Plantilla de artículo 2013
Andean Geology 50 (3): 403-435. September, 2023
Andean Geology
doi: 10.5027/andgeoV50n3-3453
Taphonomic analysis of Spinicaudata from two Triassic lacustrine systems affected by different volcanic processes
*Evelyn Luz Bustos-Escalona1, Adriana Cecilia Mancuso1,
Cecilia Andrea Benavente
1, 2, Andrea Beatriz Arcucci3

1 Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CCT-CONICET-Mendoza, Adrián Ruiz Leal s/n -Parque Gral. San Martín, (5500), Mendoza, Argentina.,,

2 Geología, Facultad de Ciencias Exactas y Naturales (FCEN), Universidad Nacional de Cuyo (UNCuyo), CP 5500 Mendoza, Argentina.

3 IMIBIO-CONICET, Área de Zoología, Facultad de Química, Bioquímica y Farmacia (FGByF), Universidad Nacional de San Luis (UNSL), Ejército de los Andes 950, (5700), San Luis, Argentina.

* Corresponding author:

Spinicaudata are freshwater branchiopods. The growth and development of spinicaudata populations in freshwater environments depend on various physical, chemical, and biological factors. The action of volcanism modifies the limnological parameters in the process of fossilization of lacustrine deposits. This study identifies spinicaudatan assemblages in two Triassic lake systems in Argentina, and analyzes the dynamics of shell preservation according to the volcanic activity involved.  Two taphonomic grades divided into three different second taphonomic grades were defined. The focus on taphonomic and chemistry studies addresses changes in spinicaudata abundance and richness through time, in different paleoenvironmental conditions, and shell preservation and chemical composition. The discussion highlights the importance of lava flow inflow and volcanic ash fall in the different lake systems. We emphasize the importance of taphonomic studies on the spinicaudata groups to understand their autochthonous and allochthonous populations in Triassic lake systems. In turn, we conclude that the post mortem processes experienced by the shells, the exposure time at the water-sediment interface, and the chemical alteration of the water into the paleolakes produced by the surrounding volcanic activity can explain the development and preservation of spinicaudata in these two Triassic lacustrine systems.

Keywords: Freshwater branchiopods, Los Rastros Formation, Agua de la Zorra Formation, Triassic, Argentina.



1. Introduction

Spinicaudata are small branchiopods that inhabit small, shallow, temporary ponds, being typically found in the coastal and photic zone of lakes and lagoons (e.g., Tasch, 1969; Webb, 1979). Spinicaudata can be considered r-strategist species because they produce resistant eggs. The growth and development of spinicaudata populations in these environments depend on physical, chemical, and biological factors. However, they have a high tolerance to variations in environmental conditions during the growth phase, such as temperature (from 1 to 41°C), oxygenation, pH (from 7 to 9.7), and salinity (from 6 to 35), which facilitates the development of populations in different environments (Gislén, 1937; Massal, 1954; Rzoska, 1961; Tasch and Zimmerman, 1961; Bishop, 1967; Horne, 1967; Moore and Burn, 1968Kobayashi, 1975; Webb, 1979; Tasch, 1987; Gore, 1988; Fryer, 1996; Thiéry, 1996; Cáceres and Tessier, 2003; Ferreira Olivera, 2007; Benvenuto et al., 2009; Astrop et al., 2012; Tassi, 2015). The co-occurrence of two or more species in the same environment is favored by morphologic and phylogenetic differences among them (Orr and Briggs, 1999; Dumont and Negrea, 2002), being more frequently monospecific communities (Spencer and Hall, 1896; Tasch and Shaffer, 1964; Moore, 1965; Tasch, 1969; Ferreira-Olivera, 2007; Astrop et al., 2012; Tassi, 2015). The nutrient availability, such as the variety and quantity of algae, microplankton, and bacteria, is one of the biological limiting factors for developing a diverse spinicaudata population (Kapler, 1960; Petr, 1968; Royan, 1976).

Taphonomic studies on spinicaudata are mainly based on mechanical features (Krishnan, 1958; Rieder et al., 1984; Zhang et al., 1990; Orr et al., 2002, 2008; Olempska, 2004; Mancuso and Marsicano, 2008; Astrop and Hegna, 2015; Astrop et al., 2015; Bustos Escalona et al., 2017; Jenisch et al., 2017), or chemical composition of post-buried shells (Briggs, 2003; Stigall et al., 2008; Astrop and Hegna, 2015; Astrop et al., 2015; Monferran et al., 2018; Hu et al., 2020). The spinicaudata shell is mainly composed of a calcium-phosphate complex (e.g., Astrop et al., 2015), and the multi-laminar structure of shells makes it resistant to taphonomic processes such as decay, desiccation, and compaction (Krishnan, 1958). Astrop and Hegna (2015) suggest that the mechanical features of spinicaudata shells are specific to taxonomic families. Moreover, the chemical composition of the shell generates differences in the fossilization process, also favoring the prevalence of some taxa in the fossil record (Briggs, 2003; Astrop and Hegna, 2015; Astrop et al., 2015; Hegna et al., 2020). However, Astrop et al. (2015) conclude that despite variation in cuticle chemistry, the degree of mineralization alone does not predict the mechanical properties of shells and that post-depositional taphonomic processes such as permineralization and dissolution also need to be analyzed.

Post mortem modifications reflect the alterations suffered by the remains, including cracking, surface marks, abrasion, polishing, and fracture (e.g., Behrensmeyer and Hook, 1992; Behrensmeyer et al., 2000; Mancuso and Marsicano, 2008; Astrop et al., 2015; Karr and Clapham, 2015). Each type of these modifications indicates a taphonomic process and may be diagnostic of a specific agent. Mechanical abrasion on carbonate skeletons can occur either as a result of repeated reworking within a high-energy environment or during long-distance transport (e.g., Behrensmeyer and Hook, 1992; Behrensmeyer and Kidwell, 1985; Behrensmeyer et al., 2000; Mancuso and Marsicano, 2008; Astrop et al., 2015; Karr and Clapham, 2015), as observed on marine mollusk shells (Behrensmeyer et al., 2000; Brett and Baird, 1986; Behrensmeyer and Hook, 1992). Bioerosion has also been recorded in carbonate rocks from marine environments (Golubic and Schneider, 1979) and spinicaudatan shells from lacustrine paleoenvironments (Bustos Escalona et al., 2019). The abrasion, combined with a long time at the sediment-water interface before burial, promotes the dissolution of the carbonate shells (Brett and Baird, 1986). Likewise, dissolution is a taphonomic alteration that is highly selective in terms of chemical and mineralogical composition, size, and microstructure of the fossil particles or fragments. The resistance of the elements to dissolution is related to the stability of their mineral components, effective surface area, and other factors such as their microstructure and permeability (e.g., Plotnick, 1986; Wilson, 1988; Behrensmeyer, 1991; Behrensmeyer and Hook, 1992; Behrensmeyer et al., 2000; Astrop et al., 2015; Karr and Clapham, 2015). Most of the shell spinicaudatan fossils, which were chemically analyzed, have been preserved as recrystallized calcium phosphate or silica replaced molds (Stigall and Hartman, 2008; Stigall et al., 2008, 2014; Hethke et al., 2013; Monferran et al., 2018; Hu et al., 2020).

