1. Introduction

Ordovician faunas from the Eastern Cordillera in northwest Argentina exhibit endemism, which complicates precise biostratigraphic intercontinental correlations (e.g., Waisfeld et al., 2023). This is particularly evident in the case of conodonts, where the analysis of isolated, discontinuous outcrops and differing taxonomic approaches have led to conflicting biostratigraphic interpretations (e.g., Carlorosi et al., 2017; Albanesi et al., 2021). These issues are especially compounded regarding Baltoniodus triangularis (Lindström, 1954), the index marker for the Lower-Middle Ordovician Series boundary (Wang et al., 2009) and earlier representatives of the Balognathidae family (e.g., Sweet, 1988; Stouge and Bagnoli, 1999; Wang et al., 2003; Dzik, 2015). Controversies primarily concern the conodont records of the Acoite Formation in the La Ciénaga de Purmamarca, located around eight kilometers west of the town of Purmamarca in the Jujuy Province (Fig. 1). This study therefore aims to correct and refine the biostratigraphy of this area based on a new conodont assemblage from a succession tightly integrated with graptolite and trilobite data, providing a more precise biostratigraphic correlation of the Acoite Formation at both regional and global scales.

Fig. 1. A. Ordovician siliciclastic outcrops of the Central Andean basin of northwestern Argentina with location of classical stratigraphical sections involving Tremadocian-Floian deposits. 1: Santa Victoria area (Chulpíos Creek), 2: Espinazo del Diablo, 3: El Aguilar, 4: Cajas, 5: Los Colorados-Chamarra, Aguas Blancas Creek, Alto de Lipán, 6: Angosto del Moreno, and 7: La Ciénaga de Purmamarca (map modified from Waisfeld et al., 2023). B. Simplified geological map of the La Ciénaga de Purmamarca study area, showing the location of the fossiliferous samples (yellow star) and of the measured stratigraphic section (red lines). C. Stratigraphic column of the Acoite Formation at the La Ciénaga de Purmarmarca, indicating the position of the fossiliferous conodont sample (yellow star) and previous graptolite records (black circles) (modified from Toro et al., 2017). Abbreviations, sh: shale, s: siltstone, f: fine-grained sandstone, m: medium-grained sandstone, and c: coarse-grained sandstone. In A, words in italics refer to geographic elements and province names.

2. Geological setting

Early Paleozoic marine sedimentary successions are widely exposed in the southern part of the Central Andean basin of Argentina and Bolivia. These are collectively referred to as the Santa Victoria Group, comprising the Santa Rosita (Furongian-Tremadocian) and the Acoite (Floian, possibly upper Tremadocian-lower Dapingian) formations (Turner, 1960). In the Purmamarca area of northwestern Argentina, however, the intricate interplay of rapid lateral facies changes and widespread Cenozoic Andean deformation has resulted in a complex local stratigraphic nomenclature. This includes, in stratigraphically ascending order, the Purmamarca shales, Chañarcito limestones, Coquena shales, Cieneguillas shales, and Sepulturas limestones (Harrington and Leanza, 1957). This stratigraphic scheme has been extrapolated to other sectors of the basin with varying degrees of confidence, often relying on single fossil samples obtained from discontinuous outcrops. Currently, the use of Cieneguillas shales has become obsolete, as it is considered a lateral equivalent to the widely accepted Acoite Formation, and to avoid confusion with the Tremadocian Cieneguillas Formation of Bolivia (e.g., Vaccari et al., 2006; Toro et al., 2017; Waisfeld et al., 2023).

At the La Ciénaga de Purmamarca, the Acoite Formation is composed of >400 meters of dark grey shales, subordinate sandstones, calcarenites, and calcareous concretions (Toro et al., 2017) (Fig. 1B, C). It developed in a relatively deep, low energy, largely dysaerobic, restricted foreland shelf ramp (Waisfeld and Asitini, 2003). The entire stratigraphic succession is strongly deformed, with both its lower and upper sections delimited by faults.

