Geological setting

The Ischigualasto-Villa Unión Basin is located between ~29°S and ~31°S in northeastern San Juan and southwestern La Rioja provinces, Argentina (Golonka, Reference Golonka2007; Césari and Colombi, Reference Césari and Colombi2016; Fig. 1). This basin corresponds to an extensional half-graben, part of a series of Triassic basins that developed parallel to and cratonward of the proto-Andean arc (Uliana et al., Reference Uliana, Biddle, Cerdan, Tankard and Balkwill1988; Ramos and Kay, Reference Ramos, Kay, Harmon and Rapela1991; López-Gamundí et al., Reference López-Gamundi, Espejo, Conaghan, Powell, Veervers and Powell1994). The filling of the Ischigualasto-Villa Unión Basin began during the late Permian and continued until the Late Triassic and is represented by a 2,500 to >4,000 m thick succession of continental deposits subdivided into a lower red alluvial package of the Talampaya and Tarjados formations and the overlying Agua de la Peña Group (Melchor, Reference Melchor2007; Gulbranson et al., Reference Gulbranson, Ciccioli, Montañez, Marenssi, Limarino, Schmitz and Davydov2015; Césari and Colombi, Reference Césari and Colombi2016). The Agua de la Peña Group comprises, from base to top, the Chañares, Ischichuca, Los Rastros, Ischigualasto, and Los Colorados formations.

The thickness of the Ischigualasto Formation varies between 300 m and 700 m (Currie et al., Reference Currie, Colombi, Tabor, Shipman and Montañez2009); nevertheless, the lack of intraformational unconformities indicates that deposition was relatively continuous (Milana and Alcober, Reference Milana and Alcober1994). The formation includes deposits of fluvial channels characterized by massive and trough cross-stratified sandstones and conglomerates intercalated with finer-grained proximal and distal floodplain facies, mainly sandstones and mudstones (Colombi and Parrish, Reference Colombi and Parrish2008; Currie et al., Reference Currie, Colombi, Tabor, Shipman and Montañez2009; Césari and Colombi, Reference Césari and Colombi2016; Fig. 2.1). The proximal floodplain deposits are composed of fine-grained sandstones and mudstones with horizontal and ripple lamination and are frequently disturbed by paleosol development; the distal floodplain facies consist of massive or finely laminated siltstones and mudstones that are always overprinted by mature paleosol development (Césari and Colombi, Reference Césari and Colombi2016). At the base of the formation there are many ash layers and a basalt (Colombi and Parrish, Reference Colombi and Parrish2008). The Ischigualasto Formation was subdivided into four members (Fig. 2.1), from base to top: La Peña, Cancha de Bochas, Valle de la Luna, and Quebrada de la Sal. These subdivisions are based on variations in the sedimentary architecture and paleosol development (Currie et al., Reference Currie, Colombi, Tabor, Shipman and Montañez2009).

Figure 2.

(1) General sedimentological section of the Ischigualasto Formation. (2) Detailed sedimentological section of the Valle de La Luna Member showing the stratigraphic position of the PFLs. Modified from Currie et al. (Reference Currie, Colombi, Tabor, Shipman and Montañez2009).

The Ischigualasto Formation is Carnian to Norian (231.4 to 225.9 Ma) according to radioisotopic information (Rogers et al., Reference Rogers, Swisher, Sereno, Monetta, Forster and Martínez1993; Martínez et al., Reference Martínez, Fernandez and Alcober2011). The Carnian Pluvial Event, also known as Carnian Humid Episode (Ruffell et al., Reference Ruffell, Simms and Wignall2016), was recognized in the underlying Los Rastros Formation at 234.47 ± 0.44 Ma (Dal Corso et al., Reference Dal Corso2020; Mancuso et al., Reference Mancuso, Benavente, Irmis and Mundil2020). The Carnian Pluvial Event was linked to a major extinction event and might have been the trigger to major diversifications and originations of conifers, insects, dinosaurs, crocodiles, lizards, turtles, and mammals on land (Dal Corso et al., Reference Dal Corso2020). The appearance and rapid diversification of dinosaurs in the overlying Ischigualasto Formation represent a major biotic event recorded in the basin (Martínez et al., Reference Martínez, Fernandez and Alcober2011, Reference Martínez, Apaldetti, Alcober, Colombi, Sereno, Fernandez, Santi Malnis, Correa and Abelin2012), corresponding with the Dinosaur Diversification Event (Benton et al., Reference Benton, Bernardi and Kinsella2018). Martínez et al. (Reference Martínez, Fernandez and Alcober2011, Reference Martínez, Apaldetti, Alcober, Colombi, Sereno, Fernandez, Santi Malnis, Correa and Abelin2012) characterized vertebrate biozones and recognized a crucial turnover between the Hyperodapedon–Exaeretodon–Herrerasaurus (HEH) and the Exaeretodon (E) biozones after the Carnian Pluvial Event and before the End-Triassic Extinction. Concerning the Ischigualasto Formation plant assemblages, including impressions, cuticles, and permineralized trunks, they were referred to as the BNP Biozone (Yabeiella brackebuschiana/Scytophyllum neuburgianum/Rhexoxylon piatnitzkyi; Spalletti et al., Reference Spalletti, Artabe, Morel and Brea1999) assigned to the Carnian (Morel et al., Reference Morel, Artabe and Spalletti2003).