In tectonically active areas, the record of volcanic material and sediment is widespread. Volcanoes produce large amounts of loose sediments that affect local deposit systems, destroy surrounding biotic habitats, change river flows, transform topography, and cause changes in the physical and chemical conditions of sedimentary environments (Lipman and Mullineaux, 1982; Smith, 1987; Lockley and Rice, 1990; Behrensmeyer and Hook, 1992; Dale et al., 2005; Payne and Egan, 2017). The volcanic eruption impact depends on the explosion magnitude and varies according to the distance from the center of the volcanic activity (Payne and Egan, 2017). Particularly in lacustrine systems, the volcanic activity increases the rate of sediment supply by the unusual ash fall and lava flow supply, which also triggers changes in the original chemical composition of lake waters and microenvironmental diagenetic conditions (Behrensmeyer and Hook, 1992). The lava flows cause alteration of the limnological parameters, both physical (e.g., temperature and habitat reduction) and chemical (e.g., pH and salinity). The entry of volcanic ash into aquatic ecosystems also produces alterations in limnological parameters (e.g., increase in turbidity, changes in pH, conductivity, variations in nutrient concentration), which directly affect the surrounding biota from a basal trophic level (e.g., decrease in primary productivity due to modification of the microbial community) and thus modify the trophic state (Barker et al., 2000; Okorafor, 2011; Massaferro et al., 2018; Pedernera et al., 2021). Despite the evident adverse effects of volcanic activity on the surrounding ecosystems, the fossil record in lake deposits could be favored by the action of volcanism (Beherensmeyer and Hook, 1992; Pan et al., 2011).

This work therefore aims to identify the taphonomic processes that affect the spinicaudata fossil assemblages found in two Triassic lacustrine systems, in Argentina, and investigates how the spinicaudata shells preservation was affected by volcanic activity (i.e., ash fall and basaltic lava flows) in the Triassic lacustrine environments.

2. Geological setting

Triassic sedimentation in Argentina occurred in extensional basins associated with the breakup of Pangea, which began in the latest Permian-earliest Triassic (Uliana and Biddle, 1988; Lovecchio et al., 2020). Two of the largest basins were developed along the southwestern margin of Gondwana (Fig. 1), with NNW-SSE trends related to basement fabric control (Martínez et al., 2011). The basins were filled by thick, entirely continental successions of predominantly fluvial and lacustrine sediments that record almost the whole Triassic.


FIG. 1. A. Map of PDC and lahar deposits of the 2015 eruption of Calbuco volcano (after Mella et al., 2015). B. Location of stratigraphic stations. C-E. Correspond to photographs of the 1st (C and D) and 2nd (E) eruptive pulses on 22 and 23 April 2015, respectively, as seen from the NW (Credits: A. Becerra and G. Cifuentes).


In the Ischigualasto‐Villa Unión Basin (San Juan and La Rioja provinces) (Fig. 1A), the Los Rastros Formation (LRF) represents a lacustrine-deltaic succession characterized by shallowing-upwards cyclic deposits of dark gray to black mudstone, siltstone, and tabular fine to coarse-grained sandstone. At the Gualo locality (Talampaya National Park, La Rioja province) (Fig. 1B-C), the outcrops are characterized by five lacustrine-deltaic cycles (Fig. 2A). Each cycle is characterized by horizontally laminated gray-black claystone (Fl) occasionally interbedded with iron mudstone (SFm), which represents the distal lacustrine and prodelta deposits (LR-A facies association-Table 1). The distal lacustrine and prodelta sediments pass upward to mudstone (Fm) and fine to coarse-grained sandstone (Sr, Sh, Sp, St, SG) deposited by progradation of a mouth-bar complex in an upward-shallowing delta front environment (LR-B facies association-Table 1). Then, medium- to coarse-grained sandstone interbedded occasionally with coal mudstone (FCS) is deposited by a distributary fluvial system with delta swamps on a deltaic plain (LR-C facies association-Table 1) (Mancuso and Marsicano, 2008; Mancuso and Caselli, 2012; Mancuso et al., 2020). The LRF records a syn-rift phase in the Ischigualasto-Villa Unión rift Basin (Mancuso and Caselli, 2012; Mancuso et al., 2020). The stratigraphic relation with the isotopically dated Chañares and Ischigualasto formations and the recent CA (Chemical Abrasion)-TIMS analysis of a tuff level from the LRF at Cerro Bola locality (234.47±0.44 Ma), suggest that LRF covers a significant part of the Carnian (Mancuso et al., 2020).


FIG. 2. Stratigraphy of PDC deposits in the Frío and Tepu valleys, including the location of stratigraphic sites. Correlation of profile descriptions in the Frío and Tepu study areas are presented. 


The Cuyana Basin (Mendoza, San Juan, and San Luis provinces), includes the Agua de la Zorra Formation (AZF), exposed at the Paramillos de Uspallata locality (Fig. 1D-E), is interpreted as deposited in a lacustrine-deltaic setting with episodic incursions of lava flows into the aquatic environment (Ottone et al., 2011; Pedernera et al., 2019). The succession is characterized by mudstone (Fm) interbedded with fine to coarse-grained sandstone (Sm, Sr, St) deposited by progradational deltas with development of mouth bars (AZ-A facies association-Table 2). Up section, black, olive black, greenish-red, and greenish-black mudstone (Fl), occasionally interbedded with muddy siltstone (Fm) and fine to coarse-grained sandstone (Sh), were deposited in a distal lacustrine and prodelta setting (AZ-B facies association-Table 2). The lower section dominated by deltaic deposits is characterized by interlayers of peperitic, olivine basalts interpreted as episodes of lava flow incursions into the aqueous environment (Fig. 2B) (Harrington, 1971; Brea and Artabe, 1999; Ottone et al., 2011; Pedernera et al., 2019). Deposits of the AZF took place in a continental rift tectonic environment, particularly associated with the syn-rift phase in the evolution of the Cuyana rift Basin (Kokogian and Boggetti, 1987; Uliana and Biddle, 1988; Kokogian and Mansilla, 1989; Ramos and Kay, 1991). Isotopic dating of the basalts of the AZF suggests that the unit was deposited in the Middle-Upper Triassic (240±10 to 235±5 Ma) (Massabie et al., 1986; Ramos and Kay, 1991; Linares, 2007).

Volcanic activity was already recognized in previous stratigraphic studies of both units (Mancuso and Caselli, 2012; Pedernera et al., 2019). In the LRF, several tuffaceous levels were interpreted as ash fall and reworked tuff deposits (Mancuso and Caselli, 2012). While, in the AZF, basalt lava flows were identified, including mudstone with peperitic basalt alteration, tuffaceous sandstone, thermally altered sandstone (in the lower section), and ash fall layers interbedded with distal lacustrine facies (in the upper section) (Pedernera et al., 2019). Based on the volcanic deposits, the Los Rastros paleolake was located more than 100 km away from the volcanic center (Pedernera, 2020), while the Agua de la Zorra paleolake was developed in the volcano vicinity, approximately within the first 10 km from the eruption zone (Pedernera, 2020).

3. Material and methods

Systematic sampling was carried out every 20 cm throughout the stratigraphic columns corresponding to the Los Rastros (Gualo locality) and Agua de la Zorra (Paramillos de Uspallata locality) formations (Fig. 2). A total of 3,149 spinicaudata remains (1,500 from the LRF and 1,649 from the AZF) were collected mainly from horizontally laminated mudstone (Fl) and mudstone and fine-grained sandstone (SFm) of the distal lacustrine facies association, and in less quantity from siltstone and fine-grained sandstone (Fm/Sh) of the distal delta front facies association. The remains were found predominantly disarticulated, preserving only one of the two shells that constituted the live spinicaudata body. They were positioned on the sediment surface according to their most stable equilibrium state, leaving the external face of the shell up. This position is called the convex-up position in this contribution. The shells are found concordant to the top of the bearing bed, without preferential orientation. A few remains were observed with articulated shells. In this case, the specimens were deposited on the top of the bearing bed with the shells open according to their most stable equilibrium position and also, leaving the external faces of the shell up. In general, the spinicaudata remains were found as compression or shell impressions.