3. Previous biostratigraphic studies

The first reference to the age of the Ordovician outcrops at the La Ciénaga de Purmamarca was provided by Harrington (in Harrington and Leanza, 1957, p. 13). He attributed the Cieneguillas shales to the Arenigian based on the presence of Thysanopyge argentina Kayser (1898) and Megalaspidella kayseri Kobayashi (1937) and assigned the Sepulturas limestones to the Llanvirnian due to the presence of Hoekaspis schlaginweiti Harrington and Leanza (1942). A review of the material assigned to H. schlaginweiti from the La Ciénaga de Purmamarca by Harrington and Leanza (1957, p. 235), along with the revision of the trilobites housed at the Centro de Investigaciones en Ciencias de la Tierra (CICTERRA)-Universidad Nacional de Córdoba (N.E. Vaccari collection), however, suggests that this taxon may instead correspond to Niobides armatus Harrington and Leanza (1957). At its type locality, in the Santa Victoria area (Acoite Formation), N. armatus is associated with T. argentina. Aceñolaza (2003), on the other hand, pointed out the presence of Pliomeridius sulcatus Leanza and Baldis (1975) in the Sepulturas Formation (another name given to the Sepulturas limestones) at the La Ciénaga de Purmamarca, suggesting a late early Arenigian age. In our collection, the trilobite association is composed of P. sulcatus, Thysanopyge clavijoi Harrington and Leanza (1957), N. armatus and Pytine wirayasqa Vaccari et al. (2006), which are indicative of the Early Ordovician Thysanopyge fauna (stage slices Tr 3-Fl 2 sensu Bergström et al., 2009).

The first ones to document conodonts from the Acoite Formation at the La Ciénaga de Purmamarca were Rao et al. (1994). They referred the conodonts to the Billingenian Regional Stage (upper Latorpian) of the Oelandian Series of southern Sweden. The Billingenian Stage comprises the Oepikodus evae, Trapezognathus diprion and Microzarkodina russica conodont interval zones (Bagnoli and Stouge, 1997; Stouge et al., 2020), representing the upper part of the Floian Global Stage in Scandinavia and the East Baltic area (Goldman et al., 2023; Nielsen et al., 2023). Quoting Rao et al. (1994), the graptolites “Didymograptus (s.l.) aff. demissus (Törnquist, 1901), D. (s.l.) cf. simulans Elles and Wood (1901) and D. (Corymbograptus) aff. vacillans (Tullberg, 1880)” from the La Ciénaga de Purmamarca could represent an age equivalent to the Tetragraptus phyllograptoides Zone (Fl 1). However, the presence of Baltograptus vacillans, index species of the homonymous graptolite biozone in Scandinavia (Maletz, 2023), suggests a slightly younger age for the graptolite bearer levels (Ortega et al., 2003). Based on a similar conodont assemblage, Rao (1999) established the Baltoniodus crassulus andinus-Drepanoistodus pitjanti Association Zone of early Arenigian age in the Espinazo del Diablo and Cajas sectors. Although Rao (1999) referred the conodont-bearing levels to the Sepulturas Formation, they correspond to the Acoite Formation instead, following the nomenclatural revision of Vaccari et al. (2006) and Toro et al. (2017).

Based on the La Ciénaga de Purmamarca fauna described by Rao et al. (1994), Albanesi et al. (2008) established the Gothodus Zone for the Eastern Cordillera, replacing the equivalent Oepikodus evae Zone of Albanesi and Ortega (2002), which encompasses the middle to upper late Floian. Albanesi et al. (2014) reported a similar conodont association from the La Ciénaga de Purmamarca, in the Aguas Blancas Creek, near Altos de Lipán, which they ascribed to the late Floian. Toro et al. (2015), on the other hand, considered that the Gothodus Zone of northwestern Argentina begins at least in the early Floian Paratetragraptus akzharensis Zone, which aligns with the recently estimated early-to-middle Floian biostratigraphic range of the Gothodus andinus Zone in the Chulpíos Creek section (Voldman et al., 2017). Albanesi et al. (2021) referred the Aguas Blancas fossil assemblage to the early-middle Floian based on its associated graptolite records (upper P. akzharensis Zone, or possibly the lower Baltograptus cf. B. deflexus Zone), and suggested that the Aguas Blancas section is stratigraphically younger than the outcrops exposed at the La Ciénaga de Purmamarca.

Aceñolaza et al. (2008) described the index species Trapezognathus diprion (Lindström, 1954) from the La Ciénaga de Purmamarca. They suggested that part of the elements described as T. argentinensis Rao et al. (1994) from the La Ciénaga de Purmamarca and Espinazo del Diablo (Rao et al., 1994; Rao, 1999) should be assigned to T. diprion, while others could represent an undescribed species of the same genus. Aceñolaza et al. (2008) also reported from the same sample a fragmentary element of the index species Oepikodus intermedius Serpagli (1974) and assigned the assemblage to the late Early Ordovician.