Paleobotanical background

The first paleobotanical contribution about Ischigualasto Formation was made by Bodenbender (Reference Bodenbender1911), who indicated the presence of fossil leaves assigned to Thinnfeldia, Cladophlebis mesozoica (Kurtz) Frenguelli, 1947, and Podozamites elongatus (Morris) Feistmantel, Reference Feistmantel1889 and “Araucarites” fossil trunks. Afterward, Frenguelli (Reference Frenguelli1948) published the first detailed study on the stratigraphy, paleontology, and correlation of the Triassic succession that crops out at Ischigualasto Provincial Park. This author presented a list of fossil plants, mentioning the following taxa for the Ischigualasto Formation: Neocalamites carrerei (Zeiller) Halle, Reference Halle1908, Dicroidium lancifolium (Morris) Gothan, Reference Gothan1912, D. odontopteroides (Morris) Gothan, Reference Gothan1912, Desmiophyllum sp. (=Heidiphyllum elongatum (Morris) Retallack, Reference Retallack1981), and “Araucarites” fossil trunks.

In the 1960s, the paleobotanical studies were resumed by Archangelsky (Reference Archangelsky1960, Reference Archangelsky1968), Archangelsky and Brett (Reference Archangelsky and Brett1961, Reference Archangelsky and Brett.1963), Bonetti (Reference Bonetti1966), and Brett (Reference Brett1968), focusing on permineralized trunks and leaf cuticles. Their investigations described new species of great systematic, paleophytogeographic, and biostratigraphic importance (i.e., Rhexoxylon piatnitzkyi Archangelsky and Brett, Reference Archangelsky and Brett1961 emend. Brett, Reference Brett1968 [corystosperm], Michelilloa waltonii Archangelsky and Brett, Reference Archangelsky and Brett.1963 [cycad], and Protojuniperoxylon ischigualastense Bonetti, Reference Bonetti1966 emend. Bodnar and Artabe, Reference Bodnar and Artabe2007 [conifer]) and proposed for the first time that the Dicroidium leaves and Rhexoxylon stems corresponded to the same plant. Petriella (Reference Petriella1978) followed this idea in his whole-plant reconstruction of the tree corystosperm dominant in the Ischigualasto Formation. Baldoni (Reference Baldoni1980) studied leaves with preserved cuticles from that unit and confirmed the occurrence of the corystosperm species Xylopteris argentina (Kurtz) Frenguelli, Reference Frenguelli1943 and X. elongata (Carruthers) Frenguelli, Reference Frenguelli1943.

In addition to the megafloras, the palynofloras of the Ischigualasto-Villa Unión Basin have been studied since the 1970s. Herbst (Reference Herbst1970, Reference Herbst1971) stated that in the Ischigualasto Provincial Park area, well-preserved palynomorphs occur only in the Los Rastros Formation. However, in the Cerro Bola area at La Rioja Province, Yrigoyen and Stover (1970) recognized a palynological assemblage at the base of the Ischigualasto Formation composed of trilete spores and bisaccate, monosulcate pollen grains. In the same stratigraphic level, these authors identified poorly preserved impressions of Podozamites, Yabeiella, Pterophyllum, and Phyllotheca. From this information, Yrigoyen and Stover (Reference Yrigoyen and Stover1970) correlated the lower section of the Ischigualasto Formation with the Quebrada de la Mina Formation and the upper section with the Carrizal Formation; both units crop out at the Marayes-El Carrizal Basin located at the southeast of Ischigualasto-Villa Unión Basin.