3.1. Taphonomic analyses

The collected samples were characterized into (1) Isolated shells, or (2) Densely Packed shells. The first category comprises rock samples with one or more spinicaudata remains, which could be seen individually, without overlapping each other. The second category includes densely packed shells in mudrocks without condensed time features preserving massive amounts of spinicaudata remains, overlapping each other, and with similar preservation features that discard time-averaging.

All spinicaudata remains were evaluated using a standard semi-quantitative method of scoring preservation quality. The method allows us to define taphonomic grades (TG) to express preservation quality by evaluating the integrity and articulation of fossils. This method has been widely used in the taphonomic analysis of insect and vertebrate assemblages (e.g., Wedmann, 1998; Smith and Moe-Hoffman, 2007; McNamara et al., 2011, 2012; Wang et al., 2013).

A first classification was made according to the fragmentation presented by the spinicaudata shells, resulting in two main TGs: (1) entire shell, and (2) fragmented shell (Table 3). Accordingly, in previous studies, the TGs would indicate the procedence of the spinicaudata shells and their transport time (Fürsich et al., 2016; Jenisch et al., 2017). In a second-level division, each TG was divided into three different second taphonomic grades (STG) according to the integrity of the morphological characters retained in the shells (STG-a, STG-b, and STG-c) (Table 3). Each STG indicates a progressive loss of definition in the morphological characters such as edges, growth lines, and ornamentation (Table 3). These STGs can thus indicate the time period that the spinicaudata shells laid on the water-sediment surface before burial (Fürsich and Pan, 2015; Hu et al., 2020).

3.2. Chemical analyses

The elemental compositions of 52 LRF samples and 19 AZF samples were analyzed by using a JEOL JSM-6610 LV scanning electron microscope (SEM), equipped with an energy dispersive spectrometer (Thermo Scientific Ultra Dry Noran System 7) at the MEBYM Lab, IANIGLA-CONICET. All samples were carbon-coated. We used a live acquisition time of 30 s, nominal incident beam energy E=15 keV, and 10 mm working distance. The results are semi-quantitative. Scanmaps (EDS) were performed from the shell and at the shell-rock matrix interface. The weight percentage of 13 major elements (C, O, F, Al, Si, P, K, Ca, Mn, Mg, Fe, Na, S) was determined in all samples in order to include major cations for rock-matrix minerals (e.g., silicates) and chemical compounds representing the original shell composition (e.g., carbonates, phosphates). A box plot was made with the percentages of the chemical elements obtained in the EDS mapping of the shells. The main element percentages that compose the spinicaudata shell (Ca and P) and of the host substrate (Si and Al) were plotted to identify a relationship between the chemical compositions of the rock and of the morphological characters (STG) observed.

4. Spinicaudata abundance and richness

Ten families of spinicaudata have so far been described for the Triassic successions in Argentina (Gallego, 1992, 1996, 1999a, b, 2001, 2005, 2010; Gallego and Melchor, 2000; Gallego et al., 2004, 2008; Mancuso, 2005; Gallego and Martins-Neto, 2005, 2006; Tassi et al., 2013, 2015; Tassi, 2015). Particularly, in the LRF, the genera Euestheria (Euestheriidae), Triasoglypta (Loxomegaglyptidae), and Triasulugkemia (Ulugkemiidae) have been documented (Gallego 1999a,b; Gallego and Melchor, 2000; Mancuso, 2005; Tassi, 2015). Recently, Bustos Escalona (2020) described the presence of four new genera for the LRF: Menucoestheria (Eosestheriidae), Cornia (Vertexiidae), Endolimnadiopsis (Palaeolimnadiopseidae), and Challaolimnadiopsis (Pemphilimnadiopseidae). Thus, the richness of spinicaudata for this formation has been increased to 7 genera corresponding to 7 different families (Fig. 2A). On the other hand, in the AZF, the genera Euestheria, Triasoglypta, Triasulugkemia, and Endolimnadiopsis were previously described (e.g., Gallego, 1999b; Gallego and Melchor, 2000; Gallego et al., 2001; Gallego and Martins-Neto, 2005, 2006; Tassi, 2015), whereas the genera Dendrostracus (Polygraptidae), Estheriellites (Fushunograptidae), and Menucoestheria were recently identified (Bustos Escalona, 2020) (Fig. 2B), increasing the richness to 7 families as well.

In the LRF, the genus Euestheria has a wider stratigraphic distribution when compared to other families, although it was recorded more frequently in cycles three to five (Fig. 2A). Triasoglypta is the second most abundant genus, although significantly less than Euestheria, and it is present from the second cycle onwards. The record of the other genera (Challaolimnadiopsis, Endolimnadiopsis, Menucoestheria, Triasulugkemia, and Cornia) is variable, ranging between cycles 2 and 5, with their best representation in cycle 3 (Fig. 2A). Euestheria and Triasoglypta remains were found together on several stratigraphic levels; however, the association of three or more genera at the same level was observed only in very few and specific sections of cycles 2 and 3. Also, particularly in cycle 3, two levels with remains of 5 and 6 genera were observed (Fig. 2A). Ten densely packed shell levels were identified along the LRF section. They are recorded in lacustrine sediments of cycles 1, 3, 4, and 5 but are more common in cycle 3 (Fig. 2A). Half of these levels are monogeneric, and the other levels include two or, in some cases, three genera.

In the AZF, the spinicaudata remains were found in the lacustrine facies (Fig. 2B). Euestheria is the genera with the widest distribution throughout the section, followed by Triasoglypta, just as in the LRF. The other genera are concentrated in the lower section of the lacustrine facies interbedded with deltaic facies and basalt flows (Fig. 2B). Euestheria and Triasoglypta genera were found together at several levels, and the record of 3 to 4 genera at the same level occurs in the lower section. The remains of Dendrostracus appear only in three levels, and in two of them is associated with Euestheria (Fig. 2B).

Twelve levels of the densely packed shells were identified along the AZF section (Fig. 2B). They are very recurrent in the lower section, where the basalt levels are interbedded. In the lower section, they include two to five genera, whereas in the upper section are generally monogeneric. 

5. Taphonomy

5.1. Taphonomic grades

The remains preserved according to the TG-1 are disarticulated shells (Fig. 3A-B, D, G-J) or articulated shells (Fig. 3C) located convex-up on top of beds. On other hand, the remains preserved according to the TG-2 are shell fragments, which are generally one-third of the shells, found either as isolated remains (Fig. 3E-F) or less frequently as clusters on top of beds (Fig. 3K-L). In the LRF, the highest percentage of shells was preserved according to TG-1 (see Table S2 in the Supplemental file), with a richness of seven genera (Euestheria, Triasoglypta, Challaolimnadiopsis, Endolimnadiopsis, Menucoestheria, Triasulugkemia, Cornia) (Table 4). The rest of the samples were preserved according to TG-2 (Euestheria and Triasoglypta) (Table 4). In the AZF, the percentage of shells preserved according to TG-1 is lower than in the LRF, and TG-2 exceeds 40% of the total samples (see Table S2 in the Supplemental file). In the AZF, seven genera were identified for TG-1, and four genera for TG-2 (Table 4). Statistical analyses indicate that the observed percentages of TG-1 and TG-2 between formations are significantly different (see Supplemental file).