Subsequent studies interpreted T. argentinensis as a mixture of other species and partly synonymized it with T. diprion (Carlorosi and Heredia, 2013) and with the index species for the base of the Dapingian, Baltoniodus triangularis (Carlorosi, 2013; Carlorosi et al., 2013). Although T. diprion and O. intermedius coexist throughout most of the O. evae Zone, their last appearance does not reach the base of the Dapingian in the Huanghuachang section of South China (Wang et al., 2009). Conversely, O. intermedius was documented in the lower Dapingian of the Argentine Precordillera (e.g., Serpagli, 1974; Albanesi et al., 1998; Mango and Albanesi, 2020), with a closely related species also recorded in North America (Stouge and Bagnoli, 1988). In addition, O. evae (Lindström, 1954) occurs in some offshore stratigraphic sections of Scandinavia within the B. triangularis Zone (e.g., Rasmussen, 2001; Bergström and Löfgren, 2009; Stouge et al., 2020, and references therein). In summary of the conodont taxonomic analysis, the same specimens studied by Rao et al. (1994) and Rao (1999) from the La Ciénaga de Purmamarca and Espinazo del Diablo were referred to both the Floian and Dapingian ages (Carlorosi, 2013; Carlorosi and Heredia, 2013; Carlorosi et al., 2013).

The graptolite records from the La Ciénaga de Purmamarca suggest an early to middle Floian age for the bearer levels. For instance, Toro and Vento (2013) recognized there early Floian levels after recording Baltograptus vacillans (Tullberg, 1880) and P. akzharensis (Tzaj, 1968). However, in the Cow Head Group of Newfoundland, Canada, the P. akzharensis Zone of Williams and Stevens (1988) correlates with the top of the Prioniodus elegans Zone, reaching the base of the O. evae Zone (cf. Stouge and Bagnoli, 1988; Johnston and Barnes, 1999) and possibly implying that age limits vary depending on the location. Some years later, Toro et al. (2017, 2024) and Navarro et al. (2019) updated the graptolite biostratigraphy at the La Ciénaga de Purmamarca and determined the Tetragraptus phyllograptoides (Fl 1), P. akzharensis (Fl 1) and the lower part of the Baltograptus cf. deflexus (Fl 2) biozones in tectonically truncated sedimentary packages in the Acoite Formation. These Floian graptolite biozones have also been recognized in the Aguas Blancas, El Aguilar, Angosto del Moreno, Cajas, and Los Colorados sectors in the western flank of the Eastern Cordillera, in the Cieneguillas section in Bolivia, as well as in the northern Santa Victoria area (Egenhoff et al., 2004; Albanesi et al., 2008, 2014; Toro and Maletz, 2008; Toro and Vento, 2013; Toro et al., 2015; Voldman et al., 2017), providing a good temporal constraint for the low-diversity conodont faunas along the Eastern Cordillera.

4. Material and methods

The La Ciénaga de Purmamarca is a classical locality for the study of the Ordovician of northwestern Argentina (Cecioni, 1953, 1965; Harrington and Leanza, 1957; Rao, 1994). In this contribution, the analyzed rock sample (geographical coordinates: 23°42’ S, 65°32.7’ W; Fig. 1B) is a phosphatized, highly fossiliferous matrix-supported calcareous coquina that includes conodonts, cnidarians, linguliform and rhynchonelliform brachiopods, hyoliths, mollusks (gastropods, bivalves, and cephalopods), as well as remnants of trilobites, ostracods and echinoderms. From this same bed, the raphiophorid trilobite Pytine wirayasqa Vaccari et al. (2006) was originally defined. 1,700 g of sample were fragmented and dissolved in buffered acetic acid following the standard conodont recovery techniques (Stone, 1987; Jeppsson et al., 1999), obtaining 636 conodont elements (Table 1).

The conodont specimens are generally smaller than 500 μm, complete to moderately fragmented, and exhibit a Color Alteration Index (CAI; Epstein et al., 1977) of 3, indicating burial temperatures of 110-200 °C. For the morphotype classification, the terminology of Sweet (in Clark et al., 1981) was adopted, which includes P, M, and S elements and their subdivisions. Images of the microfossils were obtained with a MC170 HD camera attached to a Leica DM4500 petrographic microscope and with a ZEISS Sigma scanning electron microscope (CICTERRA and LAMARX, Universidad Nacional de Córdoba, Argentina). The systematic taxonomy of Kallidontus nodosus Pyle and Barnes (2002) and of Zentagnathus argentinesis (Rao et al., 1994) is provided in the Appendix. The specimens are housed in the CICTERRA Institute under the repository code prefix CEGH-UNC.