Zamuner (Reference Zamuner1992) completed a doctoral thesis on a paleofloristic assemblage from the Ischigualasto Formation at Ischigualasto Provincial Park, in which she analyzed in detail the anatomy and development of Rhexoxylon piatnitzky trunks and rhizomes, peltasperm leaves with preserved cuticles (Lepidopteris stormbergensis (Seward) Townrow, Reference Townrow1956 and Scytophyllum neuburgianum Dobruskina, Reference Dobruskina1969), and conifer wood assigned to a new species of Araucarioxylon. However, only the studies on Scytophyllum neuburgianum were published (Zamuner and Artabe, Reference Zamuner and Artabe1990), while the other results remain unpublished. Later, Zamuner et al. (Reference Zamuner, Zavattieri, Artabe, Morel, Artabe, Morel and Zamuner2001) and Artabe et al. (Reference Artabe, Morel, Spalletti, Artabe, Morel and Zamuner2001) summarized the paleobotanical content and interpreted the preserved paleocommunities of the Ischigualasto Formation, respectively. Bodnar and Artabe (Reference Bodnar and Artabe2007) revised the anatomy of the fossil wood Protojuniperoxylon ischigualastense and analyzed the growth rings, inferring that the species was an evergreen cupressaceous conifer.

Subsequent contributions included taphonomic, paleobiogeographic, and paleoclimatic analyses. The taphonomic characterization of the Ischigualasto Formation paleoflora was carried out by Colombi and Parrish (Reference Colombi and Parrish2008), who defined nine preservation styles (including root impressions, silicified trunks and stumps, cuticles, and palynomorphs) and seven taphofacies. Césari and Colombi (Reference Césari and Colombi2013, Reference Césari and Colombi2016) reported the first Argentinian occurrence of a rich palynofloral assemblage with European Tethys and Northwestern Australia characteristic species (Onslow Microflora) and a dominance of species indicative of humid climatic conditions. Later, Pérez Loinaze et al. (Reference Pérez Loinaze, Vera, Fiorelli and Desojo2018) conducted a cluster analysis of palynological assemblages from Gondwana (including the Chañares, Ischigualasto, and Los Rastros formations), which recovered the Ischigualasto Formation palynoflora as transitional between Ipswich and Onslow subprovinces (Pérez Loinaze et al., Reference Pérez Loinaze, Vera, Fiorelli and Desojo2018). The authors inferred a warm climatic belt that resulted in mixed Onslow–Ipswich microfloras for the Chañares and Ischigualasto formations, while the Los Rastros Formation would correspond to a pure Ipswich microflora of cooler and wetter conditions (Pérez Loinaze et al., Reference Pérez Loinaze, Vera, Fiorelli and Desojo2018).

Systematic paleontology

Type species

Agathoxylon cordaianum Hartig, Reference Hartig1848, p. 188, by simultaneous publication of a single species name. Triassic, Coburg, Germany.

Agathoxylon argentinum new species
Figures 3–6

Reference Zamuner1992

Araucarioxylon argentinum Zamuner, p. 57, pl. 24, figs. 1–7.

Reference Kokogián, Spalletti, Morel, Artabe, Martínez, Alcober, Milana, Zavattieri and Papú1999

Araucarioxylon argentinum; Kokogian et al., p. 395.

Reference Spalletti, Artabe, Morel and Brea1999

Araucarioxylon argentinum; Spalletti et al., p. 448.

Reference Zamuner, Zavattieri, Artabe, Morel, Artabe, Morel and Zamuner2001

Araucarioxylon sp. A, Zamuner et al., p. 184.

Reference Morel, Artabe, Zavattieri, Bonaparte, Artabe, Morel and Zamuner2001

Araucarioxylon sp. A, Morel et al., p. 238.

Reference Artabe and Zamuner2007

Araucarioxylon sp. A, Artabe et al., p. 82.

Reference Césari and Colombi2016

Araucarioxylon sp., Césari and Colombi, p. 380.

Figure 4.

Agathoxylon argentinum n. sp. (1) General view of the transverse section of the secondary xylem, holotype LPpm 1367. (2) Detail of the growth-ring boundary in transverse section, holotype LPpm 1367. (3) Detail of transverse section of tracheids, paratype LPpm 1375. (4, 5) Tracheid radial pitting, with uniseriate to triseriate, contiguous to compressed and alternating pattern; (4) holotype LPpm 1370; (5) paratype LPpm 1376. (6) Detail of the tracheid radial pitting and the resin plate in a tracheid (arrow), holotype LPpm 1180. (7) Detail of tracheids with truncate terminal walls, holotype LPpm 1181. (8) General view of radial section of the secondary xylem, paratype LPpm 1376. (1) Scale bar = 650 μm; (2) scale bar = 250 μm; (3–5) scale bars = 120 μm; (6) scale bar = 60 μm; (7) scale bar = 70 μm; (8) scale bar = 200 μm.