FIG. 3. Photographs of PDC and lahar deposits and associated tephra fall units in the Tepu (A-D) and Frío (E-H) valleys. A. Full sequence of pyroclastic units at section T1. B. Basal units overlying pre-eruptive soil at T5. C. Subrounded boulder-sized accidental lithics at T3. D. Standing tree buried by the PDC at T6. The upper part of the section corresponds to tephra fall and lahar deposits. E. Middle PDC section at the upper Frío river (F2) showing laterally discontinuous units, one of them exhibiting intensive bulking (PDC-6). F. Fine-grained and cross-stratified PDC beds intercalated by a coarser-grained PDC-8 unit at the upper Frío river (F1). G. Upper PDC section showing two lahar beds to the top (LHR1 and 2) at section F7. H. Composite stratigraphy of the uppermost PDCs at section F8 (Blanco river). Credits: J. Romero, with the exception of inset f (F. Sánchez, lines show cross stratification). PDC labels are correlated to these of figure 2.


By the second taphonomic grade (STG), the remains preserved according to STG-a show most of the shells are whole (TG1) with clearly defined growth lines and remarkable ornamentation in the central zone bands. However, in the distal and umbonal zones, the ornamentation becomes diffuse (Fig. 3A-B, G-H). The shell fragments (TG2) show well-preserved morphological characters (Fig. 3E, K). In the STG-b, the edge of the shell is poorly differentiated, and the growth lines are delimited to a strip that covers the middle anterior, central, and middle posterior zones of the shell (Fig. 3C, F, I, L). In a few cases, the growth lines of the ventral zone cannot be distinguished. The growth bands were observed with diffuse ornamentation patterns (Fig. 3C, I). Finally, in the STG-c, shells with poor preservation were observed. The ventral-anterior, ventral-posterior, and ventral edges of the shell are diffuse, and the ornamentation of the bands is not visible (Fig. 3D, F, J, L).

In the LRF, STG-a is present in very low proportions, while STG-b and STG-c are found in similar percentages, representing almost half of the remains (Fig. 4; Table S2 in the Supplemental file). In the AZF, STG-a is present in a higher proportion than in the LRF, while STG-c is present in more than half of the samples analyzed (Fig. 4; Table S2 in the Supplemental file).


FIG. 4. 2015 lahar deposits in both Frío (A-B) and Tepu (C-D) rivers, lying on top of PDCs and in gullies generated by the erosion into these PDC units.


As mentioned, the mainly spinicaudata remains were recovered from horizontally laminated claystone (Fl) and mudstone, and fine-grained sandstone (SFm) of the distal lacustrine facies association. The remains collected from the lacustrine facies association are preserved according to every TG and STG for the AZF and the LRF, except STG-2a in the LRF. The spinicaudata remains recovered from the siltstone and sandstone (Fm/Sh) of the distal delta front facies association are preserved according to every TG and STG for the LRF, except STG-2b. The deltaic facies association in AZF has not recorded spinicaudata.

Both the LRF and the AZF record densely packed shell levels. The LRF shows ten levels mainly concentrated in cycles 3 to 5, with a dominance of shells preserved in TG-1, particularly in STG-b and STG-c, and scarce remains with TG-2 and STG-a preservation. The AZF presents twelve levels mainly concentrated in the lower section, also with the shells preserved mainly according to TG-1, STG-b and STG-c, and a few individuals corresponding to TG-2 and STG-a.

5.1.1. Interpretation of the taphonomic grades

Studies conducted on current spinicaudata shells show they are flexible and resistant to destructive physical taphonomic effects in high-energy environments (e.g., river transport) (e.g., Astrop et al., 2015). However, Astrop et al. (2015) investigated shells reared in the laboratory, that is, under favorable confinement and controlled according to the protocol of Weeks and Zucker (1999). In addition, Astrop et al’s study did not contemplate periods of post-mortem spinicaudata shell dissolution (cf. Jenisch    et al., 2017; Hu et al., 2020). Therefore, based on the features observed in the spinicaudata remains studied in this work, included in the TG, and according to other authors (e.g., Jenisch et al., 2017; Hu et al., 2020), the degree of fragmentation of the shells can be used to interpret their habitat.

The spinicaudata remains preserved according to TG-1 are entire shells found in the Fl and Sh facies (Fig. 5). Based on the integrity of shells, we interpret that they correspond to autochthonous paleolake spinicaudatans, inhabiting the calm and sheltered margin of the lake and sheltered delta zone. The recently dead animals were likely transported by the natural circulation of the lake from the inhabited zone to the open water, where they sank to the lake bottom or distal delta front layers.


FIG. 5. Fumarole activity and progress of fluvial erosion of the 2015 PDC and lahar deposits in the Frío and Tepu river valleys. Google EarthTM satellite images of both rivers on 23 April 2015 are shown in A and B. C. Aerial view of PDC deposits and their associated fumaroles in the Frío river (Diego Spatafore). D. Fumaroles in the Tepu valley by 27 April 2015 (Bárbara Corrales). E. Fumarole activity and fluvial erosion in the Frío valley during April-June 2015 (D. Morgavi and C. Valenzuela). F. Detail of degassing pipes in the surface of the PDC deposit in the Frío valley. The diameter is about 1 m. G. Steaming ground in reworked tephra fall deposits covering PDC deposits of the upper Frío valley. H. Fumarolic patches in the upper Frío River.


The TG-2 reveals a more complex history due to the two distinct fracture types recognized. The shell fragments, characterized by sharp or marked fractures with well-defined edges, suggest episodes of predation or scavenging by the fish that inhabited the paleolake (e.g., McKenzie, 1981; Mancuso and Marsicano, 2008). The remains preserved according to TG-2 with sharp fractures are found in Fl of the distal lacustrine and Fm/Sh of the distal delta front facies association (Fig. 5). Based on this evidence, we interpret that the remains correspond to autochthonous paleolake spinicaudatans, inhabiting the margin zone, and the fragment arrived to the lake bottom and distal delta front layers after predation or scavenging episodes. On the other hand, spinicaudata remains fragmented following the weak zones of the shell (growth lines) were recorded in the Fl of the distal lacustrine (TG-2 and STG-b and STG-c), and Fm/Sh of the distal delta front facies association (STG-c) (Fig. 5). The exposure of the shells at the sediment-water interface after death favors the dissolution process of the outer layers (see Hu et al., 2020). Thus, the weakened shells become susceptible to break when they are subjected to mechanical forces (e.g., Flow transport) (Brett and Bair, 1986; Plotnick, 1986; Farinati and Zavala, 1995; Fernández-López, 2000; Bustos Escalona et al., 2017; Jenisch et al., 2017; Hu et al., 2020). Based on the weakened fractures, we interpreted that these remains correspond to allochthonous spinicaudatan, possibly inhabiting ponds upstream of the paleolake. The shells, after death, were exposed for some time at the sediment-water interface, and were then transported to the paleolake, where they sank to the lake bottom and distal delta front layers. 