5. Conodont biostratigraphy and implications

In the original conodont collection from the La Ciénaga de Purmamarca area, Rao et al. (1994) described >1,400 elements, consisting of Baltoniodus crassulus andinus Rao et al. (1994), Cornuodus longibasis (Lindström, 1954), Drepanodus? sp., Drepanoistodus basiovalis (Sergeeva, 1963), Drepanoistodus pitjanti Cooper (1981) (labelled as D. aff. pitjanti in the table and figure captions of Rao et al., 1994), Drepanoistodus sp. 2, Protopanderodus sp. cf. P. n. sp. A McCracken (1989), Trapezognathus argentinensis Rao et al. (1994) and Scandodus? sp. The taxonomy of the species Gothodus andinus and Zentagnathus argentinensis were subsequently discussed in Voldman et al. (2013a, 2017). Note that Rasmussen et al. (2021) revised the genus Drepanoistodus and constrained D. basiovalis to a species with a first appearance in the Darriwilian. Accordingly, in the present collection this taxon was determined as Drepanoistodus sp. A.

The conodont sampling conducted in the present study allows for a biostratigraphic refinement of the original collection (Table 1). Based on its yield, faunal content, and field observations, our sample was obtained from the same beds from which the LC9 sample of Rao et al. (1994) was derived, sharing records of G. andinus (Fig. 2), D. pitjanti (Fig. 3A-F), D. sp. A (Fig. 3L-O), and Z. argentinensis (Fig. 4A-K). The species Drepanodus? sp. and Drepanoistodus sp. 2 of Rao et al. (1994) were assigned in the present contribution, respectively, to Drepanodus arcuatus Pander (1856) (Fig. 3R-T) and Drepanoistodus cf. andinus Voldman et al. (2013b) (Fig. 3G-K). It was not possible to recognize Protopanderodus sp. cf. P. n. sp. A McCracken (1989) and Scandodus? sp. in our collection, although Paroistodus sp. could indeed be identified (Fig. 3P-Q). The genus Paroistodus, in fact,occurs sporadically in the Early Ordovician of the Central Andean basin (e.g., Voldman et al., 2017; Albanesi et al., 2021). Based on its faunal composition, the fossil assemblage described here corresponds to the Southwestern Gondwana Province, characterized by a mixture of endemic, Baltic, and Laurentian taxa (Zeballo and Albanesi, 2013a).

Fig. 2. SEM images (unless otherwise stated) of conodonts from the La Ciénaga de Purmamarca area. A-P. Gothodus andinus (Rao et al., 1994). A. M element, CEGH-UNC 27689; B. M element, CEGH-UNC 27690; C. M element, CEGH-UNC 27691; D. Pb element, CEGH-UNC 27692; E. M element, CEGH-UNC 27693; F. Pa element, CEGH-UNC 27694, optical image; G. Pb element, CEGH-UNC 27695; H1-H2. Pb element, CEGH-UNC 27696, H1: SEM image, H2: optical image; I1-I2. Pa element, CEGH-UNC 27697, I1: optical image, I2: SEM image; J1-J2, Pa element, CEGH-UNC 27698, J1: optical image, J2: SEM image; K. Sa element, CEGH-UNC 27699; L. Sb element, CEGH-UNC 27700; M. Sb element, CEGH-UNC 27701; N. Sc element, CEGH-UNC 27702; O. Sc element, CEGH-UNC 27703; P. Sd element, CEGH-UNC 27704. All scale bars are 100 μm.

