tubes that slightly expand towards the aperture, with slightly concave apertures and a short rutellum, and show an overlap of about 2/3. There are about 14 thecae in 10 mm (2TRD is 1.32–1.4 mm at th21), remaining apparently constant toward the distal part.
Remarks. Baltograptus sp. resembles Baltograptus jacksoni in the proximal width of the stipes (0.8–0.9 mm at the aperture of th1 sensu Rushton, Reference Rushton2011), but differs in having a shorter and wider sicula (length about 1.7 mm and aperture width 0.3–0.4 mm), less robust distal stipes compared to British specimens (1.3 mm wide at th3 and 1.4–1.5 mm at th5), as well as a lower 2TRD value (1.7 mm proximally sensu Maletz, Reference Maletz2023). These specimens are tentatively referred to the genus Baltograptus, although the sicula is shorter and wider than characteristic of this genus, and the origin of the first theca is not visible. These specimens resemble Baltograptus sp. cf. B. deflexus as described by Zhang & Zhang (Reference Zhang and Zhang2014) in having a short sicula (1.2–1.6 mm) and stipe widths (0.8–0.9 mm at the aperture of th1 and 1.0–1.1 mm at th4–th5).
7. Conclusions
Two graptolites assemblages from the Acoite Formation in the El Moreno section, Cordillera Oriental of Argentina, are described. These assemblages include Acrograptus spp., Baltograptus jacksoni, B. cf. jacksoni, B. minutus, B. deflexus?, Baltograptus sp., as well as tetragraptids tentatively assigned to Tetragraptus reclinatus and Tetragraptus amii.
The biostratigraphic analysis indicates a middle to late Floian (Early Ordovician) age for the studied levels, which we refer to the Baltograptus jacksoni and Baltograptus minutus zones.
Together with graptolites, conodonts and trilobites were found, allowing the identification of the Gothodus andinus and Trapezognathus diprion conodont zones and trilobites of the Famatinolithus Fauna.
Graptolite and conodont records show that the lower part of the Baltograptus jacksoni Zone correlates with the Gothodus andinus Zone, whereas the upper part of the Baltograptus jacksoni Zone and the basal portion of the Baltograptus minutus Zone correlate with the Trapezognathus diprion Zone.
The graptolite assemblages from the El Moreno section allow for correlation with other sections in Argentina, as well as with equivalent successions in Peru, Bolivia, southern Morocco, Scandinavia and Britain.
Acknowledgements
The authors thank for the continuous support of CONICET and Universidad Nacional de Córdoba, Argentina, for conodont and graptolite studies and specially thank the CIGEA (FCEFyN, UNC) for the infrastructure and equipment support. We thank Dr. Franco Tortello for his revision on the taxonomic identification of trilobites. The comments from J.C. Gutiérrez-Marco and an anonymous reviewer were invaluable for improving our original manuscript and are sincerely appreciated. This work was supported by CONICET 2021–2023 (G.O., 11220200100298 CO) and the Consolidar SeCyt-UNC 2018–2023 (G.L.A., 33620230100151 CB). This study is part of the PhD thesis of one of the authors (E.K.R).
Competing interests
The authors declare none.
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