The second taphonomic grade (STG) is characterized by the integrity of shell morphological characters. The different grades represented from STG-a to STG-c show a progressive loss of morphological character integrity (Table 3). Based on the dissolution process suffered by shells during exposure at the sediment-water interface (Hu et al., 2020), we interpret that remains included in STG-a were buried relatively quickly after death, experiencing a short time at the sediment-water interface (Krishnan, 1958; Brett and Baird, 1986; Mancuso and Gallego, 2000; Cohen, 2003; Jenisch et al., 2017; Hu et al., 2020). The remains preserved according to STG-b show diffuse edges, growth lines, and ornamentation (Table 3; Fig. 3F, L).  Based on the moderate loss of morphological features, we interpret that these spinicaudata remains underwent a moderate time of exposure at the sediment-water interface in the aqueous environment after death, which favored the dissolution of the edge of the shell (Brett and Baird, 1986; Mancuso and Gallego, 2000; Birggs, 2003; Mancuso and Marsicano, 2008; Astrop et al., 2015; Jenisch et al., 2017; Hu et al., 2020). Particularly, the shell fragments preserved according to STG-b, which show fractures in the weak zones (growth lines) of the shell and with diffuse growth bands, show that the shells experienced dissolution after death in the ponds upstream of the paleolake, weakening the shell and favoring fragmentation during transport (Driscoll, 1970; Driscoll and Weltin, 1973; Seilacher et al., 1985; Brett and Baird, 1986; Mancuso and Gallego, 2000; Jenisch et al., 2017; Hu et al., 2020). Finally, the specimens included in STG-c did not preserve morphological characters. They represent both autochthonous (TG-1) and allochthonous (TG-2) individuals, exposed for a long time at the water-sediment interface before their final burial, either in ponds, or in the paleolake.

5.2. Chemical analyses

Seventy-one samples selected for SEM-EDS analysis were collected from the Fl of the distal lacustrine (67 samples) and Fm/Sh of the distal deltaic front (4 samples) facies association from different stratigraphic levels along the LRF and the AZF sections (Fig. 5). EDS maps of the spinicaudata remains show trends in elemental distributions. EDS allowed the recognition of several elements, but only the main elements composing the shell (P, Ca) and the host rock (Si, Al) are shown in Table 5. The EDS results show, in both formations, a general trend with the main concentration of Ca and P in the shell zone in contrast to the main concentration of Si and Al in the matrix zone (Fig. 6). They also show that the concentration of the original shell components (Ca and P) decreases from STG-a to STG-c, so the distribution of Ca and P in STG-c is more homogeneous between shell and matrix than in STG-a and STG-b (Fig. 6; Table 5).


FIG. 6. Records of cumulative A and daily B precipitation at three gauging stations: Ensenada 10 km northeast of Calbuco (41°13’39.60” S; 72°34’4.53” W, 62 m a.s.l.), Puerto Montt 32 km southwest of Calbuco (671,522 E, 5,407,833 N, 90 m a.s.l.) and Puelo 45 km southeast of Calbuco (41°27’45.97” S; 72°56’46.17” W, 5 m a.s.l.). Mean annual rainfall at these stations is 2,473, 2,046 and 2,956 mm, respectively. Source: Servicio Hidrométrico Nacional.


The box plots (Fig. 7) highlight the particularities of the trends observed in both the LRF (Figs. 7A-B) and the AZF (Fig. 7C-D). The LRF samples, especially in the Fl of the distal lacustrine facies association of cycles 1, 2, 3, and 5, show differences in the Si, Al, Ca, and P concentrations in each STG (Fig. 7A), with higher, Ca and P concentrations in the shells for STG-a (Figs. 6, 7A). However, the samples LR85-1, LR85-2, and LR85-3, that were found also in the Fl of the distal lacustrine facies association but between the tuff levels of cycle 4 (Fig. 5), record a higher Si concentration, lowest Ca concentration, and absence of P in the shells.


FIG. 7. Evolution of the fluvial incision in the Frío (A) and Tepu (B-C) river valleys and sediment delivery at the Blanco-Este River and Llanquihue Lake (D-E) some years after the eruption. (Photos a to d by J. Romero and photo e by C. LeRoy). 


The samples preserved in Fm/Sh of the distal delta front facies association: LR80 according to STG-a,LR81-1 according to STG-c, LR81-2 according to STG-b (cycle 3), and LR93 according to STG-c (cycle 4), also show higher Ca values than Si (Table 5) in the shell for STG-a and the lowest Ca and P concentration, with a higher and more homogeneous Si concentration, in the shell and in the rock matrix around the shell for STG-c (Table 5; Figs. 5, 6).

In the AZF, the box plots show that Ca, P, Si, and Al concentrations in the shell zone are as diverse as in the LRF lacustrine facies samples (Table 5; Fig. 7C). As occurs in the LRF, a higher concentration of Ca and P is observed in the samples of the Fl of the distal lacustrine facies association from the AZF upper section for the STG-b samples compared to the STG-c samples, which present higher concentration of Si and Al in the preserved zone of the shell (Figs. 6, 7C). Moreover, sample AZ19, preserved according to STG-a from Fl of the distal lacustrine facies association, but between tuff levels (Tf) (Fig. 5B), shows elevated Si and Al concentrations (Table 5). In the AZF lower section, samples preserved according to TG-1, STG-a, and STG-b (AZ1-AZ9`) are found in Fl of the distal lacustrine facies association below the basaltic flow level (Fig. 5B) and show a higher Si and Al concentration in the shell zone (Table 5).

5.2.1. Interpretation of the chemical analysis

The comparison of the general trend of the three STG for the LRF and the AZF suggests a relationship between the decreasing Ca and P concentration and the integrity of the morphological characters (Figs. 6, 7). The longer time exposed at the water-sediment interface that the shells underwent favored not only the loss of morphological characters but also the dissolution of the original shell components (Ca and P) (Golubic and Schneider, 1979; Behrensmeyer et al., 2000; Briggs, 2003; Cohen, 2003; Hu et al., 2020). 

The variations in the general trends in lacustrine samples are similar between both formations. The LRF and the AZF (upsection) lacustrine samples show the best original component preservation (Fig. 7A), with a clearer trend in the AZF lacustrine samples (upsection) that show an increase of Si in shell and matrix areas with a decrease of Ca and P (Fig. 7C). This observation suggests that the Fl of the distal lacustrine deposits favored the preservation of the original component (Fig. 5, 7A, 7C; Table 5) (Behrensmeyer and Hook, 1992; Behrensmeyer et al., 2000; Cohen, 2003; Hu et al., 2020). In distal delta front samples of the LRF, the concentration of the original components of the shell (Ca and P) decreases from STG-a to STG-c, while the concentration of the matrix components (Si and Al) increases. Furthermore, these matrix components become homogeneously distributed in the shell area (Fig. 7A; Table 5).

As mentioned above, both paleolakes recorded volcanic influence, which were likely to affect the preservational condition of shells. Some authors proposed that the alkaline microenvironment, produced by volcanic ash input and decomposition of the volcanic glass particles, can promote phosphate precipitation (e.g., Berner, 1968; Gulbrandsen, 1969; Allison, 1988; Hu et al., 2020), one of the most important biomineralization component of the spinicaudatan shells (Klug et al., 2005; Stigall and Hartman, 2008; Astrop, 2014). However, the decay of the shells decreases the pH of the microenvironment around them, and the emission of CO2 by volcanic processes increases the water acidity, inhibiting the precipitation of phosphate (Gabbott, 1998; Briggs, 2003; Hu et al., 2020). In both paleolakes, the shells interbedded with basalt flow levels (AZ1-AZ9`, AZ19) or ash fall (AZ19) in the AZF, and that from the LRF cycle 4 (LR85) show a decrease or even absence of the original component (Ca and P) in favor of Si (Table 5), resulting in almost an entire silicification of the remains (STG-a and STG-b) (Fig. 5). This evidence suggests that in both paleolakes, the alkalinity produced by volcanic influence could be neutralized at least in the microenvironment around shells by their decay and by the emission of CO2 that produces a decrease in pH, inhibiting the phosphatization and favoring the silicification (Hu et al., 2020).