Fig. 3. SEM images (unless otherwise stated) of conodonts from the La Ciénaga de Purmamarca area. A-F. Drepanoistodus pitjanti Cooper (1981). A. M element, CEGH-UNC 27719; B. Sa element, CEGH-UNC 27720; C. Sc element, CEGH-UNC 27721; D. Sb element, CEGH-UNC 27722; E. Sd element, CEGH-UNC 27723; F. Sd element, CEGH-UNC 27724. G-K. Drepanoistodus cf. andinus Voldman et al. (2013b). G. Sb element, CEGH-UNC 27725; H. M element, CEGH-UNC 27726; I. Sa element, CEGH-UNC 27727; J. M element, CEGH-UNC 27728; K. M element, CEGH-UNC 27729. L-O. Drepanoistodus sp. A. L. Sc element, CEGH-UNC 27730; M. Sb element, CEGH-UNC 27731; N. M element, CEGH-UNC 27732; O. Pa element, CEGH-UNC 27733, optical image. P-Q. Paroistodus sp. P. S element, CEGH-UNC 27734, optical image; Q. S element, CEGH-UNC 27735. R-T. Drepanodus arcuatus Pander (1856). R. Pb element, CEGH-UNC 27736; S. Pa element, CEGH-UNC 27737; T. Pa element, CEGH-UNC 27764. All scale bars are 100 μm.

Fig. 4. SEM images (unless otherwise stated) of conodonts from the La Ciénaga de Purmamarca area. A-K. Zentagnathus argentinensis (Rao et al., 1994). A. Pa element, CEGH-UNC 27705; B. Pa element, CEGH-UNC 27706, optical image; C1-C2. Pa element, CEGH-UNC 27707, optical images in lateral (C1) and upper view (C2); D1-D2. Pa element, CEGH-UNC 27708, optical images in lateral (D1) and upper view (D2); E. Pb element, CEGH-UNC 27709; F. Pb element, CEGH-UNC 27710; G. M element, CEGH-UNC 27711; H. Sa element, CEGH-UNC 27712; I. Sb element, CEGH-UNC 27713; J. Sb element, CEGH-UNC 27714; K. Sd element, CEGH-UNC 27715. L. Balognathidae indet., Pb element (basal filling?), CEGH-UNC 27716. M-N. Kallidontus nodosus Pyle and Barnes (2002), M. Pb element, CEGH-UNC 27717; N. Pa element, CEGH-UNC 27718. All scale bars are 100 μm.

Notably, the new conodont sampling yielded Kallidontus nodosus (Fig. 4M-N), an index species of the Midcontinent Realm in the shallow-water platform facies of the northern Canadian Cordillera (Pyle and Barnes, 2002) (Fig. 5). Originally, K. nodosus was introduced from the Mount Sheffield Member of the Kechika Formation in northeastern British Columbia, where it was restricted to the K. nodosus Subzone of the upper Acodus kechikaensis Zone (Pyle and Barnes, 2002, 2003) (Fig. 5). The K. nodosus Subzone is approximately coeval with the lower half of the Prioniodus elegans Zone of Baltoscandia, extending from the first appearance of K. nodosus to the first appearance of Oepikodus communis, the nominate species of the overlying biozone (Pyle and Barnes, 2002, 2003) (Fig. 5). Other typical components of the K. nodosus Subzone in the Kechika Formation include Acodus kechikaensis Pyle and Barnes (2002), A. neodeltatus Pyle and Barnes (2002), Drepanoistodus amoenus (Lindström, 1954), Drepanoistodus latus Pyle and Barnes (2002), Fahraeusodus marathonensis (Bradshaw, 1969), K. serratus Pyle and Barnes (2002), Oelandodus elongatus van Wamel (1974), Parapanderodus striatus (Graves and Ellison, 1941), Paroistodus parallelus (Pander, 1856), Scolopodus krummi (Lehnert, 1995), and Tropodus australis (Serpagli, 1974).

In the Precordilleran scheme, the Prioniodus elegans Zone is well documented in the San Juan Formation, where it has been subdivided into the Tropodus sweeti and the O. communis subzones (Albanesi et al., 1998; Albanesi and Ortega, 2016; Mango and Albanesi, 2020). These subzones approximately correlate with the upper part of the Paratetragraptus approximatus Zone and the P. akzharensis zones of the North America graptolite scheme (e.g., Stouge and Bagnoli, 1988; Goldman et al., 2023; Maletz, 2023), respectively. Stratigraphically above, the base of the overlying Oepikodus evae Zone marks the base of the middle Floian (Fl 2) and partly correlates with the base of the Baltograptus. cf. deflexus Zone in the Eastern Cordillera (Bergström et al., 2009; Waisfeld et al., 2023) (Fig. 5).