Figure 5.

Agathoxylon argentinum n. sp. (1) General view of the radial section of the secondary xylem showing the ray parenchyma cell types, the cross fields, and resin contents in ray cells, holotype LPpm 1370. (2, 3) Detail of araucarioid cross-field pitting; (2) paratype LPpm 1376; (3) holotype LPpm 1370. (4) General view of tangential section showing the mainly uniseriate rays and occasional biseriate in part rays, paratype LPpm 1377. (5, 6) Detail of rays and tracheid with spiral fissures and resin plates (arrows), holotype LPpm 1372. (7) Leaf trace in tangential section, holotype LPpm 1182. (8) Detail of leaf trace; arrows indicate tracheids. (9) Leaf trace in transverse section; arrows indicate tracheids, LPpm 1380. (1) Scale bar = 100 μm; (2) scale bar = 60 μm; (3) scale bar = 50 μm; (4) scale bar = 450 μm; (5, 6, 8, 9) scale bars = 250 μm; (7) scale bar = 350 μm.

Figure 6.

Agathoxylon argentinum n. sp., scanning electron microscope images LPPB 492. (1) General view of the tracheid radial pitting. (2) Detail of biseriate, compressed, and alternating tracheids pits. (3) Detail of uniseriate, compressed, and alternating tracheids pits. (4) Detail of a cross field. (5) Detail of a uniseriate ray in tangential section. (6) Spiral fissures in tracheid tangential walls. (1, 5, 6) Scale bars = 50 μm; (2) scale bar = 60 μm; (3) scale bar = 40 μm; (4) scale bar = 30 μm.

Remarks

Fossil woods with the characters observed in the studied specimens, known as “Araucaria-like anatomy” (i.e., pycnoxylic secondary xylem, araucarian tracheid radial pitting, and araucarioid cross fields), have been assigned to three commonly known genera, Araucarioxylon Kraus, Reference Kraus and Schimper1870, Dadoxylon Endlicher, Reference Endlicher1847, and Agathoxylon Hartig, Reference Hartig1848. Philippe (Reference Philippe1993, Reference Philippe2011) and Bamford and Philippe (Reference Bamford and Philippe2001) pointed out that Araucarioxylon and Dadoxylon are both illegitimate and superfluous synonyms of Pinites Lindley and Hutton, Reference Lindley and Hutton1831, whereas Agathoxylon is a legitimate name having priority over the two other genera. Later, Rößler et al. (Reference Rößler, Philippe, van Konijnenburg-van Cittert, McLoughlin, Sakala and Zijlstra2014) argued for the use of Agathoxylon for fossil woods with Araucaria-like anatomy. Here we treat Agathoxylon as the appropriate genus for this type of fossil wood.

Agathoxylon is a widely distributed wood genus according to a brief list of characteristics that include araucarian tracheid pitting, araucarioid cross-field pits, axial parenchyma present or absent, and mainly uniseriate rays and smooth walls in the parenchyma cells (Hartig, Reference Hartig1848; Philippe, Reference Philippe1995). The studied fossil woods were compared with species of Araucaria-like tracheidoxyls of Mesozoic age from Gondwana to assess their identity (see Table 2). The wood anatomy of Ischigualasto specimens differs from the species of Agathoxylon previously recorded in the Argentinean Triassic—that is, A. amraparense (Sah and Jain, Reference Sah and Jain1964) Crisafulli and Herbst, Reference Crisafulli and Herbst2011, A. cozzoi Gnaedinger and Zavattieri, Reference Gnaedinger and Zavattieri2020, A. dallonii (Boureau, Reference Boureau1948) Crisafulli and Herbst, Reference Crisafulli and Herbst2010, A. lamaibandianus Crisafulli and Herbst, Reference Crisafulli and Herbst2011, and A. protoaraucana (Brea, Reference Brea1997) Gnaedinger and Herbst, Reference Gnaedinger and Herbst2009—because of the presence of resin plates (see Table 2). Among other Mesozoic species, the here described tracheidoxyls show more similarities with Agathoxylon pseudoparenchymatosum (Gothan) Pujana et al., Reference Pujana, Santillana and Marenssi2014, and A. santalense (Sah and Jain, Reference Sah and Jain1964) because of the cupressoid type of individual cross-field pits; the absence of tangential tracheid pitting, septate tracheids, and axial parenchyma; and the presence of resin plates and occasional partially biseriate rays. However, the Ischigualasto fossil wood differs from these species due to the tracheid transverse section (both polygonal and circular in the studied specimens and exclusively polygonal in the other species), the presence of occasional subopposite and spaced tracheid radial pitting in the studied material, and the higher rays (up to 27 cells high in the studied material and up to 10 cells high in the other species), and the number of pits per cross field (up to nine in the Ischigualasto wood and up to six in A. pseudoparenchymatosum and A. santalense) (Table 2). Another difference with A. pseudoparenchymatosum is the presence of triseriate tracheid radial pitting in the studied material. Agathoxylon santalense also can be differentiated from the Ischigualasto wood by the indistinct growth rings in Agathoxylon santalense (Table 2). All these characteristics have taxonomic value in extant conifer woods (García Esteban et al., Reference García Esteban, de Palacios, Guindeo Casasús and García Fernández2004) and are used to delimit Agathoxylon species (see Del Fueyo et al., Reference Del Fueyo, Carrizo, Poiré and Lafuente Díaz2021). Because of these differences, we proposed a new specific taxon for the Ischigualasto material.