6. Stratigraphic distribution and taphonomic history of spinicaudatans

6.1. Los Rastros Formation

In the LRF, the stratigraphic distribution and TG of the spinicaudata taxa (Figs. 2A, 5A), show an evident change in abundance and richness between cycles 1-2 and cycles 3-5. During cycles 1 and 2, the presence of individuals mainly of TG-1 in Fl facies indicates the autochthony of the spinicaudata to the main water body, inhabiting the coasts and oxygenated areas of the paleolake (Fig. 5A) as postulated by Mancuso and Marsicano (2008). Both cycles preserved a few specimens included in TG-1 (Sh facies) and TG-2 (Fl facies), suggesting the transport of spinicaudata by tractive flows that incorporated shell remains into the main water body, indicating a low input of allochthonous spinicaudata (Fig. 5A) (Mancuso and Marsicano, 2008). Also, in both cycles, Euestheria is the most common taxon, followed exclusively in cycle 2 by Triasoglypta and Triasulugkemia (Fig. 2A).

During the transition between cycles 2 and 3, a second half-graben controlled independently by a secondary fault was proposed based on the sedimentological evidence, the architectural patterns, stratigraphic and paleogeographic distribution of the deposits, and source areas at the Gualo-Chañares área (Mancuso and Caselli, 2012; Benavente et al., 2021). Thus, it resulted in a division of the major Los Rastros paleolake and the development of a smaller isolated paleolake in the Gualo-Chañares area (Mancuso and Caselli, 2012). Pedernera et al. (2020) suggest that the decrease in plant taxa recorded between cycles 3-4 would be related to a change or reduction of input areas resulting from the isolation suffered by the activity of a secondary fault. From a taphonomic point of view, the recorded increase of spinicaudatans taxa and the increase of remains in cycle 3 (Fig. 2A) could be correlated with changes in the LRF paleolake for the Gualo-Chañares area resulting from this tectonic event. Cycle 3 shows the highest abundance and taxa richness. There is a high increase in the richness and abundance of paleolake autochthonous spinicaudata (TG-1, Fl facies) and a moderate increase in abundance of the allochthonous spinicaudata (TG-2, Fl and Sh facies). This cycle preserved multi-taxa levels with up to six different taxa together (Fig. 2A). The presence of multi-taxa levels suggests the main paleolake included a variety of subenvironments and limnological conditions (e.g., pH, temperature, salinity, vegetation) for the development of diverse spinicaudata assemblages within the trophic zones (Kapler, 1960; Petr, 1968; Royan, 1976; Orr and Briggs, 1999; Dumont and Negrea, 2002). Even in a few meters, four events of the densely packed shells are concentrated with equal preservation grade, suggesting a sudden change in the limnologic parameters, such as pH, nutrients, and/or solute concentration, resulting in unfavorable conditions to the growth of the spinicaudata assemblages (Kapler, 1960; Baird, 1862; Klunzinger, 1864; Bishop, 1967; Petr, 1968; Royan, 1976; Gislén, 1937; Massal, 1954; Tasch and Zimmerman, 1961; Moore, 1965; Horne, 1967; Mancuso and Marsicano, 2008).

Cycle 4 shows a decrease of almost half of the spinicaudata richness and abundance compared to cycle 3 (Fig. 2A). The autochthonous spinicaudata (TG-1) continues to be dominant in the assemblage (Fig. 7A) with a low abundance of allochthonous specimens (TG-2). Surprisingly, Triasoglypta is absent in this cycle, with an absolute dominance of  Euestheria with a few representatives of Endolimnadiopsis, Menucoestheria, and Triasulugkemia (Fig. 2A). This cycle includes several ash levels interbedded with distal lacustrine facies rich in autochthonous spinicaudata (TG-1) (Figs. 2A, 7A). The relation between the ash fall levels with the increase of spinicaudatan specimens suggests a volcanic influence in chemical limnological parameters, but not enough to produce a mortality event. One densely packed shell level is preserved in cycle 4 close to the distal delta front facies (Fig. 2A). The event includes autochthonous spinicaudata in an excellent state of preservation (TG-1) (Mancuso and Marsicano, 2008). Therefore, the physical factors that generated this shell packing occurred in the main paleolake. The allochthonous specimens (TG-2) were found in sediments at the distal delta front, evidencing the input of spinicaudata remains from ponds upstream from the main paleolake (Mancuso and Marsicano, 2008).

Finally, cycle 5 shows an increase in the spinicaudata abundance, approximately twice as much as in cycle 4, while the richness keeps on low (Fig. 2A). The most abundant spinicaudatan remains, in facies Fl, are considered autochthonous specimens (TG-1) with good preservation, while a small percentage are allochthonous specimens (TG-2). As occurred in the rest of the unit, Euestheria is the most abundant taxa, followed by Triasoglypta, and in this cycle Endolimnadiopsis is observed (Fig. 2A). Three densely packed shell levels were found in this cycle dominated by autochthonous spinicaudata. Two of these preserved the shells on distal lacustrine deposits (Fl facies), and the last on turbidity flow deposits (SFm facies) (Table 1) that probably removed the death spinicaudatan remains previously deposited on the deltaic front. These spinicaudata are considered autochthonous to the deltaic environment (Mancuso and Marsicano, 2008).

6.2. Agua de la Zorra Formation

The AZF unit was divided into two sections, the lower section included the interbedded distal lacustrine facies with delta facies and basalt lava flows, and the upper section represented the distal lacustrine deposits. The lower section concentrated the highest abundance and taxa richness for the unit (Fig. 2B). The highest concentration of the autochthonous spinicaudata (TG-1) is between basalt lava flows (Fig. 7B), although these levels also concentrated most densely packed shells (Fig. 2B). The recorded volcanic activity could have modified the physical conditions (temperature and spatial reduction of habitat) in the short term, as well as in the long-term chemical conditions (pH, salinity) of the paleolake (Behrensmeyer and Hook, 1992; Fürsich and Pan, 2015; Hu et al., 2020). Also, the lower section levels are bearing multi-taxa assemblage suggesting a variety of subenvironments and limnologic conditions that favored the development of complex spinicaudatan assemblages within the trophic zones (Kapler, 1960; Petr, 1968; Royan, 1976; Orr and Briggs, 1999; Dumont and Negrea, 2002). We believe that the recurrent fluctuations in limnological parameters alternately favored and/or disfavored the growth of the spinicaudatan assemblages.

The upper section shows a lower abundance and taxa richness (Fig. 2B), with a similar number of autochthonous (TG-1) and allochthonous (TG-2) spinicaudata (Fig. 7B). The record of only one level of densely packed shells (AZ19), with the lower taxa richness and mono-taxa levels (Figs. 2B, 5B), suggests a stable water body with a low variety of subenvironment and limnologic conditions that favored the development of monospecific spinicaudatan assemblages.

7. Comparison of Los Rastros and Agua de la Zorra formations

The LRF and the AZF share five spinicaudata taxa (Table 4), two of which are autochthonous and allochthonous (Euestheria and Triasoglypta). The presence of the genera Euestheria, Triasoglypta, Triasulugkemia, Menucoestheria, and Endolimnadiopsis as autochthonous paleofauna in the Los Rastros and Agua de la Zorra paleolakes, indicates that the limnologic conditions in the trophic zones of the main water bodies were in the ecological range for the development of these groups of taxa. Both the LRF and the AZF recorded the genera Euestheria and Triasoglypta as allochthonous paleofauna too, suggesting that the inhabited ponds also presented limnologic conditions in the ecological range that favored their development.