Considering the stratigraphic position of the conodont sample, its composition, and the associated graptolite fauna (indicative of the P. akzharensis Zone), the presence of K. nodosus verifies an early Floian (Fl 1) age for the lower part of the G. andinus Zonein the study area (Fig. 5). The large difference in size of the two morphotypes of K. nodosus recovered (Fig. 4M, N), as well as the absence of reworked elements both in Rao’s and in our collection (>2.000 elements in total), diminishes the possibility that K. nododus was reworked from older strata. An early Floian age for the lower levels of the G. andinus Zone aligns with the previous assessment of Voldman et al. (2017) and contradicts suggestions of a Dapingian age raised by others. In addition, the occurrence of Erraticodon patu Cooper (1981) in the upper portion of the Acoite Formation at the Chulpíos Creek and the Aguas Blancas sectors (Voldman et al., 2017; Albanesi et al., 2021) suggests a younger stratigraphical age for those levels, although still Floian, as indicated by the presence of graptolites characteristic of the lower B. cf. deflexus Zone.

The early Floian age of the endemic species G. andinus and Z. argentinensis at the La Ciénaga de Purmamarca provides an additional temporal constraint for the early evolution of prioniodontid conodonts (e.g., Sweet, 1988; Stouge and Bagnoli, 1999; Dzik, 2015; Stouge et al., 2020; Zhen et al., 2023). In the Chulpíos Creek, Gothodus vetus Voldman et al. (2017), which exhibits rudimentary denticulation, evolved directly from an adentate species, likely Acodus triangularis (Ding in Wang, 1993), and was subsequently replaced by G. andinus. Voldman et al. (2017) noted a parallel evolution between G. vetus and Prioniodus? transitans (McTavish, 1973) from the Emanuel Formation in Australia, although they interpreted a slightly younger age for the former, as it succeeds A. triangularis in the stratigraphic column. Along with Prioniodus antiquus Zhen et al. (2023) and P. gilberti Stouge and Bagnoli (1988), all these forms share the development of short and rudimentary denticles in septimembrate apparatuses around the Tremadocian-Floian boundary (Zhen et al., 2023).

The appearance of Zentagnathus in the early Floian places it among the oldest balognathids. In agreement, Heward et al. (2019) considered that the balognathids from the Middle Shale Member of the Amdeh Formation from the Sultanate of Oman, Arabian margin of Gondwana, are no older than early Floian. Zhen et al. (2023) suggested that the appearance and evolution of Prioniodus represented a significant adaptive strategy for conodont animals, enabling them to occupy and thrive in the increasingly diverse environments and complex food webs during the Great Ordovician Biodiversification Event (e.g., Goldman et al., 2020; Moreno et al., 2024). The current record of K. nodosus in the G. andinus Zone at the La Ciénaga de Purmamarca marks a significant step towards refining the temporal framework of conodonts in the Early Ordovician of the Eastern Cordillera of Southwest Gondwana.

6. Conclusions

The new conodont assemblage from the Acoite Formation at the La Ciénaga de Purmamarca, northwestern Argentina, provides refined constraints on its depositional age and enhances regional and global correlation. The assemblage comprises Balognathidaeindet., Drepanodus arcuatus, Drepanoistodus pitjanti, D. cf. andinus, D. sp.A, Gothodus andinus, Kallidontus nodosus, Paroistodus sp., and Zentagnathus argentinensis. The presence of K. nodosus allows for a precise correlation of the lower Gothodus andinus Zone with the early Floian Prioniodus elegans Zone (Fl 1). These new findings highlight the need for caution in the identification of Baltoniodus triangularis, the index fossil for the Lower-Middle Ordovician Series boundary, both in the La Ciénaga de Purmamarca and other sectors along the Eastern Cordillera.

Fig. 5. Early-Middle Ordovician biostratigraphic scheme highlighting in pale yellow the approximate age of the conodont fauna from the Acoite Formation exposed at the La Ciénaga de Purmamarca and described in this work. Additional columns indicate its regional and global correlation (figure adapted from Ross et al., 1997; Bagnoli and Stouge, 1997; Pyle and Barnes, 2002; Li et al., 2010; Albanesi and Ortega, 2016; Voldman et al., 2017; Goldman et al., 2023; Waisfeld et al., 2023).

Acknowledgments
We thank F. Zeballo (LAMARX) for the scanning electron microscope images and M.J. Mango (CICTERRA) for insightful taxonomic discussions. G. Bagnoli, C. Barnes, J. Maletz, and S. Stouge helped reviewing the manuscript. We appreciate corrections by the Editor, D. Bertin. The project was funded by Proyectos de Investigación Plurianuales, Consejo Nacional de Investigación Científica y Técnica (PIP CONICET) 11220200101361CO (Argentina). This article is a contribution to the International Geoscience Programme (IGCP) Project 735 “Rocks and the Rise of Ordovician Life”.