Table 2.

Comparison among main Mesozoic Agathoxylon species recorded in Gondwana. SGD = stratigraphic and geographic distribution; GR = growth rings; TEL = transition from earlywood to latewood; TS = tracheid section; TTD = tangential diameter of earlywood tracheids (mean value or range when the mean value was not provided); TRPSE = seriation of tracheid radial pitting; TRPA = arrangement of (two or more seriate) tracheid pitting in radial walls; TRPSP = spacing of tracheid radial pitting; TRPD = diameter of tracheid radial pits (mean values or range when the mean value was not provided); TTP = tracheid tangential pitting; TRP = tracheids with resin plates or plugs; ST = septate tracheids; AP = axial parenchyma; RW = ray width; RH = ray height; CP = type of individual cross-field pits; CA = arrangement of cross-field pits; CPN = cross-field pit number; CPD = cross-field pit diameter (mean values or range when the mean value was not provided). Between () characters which appear less frequently. *Information not provided in the species protologue and taken from a subsequent contribution. ¹We used the combination Agathoxylon antarticum (Poole and Cantrill, Reference Poole and Cantrill2001) Pujana et al., Reference Pujana, Santillana and Marenssi2014, but we follow the original diagnosis of the basionym provided by Poole and Cantrill (Reference Poole and Cantrill2001). ² The presence of resin plates is not mentioned in the diagnosis or in the description of Poole and Cantrill (Reference Poole and Cantrill2001), but some probable resin plates are observed in the photos (Poole and Cantrill, Reference Poole and Cantrill2001, pl. 1, fig, 8). ³Agathoxylon dallonni was previously cited in the Upper Triassic of Argentina by Crisafulli and Herbst (Reference Crisafulli and Herbst2010), but the specimens were reassigned to Agathoxylon lamaibandianus by Gnaedinger and Zavattieri (Reference Gnaedinger and Zavattieri2020). ⁴In the original diagnosis of Boureau (Reference Boureau1948), the cross-field pits are described as simple and in the photos seems to be bordered (Boureau, Reference Boureau1948, pl. 1, fig. 5). ⁵Although Pujana et al. (Reference Pujana, Santillana and Marenssi2014) proposed that Agathoxylon matildense is a junior synonym of Agathoxylon antarticum, we considered that the protologues of both species show enough differences to segregate them (i.e., tracheid section, the ray height, and the number of pits per cross field, see Poole and Cantrill, Reference Poole and Cantrill2001, p. 1086, pl. 1, 2–10; Zamuner and Falaschi, Reference Zamuner and Falaschi2005, p. 340, fig. 2).

Evidence of fungal activity

Structures related to the degradation of wood by fungi are preserved. It is common to observe resinous contents in the ray cells (Fig. 7.1, 7.2, 7.4). Two types of degradation patterns can be recognized: in some areas of the wood, differential degradation of cellulose and hemicellulose is observed, while lignin is at least partially preserved (Fig. 7.1, 7.2). This pattern is frequently associated with the boundary between earlywood and latewood (Fig. 7.1, 7.2) and is called brown rot (Schwarze et al., Reference Schwarze, Mattheck and Engels2004; Schmidt, Reference Schmidt2006; Schwarze, Reference Schwarze2007). The differential degradation of the polysaccharide layers of the lignified ones can also be observed by the detachment of the S3 layer, which folds toward the lumen of the tracheids (Fig. 7.3). Alternatively, the S3 layer can appear strongly thickened.