The autochthonous spinicaudata taxa unique to each formation (Cornia and Challaolimnadiopsis) (Table 4) are recorded in the multi-taxa assemblage (Fig. 2A). These findings are interpreted as the result of the development of complex spinicaudata assemblages, which are associated with the diverse subenvironments and limnological conditions within the trophic zone. The genera Cornia is widely recorded in the Lower Triassic of India (Ghosh et al., 1987a, b; Ghosh, 2011), Germany (Kozur and Seidel, 1983a, b; Kozur, 1993; Bachmann and Kozur, 2004; Kozur and Weems, 2007, 2010), Early Triassic of Russia (Novojilov, 1970), and Australia (Tasch, 1987), the Upper Triassic of Africa and Brazil (Tasch, 1987). The larger forms are most common in the Lower Triassic of South America (Ghosh and Dutta, 1996; Ghosh, 2011; Tassi et al., 2013). This genus, in Argentina, has been recorded in the Middle Triassic (Tassi et al., 2013; Tassi, 2015). The small size of specimens of the genera Cornia, and the presence of the umbonal spine in this taxon, has been proposed as an indicator of arid environments, prone to drying out, and of water bodies with high salinity (Tasch, 1963; Ghosh, 1980, 1984; Ghosh and Dutta, 1996). The genera Challaolimnadiopsis has been recorded in Argentina in the Cerro de las Cabras Formation (Middle Triassic) and the Potrerillos Formation (Middle to Early Upper Triassic) in the Mendoza province (Shen et al., 2001; Tassi, 2015). Challaolimnadiopsis remains have been recorded in low energy and/or stable environments under semi-arid seasonal climate conditions (Tassi, 2015). Both taxa (Cornia and Challaolimnadiopsis) have been recorded as autochthonous spinicaudata in Fl facies on Cycle 3 in the LRF (Table 4; Fig. 2A). There is no evidence of arid or desiccation conditions in the LRF at the Gualo area; moreover, the multiproxy climatic analyses of the LRF support that it was a humid environment (Mancuso et al., 2020). Therefore, Cornia and Challaolimnadiopsis are probably opportunistic taxa that can inhabit more environments.

The genera Estheriellites and Dendrostracus were only recorded in sediments from the AZF paleolake as autochthonous and allochthonous specimens (Fig. 2B; Table 4). Estheriellites is a genus that registers only two species for the Triassic in Argentina (Gallego, 1999b, 2001; Tassi, 2015) inhabiting a stable low-energy lake environment (Tassi et al., 2015). The genus Dendrostracus has been previously described for the Upper Triassic (Tassi, 2015) and Lower Cretaceous of Argentina (Pramparo et al., 2005), and the Middle Jurassic of Scotland (Chen and Hudson, 1991). Individuals in this genus have been associated with semi-permanent to ephemeral shallow lake deposits associated with river systems (Chen and Hudson, 1991; Pramparo et al., 2005). However, Tassi (2015) associates it with spinicaudata assemblages of the permanent lake, as they are found without transport marks in distal lacustrine sediments, along with individuals of the genus Triasoglypta. In our study, Estheriellites shells were found only in the lower section of the AZF. These remains were preserved as TG-1 STG-b and TG-2 STG-a. On the other hand, remains assigned to Dendrostracus, preserved as TG-1 STG-a and TG-2 STG-b, were identified in the lower section of the AZF (distal lacustrine facies). Also, Dendrostracus remains, assigned to TG-1 STG-a, were recorded in the upper section of the AZF (Fig. 2B; Table 4). As mentioned above, the AZF paleolake is characterized as a deltaic environment with evidence of volcanic activity, suggesting that conditions in the trophic zones and areas surrounding the paleolake were not stable in the long-term (Pedernera et al., 2021). The record of the genera Estheriellites and Dendrostracus is interpreted here as autochthonous and allochthonous elements, suggesting that life assemblages of these genera could have exhibited a wide tolerance to environmental factors, being able to inhabit and develop also in disturbed environments.

The large number of levels with densely packed shells, in both formations, suggests an increased mortality possibly related to abrupt environmental changes that modified the conditioning factors for the development and growth of spinicaudata paleofauna and the area of inhabited space (Mancuso and Marsicano, 2008). While in the AZF, the recurrent fluctuation in the paleolimnological parameters that are the source of increased mortality is mainly concentrated around the basaltic lava levels of the lower section, in the LRF this fluctuation is not linked to significant volcanic episodes.

As mentioned above, EDS and box plot graphics show a relationship between the loss of morphological character integrity and the decrease in the concentration of original components (Ca and P) of the spinicaudata shells (Figs. 6, 7; Table 5). P is an element considered a limiting factor for life development in lake systems (e.g., Schindler, 1977; Wetzel, 2001; Sterner, 2008). The concentration of P in living spinicaudatan shells depends on the concentration of dissolved P in the lake system (Stigall et al., 2008; Astrop et al., 2015). Likewise, a lake system with low P concentration tends to rapidly dissolve P from dead organisms, increasing the level of dissolved P in the lake system (Warren, 1986). The high Si percentages in shells preserved according to STG-b and STG-c in both formations suggest that both paleolakes contained water poor in available dissolved P, making silicification the main diagenetic process in the fossilization of spinicaudata shells (Hu et al., 2020). However, the distal lacustrine facies (Fl) of the upper section of the AZF records a lower P concentration than the distal lacustrine facies (Fl) of the LRF, suggesting different diagenetic conditions and dissolved P concentration in the water.

Si is the component representative of the rock matrix, replacing the original shell components. Si-rich waters can favor the silicification process by directly interacting the organic remains with microbial mats (Konhauser et al., 2001). However, in the Triassic paleolakes studied here, the shells with the highest concentration of Si are associated with volcanic activity. Therefore, the high Si concentrations in the shells could have been favored by the high Si contribution to the system through volcanic ash supply, basalt lava flows, and/or volcanic fluids (Renaut et al., 2002, 2012; Mancuso et al., 2010; Benavente et al., 2014). The SEM images and EDS results suggest that in both formations, the preserved shells with high Si concentrations are mainly molds of the external layer of the shells. Only a few shells were subjected to mineralization processes of volcanic origin, suggesting that the dissolution of the original shell components was mainly favored by the low P concentration in the paleolakes.

8. The volcanic influence compared between formations

As mentioned before, part of the LRF and the AZF stratigraphic record shows evidence of nearby volcanic activity. The LRF includes several ash fall layers throughout its section, particularly in cycle 4 (Figs. 2A, 5A) (Mancuso and Caselli, 2012), and the AZF presents lava flow levels in the lower section and ash fall levels in the upper section (Figs. 2B, 5B) (Ottone et al., 2011; Pedernera et al., 2019). Therefore, the LRF was located much farther from the volcanic center than the AZF (Fig. 8).


FIG. 8.  Vegetation affected by PDCs. A. Dead trees along the margin of the Frio river, in the front of a PDC deposit; down-valley direction is toward the viewer (Nov 2016). B. Standing dead trees to the west side of Tepu valley, partially buried in the front of a PDC deposit (Dec 2019). C. Tepu Plot B area, small stems tipped down-valley and truncated ~1 m above the former ground surface (located at the feet of the person). The PDC deposit was eroded and stems are exposed next to the current river bed. D. Toppled trees above the surface of the PDC deposit in Plot B area at Tepu River. Most of the tree tops fell down the valley direction and charcoal is observed at their bases. E. New vegetation in the lower Tepu valley between 2017 and 2019. View looking upstream. F. Aerial view of toppled, dead and sprouting trees near a waterfall in the Tepu River (Pablo Saummann). G. Hillside with vegetation toppled by dilute PDCs in the upper part of the Tepu study section. View is looking upstream. Note sprouting on toppled trees during Jan. 2017. H. Dilute PDC between two concentrated PDC units at the Tepu site showing their internal architecture (white lines), including cross stratification and lenses of coarser particles.