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Appendix

Systematic Paleontology

Class Conodonta Pander (1856)
Order Panderodontida Sweet (1988)
Family Fryxellodontidae Miller (1980)
Genus Kallidontus Pyle and Barnes (2002)

Type species: Kallidontus serratus Pyle and Barnes (2002)

Referred species: Kallidontus corbatoi (Serpagli, 1974), Kallidontus? lofgreni (Stouge and Bagnoli, 1988), K. gondwanicus Zeballo and Albanesi (2013b), K. princeps Pyle and Barnes (2002), K. nodosus Pyle and Barnes (2002).

Remarks: Many platform conodont elements with transverse ridges and node-like denticles first appeared in the Early Ordovician (e.g., Bergström, 1983; Dzik, 1983; Löfgren, 1985; Bagnoli et al., 1988). However, their fossil record is relatively sparse and most specimens are fragmentary, complicating efforts to reconstruct taxonomic relationships. The genus Kallidontus is characterized by large, antero-posteriorly compressed Pb platform elements, with variable denticulation and ridge development (Pyle and Barnes, 2002). The first representative of the Kallidontus lineage is K. gondwanicus Zeballo and Albanesi (2013b), which occurs in the Paltodus deltifer Zone (Tr2) of the underlying Coquena Formation and the Santa Rosita Formation of the Santa Victoria Group, Eastern Cordillera, Argentina, and the Tiñú Formation of Oaxaca, Mexico (Zeballo and Albanesi, 2013b; Voldman et al., 2013b). During the upper Tremadocian-lower Floian interval, the lineage is progressively succeeded by K. princeps Pyle and Barnes (2002), K. serratus Pyle and Barnes (2002), and K. nodosus Pyle and Barnes (2002).

Based on surface ornamentation, Stouge and Bagnoli (1988) reclassified Fryxellodontus? corbatoi Serpagli (1974) as Polonodus? corbatoi and described Polonodus? lofgreni Stouge and Bagnoli (1988) from the P. elegans Zone of Newfoundland. Subsequently, these species were assigned to Polonodus (e.g., Lehnert, 1995; Albanesi, 1998), and later to Kallidontus without formal taxonomic discussion (e.g., Mango and Albanesi, 2020; Rueda and Albanesi, 2023). It remains possible that Kallidontus is in turn a junior subjective synonym of the obscure genus Nericodus, described from the Paroistodus proteus Zone of Sweden (Lindström, 1954; Miller, 1980; Dzik, 1983, 2024).

Kallidontus nodosus Pyle and Barnes (2002)
Fig. 4M-N

2002. Kallidontus nodosus n. sp. Pyle and Barnes, pp. 53-54, pl. 8, figs. 1-15.
Kallidontus nodosus Pyle and Barnes, fig. 10: 12-14.

Material: 2 elements.

Remarks. The specimens at hand closely resemble the holotype of K. nodosus from British Columbia by its shape and the presence well-developed transverse ridges. In contrast, the transverse ridges are weakly developed in K. serratus Pyle and Barnes (2002) and barely visible in K. princeps Pyle and Barnes (2002). The holotype of K. lofgreni (Stouge and Bagnoli, 1988, pl. 11, fig. 1) from the lower Floian of Newfoundland also shows some resemblance to K. nodosus. However, K. lofgreni is more antero-posteriorly compressed and presents surface reticulation, while the paratypes may exhibit subtle radial ridges (Stouge and Bagnoli, 1988, pl. 11, fig. 3) not observed in K. nodosus. The discovery of additional specimens would certainly help clarify whether K. nodosus and K. lofgreni are distinct species or simply variations of a single species.

Order Prioniodontida Dzik (1976)
Family Balognathidae Hass (1959)
Genus Zentagnathus Voldman and Albanesi (in Voldman et al., 2017)

Type species: Trapezognathus? primitivus Voldman et al. (2013a)

Referred species: Z. argentinensis (Rao et al., 1994), Z. gertrudisae Albanesi et al. (2023).