Figure 7.

Structures related to the degradation of wood by fungi recognized in Agathoxylon argentinum n. sp. (1, 2) Transverse section showing resinous contents in the ray cells and tracheid walls with differential degradation of cellulose and hemicellulose and detached and strongly thickened S3 layer, LPpm 1380. (3) Collapsed tracheids in transverse section with detached S3 layer folded toward the lumen, LPpm 1380. (4) Complete degradation of the cell wall with small groups of spherical structures adhered to the cell wall (white arrow), LPpm 1380. (5) Tracheid walls with detached and strongly thickened S3 layer; white arrow indicates isolated spherical structures, LPpm 1379. (6) Isolated spherical structure adhered to a ray parenchyma cell; white arrow indicates a probable process for anchoring, LPpm 1379. (1) Scale bar = 70 μm; (2) scale bar = 50 μm; (3, 4) scale bars = 80 μm; (5) scale bar = 50 μm; (6) scale bar = 30 μm.

Figure 8.

(1–10) Mean radial cell diameter (μm) curves (a and orange line) and CSDM curves (b and blue line) in Agathoxylon argentinum n. sp. from the measured ring increments 1 to 10. Vertical dotted gray line shows the zenith of each CSDM curve.

In turn, areas with completely degraded secondary xylem cells were observed, which is evidenced as empty spaces, sometimes with associated resinous contents (Fig. 7.4). Complete degradation of the cell wall is called white pocket rot (Schwarze et al., Reference Schwarze, Mattheck and Engels2004; Schmidt, Reference Schmidt2006; Schwarze, Reference Schwarze2007).

Although the presence of hyphae was not recognized, structures 3–6 μm in diameter that adhere to the cell wall can be observed in both areas affected by white rot and areas affected by brown rot (Fig. 7.4–7.6). These structures are approximately spherical and appear isolated (Fig. 7.5, 7.6) or in small groups (Fig. 7.4). The walls of these spheres are smooth, and they are surrounded by a thick translucent outer wall. The preservation of the samples did not allow us to observe the presence of openings or discharge papillae, although some of these spheres seem to present some type of process for anchoring (Fig. 7.6).

Growth-ring analysis

In total, 10 ring increments were included in this analysis. The results are summarized in Table 3 and Figure 8. Except for one curve, left-skewed CSDM curves correspond to ring increments smaller than 40 cells in width, while right-skewed curves belong to ring increments greater than 50 cells in width. The percentage skews of the CSDM curve are highly variable, from −27% to +75.6% (mean = +11.5). The percentage diminution ranges from 56.4% to 76.3% (mean = 62.9%). The percentage of latewood varies between 5.4% and 33.3% (mean = 22%). The RMI value varies from 3.2% to 25.4% (mean = 14.1%).

Table 3.

Results of growth-ring analysis in Agathoxylon argentinum n. sp. using the Falcon-Lang (Reference Falcon-Lang2000a, Reference Falcon-Langb) method.

Composition of plant fossil assemblages

Fourteen species of plants are present in the Valle de la Luna Member of the Ischigualasto Formation as megafossils (Table 1). These include sphenophytes, corystosperms, and peltasperms and voltzialean, cupressaceous, and araucariaceous conifers. Other groups frequent in the Argentinean Triassic are not present, such as pleuromeialean lycophytes, ferns, petriellaleans, ginkgoaleans, and gnetaleans. The PFLs have low diversity, with one to six species in each one. The level PFL 1 is composed of only corystosperms, while levels PFL 2 and PFL 3 comprise both corystosperms and peltasperms. In PFL 4, cupressaceous conifers are recorded together with fossils of corystosperms and peltasperms. The level PFL 5 differs from the others since it bears exclusively sphenophyte remains. Corystosperms, cycads, and cupressaceous conifers occur in the level PFL 6, whereas only corystosperms and cupressaceous conifers continue at the level PFL 7. Finally, the PFL 8 consists of taxa that are not recorded in the previous levels (i.e., the corystosperm species Dicroidium lancifolium and D. odontopteroides and voltzialean, cupressaceous, and araucariaceous conifers) together with the sphenophyte Neocalamites carrerei, also present in PFL 5.