From a preservation point of view, the volcanic deposits show the lowest preservation of original shell components linked with basalt lava levels and ash fall levels. Notably, the AZF peperitic levels, which represent the interaction between lava and sediments, suggest a low temperature of the basalts, not able to significantly affect the lake bottom sediments (Ottone et al., 2011). Therefore, the possible thermal alteration of these lava flows can preliminarily be discarded. The alteration of the paleolake water due to volcanic activity and their microenvironmental diagenetic conditions can be explained mainly by pH modification (Jiang et al. 2011, 2012; Fürsich and Pan, 2015). Probably the acidic conditions favored by ash fall and lava flow input in the paleolakes enhanced the exchange of the main components of the shell (Ca and P) for those of the altered substrate (Holdaway and Clayton, 1982; Loope and Watkins, 1989; Butts and Briggs, 2011; Jiang et al., 2011, 2012). The opposite was reported for some plant remains from the AZF, where the volcanic deposits enhanced the preservation of the original organic components possibly due to the increase of sediment supply, carrying the plant remains to the active taphonomic zone, and/or affecting the microenvironmental conditions favoring plant preservation by diagenetic processes (Pedernera et al., 2020). However, the diagenetic conditions that favor the preservation of plant or shell remains are different. While low pH conditions favor the preservation of plant remains, the shells undergo the dissolution of their original components (Holdaway and Clayton, 1982; Loope and Watkins, 1989; Butts and Briggs, 2011; Fürsich and Pan, 2015). The influx of volcanic material can also increase the dissolved silica in paleolake waters, allowing the replacement of the original shell components by Si under specific microenvironmental diagenetic conditions.

From an ecological point of view, the LRF ash fall levels recorded in cycles 1, 2, and 4 show a lower taxa diversity and abundance (Fig. 2A) with a clear dominance of autochthonous specimens (Fig. 5A). In the AZF, the ash fall levels are recorded in the upper section, where the lower taxa richness and abundance are observed for this unit. In contrast, the AZF basalt lava levels recorded in the lower section show the highest abundance and richness (Fig. 2B) with a dominance of autochthonous specimens (Fig. 5B). Therefore, we suggest the ash fall levels in both paleolakes increased the depositional rates, the turbidity and the pH, disrupting the aquatic ecosystem. The increased level of densely packed shells of both units seems to be unrelated to this volcanic material influx. In contrast, the lava flows in the AZF enriched the aquatic ecosystem through the addition of dissolved moving elements and other nutrients. Moreover, the increase of densely packed multi-taxa levels between basalt lava levels supports recurrent paleolimnological changes in the AZF (Fig. 2B). Considering that spinicaudata assemblages are r-strategists, we assume that survival in these two paleoenvironments was favored through the resilience of their eggs, which hatched when paleolimnological conditions became favorable.

9. Conclusions

This study recognizes four new spinicaudata genera for the Los Rastros Formation (LRF) and three for the Agua de la Zorra Formation (AZF), in central-western Argentina. Euestheria is the most abundant genus in both formations, with the most extensive stratigraphic distribution, followed by Triasoglypta. The first level classification of the taphonomic grades (TG) allowed the characterization of the autochthonous and allochthonous spinicaudata assemblages in the LRF and AZF paleolakes. The second level taphonomic grade (STG), together with chemical analyses, allowed to relate the loss of morphological characters with the time of exposure of the shell in the water-sediment interface and the dissolution of the original chemical components of the shell (Ca and P). Both paleolakes contained P-poor water; however, the concentration of P in the LRF paleolake is higher than that of the AZF. The interaction of the surrounding and continuous volcanic activity with both paleolakes modified the environmental conditions necessary for developing spinicaudata assemblages and disfavored the preservation of the main chemical components of the shell. In turn, the significant increase of silica in the AZF paleolake, likely due to the surrounding volcanic activity, allowed the replacement of the original components of the shells by Si, and a better preservation of the morphological characters. In addition, the ash falls disturbed both the aquatic ecosystems of the paleolakes and the upstream section of the fluvial system. From a paleoecological perspective, the opportunistic (r-strategist) character of the spinicaudata is related to the paleolimnological modifications of the paleolakes of both formations.

We gratefully acknowledge T. Pedernera, C. Sancho, and M. Bourguet for their suggestions and assistance in the field; M. Bourguet for assistance in handling samples; S. Lassa for microphotography (LabMEB-IANIGLA-CONICET Mza). We would like to thank V. Vargas and L. Gachón (Secretaría de Cultura de La Rioja) for access to collections and working areas, we also thank, Administración de Parques Nacionales particularly and the staff of Parque Nacional Talampaya. Field research was supported by the PICT 2013-0805 (A. Mancuso) and PICT 2017-2159 (M. de la Fuente). Additional financial support was provided by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

The Editor thanks S. Martínez and one anonymous reviewer for their assistance in evaluating this paper.


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Supplemental File

Statistical analyses

Two statistical analyses were performed to determine if there is a significant difference between the Taphonomic Grades (TG) identified in two different paleolakes. The analyses were performed using data collected during systematic sampling of the Los Rastros Formation (LRF) at the Gualo area and the Agua de la Zorra Formation (AZF) at the Paramillos de Uspallata area.

In the first instance, a resampling statistic was performed with the values of the number of specimens identified according to TG-1 and TG-2, and the calculated percentages of these in each formation (LRF and AZF) (Table S1; Table S2). To verify the results obtained with the resampling statistic, a Chi-square test was performed. Both analyses were carried out with R-Studio software.

1. Resampling statistic

Resampling is a statistical procedure that generates a large number of samples simulating a resampling of the original sample. For this statistic, the number of complete (TG-1) and fragmented (TG-2) shell specimens obtained in the systematic sampling in LRF and AZF (Table S1) was used as a base. For each formation, 10,000 resamplings were programmed. This statistic allowed us to simulate the number of TG-1 and TG-2 specimens that we would obtain if we performed successive random sampling. Table S3 shows the values of the frequency in which TG-1 specimens were obtained for LRF and AZF.

For better visualization of the results, histogram graphs were made with the results obtained (Fig. S1). The resampling statistic for our sites indicates that TG-1 in AZF ranges from 51.9-64.8% with a mean frequency of 58.98%; while in LRF, TG-1 ranges from 80.8-88.6 % with a mean frequency of 85%. These results are in agreement with those obtained in the sequential sampling of both formations (see Table S2).


Fig. S1. Histogram. Frecuency distribution in which whole spinicaudatans (TG-1) are obtained by performing statistic for the Triassic paleolakes of the Agua de la Zorra (AZF) and Los Rastros (LRF) formations. %= percentage.


2. Chi-square test

The chi-square test is used to evaluate the independence between variables in my samples of different sizes (Pearson, 1900). The analysis was performed using the number of spinicaudatans collected complete (TG-1) and fragmented (TG-2) in LRF and AZF. Data are available in the supplementary materials (Table S1).

Pearson’s chi-square test with Yates’ continuity correction yielded an X-squared=258.58, with a degree of freedom (df)=1, for a p-value <2.2e-16. This statistical test yielded the following data table (Table S4). The results obtained from this test indicate that the TGs recorded for LRF are significantly different from the TGs recorded for AZF.

3. Conclusion of the statistical analysis

Both statistics used are consistent in their results. The resampling and Pearson’s chi-square test affirm that the percentages of spinicaudata shell preservations significantly differ between the AZF and LRF paleolakes.


Pearson, K. 1900. On the criterion that a given system of deviations from the probable in the case of a correlated system of variables is such that it can be reasonably supposed to have arisen from random sampling. Philosophical Magazine 50: 157-172.