Remarks: Voldman et al. (2013a) emended the diagnosis of Trapezognathus argentinensis after studying specimens from the Sierra de Zenta, although with doubts on its generic assignment. This issue became clear after the analysis of an extensive conodont collection from the Chulpíos Creek, in the Santa Victoria area, and the definition of the endemic genus Zentagnathus (Voldman et al., 2017). Zentagnathus is distinguished from Trapezognathus and early species of Baltoniodus by presenting a tertiopedate M element in their septimembrate apparatus (cf. Bagnoli and Stouge, 1997; Heward et al., 2019). Currently, it includes Z. argentinensis and Z. primitivus, both species distinguished by their degree of denticulation, from highly rudimentary to well-developed, and the more advanced, Darriwilian Zentagnathus gertrudisae Albanesi et al. (2023).

Zentagnathus argentinensis (Rao et al., 1994)
Fig. 4A-K

1994. Trapezognathus argentinensis n. sp. Rao et al., pp. 73, 75, pl. 3, figs. 7-12, 14, pl. 7, figs. 1-8.
?2003. Lenodus sp. Bultynck and Sarmiento, p. 266, pl.2, figs. 12-13 (only).
2008. Trapezognathus diprion (Lindström); Aceñolaza et al., pp. 151-153, fig. 4B.
2011. Trapezognathus diprion (Lindström); Carlorosi, fig. 4C-D.
2013a. Trapezognathus? argentinensis Rao et al.; Voldman et al., pp. 126-127, figs. 2.15-2.21.
2017. Zentagnathus argentinensis (Rao et al.); Voldman et al., pp. 409, 411, figs. 6C-E, J, M-N.
2017. Baltoniodus sp. Voldman et al., fig. 6: G-H.
2017. Baltoniodus triangularis (Lindström); Carlorosi et al., figs. 2A, B.
2021. Zentagnathus argentinensis (Rao et al.); Albanesi et al., p. 492, figs. 6Q-Y

Material. 32 elements.

Remarks: Pa elements of Z. argentinensis may resemble Pa elements of G. andinus, particularly when dealing with broken specimens, although the former are generally more denticulated and the basal cavity is slightly deeper. Z. argentinensis may look like B. triangularis (Lindström); however, its Pb element is diagnostic, as well as its non-geniculate M element, characteristic of the genus (Voldman et al., 2013a, 2017). Moreover, S-elements in Z. argentinensis present a less pronounced length/with ratio compared to those of B. triangularis, which are more acicular and more denticulated (Bagnoli and Stouge, 1997; Bergström and Löfgren, 2009).

Conversely, Carlorosi (2013) and Carlorosi et al. (2013) partly considered Z. argentinensis from the La Ciénaga de Purmamarca as a junior synonym of B. triangularis and included elements of G. andinus (Rao et al., 1994, pl. 3, fig. 2) under the same systematics. Alternatively, Carlorosi and Heredia (2013) partly synonymized Z. argentinensis from both the La Ciénaga de Purmamarca (Rao et al., 1994, pl. 3, figs. 7, 14) and Sierra de Cajas (Rao, 1999, pl. 9, fig. 6) areas with Trapezognathus diprion, and interpreted the sedimentary sequence as Floian, in agreement with Aceñolaza et al. (2008).

Ultimately, Carlorosi et al. (2018, 2023) incorrectly incorporated specimens of Z. argentinensis from Sierra de Zenta, including a paratype from Purmamarca Rao´s collection illustrated by Voldman et al. (2013b), into the synonymy of B. triangularis.Voldman et al. (2013b) diagnosed some specimens (fig. 2: 5-6) as B. cf. triangularis, although their revision suggested that they possibly represent basal fillings of Z. argentinensis. Carlorosi et al. (2018, 2023) also included Zentagnathus primitivus from the Chulpíos Creek as a junior synonym of B. triangularis, although the cited figure corresponded to Baltoniodus sp. instead (Voldman et al., 2017). Reexamination and comparison of Baltoniodus sp. with the new collection from the La Ciénaga de Purmamarca suggests it rather falls within the range of shape variability of Z. argentinensis, as juvenile specimens tend to exhibit sharper denticles than gerontic specimens (Voldman et al., 2013a). The Pb elements of Z. argentinensis are more comparable to those of Baltoniodus tetrachotomus Li and Wang in Li et al. (2004) (=Baltoniodus tetrastichus Li in Wang et al., 2003) from the lower Dawan Formation, Hubei Province, southern China, although the latter occurred later in time, in the lower O. evae Zone, but also before the base of the Dapingian.